Repetitive nerve stimulation in the differential diagnosis of congenital myotonia.

Repetitive stimulation of the ulnar nerve at 5 Hz was performed in 9 patients with congenital myotonia, 3 with dominant and 6 with recessive transmission. At this frequency the motor response differed markedly, all the patients with the recessive form presenting a clear decremental response to repeated stimulation. The size and time course of the decrease seem to be related to the degree of muscular paresis. The series includes one case of dominant congenital myotonia type II, whose response to repetitive stimulation, reported for the first time, did not differ from that of the other cases with the classic Thomsen form. Electromyography would thus seem to be a sure means of differentiating recessive from dominant forms of congenital myotonia in sporadic cases.