Peripheral Nerve Flashcards

Question

In a patient undergoing evaluation for carpal tunnel syndrome, individual muscle testing is most likely to disclose weakness of which of the following muscles? Answers: A. Flexor Pollicis Longus B. Adductor Pollicis C. Pronator Teres D. Abductor Digiti Minimi E. Abductor Pollicis Brevis

Answer 1

Abductor Pollicis Brevis ## Footnote The median nerve traverses the carpal tunnel, along with four tendons of the flexor digitorum superficialis, four tendons of the flexor digitorum profundus, and the tendon of the flexor pollicis longus. The roof of the carpal tunnel is the transverse carpal ligament, which attaches to the scaphoid and trapezium on the radial side and the pisiform and hamate on the ulnar side. Compression of the median nerve can occur due to thickening of the transverse carpal ligament, tenosynovitis, or related to a space occupying lesion in the carpal tunnel. The ulnar nerve innervates all of the hand intrinsic muscles, except for the LOAF muscles —lumbricals 1/2, opponens pollicis, abductor pollicis brevis, and flexor pollicis brevis—which are innervated by the median nerve. The median nerve sensory distribution is the palmar aspect of the radial 3.5 digits and the corresponding palm. The palm is innervated by the palmar cutaneous branch, which arises proximal to the carpal tunnel and does not pass through the carpal tunnel. Carpal tunnel syndrome presents with numbness in the radial 3.5 digits (sparing the palm), paresthesias, and/or weakness of the LOAF muscles. Pain and paresthesias often have nocturnal worsening and patients frequently describe shaking out their hand to relieve the symptoms. The median-innervated forearm muscles include the pronator teres, flexor carpi radialis, flexor digitorum superficialis, and the anterior interosseous-innervated muscles, including the flexor digitorum profundus, flexor pollicis longus, and pronator quadratus. The median-innervated forearm muscles are spared in carpal tunnel syndrome. On examination, provocative maneuvers that may prove useful in diagnosing carpal tunnel syndrome include the Tinel sign, Durkan compression test, Phalen maneuver, and reverse Phalen maneuver. Ancillary studies including electrodiagnostics and ultrasound can aid in diagnosis. Treatment options typically include bracing, carpal tunnel steroid injections, and surgical carpal tunnel release. The abductor digiti minimi and adductor pollicis are innervated by the ulnar nerve, so are not involved in carpal tunnel syndrome. The pronator teres and flexor pollicis longus are innervated by the median nerve, but proximal to the carpal tunnel, so they are not involved in carpal tunnel syndrome.

Answer 2

Rhomboids ## Footnote The rhomboid muscles (rhomboid major and minor) are innervated by the dorsal scapular nerve, which arises proximally from the C5 nerve root. In addition to the rhomboid major and minor, the dorsal scapular nerve also innervates the levator scapulae. The dorsal scapular nerve arises at the level of the nerve roots from C5, passing through the substance of the middle scalene muscle. The C5 root also gives contributions to the phrenic nerve and then the long thoracic nerve, also formed within the substance of the middle scalene. Distal to these branches, C5 and C6 join to form the upper trunk of the brachial plexus. The upper trunk then trifurcates into the suprascapular nerve, posterior division, and anterior division of the upper trunk, though this can be a bifurcation with the suprascapular nerve arising from the posterior division. Due to the very proximal location of the dorsal scapular nerve origin, in the setting of a brachial plexus injury and absent rhomboid function, one should suspect a very proximal injury, possibly pre-ganglionic. The rhomboids begin on the lower cervical and upper thoracic spinous processes and insert onto the medial border of the scapula. They function to stabilize the scapula during arm movement. The differential diagnosis for a winged scapula includes a dorsal scapular neuropathy with weakness of the rhomboid muscles, a spinal accessory neuropathy with weakness of the trapezius, and a long thoracic nerve palsy with weakness of the serratus anterior. A long thoracic nerve palsy causes medial winging, whereas dorsal scapular neuropathy and spinal accessory neuropathy cause lateral winging. The winging associated with a dorsal scapular neuropathy is typically very subtle. The serratus anterior muscle is innervated by the long thoracic nerve (C5-7). The infraspinatus muscle is innervated by the suprascapular nerve (C5-6). The subscapularis muscle is innervated by the upper and lower subscapular nerves (C5-6). The serratus posterior muscle is innervated by intercostal nerves (T2-5 for serratus posterior superior and T9-12 for serratus posterior inferior).

Answer 3

Wrist Extension ## Footnote Radial nerve injury associated with a humeral shaft fracture is a characteristic injury pattern and would cause weakness in wrist and finger extension. The radial nerve is formed from the posterior divisions of the brachial plexus (C5-T1) and is one of two terminal branches of the posterior cord, the other being the axillary nerve. The radial nerve runs behind the brachial artery and along the posterior border of the axilla. In the arm, it passes between the long head of the triceps and the shaft of the humerus, beneath the teres major muscle (triangular interval). At the mid-humeral level, the nerve crosses the posterior aspect of the humerus moving from medial to lateral, lying in the spiral groove and traveling with the profunda brachii artery. The radial nerve innervates the triceps brachii very proximally and then distal to the spiral groove innervates the brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis. In the proximal forearm, the nerve bifurcates into the superficial radial nerve (sensory) and the deep radial/posterior interosseous nerve (PIN). The posterior interosseous nerve innervates the supinator, extensor carpi ulnaris, extensor digitorum communis, extensor digiti minimi, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor indicis. Potential compression points include the radial nerve at the spiral groove by the lateral intermuscular septum, the posterior interosseous nerve at the supinator/Arcade of Frohse, and the superficial radial nerve between the brachioradialis and extensor carpi radialis longus tendons. Due to the course of the radial nerve in close association with the humerus, the radial nerve is particularly prone to injury in association with mid-shaft humeral fractures. Clinically, proximal radial nerve injuries are characterized by weakness or loss of elbow extension, wrist extension, and finger extension. The triceps branches arise very proximally such that elbow extension is preserved with injuries to the radial nerve in the mid-arm. A posterior interosseous palsy is characterized by loss of finger extension, with preserved wrist extension but radial deviation during wrist extension. This occurs due to the preservation of function in the extensor carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the extensor carpi ulnaris (innervated by the posterior interosseous nerve). While humeral shaft fractures account for only approximately 3% of all fractures, these have a high association with radial nerve palsies. This incidence has been noted to be anywhere between 7 and 17% of all humeral shaft fractures, which makes this the most common nerve injury associated with long bone fractures. There is debate regarding timing of surgical exploration. Traditionally, radial nerve injuries associated with closed humeral fractures have been treated expectantly, with ~70% spontaneous recovery. When combined with those patients who underwent delayed surgical exploration due to lack of recovery, the overall recovery rate rose to ~88%. In this series, the recovery rate when early exploration was utilized was ~88%, suggesting that early exploration and delayed exploration for those that do not spontaneously recover are equivalent. However, in some more updated series, there has been some suggestion that early exploration may be better. In a larger, more recent series, expectant management was noted to yield a 77% recovery rate, while early explorations (<3 weeks from injury) yielded a 90% recovery rate. Thus, more recent data have suggested that early exploration may provide more benefit than expectant management in radial nerve injury associated with humerus fractures. Regardless, a good outcome can be expected in the majority of patients with radial nerve injuries associated with humeral fractures. When examining for spontaneous recovery, the branch to the brachioradialis is the first branch distal to the spiral groove. Thus, the brachioradialis is the muscle that is expected to show recovery first and is critical to examine. Finger and wrist flexion weakness occur primarily with median injuries, though the ulnar nerve contributes to these movements as well. Finger abduction and adduction are innervated by the ulnar nerve. Elbow extension is mediated by the triceps brachii muscle, which is innervated by the radial nerve. However, the branches to the triceps brachii occur proximal to the spiral groove, so the triceps brachii is typically spared with injury to the radial nerve associated with mid-shaft humeral fractures.

Answer 4

Normal Sensory Nerve Action Potential, Normal Motor Nerve Action Potential, Absent Voluntary Motor Unit Action Potentials, Absent Fibrillation Potentials ## Footnote When axons are disrupted, the distal axon degenerates through a process called Wallerian degeneration. This degenerative process occurs in an anterograde fashion distal to the site of injury. Within hours of the injury, axonal and myelin fragmentation occurs with breakdown of its cytoskeletal components and subsequent loss of conductivity by 2-4 days postinjury, and the terminal, injured, axonal end sealing itself off. Due to this process taking several days, immediately after the injury action potentials can continue to be transmitted in the injured axon resulting in no detectable electrophysiological abnormality. For mixed nerves, this means that a motor action potential and sensory action potential can be detected and will be normal for several days after a nerve transection, assuming the stimulating and recording electrodes are both placed proximal to the injury or distal to the injury and not across the injury. Despite the motor action potential being normal, the action potential cannot cross the site of transection, leading to loss of voluntary motor unit action potentials, which occurs immediately at the time of the injury. Schwann cells respond to the injury immediately by proliferating and upregulating gene expression essential to their function in Wallerian degeneration. Macrophages, having migrated into the zone of injury from nearby blood vessels, phagocytose the axonal and myelin debris preparing it for axonal regeneration. With the macrophages, other inflammatory cells also gain entry into the site and begin phagocytosing the debris, which can take several weeks to several months postinjury depending on the severity of the injury. Wallerian degeneration of the distal process continues over the subsequent weeks to months. When injuries are more severe, the inflammatory response is typically more vigorous with subsequent necrosis, cellular infiltration, and proliferation that correlates with the severity of the initial injury. Endoneurial vascular disruption also occurs resulting in hemorrhage, edema, and influx of neutrophils and fibroblasts. The latter ultimately results in intraneural scarring with collagen deposition. Positive sharp waves and fibrillation potentials typically take 10-14 days to develop, as the muscle membrane becomes unstable due to denervation when Wallerian degeneration reaches the neuromuscular junction. As mentioned above, the process of Wallerian degeneration typically takes weeks to months. Because of this, in the event of an intraoperative injury, stimulating the nerve distal to the site of injury will still result in the detection and recording of action potentials due to the axons still being in continuity and conducting the action potentials toward the recording electrode. In the same vein, performing nerve conduction studies distal to the site of injury, including compound motor action potentials (CMAPs) and sensory nerve action potentials (SNAPs), immediately after injury may not detect any abnormality due to the fact that the axons conducting these action potentials have not yet degenerated through Wallerian degeneration, assuming both the stimulating electrode and recording electrode are distal to the site of injury. On the other hand, if the stimulating electrode and the recording electrode are on opposite sides of the transected nerve, then no action potentials will be recorded. In order for both SNAPs and CMAPs to be recorded, integrity of the lower motor neuron, specifically the distal axonal segment, must be preserved. As such, both SNAP and CMAP recordings will be affected in the event of a post-ganglionic injury. However, SNAPs and CMAPs differ in the setting of a pre-ganglionic injury in that CMAPs will be affected, but SNAPs will be preserved due to the injury occurring proximal to the cell body and thus the integrity of the distal axonal segment being preserved.

Answer 5

Tibialis Anterior ## Footnote The deep peroneal nerve is a terminal branch of the common peroneal nerve. After the sciatic bifurcation, the common peroneal nerve (CPN) continues deep and medial to the biceps femoris muscle and tendon as it proceeds distally into the leg. It courses around the neck of the fibula where it is usually quite superficial and as a result, susceptible to injury. The common peroneal nerve trifurcates into the superficial peroneal nerve, deep peroneal nerve, and the articular branch to the superior tibiofibular joint around the fibular tunnel inlet at the fibular neck. At the fibular tunnel inlet, the common peroneal nerve can be compressed by the deep fascia of the peroneus longus. The superficial peroneal nerve continues in the lateral compartment of the leg (most commonly) between the peroneus longus and brevis, supplying sensory innervation to the anterolateral leg and dorsal aspect of the foot (with the exception of a small area in the first web space between digits 1 and 2), as well as motor innervation to the peroneus longus and brevis for foot eversion. The deep peroneal nerve pierces the peroneus longus muscle and courses under the extensor digitorum longus to run between the tibialis anterior and extensor digitorum longus in the superior portion of the leg and between the tibialis anterior and extensor hallucis longus in the inferior leg. As it moves distally through the anterior compartment of the leg on the anterior surface of the interosseous membrane, it travels adjacent to the anterior tibial artery to the dorsal foot. Along this route, it supplies motor innervation to the tibialis anterior, extensor digitorum longus, extensor hallucis longus, extensor digitorum brevis, extensor hallucis brevis, and peroneus tertius to enable ankle dorsiflexion and toe extension. It also supplies cutaneous sensory innervation to a small area of skin in the first webspace (between digits 1 and 2). The common peroneal nerve is susceptible to compression at the fibular tunnel inlet by the deep fascia of the peroneus longus. The superficial peroneal nerve can be compressed in the distal leg where it pierces the fascia to leave the lateral compartment (typically) and enter the subcutaneous tissue. The deep peroneal nerve is susceptible to compression in the anterior tarsal tunnel by the inferior extensor retinaculum or in the mid-foot by the tendon of the extensor hallucis brevis. This should not be confused with what is commonly referred to as tarsal tunnel syndrome. Tarsal tunnel syndrome should be more appropriately referred to as posterior tarsal tunnel syndrome. Posterior tarsal tunnel syndrome involves compression of the tibial nerve by the flexor retinaculum. The common peroneal nerve is the nerve most susceptible to formation of an intraneural ganglion cyst. Synovial fluid from a degenerative superior tibiofibular joint travels along the articular branch into the common peroneal nerve to form the cyst. Due to the anatomic relationship at the trifurcation of the common peroneal nerve, peroneal intraneural ganglion cysts often preferentially affect the deep peroneal nerve over the superficial peroneal nerve. The deep peroneal nerve is anatomically adjacent to the articular branch to the superior tibiofibular joint, which is the conduit for formation of peroneal intraneural ganglion cysts. The soleus and tibialis posterior muscles are innervated by the tibial nerve. The short head of the biceps femoris is innervated by the peroneal division of the sciatic nerve in the thigh.

