Electrophysiological evaluation of cranial neuropathies

Cranial nerve abnormalities occur frequently in both focal and diffuse neurologic disorders and can be evaluated by electrophysiological techniques available in most clinical neurophysiology laboratories.

The optic nerve is evaluated by visual evoked potentials. Measurements of latency, amplitude, and waveform morphology are especially useful in detecting demyelinating lesions. Brain stem auditory evoked potentials evaluate the auditory portion of the eighth cranial nerve. Using an auditory stimulus, a number of waveforms are generated, and changes in the normal patterns of response can detect abnormalities. Assessment of the trigeminal and facial nerves is done using a series of electrical stimulation techniques including the blink, masseter, and masseter inhibitory reflexes and facial motor nerve conduction studies. The blink reflex detects lesions of the first division of the trigeminal nerve and the facial nerve. The masseter reflex evaluates the third division of the trigeminal nerve. Changes in responses are measured and, using a combination of these techniques, localization of lesions at specific sites can be made. Accessory motor nerve conduction is useful not only in focal nerve injury, but repetitive stimulation on the accessory and facial nerves is used in diagnosing neuromuscular junction disorders. In addition, many of the voluntary muscles innervated by the cranial nerves are accessible to needle electrode examination, and evaluation can aid in identification of focal nerve lesions, as well as diagnosis in diffuse nerve and muscle disorders.

Electrophysiological techniques offer reliable means of measuring the integrity of the cranial nerves and their central pathways.