Hereditary myokymia and periodic ataxia.

A kindred in which at least 11 individuals in 3 consecutive generations have continuous muscle movement, i.e., myokymia, and periodic ataxia, has been studied. Three patients, a 24-year-old woman, her 4-year-old son and her 27-year-old sister, have been studied in detail. The disorder is inherited as an autosomal-dominant trait and presents in early childhood with attacks of ataxia of 1-2 min in duration, with associated jerking movements of the head, arms and legs. Attacks are provoked by abrupt postural change, emotional stimulus, and caloric-vestibular stimulation. At the age of 12 years approximately, facial and extremity myokymia appears. Physical findings include large calves, normal muscle strength and widespread myokymia of face, hands, arms and legs with a hand posture resembling carpopedal spasm. EMG studies at rest showed continuous spontaneous activity of otherwise normal motor units. Nerve conduction velocities were normal. Gastrocnemius biopsy in 2 patients showed fiber type grouping and small angular fibers, and was consistent with denervation. Histographic analysis of the biopsies demonstrated enlargement of both fiber types, particularly of Type I fibers. These findings are consistent with chronic denervation and an abnormality of motor neuron population or firing. The myokymia described here is of interest not only because of its genetic association with a movement disorder, but also because the muscle findings support a peripheral basis for the muscle movements.