Answer 6

Perineurium ## Footnote Axons are bundled together into groups called fascicles. Fascicles are bundled together to form nerves. Axons can be myelinated or unmyelinated. Endoneurium is the intrafascicular connective tissue surrounding the axons. Perineurium is the connective tissue surrounding each fascicle. Epineurium surrounds the entire nerve. Two main peripheral nerve injury classification systems exist to describe the severity of nerve injury and are based on the severity of injury occurring to the axons and the connective tissue layers surrounding them. The first classification system was proposed by Seddon and consists of 3 grades: neurapraxia, axonotmesis, and neurotmesis. Neurapraxia is the mildest from of peripheral nerve injury and is strictly an injury to the myelin sheath, with preservation of the axon, endoneurium, perineurium, and epineurium. Axonotmesis is the next most severe grade of injury and results in demyelination, as well as injury to the axon. Axonotmesis has preservation of the epineurium, with variable degrees of injury to the endoneurium and perineurium. In contrast, neurotmesis is the most severe type of nerve injury in this schema and results in injury to both the axon and its surrounding connective tissue, including the epineurium, perineurium, and endoneurium. Subsequent to the Seddon classification being created, Sunderland elaborated on it by expanding and describing the various degrees of injury that can occur to the nerve connective tissue. The resultant Sunderland classification system consisted of 5 grades. Grade 1 injuries correspond to neurapraxic injuries, grade 5 to neurotmetic injuries, and grades 2-4 to axonotmetic injuries in the Seddon classification. Grades 2-4 are differentiated depending on the degree of connective tissue injury: Grade 2: axonal injury only, Grade 3: axonal injury with endoneurial injury, Grade 4: axonal injury with endoneurial and perineurial injury.

Answer 7

Opponens Pollicis ## Footnote In the upper extremity, as the median nerve travels from the anterior compartment of the forearm to the palmar hand, it passes through the carpal tunnel. The roof of the carpal tunnel is formed by the flexor retinaculum or transverse carpal ligament (fibrous ligament attaching from the pisiform and hamate medially to the trapezium and scaphoid laterally). The floor is made up of the scaphoid, lunate, and triquetral carpal bones. Along with the median nerve, there are 9 tendons that traverse the carpal tunnel, including the flexor digitorum superficialis and profundus tendons and the flexor pollicis longus tendon. As the median nerve approaches the carpal tunnel in the distal forearm, a palmar cutaneous branch emerges from it, piercing the deep antebrachial fascia to course superficial to the flexor retinaculum and supply cutaneous sensory innervation to the base and palmar aspect of the thenar eminence. Because this branch does not traverse the carpal tunnel, it is spared in carpal tunnel syndrome. The recurrent thenar branch of the median nerve arises under or just distal to the flexor retinaculum and winds around the distal border of the retinaculum to reach the thenar muscles and the lateral two lumbricals (lumbricals 1 and 2). All of the hand intrinsic muscles minus the LOAF muscles—lumbricals 1 and 2, opponens pollicis, abductor pollicis brevis, and flexor pollicis brevis—are innervated by the ulnar nerve, while the LOAF muscles are innervated by the median nerve. Thus, the LOAF muscles would be expected to be weak in carpal tunnel syndrome or with injury to the median nerve or recurrent thenar branch around the carpal tunnel. Once past the carpal tunnel, in addition to the aforementioned motor innervation, the median nerve supplies sensory innervation to the palmar aspect of digits 1-3 and lateral half of digit 4 by dividing into common palmar digital nerves and subsequently proper palmar digital nerves. The abductor pollicis longus is innervated by the radial nerve via the posterior interosseous nerve. The adductor pollicis is innervated by the ulnar nerve, specifically the deep branch of the ulnar nerve. The extensor pollicis longus is innervated by the radial nerve via the posterior interosseous nerve. The flexor pollicis longus is innervated by the median nerve via the anterior interosseous nerve in the forearm.

Answer 8

Parsonage-Turner Syndrome ## Footnote Parsonage-Turner syndrome, also referred to as idiopathic brachial plexopathy or neuralgic amyotrophy, classically presents with sudden onset of severe shoulder or periscapular pain that lasts from hours to a few weeks. Following resolution (or significant improvement) of the pain, weakness then begins and progresses over a few days to weeks. While Parsonage-Turner syndrome is poorly understood, it is thought to be an autoimmune inflammatory neuropathy. Often a potential precipitator can be identified in the patient history, including surgery, trauma, viral infection, vaccination, pregnancy, or significant stress. For unclear reasons, Parsonage-Turner syndrome does have a predilection for certain nerves. The most commonly involved nerves include the long thoracic nerve, suprascapular nerve, axillary nerve, musculocutaneous nerve, posterior interosseous nerve, and anterior interosseous nerve. The nerves are often affected in a patchy manner, displaying various degrees of involvement and sparing of nerves from the same root or plexus level. Electromyography can help identify muscles/nerves with subclinical involvement, which can help support the diagnosis. Oral steroids or immunotherapy have been recommended as a potential treatment early in the course of the disease (typically within the first week after onset) but remain controversial. Physical therapy remains the mainstay of treatment. The prognosis is good following a diagnosis of Parsonage-Turner syndrome, with most patients demonstrating functional recovery within 2-3 years. The differential diagnosis in this case would include suprascapular nerve entrapment at the suprascapular notch. Entrapment of the suprascapular nerve can present with weakness of shoulder abduction and external rotation, as well as pain, typically around the posterolateral shoulder. Athletes and laborers that engage in frequent overhead activity, such as volleyball, tennis, and swimming, are at risk for suprascapular nerve entrapment. Ganglion cysts, both extraneural and intraneural, can also cause suprascapular neuropathy. The typical presentation of polymyalgia rheumatica is widespread muscle aching and stiffness in the shoulders and hips, most often in the morning. Amyotrophic lateral sclerosis presents with both upper and lower motor neuron dysfunction including progressive asymmetric muscle weakness, fasciculations, increased reflexes, difficulty speaking, and swallowing difficulty. Hereditary Neuropathy with Liability to Pressure Palsies (HNPP) presents with recurrent episodes of mononeuropathies often with minor trauma or pressure mostly affecting the peroneal, ulnar, and median nerves. Charcot-Marie-Tooth disease is a hereditary polyneuropathy that often presents with weakness of the lower extremities, foot deformities, and progressive gait difficulties and, later in the course, can involve weakness of the upper extremities.

Answer 9

Plantar flexion ## Footnote The sciatic nerve has nerve root contributions originating from L4-S3. It comprises tibial (medial) and common peroneal (lateral) divisions. After exiting from the pelvis through the greater sciatic foramen, the sciatic nerve divides in the posterior thigh a variable distance from the popliteal fossa into the tibial nerve and common peroneal nerve. The tibial division of the sciatic nerve innervates the majority of the hamstring muscles (semitendinosus, semimembranosus, and long head of the biceps femoris) in the posterior thigh, with the exception of the short head of the biceps femoris (supplied by the peroneal division of the sciatic nerve). After the sciatic bifurcation, the tibial nerve then continues inferiorly through the middle of the popliteal fossa, coursing deep to the two heads of the gastrocnemius muscle. It then passes under the fibrous/tendinous arch of the soleus muscle, known as the soleal sling. The soleal sling is a potential point of compression/entrapment for the tibial nerve. As the tibial nerve courses distally, it supplies motor innervation to the gastrocnemius muscle, soleus muscle, and plantaris muscle in the proximal leg. Typically, just proximal to the popliteal fossa, the tibial nerve gives of the medial sural cutaneous nerve. The common peroneal nerve gives off the lateral sural cutaneous nerve. Though there is significant sural nerve variability, the most common configuration is for the lateral sural cutaneous nerve to give off a sural communicating branch that joins the medial sural cutaneous nerve in leg to form the sural nerve proper. Once deep to the soleus, the tibial nerve continues posterior to the tibia between the flexor digitorum longus (medially) and flexor hallucis longus (laterally), both of which it supplies in addition to the tibialis posterior muscle. At the level of the ankle, the tibial nerve passes between the medial malleolus and the Achilles tendon through the posterior tarsal tunnel. The posterior tarsal tunnel is covered superficially (roof) by the flexor retinaculum which attaches to the medial malleolus and calcaneal tuberosity. In analogous fashion to the carpal tunnel being a site of entrapment for the median nerve in the upper extremity, the posterior tarsal tunnel with its overlying flexor retinaculum can also serve as a site of entrapment for the tibial nerve in the lower extremity. In proximity to or within this tunnel, the tibial nerve terminates into the medial plantar, lateral plantar, and calcaneal nerves, which supply cutaneous innervation to the medial, lateral, and calcaneal portion of the plantar foot surface, respectively. Motor innervation to the foot intrinsic muscles also arises from the medial (abductor hallucis, flexor digitorum brevis, and flexor hallucis brevis) and lateral (adductor hallucis, flexor digiti minimi, abductor digiti minimi, interossei) plantar nerves. The lateral plantar, medial plantar, and calcaneal nerves also individually run through fibrous tunnels to reach the plantar surface of the foot. Each of these nerves is subject to possible compression within its individual fibrous tunnel. Compression of the tibial nerve or injury to the tibial nerve at the level of the popliteal fossa would result in weakness of plantar flexion, inversion, toe flexion, toe abduction, and toe adduction. Knee flexion is mediated by the hamstring muscles, which are innervated by the tibial division of the sciatic nerve predominantly (the short head of the biceps is innervated by the peroneal division of the sciatic), with the motor branches arising in the thigh proximal to the popliteal fossa. Knee extension is mediated by the quadriceps muscles, which are innervated by the femoral nerve. Dorsiflexion is mediated by the tibialis anterior, which is innervated by the common peroneal nerve via the deep peroneal nerve. Eversion is mediated by the peroneus longus and brevis, which are innervated by the common peroneal nerve via the superficial peroneal nerve.

Answer 10

A delta and C fibers ## Footnote Sensory nerve fibers can be classified into several different types depending on the type of sensory receptors activated and stimulus information they convey. A alpha fibers (type 1a and 1b fibers) convey sensory information from proprioceptors regarding muscle and joint position, speed, and force of contraction from muscle spindles and Golgi tendon organs. A beta fibers (type 2 fibers) convey information from mechanoreceptors regarding touch and pressure sensation. A delta fibers (type 3 fibers) convey information from mechanoreceptors, nociceptors, and thermoreceptors regarding light touch, well localized/sharp pain, and temperature sensation. C fibers (type 4 fibers) convey information from nociceptors and thermoreceptors regarding pain and temperature sensation that is poorly localized, and dull/aching pain. A alpha and A beta fibers are large diameter, myelinated fibers with fast conduction velocity. A delta fibers are smaller diameter, with thinly myelinated axons and slower conduction velocities. C fibers are small, unmyelinated axons and have the slowest conduction velocity. Upon stimulation beyond their depolarization and action potential threshold, the A delta and C fibers will convey a pain stimulus impulse from the periphery to their primary sensory cell body in the dorsal root ganglion. From there, they synapse with second order neurons in the Rexed laminae of the spinal cord. Projections from the second order neurons then decussate to the contralateral side of the spinal cord within a few segments through the anterior white commissure and ascend in the lateral spinothalamic tract to the ventral posterolateral (VPL) nucleus of the thalamus (note that some fibers convey these pain impulses through the ventral spinothalamic tract and to various areas of the brain stem, the details of which are beyond the scope of this description). The cell bodies of the third order neurons are within the VPL of the thalamus. These third order neurons project via the posterior limb of the internal capsule to the primary somatosensory cortex in the post-central gyrus.

Answer 11

Lateral ## Footnote Disc herniations occur as a result of disc degeneration and desiccation, which may lead to herniation of the nucleus pulposus. The pattern of symptoms depends on the anatomic location within the spinal column, but also where the disc herniation is located relative to the spinal canal. In the lumbar spine, the exiting nerve roots are named with the corresponding vertebral level. For example, the L3 nerve root passes under the L3 pedicle through the L3-4 neuroforamen. The thoracic nerve roots are named similarly. This is in contrast to the cervical spine, where the exiting nerve root is named for the level below. For example, the C5 nerve root passes under the C4 pedicle through the C4-5 neuroforamen. The cervical spine has 8 pairs of cervical nerve roots but only 7 cervical vertebrae. C1 passes between the occiput and C1, while C8 traverses the C7-T1 neuroforamen. The thoracic spine (most commonly) has 12 pairs of thoracic nerves and 12 vertebrae. The lumbar spine (most commonly) has 5 pairs of lumbar nerves and 5 vertebrae. There are typically 5 pairs of sacral nerves and 1 pair of coccygeal nerves. Disc herniations may occur in a variety of anatomic locations such as posterior central, posterior paracentral, inferior, or lateral. These are named in relation to the vertebral body. Anterior (ventral) herniations do not cause radicular symptoms by themselves. The pattern of symptoms varies with the location of the disc herniation. A lumbar paracentral disc herniation is more likely to affect the traversing nerve root rather than the exiting nerve root. Thus, a paracentral disc herniation at the L3-4 level is more likely to affect the L4 nerve root. A lumbar lateral disc herniation is more likely to affect the exiting nerve root rather than the traversing nerve root. Thus, a lateral disc herniation at the L3-4 level is more likely to affect the L3 nerve root. Surgery for lateral disc herniations uses the “Wiltse approach,” or posterolateral approach, to the spine. This may be done open or minimally invasively. The fascial plane between the medial multifidus and lateral longissimus is developed until the level of the appropriate transverse process or facet joint is identified. The discectomy can then be safely performed once the exiting nerve root is identified and protected.

