Walsh & Hoyt: Neuromyotonia

Acquired neuromyotonia, also called Isaacs syndrome, the syndrome of continuous muscle fiber activity, or pseudomyotonia, is a disorder of unknown cause that produces continuous muscle activity and delayed relaxation following active contraction. The continuous motor unit activity is caused by peripheral nerve hyperexcitability and thus persists during sleep, may or may not be eliminated by peripheral nerve block, and is abolished by curare. Neuromyotonia can develop in association with clinical or electrophysiologic evidence of a peripheral neuropathy or as a neurologically isolated syndrome. Affected patients typically experience progressive stiffness and twitching of muscles, cramping and increased stiffness with use of the extremities, and, during ambulation, difficulty relaxing the muscles following use, difficulty ambulating, and excessive perspiration. Abnormal physical findings include continuous myokymia and fasciculations of affected muscles, hypertonia, a stiff awkward-appearing gait with extended extremities, reduced muscle stretch reflexes, and prolonged contraction of muscles following exercise (pseudomyotonia). Weakness and wasting of muscle bulk are generally not present in neuromyotonia. In contrast to the stiff-man syndrome, the distal and proximal limb muscles, the facial muscles, the bulbar muscles, and the extraocular muscles can be affected. Also in contrast to stiff-man syndrome, the motor findings of neuromyotonia persist during sleep. Electrophysiologic abnormalities in this condition include continuous motor unit discharges (e.g., fasciculations and myokymia) and muscle fiber discharges (e.g., high-frequency repetitive discharges), as well as afterdischarges following direct compound muscle action potential or in response to voluntary contraction or percussion.