Cervical Spine injury and Neurophysiological Monitoring

Question 1

Describe the arterial blood supply to the spinal cord

Answer 1

There are 3 sources of supply:

1) Anterior Spinal Artery: Formed by the union of the 2 vertebral arteries at the foramen magnum. It supplies the anterior 2/3s of the cord (which covers the spinothalamic and corticospinal tracts)

2) 2 Posterior Spinal Arteries: which are formed from either the each of the vertebral arteries or the posterior inferior cerebellar arteries, and these supply the final 1/3 of the spinal cord (which covers the dorsal columns)

3) The segmental arterial supply: Numerous paired branches perfuse the spinal cord along its length. They arise from the Vertebral, deep Cervical, Intercostal, Aortic, and Pelvic Vessels.

The artery of Adamkiewicz is the biggest segmental artery and forms a major supply to the lumbosacral spinal cord, arising at a variable vertebral level between T8 and L4, although most commonly between T12 and L1. It usually arises from an intercostal artery, but less commonly from a lumbar artery.

Question 2

What forms of Neurophysical monitoring can be used during cervical spine surgery, and describe the specifics of their use.

Answer 2

Somatosensory evoked potentials:

Peripheral sensory nerves (usually ulnar / median / posterior tibial) are stimulated, and electrodes are placed on the scalp to record cortical response. They mainly test the integrity of the dorsal columns +/- the spinothalamic tracts

Motor Evoked Potentials:

The motor cortex is stimulated, and electrodes are placed on peripheral muscles to record impulses. These mainly test the integrity of the corticospinal tracts; their use does come with risks, including tongue biting, scalp burns, and seizures.

Electromyography:

Needle electrodes are placed into a specific muscle group to detect surgical irritation or damage of a particular nerve. Complete transection of the nerve will result in the loss of signal

EEG:

Topical scalp electrodes give information about the depth of anaesthesia and cerebral blood flow

Question 3

How may the use of neurophysiological monitoring affect the choice of anaesthetic technique?

Answer 3

Volatile agents reduce the amplitude of Motor Evoked Potentials; Hence, TIVA is preferred

Neuromuscular blockade results in the loss of motor evoked potentials and electromyography signals and hence should be avoided.

Ketamine can increase the amplitude of motor and sensory evoked potentials and may be used to enhance low-amplitude, poorly defined Motor Evoked Potential responses

Alpha 2 agonists may reduce motor evoked potentials and may therefore be unhelpful during monitored surgery.

Question 4

What physiological approaches are there to minimise the risk of neurological injury during cervical spine surgery

Answer 4

Physiological maintenance of ANY neurological injury, be it spinal cord, cerebral, or peripheral nerve, relates to the maintenance of oxygen delivery and perfusion. Hence, the approaches that could be used include:

Optimal Ventilation to avoid hypoxia and hypercapnia

Maintenance of MAP to ensure maintenance of Spinal cord perfusion pressure

Replacement of any significant blood loss

Maintenance of normal acid-base status

Maintenance of normothermia

Question 5

What are the surgical complications of cervical spinal surgery

Answer 5

Spinal Cord or Nerve Root Injury due to: direct injury, local haematoma, or metal work migration

Bleeding or Haematoma resulting in postoperative airway compromise

Infection leading to discitis, meningitis, or cerebral abscess

Dural Tear and CSF Leak

Damage to local structures:

An anterior approach surgery may cause damage to the oesophagus, trachea, vertebral and carotid arteries, recurrent laryngeal and hypoglossal nerves, as well as the sympathetic chain.

While the posterior approach to surgery may cause damage to the vertebral arteries

N.B. Cervical spine surgery can be performed via a posterior or anterior approach, and hence the approach plus the spinal level of the surgery would dicate some of the complications

Question 6

What are the potential complications of general anaesthesia in the prone position?

Answer 6

Airway

Accidental extubation

Airway and Tongue oedema leading to postoperative airway obstruction

Respiratory

In obese patients, lung expansion may be reduced by failure to accommodate the abdomen within the pre-cut shape of the Montreal mattress.

However, generally speaking, the prone position should improve V/Q matching

Cardiovascular

Abdominal pressure may cause IVC compression, reducing venous return and cardiac output.

Loss of IV access on turning the patient

Neurological

Abnormal neck flexion or extension can result in impaired cerebral perfusion and venous drainage

Peripheral neuropathies affecting the brachial plexus, the ulnar nerve at the elbow, and the common peroneal nerve. Due to poor positioning

Central retinal artery occlusion due to pressure on the eye, corneal abrasion, ischaemic optic neuropathy

Gastrointestinal

Increased intra-abdominal pressure if care is not taken to ensure accommodation of the abdomen in the cut out of the Montreal mattress, resulting in gastric acid reflux with consequent oral and eye irritation

Cutaneomusculoskeletal

Direct pressure effects to the face, pinna, breasts, genitalia, femoral triangle