Answer 12

Flexor Digitorum Superficialis Tendons ## Footnote The median nerve traverses the carpal tunnel, along with four tendons of the flexor digitorum superficialis, four tendons of the flexor digitorum profundus, and the tendon of the flexor pollicis longus. The roof of the carpal tunnel is the transverse carpal ligament, which attaches to the scaphoid and trapezium on the radial side and the pisiform and hamate on the ulnar side. Compression of the median nerve can occur due to thickening of the transverse carpal ligament, tenosynovitis, or a space occupying lesion in the carpal tunnel. The ulnar nerve innervates all of the hand intrinsic muscles, except for the LOAF muscles —lumbricals 1/2, opponens pollicis, abductor pollicis brevis, and flexor pollicis brevis—which are innervated by the median nerve. The median nerve sensory distribution is the palmar aspect of the radial 3.5 digits and the corresponding palm. The palm is innervated by the palmar cutaneous branch, which arises proximal to the carpal tunnel (approximately 4-8 cm proximal to the distal wrist crease) and does not pass through the carpal tunnel. Carpal tunnel syndrome presents with numbness in the radial 3.5 digits (sparing the palm), paresthesias, and/or weakness of the LOAF muscles. Pain and paresthesias often have nocturnal worsening and patients frequently describe shaking out their hand to relieve the symptoms. The median-innervated forearm muscles include the pronator teres, flexor carpi radialis, flexor digitorum superficialis, and the anterior interosseous innervated muscles, including the flexor digitorum profundus, flexor pollicis longus, and pronator quadratus. The median-innervated forearm muscles are spared in carpal tunnel syndrome. On examination, provocative maneuvers that may prove useful in diagnosing carpal tunnel syndrome include the Tinel sign, Durkan compression test, Phalen maneuver, and reverse Phalen maneuver. Ancillary studies including electrodiagnostics and ultrasound can aid in diagnosis. Treatment options typically include bracing, carpal tunnel steroid injections, and surgical carpal tunnel release.

Answer 13

Extensor Carpi Radialis Longus, Extensor Carpi Radialis Brevis, Brachioradialis ## Footnote The radial nerve arises from the posterior cord in the axilla. The nerve travels along the posterior wall of the axilla supplying motor branches to the three heads of the triceps and sensation to the posterior aspect of the arm. The nerve continues through the triangular interval between the long head of the triceps and the humerus. The radial nerve then passes down the spiral groove and wraps around the humerus to pass between the brachioradialis muscle and brachialis muscle to enter the forearm anterior to the lateral epicondyle. In the proximal forearm, the radial nerve terminates by dividing into the superficial radial (sensory) and the deep radial/posterior interosseous nerve (PIN). The radial nerve innervates the triceps brachii, brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis. The posterior cutaneous nerve of the arm and posterior cutaneous nerve of the forearm are sensory branches of the radial nerve that supply sensation to the posterior arm and forearm. The superficial radial nerve is a terminal branch of the radial nerve that is a pure sensory nerve, supplying sensation to the dorsum of the radial 3.5 digits. The other terminal branch of the radial nerve is the deep radial nerve/posterior interosseous nerve. This nerve supplies the supinator, extensor carpi ulnaris, extensor digitorum communis, extensor digiti minimi, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor indicis. Potential compression points include the radial nerve at the spiral groove by the lateral intermuscular septum, the posterior interosseous nerve at the supinator/Arcade of Frohse, and the superficial radial nerve between the brachioradialis and extensor carpi radialis longus tendons. Due to the course of the radial nerve in close association with the humerus, the radial nerve is particularly prone to injury in association with mid-shaft humeral fractures. Clinically, proximal radial nerve injuries are characterized by weakness or loss of elbow extension, wrist extension, and finger extension. The triceps branches arise very proximally such that elbow extension is preserved with injuries to the radial nerve in the mid-arm. A posterior interosseous palsy is characterized by loss of finger extension, with preserved wrist extension but radial deviation during wrist extension. This occurs due to the preservation of function in the extensor carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the extensor carpi ulnaris (innervated by the posterior interosseous nerve).

Answer 14

Spinal Accessory Nerve ## Footnote Scapular winging can be categorized as lateral or medial winging. Medial winging is caused by weakness of the serratus anterior muscle, innervated by the long thoracic nerve. Lateral winging can be caused by weakness of the rhomboid muscles, innervated by the dorsal scapular nerve, or weakness of the trapezius, innervated by the spinal accessory nerve. Winging related to a dorsal scapular neuropathy is usually subtle. Winging is more than just cosmetic; patients with significant winging often present with reduced active range of motion with shoulder abduction and/or forward flexion. The spinal accessory nerve is a pure motor nerve. It leaves the jugular foramen and innervates the sternocleidomastoid and trapezius muscles. After innervating the sternocleidomastoid, it typically emerges from under the sternocleidomastoid approximately 2/3 of the distance from the sternal notch to the mastoid and runs through the posterior triangle of the neck where it is very superficial and prone to injury during surgery in the area, including cervical lymph node dissection/biopsy. The suprascapular nerve innervates the supraspinatus and infraspinatus muscles. The axillary nerve innervates the teres minor and deltoid muscles. The upper subscapular nerve innervates the subscapularis. The lateral pectoral nerve innervates the pectoralis major muscle.

Answer 15

Upper Trunk ## Footnote The brachial plexus can be divided into five segments: roots, trunks, divisions, cords, and branches. The roots and trunks can be found in the supraclavicular space. The divisions typically are retroclavicular. The cords and branches are infraclavicular. The branches typically arise at the level of the axilla. The brachial plexus comprises the C5-T1 nerves. The C5 and C6 nerves merge to form the upper trunk. The C7 nerve continues independently as the middle trunk. The C8 and T1 nerves merge to form the lower trunk. The biceps brachii is innervated by the musculocutaneous nerve (C5/C6, upper trunk, lateral cord). The deltoid is innervated by the axillary nerve (C5/C6, upper trunk, posterior cord). The brachioradialis and supinator are innervated by the radial nerve (C5/C6, upper trunk, posterior cord). The supraspinatus and infraspinatus are innervated by the suprascapular nerve (C5/C6, upper trunk). These muscles are all commonly innervated by the upper trunk. With an injury to the upper trunk, the major motions that are lost include shoulder abduction, forward flexion, and external rotation, elbow flexion, and supination. The upper trunk ends in a trifurcation with the suprascapular nerve laterally, posterior division in the middle, and anterior division medially. As an anatomic variant, the suprascapular nerve can arise from the posterior division of the upper trunk. The anatomic arrangement of this trifurcation is often misrepresented in many textbooks. The supraspinatus and infraspinatus are innervated by the suprascapular nerve. The anterior division carries the motor innervation to the coracobrachialis, biceps brachii, and brachialis through the musculocutaneous nerve. The posterior division carries the motor innervation to the deltoid and teres minor through the axillary nerve and to the brachioradialis and supinator through the radial nerve. The rule of 3s can be used as a rough guide for the management of traumatic nerve injuries. Open, sharp nerve injuries should be repaired within 3 days of the injury. Open, ragged nerve injuries are typically repaired 3 weeks after the injury, allowing time for the zone of injury to fully demarcate. In many instances, a more acute exploration is undertaken to tag the injured nerves to be repaired later. Management of closed nerve injuries is typically delayed for 3 months, with surgical management reserved for those patients not demonstrating clinical or electrodiagnostic evidence of recovery. The management of closed injuries is the most nuanced. Gunshot wounds rarely sever the nerve, and instead typically cause a concussive injury from the velocity of the round or a heat injury related to the round traversing the tissue. Correspondingly, nerve injuries related to gunshot wounds are typically managed similarly to closed injuries.

Answer 16

Radial Nerve ## Footnote The cutaneous sensation to the dorsal forearm is supplied by three major sensory nerves: medial antebrachial cutaneous (branch of medial cord), lateral antebrachial cutaneous (continuation of musculocutaneous nerve), and the dorsal (posterior) cutaneous nerve of the forearm (branch of radial nerve, also called the posterior antebrachial cutaneous nerve). The dorsal (posterior) cutaneous nerve of the forearm arises from the radial nerve in the proximal arm and courses with the radial nerve through the mid and upper arm. The dorsal (posterior) cutaneous nerve of the forearm diverges from the radial nerve as the radial nerve pierces the lateral intermuscular septum, emerges superficially through a fascial hiatus proximal to the lateral epicondyle, gives off a branch to the lateral epicondyle, and continues to its termination in the cutaneous tissue of the dorsal forearm. The dorsal (posterior) cutaneous nerve of the forearm provides sensation to the lateral epicondyle and the skin of the dorsal midline forearm between the elbow and wrist. The sensory distribution of the radial nerve includes the posterior arm (posterior cutaneous nerve of the arm), the posterior forearm (posterior cutaneous nerve of the forearm), and the dorsal-lateral hand and dorsal radial 3.5 digits (superficial radial nerve). The sensory distribution of the ulnar nerve includes the dorsal ulnar hand (dorsal cutaneous branch), the ulnar palm (palmar cutaneous branch), and the dorsal and palmar ulnar 1.5 digits (superficial branch of the ulnar nerve). The sensory distribution of the median nerve includes the radial palm (palmar cutaneous branch) and the palmar aspect of the radial 3.5 digits. The sensory distribution of the musculocutaneous nerve includes the lateral forearm via the lateral antebrachial cutaneous nerve, which is the continuation of the musculocutaneous nerve distal to the motor branches to the biceps brachii and brachialis. The sensory distribution of the axillary nerve is the area overlying the deltoid in the lateral upper arm (superior lateral cutaneous nerve).

Answer 17

Loss of elbow flexion ## Footnote The most likely physical exam manifestation of an upper (superior) trunk injury would be loss of elbow flexion. The trunks of the brachial plexus are formed through the confluence or continuation of the C5 through T1 roots. The upper trunk is formed from the union of the C5 and C6 roots, the middle trunk is the continuation of the C7 root, and the lower trunk is the union of the C8 and T1 roots. Occasionally, the C4 root may contribute to the upper trunk or the T2 root may contribute to the lower trunk, with these variations termed a pre-fixed (C4 contribution) or post-fixed (T2 contribution) brachial plexus. The upper trunk typically trifurcates into the anterior division, posterior division, and the suprascapular nerve, though the suprascapular nerve may arise from the posterior division of the upper trunk. The upper trunk’s contribution to the lateral cord provides axons to the lateral pectoral nerve (pectoralis major), musculocutaneous nerve (coracobrachialis, biceps brachii, brachialis), lateral antebrachial cutaneous nerve, and the median nerve (pronator teres, flexor carpi radialis, sensory fibers in the median nerve). The upper trunk’s contribution to the posterior cord provides axons to the upper and lower subscapular nerves (subscapularis and teres major), thoracodorsal nerve (latissimus dorsi), axillary nerve (deltoid, teres minor) and radial nerve (brachioradialis, supinator, extensor carpi radialis longus and brevis). Neuroanatomical localization of a brachial plexus injury requires careful consideration of muscles innervated proximal and distal to the suspected level of injury. The upper trunk provides critical distal innervation for muscles of elbow flexion (biceps brachii, brachialis, brachioradialis), movement and stabilization of the shoulder joint (supraspinatus, infraspinatus, deltoid, teres major, teres minor, subscapularis), and cutaneous sensation to the lateral arm, forearm, and hand. However, nerves that branch off proximal to the formation of the upper trunk (root level), such as the dorsal scapular nerve (rhomboid, levator scapulae), phrenic nerve (diaphragm), or long thoracic nerve (serratus anterior), will not evidence injury on physical examination, imaging, or electrodiagnostic testing in an injury at the level of the upper trunk. Involvement of these muscles raises the concern for a more proximal pathology at the root level. Root level injuries may be preganglionic (such as root avulsions) or post-ganglionic. Discerning pre-ganglionic versus postganglionic injury is critical for consideration of nerve reconstruction. Nerve graft reconstruction requires a viable and accessible proximal nerve stump that is unavailable in pre-ganglionic injuries. Nerve transfers are the primary consideration for nerve reconstruction in patients with preganglionic root level injuries. CT or MRI evidence of a pseudomeningocele should raise the suspicion for pre-ganglionic injury, but presence or absence is not conclusive. Nerve conduction studies demonstrating absent motor conduction and normal sensory conduction from an anesthetic region suggests a pre-ganglionic nerve injury, as the afferent nerve fibers may be preserved despite disconnection from the spinal cord given the location of their cell bodies in the dorsal root ganglion rather than the spinal cord. Due to this, the sensory fibers do not undergo Wallerian degeneration, allowing a preserved sensory nerve action potential, despite the disconnection from the spinal cord resulting in anesthesia. Major motions that are lost in a complete upper trunk injury include shoulder abduction, shoulder external rotation, elbow flexion, and forearm supination. Elbow and wrist extension loss would be unlikely despite the upper trunk contribution to the radial nerve as the triceps, extensor carpi ulnaris, and extensor carpi radialis longus/brevis all receive significant contributions from C7/middle trunk and/or C8/lower trunk. Finger flexion loss would be unlikely as the upper trunk typically does not contribute innervation to the finger flexors (flexor digitorum superficialis, flexor digitorum profundus, lumbricals). Shoulder shrug loss would be unlikely as this is primarily mediated by the trapezius muscle, which is innervated by the spinal accessory nerve.

Answer 18

Axillary Nerve ## Footnote The axillary nerve is the nerve most likely to be injured with a posterior dislocation of the humeral head. The axillary nerve arises as a terminal branch of the posterior cord of the brachial plexus. The axillary nerve carries fibers originating from the C5 and C6 roots coursing through the posterior division of the upper trunk. Originating from the posterior cord anterior to the scapula, the axillary nerve travels posteriorly underneath the glenohumeral joint to enter the quadrangular space. The quadrangular space (also known as the quadrilateral space) is bounded by the teres minor (superior), teres major (inferior), long head of triceps (medial), and the humerus (lateral). The axillary nerve traverses the quadrangular space with the posterior humeral circumflex vessels. The axillary nerve divides within or as it exits the quadrangular space into an anterior and posterior division. The anterior division innervates the anterior and middle deltoid, while the posterior division innervates the teres minor and posterior deltoid and provides cutaneous sensation for the lateral upper arm through the superior lateral cutaneous nerve. Given the close proximity to the glenohumeral joint and its course from anterior to posterior through a constrained anatomical corridor (quadrangular space), the axillary nerve is prone to stretch injury with glenohumeral dislocations such as in this case.

