Heterozygote manifestation in recessive generalized myotonia.

The frequency of heterozygotes of recessive generalized myotonia may be estimated at about 1/108 in the German Federal Republic. Some heterozygotes can be identified by an EMG. However, apart from this, apparently 2%--5% of heterozygotes may show minor subclinical manifestations. Sporadic cases of myotonia with late onset and a history of preceding, extremely prolonged physical stress, undernourishment, and/or prolonged cold exposure may be due to heterozygote manifestations of this otherwise recessive gene. Late onset and sporadic appearance also are features in patients with myotonia associated with hypothyroidism. One male patient displayed myotonia after a protracted diabetic coma. In cases reported in the literature where myotonia developed in association with either propranolol (beta-adrenergic blocking agent) or fenoterolhydrobromide (stimulator of beta receptors) heterozygote manifestation of recessive generalized myotonia is suggested.