Nerve conduction abnormalities in chronic inflammatory demyelinating polyneuropathy (CIDP)]

Changes in compound muscle action potentials (CMAPs) were evaluated in 13 cases with CIDP by using inching-stimulation technique. The presence of electrophysiological multifocal lesion distinguished CIDP from hereditary demyelinating neuropathy (HDN). The size of each CIDP lesion was sometimes as small as a few millimeters, showing a decrease in CMAP amplitude proximally. No HDN cases revealed such focal changes. It is thus important to demonstrate small lesions with CMAP changes of demyelination-type in diagnosis of CIDP. Decrease in amplitude of proximally-evoked CMAP is essential but sometimes inadequate to determine partial conduction block, because HDN may show decrease in CMAP amplitude, up to 45%, by elbow stimulation as compare to one by wrist stimulation in our series. In determining partial conduction block a standard two-point stimulation can cause an error, because that threshold is quite high over a demyelinated segment, and that a cross stimulation may occur when high voltage stimulation is applied. Multiple-sites stimulation is the most reliable technique to demonstrate convincing changes in CMAPs over small demyelinating lesions. Revival of blocked motor fibers introduces new units to CMAP. In most cases some increase in CMAP amplitude occur first, because remyelinated fibers are slowly-conducting. Some units with shorter latency occasionally revived first, which suggests a possibility of conduction block without morphological background.