Answer 19

Median Nerve ## Footnote The median nerve arises from contributions from the lateral and medial cords of the brachial plexus, carrying fibers from C6-T1. The medial cord provides predominantly motor axons from C8 and T1, while most of the sensory contribution to the median nerve comes from the lateral cord from C6 and C7. The median nerve does not innervate any muscles in the arm. The branch to the pronator teres is the first branch off the median nerve just proximal to the elbow. The pronator teres (C6, C7) is the main muscle responsible for forearm pronation. Distal to the elbow, the median nerve gives off branches to innervate the flexor digitorum superficialis and the flexor carpi radialis. The anterior interosseous nerve (AIN) branches from the median nerve in the proximal forearm at the level of the pronator teres. The AIN is a motor nerve that innervates three forearm muscles: flexor digitorum profundus (2nd and 3rd digits), flexor pollicis longus, and pronator quadratus. An anterior interosseous nerve palsy is typified by the inability to form an OK sign secondary to the inability to flex the interphalangeal joint of the thumb (flexor pollicis longus) and the distal interphalangeal joint of the index finger (flexor digitorum profundus). After giving off the anterior interosseous nerve, the median nerve continues down the forearm eventually passing through the carpal tunnel to enter the hand. In the hand, the median nerve innervates the LOAF muscles: lumbricals 1 and 2, opponens pollicis, abductor pollicis brevis, and flexor pollicis brevis. The median nerve also provides sensory innervation to the radial 3.5 digits on the palmar surface, as well as the corresponding segment of the palm through the palmar cutaneous branch, which arises proximal to the carpal tunnel and does not run through it. Pronator syndrome results from compression of the median nerve between the two heads of the pronator teres. Patients typically present with volar forearm pain and may have a positive Tinel sign at the pronator teres. Atrophy of the muscles innervated by the median nerve is rare in this syndrome. Those at risk for developing pronator syndrome include carpenters and tennis players due to the repetitive forearm pronation. Trauma, peripheral nerve tumors, and secondary compression may also be responsible for pronator syndrome. Compression of the median nerve in the forearm can also occur related to the lacertus fibrosus and the sublimus arch of the flexor digitorum superficialis. The median nerve can also be compressed at the carpal tunnel. Carpal tunnel syndrome is the most common entrapment neuropathy. In carpal tunnel syndrome, the LOAF muscles may be weak and there may be numbness of the palmar aspect of the radial 3.5 digits. The median- and anterior interosseous-innervated forearm muscles are spared in carpal tunnel syndrome.

Answer 20

L5 ## Footnote The common peroneal nerve is one of the two divisions of the sciatic nerve and is responsible for toe extension, ankle dorsiflexion, and eversion. The tibial nerve is the other division of the sciatic nerve and is responsible for toe flexion, ankle plantar flexion, and inversion. At the nerve root level, dorsiflexion is primarily mediated by the L5 nerve, whereas plantar flexion is primarily mediated by the S1 nerve. Inversion is also primarily mediated by the L5 nerve, whereas eversion is primarily mediated by the L5 and S1 nerves. When a patient presents with a foot drop (dorsiflexion weakness), the main differential diagnosis is a common peroneal neuropathy versus an L5 radiculopathy. Dorsiflexion plus eversion weakness would be typical of a common peroneal neuropathy. Inversion is spared in a common peroneal neuropathy. Conversely, dorsiflexion weakness plus inversion weakness would be typical of an L5 radiculopathy. In this way, inversion and eversion can be utilized to help differentiate a common peroneal neuropathy from an L5 radiculopathy. The extensor hallucis longus (great toe extension) is innervated by the deep peroneal nerve, with L5 as the primary nerve root contributing to its innervation. Thus, in both L5 radiculopathy and common peroneal neuropathy, toe extension weakness can be a prominent finding. Electrodiagnostics and imaging can then be used to support the suspected diagnosis. For a common peroneal neuropathy, the typical compression point is at the fibular tunnel around the fibular neck, where the nerve can be compressed by the deep fascia of the peroneus longus muscle. L5 radiculopathy most commonly occurs related to L4-5 paracentral disc herniations, as the disc compresses the traversing L5 nerve root. Less commonly, the L5 nerve root can be compressed from an L5-S1 far lateral disc, as the disc compresses the exiting L5 nerve root.

Answer 21

Lumbricals 3/4, Palmar Interossei, Dorsal Interossei, Adductor Pollicis ## Footnote The ulnar nerve arises from the medial cord of the brachial plexus with primary nerve fibers coming from the C8 and T1 nerve roots. It then descends with the brachial artery up to the insertion point of the coracobrachialis muscle. It subsequently pierces the medial intermuscular septum and enters the posterior compartment of the arm where it then travels posteromedial to the humerus and behind the medial epicondyle to enter the cubital tunnel. It then enters the anterior compartment of the forearm between the two heads of the flexor carpi ulnaris. Once it reaches the wrist, it enters the hand superficial to the flexor retinaculum and lateral to the pisiform bone traversing Guyon’s canal. It then terminally divides into superficial and deep branches. Two major sensory branches arise in the forearm, distal to the cubital tunnel but proximal to Guyon’s canal: the palmar cutaneous branch and the dorsal cutaneous branch. The two major zones of compression for the ulnar nerve are around the elbow and around Guyon’s canal in the palm. Compression around the elbow, known as cubital tunnel syndrome, can occur related to the medial intermuscular septum, the Arcade of Struthers, bony abnormalities of the medial epicondyle, Osborne’s ligament, or Osborne’s fascia. The major sensory branches in the forearm—the dorsal cutaneous and palmar cutaneous branches—can be used to help differentiate cubital tunnel syndrome from Guyon’s canal syndrome. Sensory loss in the distribution of these nerves suggests cubital tunnel syndrome, since these branches arise proximal to Guyon’s canal and are spared in Guyon’s canal syndrome. In Guyon’s canal, compression can often occur related to the presence of a ganglion cyst or related to external compression such as can occur from a bicycle handlebar. The ulnar nerve provides sensory function to the fifth digit and medial half of the fourth digit via the terminal superficial branch of the ulnar nerve, to the corresponding area of the palm via the palmar cutaneous branch, and to the dorsal medial portion of the hand via the dorsal cutaneous branch of the ulnar nerve. The ulnar nerve supplies two muscles in the forearm: the flexor carpi ulnaris and the medial half of the flexor digitorum profundus. In the hand, the deep branch of the ulnar nerve supplies the hypothenar muscles, including the opponens digiti minimi, abductor digiti minimi, flexor digiti minimi brevis, as well as lumbricals 3 and 4, dorsal interossei, palmar interossei, adductor pollicis, and the deep head of the flexor pollicis brevis. The superficial branch provides motor function to the palmaris brevis. All of the hand intrinsic muscles except for the LOAF muscles—lumbricals 1 and 2, opponens policis, abductor pollicis brevis, and flexor pollicis brevis— are supplied by the ulnar nerve. The LOAF muscles are innervated by the median nerve. In carpal tunnel syndrome, one would expect to potentially find weakness of the LOAF muscles, sparing the rest of the hand intrinsics. In ulnar neuropathy, one would expect to potentially find weakness of the hand intrinsics sparing the LOAF muscles. In C8/T1 radiculopathy or lower trunk plexopathy, one would expect to potentially find weakness of all of the hand intrinsic muscles.

Answer 22

Axillary Nerve ## Footnote The axillary nerve is the nerve most likely to be entrapped within the quadrangular space. The axillary nerve arises as a terminal branch of the posterior cord of the brachial plexus. The axillary nerve carries fibers originating from the C5 and C6 roots coursing through the posterior division of the upper trunk. Originating from the posterior cord anterior to the scapula, the axillary nerve travels posteriorly underneath the glenohumeral joint to enter the quadrangular space. The quadrangular space (also known as the quadrilateral space) is bounded by the teres minor (superior), teres major (inferior), long head of triceps (medial), and the humerus (lateral). The axillary nerve traverses the quadrangular space with the posterior humeral circumflex vessels. The axillary nerve divides within or as it exits the quadrangular space into an anterior and posterior division. The anterior division innervates the anterior and middle deltoid, while the posterior division innervates the teres minor and posterior deltoid and provides cutaneous sensation for the lateral upper arm through the superior lateral cutaneous nerve. As originally described, quadrilateral (or quadrangular) space syndrome presents with diffuse pain around the shoulder, paresthesias in a non-dermatomal distribution, point tenderness above the quadrangular space, and occlusion or narrowing of the posterior humeral circumflex artery in a provocative position on angiogram. Fatty atrophy of the teres minor is common in this syndrome and can be detected on MRI. Athletes that perform repeated overhead activities are most susceptible to developing this syndrome, including baseball and volleyball players. The quadrangular space is separated from the triangular interval by the teres major muscle. The boundaries of the triangular interval include the teres major superiorly, long head of the triceps medially, and the humerus laterally. The radial nerve passes through the triangular interval.

Answer 23

Dorsal Interossei—Ulnar Nerve ## Footnote Abduction of digits 2-5 results from the action of the dorsal interossei, while adduction of digits 2-5 results from the action of the palmar interossei. Both dorsal and palmar interossei are innervated by the ulnar nerve, specifically via the deep branch of the ulnar nerve. The ulnar nerve arises from the medial cord, carrying fibers predominantly from the C8 and T1 roots, though there can be fibers from C7. The ulnar nerve provides motor innervation to the flexor carpi ulnaris and the ulnar half of the flexor digitorum profundus in the forearm and then innervates all of the hand intrinsic muscles except for the LOAF muscles—lumbricals 1 and 2, opponens pollicis, abductor pollicis brevis, and flexor pollicis brevis—which are innervated by the median nerve. The ulnar nerve provides sensation to the ulnar 1.5 digits through the terminal superficial branch of the ulnar nerve and also provides sensation to the dorsal ulnar hand through the dorsal cutaneous branch and to the palmar ulnar hand through the palmar cutaneous branch, both of which arise in the forearm, proximal to Guyon’s canal. Ulnar nerve entrapment occurs primarily in two places: 1) at or around the elbow, referred to as cubital tunnel syndrome, and 2) in the palm, referred to as Guyon’s canal syndrome. Entrapment of the ulnar nerve at the elbow is much more common than at the palm and represents the second most common entrapment neuropathy in the upper extremity, following carpal tunnel syndrome. Understanding the common points of compression for the ulnar nerve is important in surgical treatment of ulnar entrapment neuropathy. Commonly described points of compression around the elbow from proximal to distal include: 1. Arcade of Struthers – Musculoaponeurotic band in the upper arm extending from the medial head of triceps to the medial intermuscular septum. This is not to be confused with the Struthers ligament which is a fibrous band extending from a supracondylar process to the medial epicondyle and associated with entrapment of the median nerve. 2. Medial intermuscular septum 3. Medial epicondyle 4. Osborne’s ligament – fibrous band extending from medial epicondyle to the olecranon 5. Osborne’s fascia – fascia bridging the two heads of the flexor carpi ulnaris muscle. Entrapment at the elbow can be differentiated from entrapment at Guyon’s canal by examining the dorsal and palmar hand. The branches that innervate the dorsal hand and the palm originate distal to the cubital tunnel in the forearm and do not traverse Guyon’s canal. Thus, reduced sensation on the dorsal hand and the palm are associated with cubital tunnel but not Guyon’s canal syndrome, whereas both syndromes will have reduced sensation in the ulnar 1.5 digits. Ulnar neuropathy should also be differentiated from more proximal etiologies, including thoracic outlet syndrome and C8 radiculopathy. Ulnar neuropathy will present with sensory loss that does not extend proximal to the wrist and that reliably splits the fourth digit, whereas C8 radiculopathy will extend proximal to the wrist and will not split the fourth digit. With regard to motor function, hand weakness related to an ulnar neuropathy will spare the LOAF muscles, whereas a C8 radiculopathy will involve all of the hand intrinsic muscles (both the ulnar and median innervated muscles). Patients with severe ulnar neuropathy can present with characteristic exam findings. One such finding is the Wartenberg sign. The Wartenberg sign is the inability to adduct digit 5. When the palmar interossei are weak, the radial innervated finger extensors are unopposed, which have an abducting effect on digit 5. Another such finding is the Froment sign. The patient is asked to pinch a piece of paper between the thumb and index finger, with the interphalangeal joint of the thumb extended. When the paper is pulled, the patient will flex the interphalangeal joint of the thumb to try to maintain pinch on the paper. This occurs due to weakness of the adductor pollicis. The patient attempts to compensate by using the flexor pollicis longus, which is innervated by the anterior interosseous nerve. Finally, ulnar clawing can be observed. When asked to open the hand, the proximal and distal interphalangeal joints of digits 4 and 5 remain flexed. There is also typically hyperextension of the metacarpophalangeal joints of these digits. This occurs due to weakness of lumbricals 3 and 4. The lumbricals action is complex. They flex the metacarpophalangeal joint, while extending the proximal and distal interphalangeal joints.

Answer 24

Extensor Carpi Ulnaris ## Footnote The radial nerve arises from the posterior cord in the axilla. The nerve travels along the posterior wall of the axilla supplying motor branches to the three heads of the triceps and sensation to the posterior aspect of the arm. The nerve continues through the triangular interval between the long head of the triceps and the humerus. The radial nerve then passes down the spiral groove and wraps around the humerus to pass between the brachioradialis muscle and brachialis muscle to enter the forearm anterior to the lateral epicondyle. In the proximal forearm, the radial nerve terminates by dividing into the superficial radial (sensory) and the deep radial/posterior interosseous nerve (PIN). The radial nerve innervates the triceps brachii, brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis. The posterior cutaneous nerve of the arm and posterior cutaneous nerve of the forearm are sensory branches of the radial nerve that supply sensation to the posterior arm and forearm. The superficial radial nerve is a terminal branch of the radial nerve that is a pure sensory nerve, supplying sensation to the dorsum of the radial 3.5 digits. The other terminal branch of the radial nerve is the deep radial nerve/posterior interosseous nerve. This nerve supplies the supinator, extensor carpi ulnaris, extensor digitorum communis, extensor digiti minimi, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor indicis. Potential compression points include the radial nerve at the spiral groove by the lateral intermuscular septum, the posterior interosseous nerve at the supinator/Arcade of Frohse, and the superficial radial nerve between the brachioradialis and extensor carpi radialis longus tendons. Due to the course of the radial nerve in close association with the humerus, the radial nerve is particularly prone to injury in association with mid-shaft humeral fractures. Clinically, proximal radial nerve injuries are characterized by weakness or loss of elbow extension, wrist extension, and finger extension. The triceps branches arise very proximally such that elbow extension is preserved with injuries to the radial nerve in the mid-arm. A posterior interosseous palsy is characterized by loss of finger extension, with preserved wrist extension but radial deviation during wrist extension. This occurs due to the preservation of function in the extensor carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the extensor carpi ulnaris (innervated by the posterior interosseous nerve).

