Flashcards in Workbook questiond 6 - Nerve entrapment syndromes Deck (22):
Most peripheral nerves, such as the median nerve are commonly referred to as “mixed spinal nerves”. Identify 3 general characteristics of spinal nerves that give rise to use of the term “mixed spinal nerves”.
a) Directionality in which nerve impulses are conducted- afferents (conduction from sense organs to the CNS) and efferents (conduction from the CNS to the PNS)
b) Functional Modalities represented within the nerve, sensory, motor and autonomic
c) General or Special Modalities. This is really only true in the case of cranial nerves (e.g. visceral efferent; special sensory; general sensory
C) Myelination or lack of. Nerves are strictly divisible into two categories; either myelinated or unmyelinated. Myleinated axons can be further teased into lightly myelinated, or heavily myelinated.
D) The speed with which nerve impulses are conducted. The speed of nerve impulse conduction is heavily influenced by the level of myelination of the nerve in question. The speed of conduction in myleinated axons is directly proportional to the fibre cross-sectional diameter, whilst in unmyelinated axons it is
E) Thickness of axonal fibres. Whilst it is true that myelinated axons are more likely to have thicker fibres than unmyelinated axons, it is interesting to note that the cross-sectional diameter of an axon is directly proportional to the size of its cell body.
What is the explanation for the various waves of a compound action potential
The various waves of the compound action potential suggest that there are differences in the speed with which impulses are conducted in human peripheral nerves. In particular, it suggests that there are 3 general waves to the compound action potential, thus, implying that there must be 3 general categories with respect to relative speeds of impulse conduction.
Additionally, the first major wave is itself complex but relatively narrower with respect to the latter 2, with the last wave appearing much wider. The first general wave suggests that the speeds of conduction within the nerve fibres constituting it are tightly packed whilst for the latter two, these are more widely distributed, with the last wave, particularly wider.
When carrying out nerve conduction investigations of a specific mixed spinal nerve, which sensory modalities would you expect to be activated at just threshold intensities of stimulation (i.e. minimal intensities). Justify your answer.
Proprioceptors and in particular, muscle spindle afferents because these are the most heavily myelinated nerves in the body. Since myelin reduces the capacitance of axons, it follows that the most heavily myelinated axons would be easiest to bring to threshold by electrical stimulation since the will require relatively less current to activate them.
When carrying out nerve conduction investigations of a specific mixed spinal nerve, which sensory modalities would you expect to be activated last (i.e. maximal intensities of stimulation). Justify your answer
C-fibres (hence slow pain fibres). These are unmyelinated axons of the body. Their lack in myelin means that their axons have a relatively high capacitance, thus, making them particularly difficult to bring to threshold by electrical stimulation. As such, they are likely to be the last axons of the body to be activated by electrical stimulation.
History in this patient suggested that three weeks prior to the onset of pain, the affected hand became functionally weak. She had difficulty putting her signature to documents, gripping cutlery correctly during meals and finally her grip had weakened leading to dropping a mug full of tea. What is the explanation for these difficulties in this patient?
1) The nerve is under constant compression within the carpal tunnel by the progressively growing ganglion. This gives rise to a nerve compression syndrome. When nerves are gradually compressed, the most myelinated axons are most susceptible to the effects of compression, presumably due to disruption of their blood supply, leading to deprivation of nutritional substrate. In this case, the proprioceptive afferents are thus, being selectively knocked out of action, leading to the patient being unable to carry out skilled fine movements like putting her signature to documents. The difficulties may be arising due to lack of afferent feedback from muscle spindle afferent axons that have succumbed to compression 2) Whilst it is theoretically possible that although the motor deficits in this patient could be explained by defective signalling in proprioceptive afferents, the possibility of the compression of the nerves having progressed to also directly disrupt the signalling of motor axons has to be seen as an equally plausible explanation for the emergence of these motor deficits.
The patient described the problematic hand as not painful but as if “sometimes it was not mine”
What is the explanation for this lack of awareness of the hand here?
This is likely to have arisen from compression of proprioceptive elements of the nerves leading to a consequent lack of proprioceptive feedback from the hand. Thus, the brain was no longer aware of proprioceptive feedback from it, hence its alienation.
What is the explanation for a) Numbness? B) The sensation of pins and needles.
Anaesthesia due to compression of the nerve and its attendant blood supply.
Parasthesia due to compression. The pressure on the axons of the nerve due to compression may have led them to fire action potentials due to this non-physiological stimulus. Action potentials fired in this way would not have been encoded correctly and as such, when they occur, the nervous system is unable to make sense of them, hence the strange feeling of pins and needles.
