DIFFERENTIATION BETWEEN AXONAL AND DEMYELINATING NEUROPATHIES - IDENTICAL SEGMENTS RECORDED FROM PROXIMAL AND DISTAL MUSCLES

The presence of significant slowing of motor nerve conduction velocity is considered one of the electrodiagnostic hallmarks of demyelinating neuropathies; however, slowing of conduction velocity may also accomp any severe axonal loss. When compound muscle action potential (CMAP) a mplitudes are markedly reduced, it is frequently difficult to determin e if conduction velocity slowing is due to axonal loss with dropout of the fastest conducting fibers or demyelination, To evaluate the relat ionship between conduction velocity and axonal dropout, we compared co nduction velocities through the same segment of nerve recording from d istal and proximal peroneal muscles in patients with chronic neuropath ies, in patients with motor neuron disease, and in control subjects. I n controls and patients with motor neuron disease, conduction velociti es were normal with no significant difference between proximal and dis tal sites. In patients with axonal neuropathies, conduction velocities were preferentially slowed when recording from distal muscles and rel atively normal when recording from proximal sites. Patients with demye linating neuropathies showed marked slowing of conduction at both site s. We conclude that comparing conduction velocity obtained from proxim al versus distal muscle recordings provides a simple, reliable aid for differentiating between chronic axonal and demyelinating polyneuropat hies, especially in cases with conduction velocity slowing and low CMA P amplitudes. (C) 1995 John Wiley and Sons, Inc.