Answer 25

Extensor Digitorum Communis ## Footnote The radial nerve is formed from the posterior divisions of the brachial plexus (C5-T1) and is one of two terminal branches of the posterior cord, the other being the axillary nerve. The radial nerve runs behind the brachial artery and along the posterior border of the axilla. In the arm, it passes between the long head of the triceps and the shaft of the humerus, beneath the teres major muscle (triangular interval). This is not to be confused with the quadrangular space through which the axillary nerve passes. At the mid-humeral level, the nerve crosses the posterior aspect of the humerus moving from medial to lateral, lying in the spiral groove and traveling with the profunda brachii artery. The radial nerve innervates the triceps brachii very proximally and then distal to the spiral groove innervates the brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis. In the proximal forearm, the nerve bifurcates into the superficial radial nerve (sensory) and the deep radial/posterior interosseous nerve (PIN). The posterior interosseous nerve innervates the supinator and then passes through the arcade of Frohse, a fibrous band between the two heads of the supinator. While entrapment of the PIN is rare, this is one potential site of compression. The superficial muscle group—extensor carpi ulnaris, extensor digitorum communis, and extensor digiti minimi—and deep muscle group— abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, extensor indicis—are all innervated by the PIN. The pattern of motor and sensory loss can be particularly helpful for localizing radial nerve injuries or lesions. A high radial nerve palsy occurring in the axilla is rare. These are distinguished from more distal radial nerve injuries by the presence of triceps weakness (elbow extension) and loss of sensation to the posterior arm. A radial nerve palsy at the spiral groove presents with wrist and finger drop, weakness of brachioradialis, and loss of sensation in the posterior forearm and hand. Injury to the radial nerve in this location can occur related to humerus fractures, external compression (Saturday night palsy), or compression related to the lateral intermuscular septum. Injuries to the PIN present with intact sensation and a finger drop, without a wrist drop. There is radial deviation on wrist extension due to the preservation of function in the extensor carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the extensor carpi ulnaris (innervated by the posterior interosseous nerve). The posterior interosseous nerve can be compressed at the supinator inlet (Arcade of Frohse), but also is commonly involved in Parsonage- Turner syndrome. Entrapment of PIN is less common than Parsonage-Turner syndrome, so patients presenting with a posterior interosseous neuropathy warrant additional evaluation for a more widespread brachial plexopathy and for other etiologies, for example, tumors.

Answer 26

Nerves to the Triceps ## Footnote The radial nerve is formed from the posterior divisions of the brachial plexus (C5-T1) and is one of two terminal branches of the posterior cord, the other being the axillary nerve. The radial nerve runs behind the brachial artery and along the posterior border of the axilla. In the arm, it passes between the long head of the triceps and the shaft of the humerus, beneath the teres major muscle (triangular interval). At the mid-humeral level, the nerve crosses the posterior aspect of the humerus moving from medial to lateral, lying in the spiral groove and traveling with the profunda brachii artery. The radial nerve innervates the triceps brachii very proximally and then distal to the spiral groove innervates the brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis. In the proximal forearm, the nerve bifurcates into the superficial radial nerve (sensory) and the deep radial/posterior interosseous nerve (PIN). The posterior interosseous nerve innervates the supinator, extensor carpi ulnaris, extensor digitorum communis, extensor digiti minimi, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor indicis. Clinically, proximal radial nerve injuries are characterized by weakness or loss of elbow extension, wrist extension, and finger extension. The triceps branches arise very proximally such that elbow extension is preserved with injuries to the radial nerve in the mid-arm. A posterior interosseous palsy is characterized by loss of finger extension, with preserved wrist extension but radial deviation during wrist extension. This occurs due to the preservation of function in the extensor carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the extensor carpi ulnaris (innervated by the posterior interosseous nerve). While humeral shaft fractures account for only approximately 3% of all fractures, these have a high association with radial nerve palsies. This incidence has been noted to be anywhere between 7 and 17% of all humeral shaft fractures, which makes this the most common nerve injury associated with long bone fractures. There is debate regarding timing of surgical exploration. Traditionally, radial nerve injuries associated with closed humeral fractures have been treated expectantly, with ~70% spontaneous recovery. When combined with those patients who underwent delayed surgical exploration due to lack of recovery, the overall recovery rate rose to ~88%. In this series, the recovery rate when early exploration was utilized was ~88%, suggesting that early exploration and delayed exploration for those that do not spontaneously recover are equivalent. However, in some more updated series, there has been some suggestion that early exploration may be better. In a larger, more recent series, expectant management was noted to yield a 77% recovery rate, while early exploration (<3 weeks from injury) yielded a 90% recovery rate. Thus, more recent data have suggested that early exploration may provide more benefit than expectant management for radial nerve injuries associated with humerus fractures. Regardless, a good outcome can be expected in the majority of patients with radial nerve injuries associated with humeral fractures. When examining for spontaneous recovery, the branch to the brachioradialis is the first branch distal to the spiral groove. Thus, the brachioradialis is the muscle that is expected to show recovery first and is critical to examine.

Answer 27

Brachioradialis ## Footnote The radial nerve originates from the posterior cord of the brachial plexus. It then traverses through the triangular interval, where it ultimately reaches the radial sulcus on the posterior aspect of the humerus. It subsequently travels down the arm between the lateral and medial heads of the triceps until it reaches the lateral aspect of the arm at a point approximately 5 cm below the deltoid tuberosity. At this point, the nerves pierces the lateral intermuscular septum to reach the anterior compartment of the arm. It then crosses the elbow anterior to the lateral epicondyle and terminally branches into the superficial radial nerve and the deep radial nerve/posterior interosseous nerve in the proximal forearm. The radial nerve innervates the triceps brachii, brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis. The posterior cutaneous nerve of the arm and posterior cutaneous nerve of the forearm are sensory branches of the radial nerve that supply sensation to the posterior arm and forearm. The superficial radial nerve is a terminal branch of the radial nerve that is a pure sensory nerve, supplying sensation to the dorsum of the radial 3.5 digits and corresponding dorsal hand. The other terminal branch of the radial nerve is the deep radial nerve/posterior interosseous nerve. This nerve supplies the supinator, extensor carpi ulnaris, extensor digitorum communis, extensor digiti minimi, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor indicis. Potential compression points include the radial nerve at the spiral groove by the lateral intermuscular septum, the posterior interosseous nerve at the supinator/Arcade of Frohse, and the superficial radial nerve between the brachioradialis and extensor carpi radialis longus tendons. Due to the course of the radial nerve in close association with the humerus, the radial nerve is particularly prone to injury in association with mid-shaft humeral fractures. Clinically, proximal radial nerve injuries are characterized by weakness or loss of elbow extension, wrist extension, and finger extension. The triceps branches arise very proximally such that elbow extension is preserved with injuries to the radial nerve in the mid-arm. A posterior interosseous palsy is characterized by loss of finger extension, with preserved wrist extension but radial deviation during wrist extension. This occurs due to the preservation of function in the extensor carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the extensor carpi ulnaris (innervated by the posterior interosseous nerve). While humeral shaft fractures account for only approximately 3% of all fractures, these have a high association with radial nerve palsies. This incidence has been noted to be anywhere between 7 and 17% of all humeral shaft fractures, which makes this the most common nerve injury associated with long bone fractures. There is debate regarding timing of surgical exploration. Traditionally, radial nerve injuries associated with closed humeral fractures have been treated expectantly, with ~70% spontaneous recovery. When combined with those patients who underwent delayed surgical exploration due to lack of recovery, the overall recovery rate rose to ~88%. In this series, the recovery rate when early exploration was utilized was ~88%, suggesting that early exploration and delayed exploration for those that do not spontaneously recover are equivalent. However, in some more updated series, there has been some suggestion that early exploration may be better. In a larger, more recent series, expectant management was noted to yield a 77% recovery rate, while early explorations (<3 weeks from injury) yielded a 90% recovery rate. Thus, more recent data have suggested that early exploration may provide more benefit than expectant management in radial nerve injury associated with humeral fractures. Regardless, a good outcome can be expected in the majority of patients with radial nerve injuries associated with humeral fractures. When examining for spontaneous recovery, the branch to the brachioradialis is the first branch distal to the spiral groove. Thus, the brachioradialis is the muscle that is expected to show recovery first and is critical to examine.

Answer 28

D. Long Thoracic Nerve ## Footnote Scapular winging can be categorized as lateral or medial winging. Medial winging is caused by weakness of the serratus anterior muscle, innervated by the long thoracic nerve. Lateral winging can be caused by weakness of the rhomboid muscles, innervated by the dorsal scapular nerve, or weakness of the trapezius, innervated by the spinal accessory nerve. Winging related to a dorsal scapular neuropathy is usually subtle. Winging is more than just cosmetic, patients with significant winging often present with reduced active range of motion with shoulder abduction and/or forward flexion. The image in the question demonstrates medial winging consistent with a long thoracic nerve palsy. The spinal accessory nerve is a pure motor nerve. It leaves the jugular foramen and innervates the sternocleidomastoid and trapezius muscles. After innervating the sternocleidomastoid, it typically emerges from under the sternocleidomastoid approximately 2/3 of the distance from the sternal notch to the mastoid and runs through the posterior triangle of the neck where it is very superficial and prone to injury during surgery in the area, including cervical lymph node dissection/biopsy. The dorsal scapular nerve arises directly from the C5 nerve. The dorsal scapular nerve pierces the middle scalene muscle and then courses deep to the levator scapulae and rhomboid muscles. The long thoracic nerve forms from the merger of branches from C5, C6, and C7. The long thoracic nerve pierces the middle scalene muscle, runs over the first rib, and then descends along the chest wall between the chest wall and the latissimus dorsi muscle. The long thoracic nerve is prone to injury due to its long course along the chest wall. It is also susceptible to possible compression in the middle scalene muscle. Parsonage-Tuner syndrome, also referred to as idiopathic brachial plexopathy or neuralgic amyotrophy, classically presents with sudden onset of severe shoulder or periscapular pain that lasts from hours to a few weeks. Following resolution (or significant improvement) of the pain, weakness then begins and progresses over a few days to weeks. While Parsonage-Turner syndrome is poorly understood, it is thought to be an autoimmune inflammatory neuropathy. Often a potential precipitator can be identified in the patient history, including surgery, trauma, viral infection, vaccination, pregnancy, or significant stress. For unclear reasons, Parsonage-Turner syndrome does have a predilection for certain nerves. The most commonly involved nerves include the long thoracic nerve, suprascapular nerve, axillary nerve, musculocutaneous nerve, posterior interosseous nerve, and anterior interosseous nerve. The nerves are often affected in a patchy manner, displaying various degrees of involvement and sparing of nerves from the same root or plexus level. Electromyography can help identify muscles/nerves with subclinical involvement, which can help support the diagnosis.

Answer 29

Injury to the Sympathetic Chain ## Footnote The sympathetic chain courses over the anterior surface of the longus colli muscles in a loose fascial plane. The sympathetic chain is located posteromedial from the carotid sheath. It courses along the longus colli muscle lateral to the medial border of the musculature. During prolonged retraction of the longus colli muscles, these fibers can be stretched leading to a transient or potentially irreversible injury. Care should be taken to rest the lip of retractors under the edge of the longus colli rather than on the surface of the muscle to help avoid injury to the sympathetic chain. The findings classically associated with injury to the sympathetic chain are eponymously known as Horner syndrome. These symptoms include ptosis, anhidrosis, and miosis. The sympathetic chain is most at danger during exposure to the middle to lower aspect of the cervical spine, as the fibers are positioned more medially at these levels.

Answer 30

Axillary Nerve ## Footnote The axillary nerve arises as a terminal branch of the posterior cord of the brachial plexus. The axillary nerve carries fibers originating from the C5 and C6 roots coursing through the posterior division of the upper trunk. Originating from the posterior cord anterior to the scapula, the axillary nerve travels posteriorly underneath the glenohumeral joint to enter the quadrangular space. The quadrangular space (also known as the quadrilateral space) is bounded by the teres minor (superior), teres major (inferior), long head of triceps (medial), and the humerus (lateral). The axillary nerve traverses the quadrangular space with the posterior humeral circumflex vessels. The axillary nerve divides within or as it exits the quadrangular space into an anterior and posterior division. The anterior division innervates the anterior and middle deltoid, while the posterior division innervates the teres minor and posterior deltoid and provides cutaneous sensation for the lateral upper arm through the superior lateral cutaneous nerve. As originally described, quadrilateral (or quadrangular) space syndrome presents with diffuse pain around the shoulder, paresthesias in a non-dermatomal distribution, point tenderness above the quadrangular space, and occlusion or narrowing of the posterior humeral circumflex artery in a provocative position on angiogram. Fatty atrophy of the teres minor is common in this syndrome and can be detected on MRI. Athletes that perform repeated overhead activities are most susceptible to developing this syndrome, including baseball and volleyball players. The quadrangular space is separated from the triangular interval by the teres major muscle. The boundaries of the triangular interval include the teres major superiorly, long head of the triceps medially, and the humerus laterally. The radial nerve passes through the triangular interval.

