Neurophysiology and Peripheral Nerve Demyelination

On neurophysiologic testing with motor nerve conduction studies (NCS), the pattern of changes in the recorded responses differs when focal neurapraxia occurs to the same degree and at the same site along multiple axons within a nerve compared with differing degrees of focal demyelination among different axons within the nerve. In disorders where uniform demyelination occurs at a focal site along a nerve (conduction block), stimulation of the nerve distal to the site will elicit a normal compound muscle action potential (CMAP) response, whereas stimulation proximal to the site will elicit a CMAP that is of lower amplitude and area but of similar morphology. (A) In contrast, when multifocal demyelination occurs among the axons within the nerve, the degree of slowing or block varies among different axons. As a result, stimulation distal to the areas of demyelination will result in a normal CMAP response, but stimulation at a proximal site will elicit a response that is of lower amplitude and area as well as increased in duration (temporally dispersed) (B).

With both axonotmesis and neurotmesis, the continuity of the axon is disrupted, and the portion of the axon separated from the anterior horn cell or posterior root ganglia undergoes wallerian degeneration. Axonotmesis occurs when axonal continuity is disrupted; however, the connective tissue, including the endoneurium, is preserved. Axonal regeneration and regrowth along the endoneurial tubes is still possible as long as the connective tissue along the endoneurial tube remains intact. Neurotmesis is a more severe stage of injury, where the axon, myelin, and connective tissue sheath, including the epineurium, are disrupted and the two ends of the nerve are separated. In this stage, effective recovery is very unlikely or impossible, depending on the amount of separation of the two ends of the nerve.

When NCS are performed in this setting during the first week after an axonotmetic or neurotmetic injury, and a nerve is stimulated electrically distal to the site of injury, the portion of the axon that is separated from the cell body will temporarily continue to have the ability to propagate an action potential. However, once an entire week of axonal wallerian degeneration occurs, the disconnected segment of the axon can no longer respond to electrical stimulation to conduct an action potential. Therefore the CMAP amplitude will be reduced or absent with distal and proximal stimulation sites. The motor fibers are more sensitive and lose their ability to conduct at about 7 days, whereas one may still obtain a sensory nerve action potential (SNAP) up to about 10 days.

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