EMG in X-Linked CMT

Source: Gutierrez A, et al. Unusual electrophysiological findings in X-linked dominant Charcot-Marie-Tooth disease. Muscle Nerve 2000;23:182-188.

X-linked dominant charcot-marie-tooth disease (CMTX) is the second most common form of CMT and the result of a connexin 32, gap-junction protein gene mutation. Variable and intermediate range velocity slowing has raised uncertainty as to whether this is due to large fiber axonal dropout or primary demyelination. Nerve conduction studies of five genetically confirmed CMTX patients revealed decreased compound muscle action potentials (CMAP), prolonged F wave latencies, and, surprisingly, nonuniform conduction velocities between and within nerves, as demonstrated by differential slowing along a nerve, and temporal dispersion (> 40% increased duration of CMAP proximally compared to distally). These demyelinating features, confirmed by sural nerve biopsy, which revealed uniform loss of myelinated fibers and onion bulbs in the absence of inflammation, indicate that this hereditary neuropathy is demyelinating in nature but has the unique characteristics of acquired disease of temporal dispersion and nonuniform slowing. —mr

On electrodiagnostic studies:

a. sensory nerve conduction velocities are slowed significantly and more frequently in hereditary neuropathy with liability to pressure palsies (HNPP) than in chronic inflammatory demyelinating polyneuropathy (CIDP) or diabetes.

b. respiratory synkinesis ("breathing arm"), with bursts of motor unit activity in one or more arm muscles synchronous with respiration, is seen in 50% of syringomyelia patients.

c. complex repetitive discharges, even in the absence of positive waves or fibrillation potentials, are strongly predictive of inflammatory histology on muscle biopsy.

d. x-linked dominant Charcot-Marie-Tooth disease (CMTX) is predominantly axonal in nature but has the unique characteristics of acquired demyelinating disease of temporal dispersion and nonuniform slowing.

e. All the above are false