Answer 31

Deep Peroneal Nerve ## Footnote The deep peroneal nerve begins at the trifurcation of the common peroneal nerve into the deep peroneal nerve, superficial peroneal nerve, and articular branch to the superior tibiofibular joint. The trifurcation occurs at the inlet to the fibular tunnel around the junction of the fibular head and neck. The deep peroneal nerve then courses deep to the extensor digitorum longus where it ultimately resides on the anterior surface of the interosseous membrane. Once it reaches approximately the midway point of the leg, it travels with the anterior tibial artery. It descends with the artery anterior to the ankle joint and ultimately divides into lateral and medial terminal branches. Potential compression points along the course of the nerve include at the fibular tunnel inlet at the origin of the nerve, where compression can occur related to the deep fascia of the peroneus longus muscle, at the anterior tarsal tunnel, where the compression is related to the inferior extensor retinaculum, and at the midfoot, where compression can occur from the extensor hallucis brevis tendon. Peroneal intraneural ganglion cysts also often preferentially impact the deep peroneal nerve more than the superficial peroneal nerve due to the anatomic relationship between the deep peroneal nerve and the articular branch to the superior tibiofibular joint, where the deep peroneal nerve is closer anatomically than the superficial peroneal nerve. The deep peroneal nerve provides motor fibers to the tibialis anterior, extensor hallucis longus, extensor digitorum longus, peroneus tertius, extensor digitorum brevis, and extensor hallucis brevis. The deep peroneal nerve is primarily responsible for innervating the muscles responsible for dorsiflexion and toe extension. In addition, it provides sensation to the web space between the first and second digits. The saphenous nerve has no motor function. It is a pure sensory nerve that provides sensation to the medial aspect of the leg, the infrapatellar region, and the medial ankle. The sural nerve is also a pure sensory nerve which provides sensation to the lateral aspect of the calf and the lateral aspect of the foot. The superficial and deep peroneal nerves arise from the common peroneal nerve. The superficial peroneal nerve provides motor innervation to the peroneus longus and brevis muscles. It also provides sensory fibers to the anterolateral aspect of the leg and most of the dorsum of the foot, save the webspace between the first and second digits. The tibial nerve has important motor and sensory innervations. The motor innervations include the gastrocnemius, soleus, plantaris, tibialis posterior, flexor digitorum longus, flexor hallucis longus, abductor hallucis, flexor digitorum brevis, flexor hallucis brevis, flexor digiti minimi, adductor hallucis, abductor digiti minimi, interossei, and lumbricals. With regard to sensation, at or around the popliteal fossa, the medial sural cutaneous nerve branches off of the tibial nerve to supply the lateral aspect of the lower leg and the lateral aspect of the foot. The calcaneal nerve branch arises around the ankle and supplies sensation to the posterior aspect of the heel. Finally, the medial and lateral plantar branches supply the sole of the foot. Inversion is mediated primarily by the tibialis posterior, which is innervated by the tibial nerve. Eversion is mediated primarily by the peroneus longus and peroneus brevis, both of which are supplied by the common peroneal nerve via the superficial peroneal nerve. Dorsiflexion is mediated primarily by the tibialis anterior, which is supplied by the common peroneal nerve via the deep peroneal nerve. Plantar flexion is mediated primarily by the gastrocnemius and soleus, both of which are supplied by the tibial nerve.

Answer 32

Loss of Plantar Flexion, Loss of Dorsiflexion, Loss of Hamstring Function, Preservation of Knee Extension ## Footnote The sciatic nerve arises via the lumbosacral plexus from the L4, L5, S1, S2, and S3 spinal nerve roots. The sciatic nerve is two nerves joined together that share the same epineurium – the common peroneal (posterior divisions of L4-S2) and tibial nerve (anterior divisions L4-S3). It exits the pelvis through the greater sciatic foramen, most commonly coursing under the piriformis muscle, and runs deep to the gluteus maximus. The sciatic nerve then travels in the middle of the posterior thigh between the hamstrings. Approximately two-thirds of the way down the thigh, the sciatic nerve divides into the tibial and common peroneal nerves. The tibial nerve supplies muscles that mediate plantar flexion, foot inversion, and toe flexion. Muscles responsible for dorsiflexion, eversion, and toe extension are supplied by the common peroneal nerve. The sciatic nerve innervates the musculature of the posterior compartment of the thigh, including the hamstring muscles and a portion of the adductor magnus. The tibial division provides innervation to the semitendinosus and semimembranosus medially, the long head of the biceps femoris laterally, and the ischial half of the adductor magnus. The peroneal division supplies the short head of the biceps femoris. The hamstrings function to flex the knee. The adductor magnus, along with the obturator-innervated muscles, acts to adduct the hip. The short head of the biceps femoris is the only peroneal-innervated muscle above the fibular head. If a patient is suffering from a common peroneal neuropathy, the short head of the biceps femoris will not be affected on electrodiagnostic testing, whereas injury to the peroneal division of the sciatic nerve will involve not only the distal peroneal-innervated musculature but also the short head of the biceps femoris. Therefore, this muscle is key to distinguishing a sciatic neuropathy from a common peroneal neuropathy. Additionally, when the peroneal division of the sciatic nerve is injured, the first muscle to potentially recover is the short head of the biceps femoris. Thus, this muscle is also important in tracking recovery. Hip abductors are innervated by the superior gluteal nerve (L4-S1), which is a branch of the lumbosacral plexus. The obturator nerve (L2-L4) is primarily responsible for hip adduction. The quadriceps are primarily responsible for knee extension and are supplied by the femoral nerve (L2-4).

Answer 33

Knee Flexion ## Footnote Knee flexion weakness would be the most likely finding in a patient with sciatic nerve injury at the level of the buttock. The sciatic nerve is formed via contributions from the L4-S3 roots and composed of distinct tibial and peroneal components (divisions). The sciatic nerve descends through the posterior pelvis toward the lower limb and exits the pelvis into the gluteal region via the greater sciatic foramen. After exiting the sciatic foramen, the sciatic nerve typically courses underneath the piriformis muscle before entering the posterior compartment of the thigh. However, several anatomical variations have been described for the relationship of the sciatic nerve to the piriformis muscle. As the nerve courses through the posterior thigh, it provides branches innervating the hamstring muscles that are responsible for knee flexion (semitendinosus, biceps femoris, semimembranosus, hamstring portion of adductor magnus). The tibial division of the sciatic nerve innervates the semitendinosus, semimembranosus, hamstring portion of the adductor magnus, and long head of the biceps femoris, while the peroneal division innervates the short head of the biceps femoris. This is the only muscle innervated by the peroneal division/common peroneal nerve above the knee. The sciatic nerve courses down the posterior thigh and divides into the common peroneal nerve (lateral) and tibial nerve (medial) just proximal to the popliteal fossa. The common peroneal nerve courses laterally through the popliteal fossa, around the fibular neck. At the inlet to the fibular tunnel, the common peroneal nerve trifurcates into the superficial peroneal nerve, deep peroneal nerve, and the articular branch to the superior tibiofibular joint. The superficial peroneal nerve continues in the lateral compartment of the leg (most commonly) between the peroneus longus and brevis, supplying sensory innervation to the anterolateral leg and dorsal aspect of the foot (with the exception of a small area in the first web space between digits 1 and 2), as well as motor innervation to the peroneus longus and brevis for foot eversion. The deep peroneal nerve pierces the peroneus longus muscle and courses under the extensor digitorum longus to run between the tibialis anterior and extensor digitorum longus in the superior portion of the leg and between the tibialis anterior and extensor hallucis longus in the inferior leg. As it moves distally through the anterior compartment of the leg on the anterior surface of the interosseous membrane, it travels adjacent the anterior tibial artery to the dorsal foot. Along this route, it supplies motor innervation to the tibialis anterior, extensor digitorum longus, extensor hallucis longus, extensor digitorum brevis, extensor hallucis brevis, and peroneus tertius to enable ankle dorsiflexion and toe extension. It also supplies cutaneous sensory innervation to a small area of skin in the first webspace between digits 1 and 2. The tibial nerve after the sciatic bifurcation continues into the deep posterior compartment of the leg. It innervates the gastrocnemius and soleus for plantar flexion, the tibialis posterior for inversion, the flexor hallucis longus and flexor digitorum longus for toe flexion, as well as all of the foot intrinsic muscles except the extensor hallucis brevis and extensor digitorum brevis. The tibial nerve also provides cutaneous sensation to the plantar foot via the medial plantar nerve, lateral plantar nerve, and calcaneal nerve. With proximal sciatic injuries, sensation is lost from the knee distal, with the exception of the medial leg and the area around the instep of the foot, which are supplied by the saphenous nerve (a branch of the femoral nerve). Proximal sciatic injuries are characterized by loss of knee flexion, dorsiflexion, eversion, toe extension, plantar flexion, inversion, and toe flexion. Hip flexion is primarily mediated by the iliopsoas, which comprises the psoas major and the iliacus muscles. The psoas major is innervated by direct branches from the lumbosacral plexus, typically L1-L3, while the iliacus is innervated by the femoral nerve. Hip adduction is primarily mediated by the adductor longus and brevis, adductor portion of adductor magnus, and gracilis, which are innervated by the obturator nerve, as well as the pectineus, which is innervated by the femoral nerve. Hip abduction is primarily mediated by the gluteus medius, gluteus minimus, and tensor fascia latae, innervated by the superior gluteal nerve. Knee extension is primarily mediated by the quadriceps muscles (rectus femoris, vastus lateralis, vastus intermedius, vastus medialis), innervated by the femoral nerve.

Answer 34

Primarily use aerobic respiration ## Footnote Human skeletal muscle can be subcategorized into three main types of muscle fibers based on their type of metabolism and speed of contraction. Type 1 fibers are designated slow oxidative muscle fibers that rely on aerobic oxidative phosphorylation metabolism (and thus the use of oxygen and glucose for ATP energy production) and contract relatively slowly with respect to actin-myosin cross-link cycling (dependent on how quickly the muscle fibers hydrolyze ATP to break the actin-myosin cross-link). Type 2 fibers can be stratified into type 2a and 2b. Type 2a fibers (or intermediary fibers) are fast oxidative metabolizing fibers that also rely on aerobic metabolism. However, they are able to contract faster and produce more power for contraction than type 1 fibers and, as a result, fatigue more quickly as well. Type 2b muscle fibers utilize anaerobic glycolysis for ATP production and are able to contract through actin-myosin cross-bridge formation and cycling the fastest of the three fiber types. Accordingly, they also fatigue the quickest. Type 1 and 2a fibers are the most similar in that they use oxidative aerobic metabolism. As a result, they will be most resistant to fatigue, have a more robust capillary network for supplying glucose and oxygen, and have more mitochondria. They are also typically smaller in diameter and have less glycogen than type 2b fibers. Moreover, type 1 fibers will be used more for movement and functions that require less energy and occur frequently, such as postural maintenance or fine delicate movements in the hands. Type 2b fibers will be used for movement and functions that require a lot of energy and force over a short period of time (powerful bursts) though are less precise, for example, quadriceps contraction used for sprinting. As a result, these muscles produce and use energy quickly and will fatigue quickly as well, with less mitochondria, a less rich capillary network, and less myoglobin.

Answer 35

Supination ## Footnote The upper trunk of the brachial plexus is formed by the merger of the ventral rami of the C5 and C6 nerves. It most commonly terminally trifurcates into the anterior division of the upper trunk, posterior division of the upper trunk, and the suprascapular nerve, though the suprascapular nerve can arise from the posterior division. The upper trunk ultimately has axonal contributions to the axillary, radial, musculocutaneous, median, and lateral pectoral nerves. In addition, the nerve to the subclavius arises from the upper trunk. Supination of the forearm is controlled by two muscles, the biceps brachii and the supinator. The biceps brachii receives innervation via the musculocutaneous nerve, while the supinator receives innervation from the radial nerve. The axons innervating these muscles originate in C5 and C6 and thus are lost in an injury to the upper trunk. The classic waiter’s tip posture is characteristic of an upper trunk injury. The shoulder is adducted due to loss of the deltoid (axillary nerve) and supraspinatus (suprascapular nerve), internally rotated due to loss of the infraspinatus (suprascapular nerve) and teres minor (axillary nerve), the elbow is extended due to loss of the biceps brachii (musculocutaneous nerve), brachialis (musculocutaneous nerve), and brachioradialis (radial nerve), and the forearm is pronated due loss of the biceps brachii (musculocutaneous nerve) and supinator (radial nerve). Pronation of the forearm is mediated by the pronator teres (median nerve) and pronator quadratus (anterior interosseous nerve) muscles. Axons to the pronator teres originate predominantly in C6 and C7, whereas the axons to the pronator quadratus originate in C8 predominantly but also C7 and T1. Elbow extension is mediated by the triceps brachii, innervated by the radial nerve. Axons to the triceps brachii originate in C7 and C8 predominantly, usually with some contribution from C6. Finger abduction is mediated by the dorsal interossei, innervated by the ulnar nerve, while the palmar interossei mediate finger adduction, also innervated by the ulnar nerve. Axons to the dorsal and palmar interossei originate from C8 and T1.

Answer 36

Tibialis Anterior ## Footnote Dorsiflexion is mediated primarily by the tibialis anterior muscle, which is innervated by the deep peroneal nerve. The tibialis posterior muscle is primarily an invertor of the ankle and is innervated by the tibial nerve. The peroneus longus is primarily an evertor of the ankle and is innervated by the superficial peroneal nerve. Both the gastrocnemius and soleus mediate plantar flexion and are innervated by the tibial nerve. The differential diagnosis for a patient presenting with a foot drop would include a deep peroneal neuropathy, common peroneal neuropathy, sciatic neuropathy, and L5 radiculopathy, with common peroneal neuropathy and L5 radiculopathy most common. Deep peroneal neuropathy presents with weakness of dorsiflexion and toe extension. Common peroneal neuropathy presents with deep plus superficial peroneal innervated muscle weakness, so dorsiflexion, toe extension, and eversion. Sciatic neuropathy presents with common peroneal plus tibial innervated muscle weakness, so dorsiflexion, toe extension, eversion, plantar flexion, toe flexion, and inversion. L5 radiculopathy presents with weakness of dorsiflexion and toe extension as well as inversion. Inversion and eversion can be used to help differentiate L5 radiculopathy from common peroneal neuropathy. Dorsiflexion plus inversion weakness suggests an L5 radiculopathy, whereas dorsiflexion plus eversion weakness, with sparing of inversion, suggests a common peroneal neuropathy.

