The spectrum of ectopic motor nerve behavior: from fasciculations to neuromyotonia.

Abstract

Ectopic impulses in motor nerves generate clinically and electromyographically detectable activity in muscle. These discharges occur in the form of isolated fasciculations, as persistent muscle activity in the form of myokymia and neuromyotonia, or in fulminant contractions of individual muscles in the form of a cramp. In the last 20 years, new studies have helped establish the relationship of fasciculations with cramps and neuromyotonia and have identified common pathophysiologic mechanisms. Current evidence suggests that both cramps and fasciculations originate primarily in the most distal motor nerve terminals. In this portion of the axon, the overlying Schwann cells do not form a myelin sheath and the blood-nerve barrier is relatively porous. The terminal axon is studded with receptors to monitor the release of neurotransmitters. Under certain stresses, reinnervation, ionic imbalances, motor nerve disease, or pharmacologic challenge, ectopic impulses arise and create visible, but sporadic fasciculations. Other circumstances, including muscle shortening and dehydration, give rise to more frequent and localized fasciculations that can erupt into a painful muscle cramp. The most unusual motor ectopic phenomena involves rapidly recurrent discharges in multiple motor nerves, giving rise to grouped fasciculations, including myokymia and neuromyotonia. Recent studies have implicated toxins, and autoimmune and genetic mechanisms in the generation of neuromyotonic and myokymic syndromes. Plasma exchange, carbamazepine, and phenytoin have proven helpful in select cases. These findings indicate that motor nerve hyperexcitability is a fruitful subject of clinical and laboratory investigation and that treatment based on underlying mechanisms proves beneficial.