Differentiation between uniform and non-uniform motor nerve conduction slowing

Demyelination may cause a uniform reduction of the conduction velocity of all fibres of a peripheral nerve segment, or may affect only certain nerve fibres in a non-uniform way while sparing others. This study was done to improve the detection of non-uniform conduction slowing by using the high-frequency attenuation (HFA) method.

Nerve conduction data from patients with early inflammatory demyelinating neuropathy (non-uniform demyelination, n=20), hereditary neuropathy (uniform demyelination, n=9), motor neuron disease (axon loss, n=20), and from healthy control subjects (n=20) were analysed.

HFA, compound muscle action potential (CMAP) amplitude decay, and F-wave chronodispersion correlated significantly. Among these variables both HFA and amplitude decay most sensitively identified non-uniform demyelination (35%). In the patients with uniform demyelination, the most frequent finding was a reduced nerve conduction velocity (NCV) (100%). The most specific marker of non-uniform demyelination was HFA. For uniform demyelination it was NCV.

The pattern of correlations between the variables studied confirms that NCV and F-min are indicators of uniform conduction slowing. HFA, amplitude decay, and F-wave chronodispersion indicate non-uniform conduction slowing, for which HFA is both sensitive and specific.

The HFA method improves both the diagnostic sensitivity and specificity of nerve conduction studies in patients with non-uniform demyelination.