Answer 37

Lateral Cord ## Footnote Injury to the lateral cord is the most likely to present with weakness in elbow flexion and forearm pronation after a penetrating, sharp injury at the shoulder. The lateral cord of the brachial plexus arises from the anterior divisions of the upper and middle trunks carrying fibers from the C5, C6, and C7 roots. The lateral cord gives off the lateral pectoral nerve, musculocutaneous nerve, and the lateral cord contribution to the median nerve. The lateral pectoral nerve arises from the proximal lateral cord and provides innervation to the pectoralis major. The musculocutaneous nerve provides innervation to two of the three muscles responsible for elbow flexion (brachialis, biceps brachii) and provides sensory innervation to the lateral forearm through the lateral antebrachial cutaneous nerve. The lateral cord contribution of the median nerve carries the bulk of the sensory nerve fibers for the median nerve, whereas the medial cord contribution to the median nerve carries the bulk of its motor fibers. However, the lateral cord contribution of the median nerve typically does provide the majority of the motor fibers that innervate the pronator teres and flexor carpi radialis. A lateral cord injury would typically present with weakness of elbow flexion and forearm pronation, as well as numbness in the lateral forearm and lateral hand. An isolated lateral cord injury would not result in complete loss of elbow flexion, as the third muscle of elbow flexion (brachioradialis) is innervated by the radial nerve, which arises from the posterior cord. In addition to the motor exam findings, the anatomic location of the stab injury helps guide approximating the most likely component of the brachial plexus to be injured. Approximate external landmarks for regions of the brachial plexus (roots, trunks, divisions, cords, branches) are the following: Roots – Above the clavicle at the level of the anterior scalene Trunks – Above the clavicle, lateral to the anterior scalene Divisions – At the level of the clavicle Cords – Below the clavicle, medial to the coracoid process of the scapula (shoulder level) Branches – Below the clavicle, within or distal to the axilla The rule of 3s helps guide management of nerve injuries. Open, sharp injuries are typically explored and repaired within 3 days of the injury. Open, ragged injuries are typically definitively managed 3 weeks after the injury, though acutely the injury may be explored to tag the nerves. Finally, closed injuries are typically managed non-surgically for 3 months and then evaluated for surgical intervention. Though gunshot wounds are controversial, they are typically managed as closed injuries, since gunshot injuries rarely transect the nerve but rather injure the nerves through the concussive force or heat generated as the bullet traverses the tissue. Given the suspected open, sharp nerve laceration, this patient should be managed with acute exploration within 3 days of injury. If the nerve is found in discontinuity, nerve reconstruction with tension-free primary repair is the preferred option. Primary repair is the preferred reconstruction option when healthy nerve ends can be reapproximated in a tension-free manner. Inclusion of a nerve graft reduces the likelihood of a good outcome, and the likelihood of a good outcome further declines as the graft length increases. Increasingly, nerve transfers are also being utilized for nerve reconstruction, particularly in the management of closed nerve injuries.

Answer 38

Lower amplitude ## Footnote Electromyography (EMG) is used to test the function of the peripheral nervous system and consists of measuring and recording electrical properties of muscles at rest and with activation. Testing is performed by inserting a recording needle electrode into a muscle to measure the action potentials from the muscle fibers. Motor unit potentials (MUPs) are the sum of the action potentials from all of the muscle fibers of a motor unit. Increased insertional activity, positive sharp waves, and fibrillation potentials reflect muscle membrane instability/irritability. This can occur in the setting of denervation or, alternatively, in the setting of inflammation or necrosis as can occur in some myopathies. In myopathies, individual muscle fibers are dysfunctional. This leads to a decrease in the size of the motor unit. The motor unit potentials become shorter duration, smaller amplitude, and polyphasic. Because the motor units are smaller, more motor units are needed to generate a similar amount of force, leading to an early recruitment pattern.

Answer 39

Radial Nerve around the Spiral Groove ## Footnote The brachial plexus is divided into five anatomical sections – roots, trunks, divisions, cords, and branches. The roots and trunks are considered the supraclavicular brachial plexus and are above the clavicle. The divisions lie directly posterior to the clavicle. The infraclavicular plexus is composed of the cords and branches which lie distal to the clavicle, with the branches typically arising around the axilla. The radial nerve is formed from the posterior divisions of the brachial plexus (C5-T1) and is one of two terminal branches of the posterior cord, the other being the axillary nerve. The radial nerve runs behind the brachial artery and along the posterior border of the axilla. In the arm, it passes between the long head of the triceps and the shaft of the humerus, beneath the teres major muscle (triangular interval). At the mid-humeral level, the nerve crosses the posterior aspect of the humerus moving from medial to lateral, lying in the spiral groove and traveling with the profunda brachii artery. The radial nerve innervates the triceps brachii very proximal and then distal to the spiral groove innervates the brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis. In the proximal forearm, the nerve bifurcates into the superficial radial nerve (sensory) and the deep radial/posterior interosseous nerve (PIN). The posterior interosseous nerve innervates the supinator, extensor carpi ulnaris, extensor digitorum communis, extensor digiti minimi, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor indicis. Potential compression points include the radial nerve at the spiral groove by the lateral intermuscular septum, the posterior interosseous nerve at the supinator/Arcade of Frohse, and the superficial radial nerve between the brachioradialis and extensor carpi radialis longus tendons. Due to the course of the radial nerve in close association with the humerus, the radial nerve is particularly prone to injury in association with mid-shaft humeral fractures. Clinically, proximal radial nerve injuries are characterized by weakness or loss of elbow extension, wrist extension, and finger extension. The triceps branches arise very proximally such that elbow extension is preserved with injuries to the radial nerve in the mid-arm. A posterior interosseous palsy is characterized by loss of finger extension, with preserved wrist extension but radial deviation during wrist extension. This occurs due to the preservation of function in the extensor carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the extensor carpi ulnaris (innervated by the posterior interosseous nerve). An injury to the middle trunk would typically not produce any complete loss of function due to significant overlap with the upper and lower trunks, but may present with weakness of elbow extension, wrist flexion, wrist extension, and/or pronation. Injury to the posterior cord would affect both the radial nerve, with loss of elbow extension, wrist extension, and finger extension, and the axillary nerve—the two terminal branches of the posterior cord—with weakness of shoulder abduction and forward flexion. An injury to the radial nerve proximally around the proximal humerus/axilla would affect the entire radial nerve and would present with weakness of elbow extension, wrist extension, and finger extension. An injury to posterior interosseous nerve preserves the radial nerve branches to the triceps, brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis and presents with a finger drop without a wrist drop. There is radial deviation on wrist extension due to the loss of function in the extensor carpi ulnaris, innervated by the posterior interosseous nerve.

Answer 40

Brachioradialis ## Footnote The posterior cord of the brachial plexus terminates as a bifurcation into the axillary nerve and the radial nerve. Almost immediately after its origin, one or more branches supplying the triceps brachii then emerge. The posterior cutaneous nerve of the arm also arises before the nerve exits the axilla and supplies sensation to the posterior arm. The radial nerve leaves the axilla traversing the triangular interval with the profunda brachii artery. This space is bounded by the humerus laterally, the triceps (long head) medially, and the teres major superiorly. The radial nerve then emerges posteriorly in the upper arm and winds around the humerus anteriorly in the spiral groove, passing between the lateral and medial heads of the triceps where it is prone to injury. The nerve can be externally compressed in this location as can happen with Saturday night palsy, can be compressed by the lateral intermuscular septum, and is prone to injury in this location, typically in association with mid-shaft fractures of the humerus. Humeral fractures in this area are typically treated with reduction (open or closed reduction) and fixed immobilization, either internal via surgical plating and screws or external via casting or application of an external fixator. Thorough neurological examination prior to treatment is essential to determine timing of radial nerve injury. Weakness prior to fracture treatment is almost undoubtedly related to the inciting trauma itself and can be managed with close clinical and electrodiagnostic follow-up. Radial nerve related weakness whose onset is immediately post-surgical needs to be investigated to ensure that no iatrogenic compression or laceration of the nerve has occurred from the placement of instrumentation. Delayed radial nerve related weakness after cast application also needs to be investigated to ensure no ongoing compression/entrapment is causing injury. The radial nerve gives off a branch supplying the anconeus, as well as the posterior cutaneous nerve of the forearm before piercing the lateral intermuscular septum beneath the lateral head of the triceps and entering the lateral part of the anterior compartment of the arm. The posterior cutaneous nerve of the forearm supplies sensation to the posterior aspect of the forearm. The radial nerve thencourses between the brachialis muscle and brachioradialis muscle and supplies motor innervation to the brachioradialis. Thus, the motor branch to the brachioradialis is the first motor branch distal to the spiral groove, making examination of the brachioradialis critical in detecting early recovery following radial nerve injury at the spiral groove. The nerve branch to the extensor carpi radialis longus then arises from the radial nerve several centimeters above the elbow. The radial nerve then enters the lateral antecubital fossa beneath the brachioradialis and anterior to the lateral epicondyle and gives off a motor branch to the extensor carpi radialis brevis. Immediately distal to the antecubital fossa, the radial nerve begins to migrate posteriorly and bifurcates into its two terminal branches—the deep radial nerve/posterior interosseus nerve (PIN) and the superficial radial nerve. The superficial radial nerve continues distally in the forearm between the brachioradialis above and the pronator teres below (lateral to the radial artery) to emerge between the brachioradialis and extensor carpi radialis longus tendons and supply cutaneous sensation to the dorsal aspect of the radial hand. The PIN, however, moves posteriorly immediately after its takeoff diving between the two heads of the supinator (and supplying the muscle as it does so) through the arcade of Frohse, the fibrous fascial arch of the superficial part of the supinator that can be a site of entrapment for the PIN, to enter the posterior compartment of the arm. The posterior interosseous nerve provides motor innervation to the supinator, extensor carpi ulnaris, extensor digitorum communis, extensor digiti minimi, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor indicis. Clinically, proximal radial nerve injuries are characterized by weakness or loss of elbow extension, wrist extension, and finger extension. The triceps branches arise very proximally such that elbow extension is preserved with injuries to the radial nerve in the mid-arm. A posterior interosseous palsy is characterized by loss of finger extension, with preserved wrist extension but radial deviation during wrist extension. This occurs due to the preservation of function in the extensor carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the extensor carpi ulnaris (innervated by the posterior interosseous nerve). While humeral shaft fractures account for only approximately 3% of all fractures, these have a high association with radial nerve palsies. This incidence has been noted to be anywhere between 7 and 17% of all humeral shaft fractures, which makes this the most common nerve injury associated with long bone fractures. There is debate regarding timing of surgical exploration. Traditionally, radial nerve injuries associated with closed humeral fractures have been treated expectantly, with ~70% spontaneous recovery. When combined with those patients who underwent delayed surgical exploration due to lack of recovery, the overall recovery rate rose to ~88%. In this series, the recovery rate when early exploration was utilized was ~88%, suggesting that early exploration and delayed exploration for those that do not spontaneously recover are equivalent. However, in some more updated series, there has been some suggestion that early exploration may be better. In a larger, more recent series, expectant management was noted to yield a 77% recovery rate, while early explorations (<3 weeks from injury) yielded a 90% recovery rate. Thus, more recent data have suggested that early exploration may provide more benefit than expectant management in radial nerve injury associated with humerus fracture. Regardless, a good outcome can be expected in the majority of patients with radial nerve injuries associated with humeral fractures.

Answer 41

Tibial Nerve at the knee ## Footnote The correct answer is the tibial nerve injury at the level of the knee. Primary re-approximation (8-0 or 10-0 Prolene) is recommended following acute nerve sharp transection. The primary factor affecting functional recovery is the length of nerve distal to the injury (following Wallerian degeneration, nerve fibers regenerate approximately 1 inch per month). Proximally innervated muscles have a better chance for recovery than small distal muscles performing more delicate movements. Motor-fiber predominant nerves tend to have better prognosis versus mixed motorsensory nerves. Good tibial nerve recovery (100%) compared to common peroneal nerve (84%) is also thought to be related to better blood supply and better nerve elasticity. Good recovery for radial and median nerves (91%) was reportedly better than ulnar nerve recovery (73%). Of note, secondary repair for median nerve outcomes (78%) were better than radial nerve (69%).

Answer 42

Posterior Interosseous Nerve ## Footnote Injury to the posterior interosseous nerve is the most likely to present with metacarpophalangeal joint extension weakness after a blunt injury at the mid-forearm. In the proximal forearm, the radial nerve divides into the superficial radial nerve and the deep radial nerve/posterior interosseous nerve. The nomenclature for the posterior interosseous nerve can be confusing, with some articles using the terms deep radial nerve and posterior interosseous nerve interchangeably and some using the term deep radial nerve for the portion of the nerve proximal to the supinator and the term posterior interosseous nerve for the continuation of the deep radial nerve distal to the supinator. The deep radial/posterior interosseous nerve is susceptible to compression at the supinator inlet by the Arcade of Frohse and is also one of the nerves that is commonly involved in Parsonage-Turner syndrome. The deep radial/posterior interosseous nerve innervates the supinator, extensor carpi ulnaris, extensor digitorum communis, and extensor digiti minimi, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor indicis. The nerve terminates as an articular branch to the wrist joint and typically does not supply any cutaneous sensation. Deep radial/posterior interosseous neuropathy presents with a finger drop, without a wrist drop. There is radial deviation on wrist extension. This occurs because the extensor carpi radialis longus and brevis are innervated more proximally by the radial nerve and are spared, whereas the extensor carpi ulnaris is innervated by the posterior interosseous nerve and is lost. It is important to recognize that the posterior interosseous-innervated finger extensors primarily act at the metacarpophalangeal joints. Extension of the proximal and distal interphalangeal joints will be preserved in a posterior interosseous neuropathy, since this movement is driven by the lumbricals, which are innervated by the median and ulnar nerves. The rule of 3s can be used as a rough guide for managing nerve injuries. An open, sharp laceration should be explored and repaired within 3 days of the injury. An open, ragged injury should be definitively repaired approximately 3 weeks after the injury. In some circumstances, acute exploration is useful to tag the nerves to be repaired later. Finally, closed, blunt injuries are considered for surgical intervention if there is no clinical or electrodiagnostic evidence of recovery at 3 months following the injury. This decision is the most nuanced.

