Variably Defined Conduction Block, Temporal Dispersion and Other Electrophysiological Abnormalities in Multifocal Motor Neuropathy: A Multicentre Study

Background The definition of conduction block (CB) is variable in multifocal motor neuropathy (MMN). In current criteria, excessive temporal dispersion (TD) may preclude the recognition of CB and the diagnosis of MMN. Methods We retrospectively studied the electrophysiological data of 47 consecutive subjects with MMN and of 69 consecutive controls with upper limb‐onset motor neuron disease, from three neuromuscular centres in the United Kingdom and Korea. Results Compared to CB defined by compound muscle action potential (CMAP) area reduction (CB‐Area) > 30%, CB defined by CMAP amplitude reduction (CB‐amp) > 30% was more sensitive (78.7% vs. 63.8%; McNemar's Test: p = 0.008), but less specific versus controls (90.1% vs. 96.7%; McNemar's Test: p = 0.001). CB‐amp > 30% offered greater diagnostic accuracy than CB‐Area > 30% (Youden's Index: 0.688 vs. 0.606). TD showed a sensitivity of 59.6% and specificity of 94.3%. F‐wave prolongation or absence showed a sensitivity of 42.6% and specificity of 96.9%. Considering CB‐amp > 30% or TD or F‐wave prolongation or absence, as independent electrodiagnostic markers of MMN, improved diagnostic sensitivity from 78.7% to 91.5% compared to CB‐amp > 30% alone (McNemar's Test: p = 0.031), also offering optimal accuracy (Youden's Index: 0.816). Within this three‐parameter combination, CB defined by CMAP amplitude reduction > 30% offered similar sensitivity, specificity and accuracy to when defined by CMAP amplitude reduction > 50%. Conclusions CB‐amp has higher sensitivity and accuracy than CB‐Area for the electrodiagnosis of MMN. Consideration of TD or F‐wave prolongation or absence, as independently diagnostic of MMN, in addition to CB‐amp > 30% alone, may improve electrophysiological sensitivity as well as accuracy.