Dispersion of the distal compound muscle action potential in chronic inflammatory demyelinating polyneuropathy and carpal tunnel syndrome.

Median neuropathy at the wrist can confound the electrodiagnosis of chronic inflammatory demyelinating polyneuropathy (CIDP), since both conditions can prolong median distal motor latency. Dispersion of the distal CMAP (DCMAP) has recently emerged as a potentially useful adjunct in the electrodiagnosis of CIDP, with good specificity in distinguishing CIDP from certain axon-loss disorders. However, it is uncertain whether focal compression neuropathies produce dispersion of the DCMAP in a manner similar to CIDP. In this study we compared median DCMAP duration in 27 patients with CIDP and 86 with carpal tunnel syndrome, using 39 patients with non-neuropathic musculoskeletal pain syndromes as electrophysiologic controls. We found that, in contrast to CIDP, dispersion of the median DCMAP is uncommon, even in advanced carpal tunnel syndrome, being seen in only 8 of 103 (7.8%) hands. Although the pathophysiologic reasons for a differential effect of focal compression-mediated demyelination and multifocal immune-mediated demyelination (CIDP) on DCMAP duration are uncertain, our findings suggest that the presence of dispersion of the median DCMAP may prove useful in distinguishing immune-mediated demyelination from compression neuropathy alone.