Answer 43

Deep Peroneal Nerve ## Footnote The deep peroneal nerve is a terminal branch of the common peroneal nerve. After the sciatic bifurcation, the common peroneal nerve (CPN) continues deep and medial to the biceps femoris muscle and tendon as it proceeds distally into the leg. It courses around the neck of the fibula where it is usually quite superficial and, as a result, susceptible to injury. The common peroneal nerve trifurcates into the superficial peroneal nerve, deep peroneal nerve, and the articular branch to the superior tibiofibular joint around the fibular tunnel inlet at the fibular neck. At the fibular tunnel inlet, the common peroneal nerve can be compressed by the deep fascia of the peroneus longus. The superficial peroneal nerve continues in the lateral compartment of the leg (most commonly) between the peroneus longus and brevis, supplying sensory innervation to the anterolateral leg and dorsal aspect of the foot (with the exception of a small area in the first web space between digits 1 and 2), as well as motor innervation to the peroneus longus and brevis for foot eversion. The deep peroneal nerve pierces the peroneus longus muscle and courses under the extensor digitorum longus to run between the tibialis anterior and extensor digitorum longus in the superior portion of the leg and between the tibialis anterior and extensor hallucis longus in the inferior leg. As it moves distally through the anterior compartment of the leg on the anterior surface of the interosseous membrane, it travels adjacent the anterior tibial artery to the dorsal foot. Along this route, it supplies motor innervation to the tibialis anterior, extensor digitorum longus, extensor hallucis longus, extensor digitorum brevis, extensor hallucis brevis, and peroneus tertius to enable ankle dorsiflexion and toe extension. It also supplies cutaneous sensory innervation to a small area of skin in the first webspace between digits 1 and 2. The common peroneal nerve is susceptible to compression at the fibular tunnel inlet by the deep fascia of the peroneus longus. Risk factors for common peroneal neuropathy include habitual leg crossing, diabetes mellitus, and rapid weight loss. The superficial peroneal nerve can be compressed in the distal leg where it pierces the fascia to leave the lateral compartment (typically) and enter the subcutaneous tissue. The deep peroneal nerve is susceptible to compression in the anterior tarsal tunnel by the inferior extensor retinaculum or in the mid-foot by the tendon of the extensor hallucis brevis. This should not be confused with what is commonly referred to as tarsal tunnel syndrome. Tarsal tunnel syndrome should be more appropriately referred to as posterior tarsal tunnel syndrome. Posterior tarsal tunnel syndrome involves compression of the tibial nerve by the flexor retinaculum. The common peroneal nerve is the nerve most susceptible to formation of an intraneural ganglion cyst. Synovial fluid from a degenerative superior tibiofibular joint travels along the articular branch into the common peroneal nerve to form the cyst. Due to the anatomic relationship at the trifurcation of the common peroneal nerve, peroneal intraneural ganglion cysts often preferentially affect the deep peroneal nerve over the superficial peroneal nerve. The deep peroneal nerve is anatomically adjacent to the articular branch to the superior tibiofibular joint, which is the conduit for formation of peroneal intraneural ganglion cysts.

Answer 44

Weakness of left shoulder external rotation ## Footnote The suprascapular nerve arises from the upper trunk carrying contributions from the C5 and C6 nerve roots. The suprascapular nerve arises either as a trifurcation of the upper trunk or can arise directly from the posterior division of the upper trunk. Many anatomic diagrams misrepresent the anatomic arrangement in this location. The suprascapular nerve is always the most posteriorlateral, with the posterior division of the upper trunk next to it and the anterior division of the upper trunk most anterior-medial. The suprascapular nerve courses dorsally, parallel to the inferior belly of the omohyoid and deep to the trapezius toward the upper border of the scapula. At the upper border of the scapula, the suprascapular nerve enters the supraspinous fossa of the scapula via the suprascapular notch, traversing under the suprascapular ligament, and provides a branch innervating the supraspinatus muscle. The supraspinatus functions to abduct the upper arm at the shoulder and stabilize the shoulder joint as a component of the rotator cuff. The supraspinatus is the main abductor of the shoulder over the first 15 degrees and then assists the deltoid beyond that. The suprascapular nerve then courses laterally and caudally into the infraspinous fossa of the scapula via the spinoglenoid notch and innervates the infraspinatus muscle. The infraspinatus muscle functions to externally rotate the shoulder and stabilize the shoulder joint as a component of the rotator cuff. Lesions of the suprascapular nerve at or proximal to the suprascapular notch, which is a potential site of entrapment, present with weakness in both shoulder abduction (supraspinatus) and external rotation (infraspinatus). However, lesions of the suprascapular nerve can also present with isolated shoulder external rotation weakness due to injury distal to the branch to supraspinatus, such as entrapment at the spinoglenoid notch. Suprascapular neuropathy should be differentiated from a C5 radiculopathy or upper trunk plexopathy. In a C5 radiculopathy or upper trunk plexopathy, the supraspinatus and infraspinatus may be weak, but the deltoid and biceps will also be weak (typicall In addition to entrapment neuropathies or suprascapular nerve lesions, the differential diagnosis for patients presenting with weakness of muscles of the shoulder girdle, such as shoulder external rotation, should include idiopathic brachial plexitis (Parsonage-Turner Syndrome or Neuralgic Amyotrophy). Parsonage-Turner syndrome classically presents with acute onset, severe, temporary upper extremity (usually shoulder or periscapular) pain that is followed by development of weakness in shoulder girdle musculature. Though any nerve can be affected by Parsonage- Turner syndrome, the long thoracic nerve, suprascapular nerve, and axillary nerve were specifically noted to be commonly affected in the syndrome’s original description. The pathophysiology of Parsonage-Turner Syndrome remains unclear but is thought to be an immunemediated inflammatory condition. In many cases a potential precipitating event can be identified, such as a viral illness, vaccine, pregnancy, trauma, or surgery. Internal rotation of the shoulder is mediated by the pectoralis major (lateral pectoral nerve), subscapularis (upper and lower subscapular nerves), teres major (lower subscapular nerve), latissimus dorsi (thoracodorsal nerve) and the anterior deltoid (axillary nerve). The major internal rotator is the subscapularis. Elbow flexion is mediated by the brachialis (musculocutaneous nerve), biceps brachii (musculocutaneous nerve), and brachioradialis (radial nerve).y deltoid more than biceps if secondary to a C5 radiculopathy). In an isolated suprascapular neuropathy, the weakness will be limited to the supraspinatus and infraspinatus.

Answer 45

Fibrillation Potentials ## Footnote When axons are disrupted, the distal axon degenerates through a process called Wallerian degeneration. This degenerative process occurs in an anterograde fashion distal to the site of injury. Within hours of the injury, axonal and myelin fragmentation occurs with breakdown of its cytoskeletal components and subsequent loss of conductivity by 2-4 days postinjury, and the terminal, injured, axonal end sealing itself off. Due to this process taking several days, immediately after the injury action potentials can continue to be transmitted in the injured axon resulting in no detectable electrophysiological abnormality, as long as the stimulating electrode and recording electrode are either both proximal to the injury or both distal to the injury. Immediately after the injury, conductivity across the injury will be lost. For mixed nerves, this means that a motor action potential and sensory action potential can be detected and will be normal for several days after a nerve transection, assuming the stimulation and recording electrodes are both placed distal to the site of injury and not across the site of injury. Despite the motor action potential being normal, the action potential cannot cross the site of transection, leading to loss of voluntary motor unit action potentials, which occurs immediately at the time of the injury. Schwann cells respond to the injury immediately by proliferating and upregulating gene expression essential to their function in Wallerian degeneration. Macrophages, having migrated into the zone of injury from nearby blood vessels, phagocytose the axonal and myelin debris preparing it for axonal regeneration. With the macrophages, other inflammatory cells also gain entry into the site and begin phagocytosing the debris; this can take several weeks to several months postinjury, depending on how severe the injury is. Wallerian degeneration of the distal process continues over the subsequent weeks to months. When injuries are more severe, the inflammatory response is typically more vigorous with subsequent necrosis, cellular infiltration, and proliferation that correlates with the severity of the initial injury. Endoneurial vascular disruption also occurs, resulting in hemorrhage, edema, and influx of neutrophils and fibroblasts. The latter ultimately results in intraneural scarring with collagen deposition. Positive sharp waves and fibrillation potentials typically take 10-14 days to develop, as the muscle membrane becomes unstable due to denervation when Wallerian degeneration reaches the neuromuscular junction. As mentioned above, the process of Wallerian degeneration typically takes weeks to months. Because of this, in the event of an intraoperative injury, stimulating the nerve distal to the site of injury will still result in the detection and recording of action potentials due to the axons still being in continuity and conducting the action potentials toward the recording electrode. In the same vein, performing nerve conduction studies distal to the site of injury, including compound motor action potentials (CMAPs) and sensory nerve action potentials (SNAPs), immediately after injury may not detect any abnormality due to the fact that the axons conducting these action potentials have not yet degenerated through Wallerian degeneration, assuming both the stimulating electrode and recording electrode are distal to the site of injury. On the other hand, if the stimulating electrode and the recording electrode are on opposite sides of the transected nerve, then no action potentials will be recorded. In order for both SNAPs and CMAPs to be recorded, integrity of the lower motor neuron, specifically the distal axonal segment, must be preserved. As such, both SNAP and CMAP recordings will be affected in the event of a post-ganglionic injury. However, SNAPs and CMAPs differ in the setting of a pre-ganglionic injury in that CMAPs will be affected, but SNAPs will be preserved, due to the injury occurring proximal to the cell body and thus the integrity of the distal axons being preserved. At 6 weeks out from an upper trunk avulsion injury, we would expect to find no voluntary motor unit action potentials and present fibrillation potentials and positive sharp waves in the deltoid on electromyography. The fibrillation potentials and positive sharp waves typically take 2-3 weeks to appear on EMG, whereas the lack of voluntary motor unit action potentials would be present immediately after the injury.

Answer 46

Digits 1 and 2 ## Footnote The deep peroneal nerve is a terminal branch of the common peroneal nerve. After the sciatic bifurcation, the common peroneal nerve (CPN) continues deep and medial to the biceps femoris muscle and tendon as it proceeds distally into the leg. It courses around the neck of the fibula where it is usually quite superficial and, as a result, susceptible to injury. The common peroneal nerve trifurcates into the superficial peroneal nerve, deep peroneal nerve, and the articular branch to the superior tibiofibular joint around the fibular tunnel inlet at the fibular neck. At the fibular tunnel inlet, the common peroneal nerve can be compressed by the deep fascia of the peroneus longus. The superficial peroneal nerve continues in the lateral compartment of the leg (most commonly) between the peroneus longus and brevis, supplying sensory innervation to the anterolateral leg and dorsal aspect of the foot (with the exception of a small area in the first web space between digits 1 and 2), as well as motor innervation to the peroneus longus and brevis for foot eversion. The deep peroneal nerve pierces the peroneus longus muscle and courses under the extensor digitorum longus to run between the tibialis anterior and extensor digitorum longus in the superior portion of the leg and between the tibialis anterior and extensor hallucis longus in the inferior leg. As it moves distally through the anterior compartment of the leg on the anterior surface of the interosseous membrane, it travels adjacent to the anterior tibial artery to the dorsal foot. Along this route, it supplies motor innervation to the tibialis anterior, extensor digitorum longus, extensor hallucis longus, extensor digitorum brevis, extensor hallucis brevis, and peroneus tertius to enable ankle dorsiflexion and toe extension. It also supplies cutaneous sensory innervation to a small area of skin in the first webspace, between digits 1 and 2. The common peroneal nerve is susceptible to compression at the fibular tunnel inlet by the deep fascia of the peroneus longus. The superficial peroneal nerve can be compressed in the distal leg where it pierces the fascia to leave the lateral compartment (typically) and enter the subcutaneous tissue. The deep peroneal nerve is susceptible to compression in the anterior tarsal tunnel by the inferior extensor retinaculum or in the mid-foot by the tendon of the extensor hallucis brevis. This should not be confused with what is commonly referred to as tarsal tunnel syndrome. Tarsal tunnel syndrome should be more appropriately referred to as posterior tarsal tunnel syndrome. Posterior tarsal tunnel syndrome involves compression of the tibial nerve by the flexor retinaculum. Around the area of the tarsal tunnel, the tibial nerve branches into the calcaneal branch, lateral plantar nerve, and medial plantar nerve, all of which individually enter fibrous tunnels in which these branches can be compressed. The common peroneal nerve is the nerve most susceptible to formation of an intraneural ganglion cyst. Due to the anatomic relationship at the trifurcation of the common peroneal nerve, peroneal intraneural ganglion cysts often preferentially affect the deep peroneal nerve over the superficial peroneal nerve. The deep peroneal nerve is anatomically adjacent to the articular branch to the superior tibiofibular joint, which is the conduit for formation of peroneal intraneural ganglion cysts.

Answer 47

Pronator Teres ## Footnote The median nerve arises from contributions from the lateral and medial cords of the brachial plexus, carrying fibers from C6-T1. The medial cord provides predominantly motor axons from C8 and T1, while most of the sensory contribution to the median nerve comes from the lateral cord from C6 and C7. The median nerve does not innervate any muscles in the arm. The branch to the pronator teres is the first branch off the median nerve just proximal to the elbow. The pronator teres (C6,7) is the main muscle responsible for forearm pronation. Distal to the elbow, the median nerve gives off branches to innervate the flexor digitorum superficialis and the flexor carpi radialis. The anterior interosseous nerve (AIN) branches from the median nerve in the proximal forearm at the level of the pronator teres. The AIN is a motor nerve that innervates three forearm muscles: flexor digitorum profundus (2nd and 3rd digits), flexor pollicis longus, and pronator quadratus. An anterior interosseous nerve palsy is typified by the inability to form an ok sign secondary to the inability to flex the interphalangeal joint of the thumb (flexor pollicis longus) and the distal interphalangeal joint of the index finger (flexor digitorum profundus). The anterior interosseous nerve arises from the radial side of the median nerve, whereas the branches to the flexor carpi radialis and flexor digitorum superficialis arise from the ulnar side of the median nerve. After giving off the anterior interosseous nerve, the median nerve continues down the forearm, eventually passing through the carpal tunnel to enter the hand. In the hand, the median nerve innervates the LOAF muscles: lumbricals 1 and 2, opponens pollicis, abductor pollicis brevis, and flexor pollicis brevis. The median nerve also provides sensory innervation to the radial 3.5 digits on the palmar surface, as well as the corresponding segment of the palm through the palmar cutaneous branch, which arises proximal to the carpal tunnel and does not run through it. The pronator quadratus and flexor pollicis longus are innervated by the anterior interosseous nerve. The flexor pollicis brevis is innervated by the median nerve distal to the carpal tunnel. The supinator is innervated by the deep radial/posterior interosseous nerve.

Peripheral Nerve Flashcards

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