Neurophysiological tests of hereditary neuropathy

Abstract

Treatment of Charcot–Marie–Tooth disease (CMT) has been challenging. However, appropriate diagnosis between CMT and other treatable conditions such as inflammatory neuropathies is important. Before pursuing genetic studies, workup should include electrophysiological and imaging studies.

Traditionally, CMT has been divided into demyelinating and axonal subtypes, that are electrophysiologically distinguished by conduction velocities (CV) of the upper limb (i.e., demyelinating if CV is slower, and axonal if CV is faster than 38m/s in the forearm segment). Of note, axonal neuropathy could demonstrate slow conduction velocity due to loss of fast–conducting fibers, such that careful evaluation is necessary in case of a low amplitude of compound muscle action potential (CMAP). If axonal degeneration is too severe to record CMAP, one should consider to record from proximal muscles, such as tibialis anterior and biceps brachii, and even blink reflex to find any demyelinating feature.

On the other hand, imaging studies has been utilized to visualize peripheral nerves. In demyelinating neuropathies, peripheral nerves are thickened and can be visualized by MRI and ultrasound. Among demyelinating neuropathies, CMT1A shows by far the largest nerve size. Nerve enlargement in CMT1A can be visualized in infancy, even before 6 months of age and become larger with age. This information is important in pre–symptomatic detection of demyelinating neuropathy and sonography could be used as a biomarker for therapeutic intervention.