[Midmotoraxonal reexcitation observed in a patient with hereditary motor and sensory neuropathy (HMSN) type IA]

We report a unique late motor unit potential observed in a patient with hereditary motor and sensory neuropathy (HMSN) type IA. The characteristics of the potential were as follows; (1) constant appearance with more than 20 mA of the stimuli, without disappearance with supramaximal stimuli, (2) constant wave-form and amplitude, (3) shorter latency with more proximal stimulation (indirect pathway), (4) remarkable fluctuation of the latency, and (5) disappearance with paired stimuli. The characteristics (1)-(3) correspond to Tomasulo's "peripheral late wave". But our late potential had fluctuation in its latency and was cancelled by paired stimuli, which indicates that this potential originated from the reflection at the midmotoraxonal demyelinating portion. Demyelinating portion is electrically unstable, where the duration of action potential is elongated. Consequently, nerve action potential at that site continues even after the refractory period is over at the adjacent distal portion, which might be the reason that this reflecting potential was evoked.