History in this case indicates that when pain started to be noticeable, it seemed tolerable for a while but gradually worsened. What is the explanation for this progression in the perception of pain in this patient?
The gradual compression of the nerve led to a gradual and progressive increase in the recruitment of pain fibres. This would have given rise to the progressive increase in the severity of pain. (NB: The central phenomenon of acute pain progressing to chronic pain due to changes in the central wiring across pain pathways in the brain and spinal cord is avoided here as this may complicate this simple case scenario)
What does the term “ganglion” refer to in a carpal tunnel case?
A tumour growing axons of a peripheral nerve
What treatment strategies might you suggest would resolve this problem a) acutely and b) long-term?
Acutely- conservatively by trying to manage the pain in the hope that the condition stabilises.
Chronically- if this case does not settle due to progression of the growth, surgical removal of the growth may be inevitable for thre reasons: a) To relieve pain and suffering b) Reduce the possibility of this acute pain becoming a case of chronic pain; c) Surgical removal of the growth would avail it for biopsy.
How common are soft tissue injuries as the cause of lower back pain?
Identify soft tissues of the back that are likely to be implicated in such a case.
Most Common. Muscle Tissue; Ligaments of the sacro-iliac joint; tendons of muscles
What would be the characteristics of radiation of pain in lower back pain?
Local tenderness; Stiffness of back muscles: Stiffness of trunk musculature.
What would be the characteristics of radiation of lower back pain in entrapment syndrome which involved the sciatic nerve?
Shooting pain radiating down the leg and buttock. The pain here would trace the physical course taken by the anatomical layout of the nerve
Name the likely sites in which the sciatic nerve or its roots might be trapped.
a) Intervertebral Foramen of the spinal cord (Root values L2-L4);
b) Greater Sciatic Foramen;
c) As the nerve passes though the substance muscle of the buttock and in particular, pyrifromis
What further tests (non-imaging) might you carry out in order to ascertain involvement of the sciatic nerve?
Straight-Leg raise test
Why is cardiac pain referred the left arm?
Visceral pain fibres from the heart, travel in the cardiac nerves along with sympathetic afferent fibres to the superior cervical ganglion of the sympathetic trunk. As the heart develops at the same segmental level as structures within the dermatome T1, the pain fibres enter the spinal cord along with somatic afferents from the T1 dermatome of the left side. Within the spinal grey matter the visceral and somatic afferents converge on secondary fibres in the spinothalamic tracts so that pain originating in the heart is perceived within the cortex as if it comes from T1 - the left breast and the medial portion of the left arm
Where on the body surface do we experience pain originating in the diaphragm?
Visceral pain arising from the diaphragm may be felt in the region of the costal margins and in the shoulder region.
Why might this [diaphragm] pain be referred to two different regions?
Referred pain in the diaphragm is felt in two places because its peripheral regions are innervated, along with the skin over the costal margins, by inferior intercostal nerves arising from segments T5 - T11. The central portion along with skin over the shoulders is innervated by fibres in the phrenic nerve from segment C5.
Why may an amputated limb still be felt and still give pain?
Following the removal of an arm or leg, patients report sensations from the missing limb - phantom sensations.
These sensations usually involve the distal structures the hand or foot, where the sensory receptor density was the greatest rather than from the intervening arm or leg, although the phantom limb usually feels normal in size. For some patients the missing hand or foot appears to grow directly out of the stump.
Phantom sensations are usually associated with limbs but such sensations can arise from any part of the body e.g. phantom breasts after mastectomy, or a phantom body after transection of the spinal cord.
Phantom pain originates within the somatosensory regions of the cerebral cortex and can not normally be managed by opiate analgesia.
In severe injury such as in a RTA, victims may initially feel a mild pain only. Why?
In critical/stressful situations high order regions of the CNS including the frontal cortex and the somatosensory cortex can interact with the nociceptive pathway to reduce the sensation of pain. Fibres from these regions release opiate like neuropeptides including the enkephalins and the endorphins which act upon cells in the periaqueductal grey matter (PAG) of the midbrain.
Descending projections from the PAG activate serotoninergic fibres and noradrenergic fibres in the medulla which in turn activate enkephalinergic neurones in the dorsal horn of the spinal cord and trigeminal nucleus which moderate the nociceptive pathway.
The opioid receptors in the PAG are engaged by the ascending nociceptive fibres forming a pain modulating feedback loop and by cells in the hypothalamus.
In stressful situations the release of the hormone ACTH from the anterior pituitary is accompanied by the release of endorphin like chemicals
Which nerve transmits the sensation of toothache?
Sensory fibres from the upper teeth are carried in the maxillary branch (V2) and from the lower teeth the mandibular branch (V3) of the trigeminal nerve.