Sustained atypical myokymia of the abductor pollicis brevis with a focal slowing of the median nerve motor axons at the wrist

摘要

ObjectiveWe report a case of sustained atypical myokymia associated with short bursts of neuromyotonic discharges involving the abductor pollicis brevis (APB) muscle and describe a useful way of detecting a focal slowing involving a small number of median nerve motor fibers with a concentric needle using the filter setting for single fiber electromyography (EMG).Methods and ResultsA 62-year-old woman developed right thumb twitches at regular interval of 1.7–3.3?s (0.6–0.3?Hz), which continued for more than four months. Muscle twitches remained the same during altered hand position, psychological stress, or sleep. A concentric needle inserted in the active zone of the APB muscle revealed myokymic bursts with a characteristic of neuromyotonic discharges. Inching study, stimulating at 5?mm increment along the median nerve and recording with a concentric needle using a filter setting for single fiber EMG, revealed a focal slowing of the motor fibers at a point 5–10?mm distal from the distal crease of the wrist, an entrapment site occasionally seen in the carpal tunnel syndrome. One injection of botulinum toxin type A eliminated the myokymia, which then recurred two and a half years later, showing less prominent muscle twitches.ConclusionsSustained atypical myokymia seen in our case represented bursts of neuromyotonic discharges originated from a focal demyelinating lesion involving a few median nerve motor fibers.