Myotonia fluctuans.

Autosomal-dominantly inherited nondystrophic myotonic disorders are an interesting group of muscle diseases that provide considerable opportunity for future molecular genetic studies to identify the genes responsible for specific membrane functions. A family with such a myotonic disorder is described with features that are distinctly different from myotonia congenita and paramyotonia congenita. Five members were affected in three generations. The myotonia fluctuated to an unusual degree. It did not worsen with cold but increased markedly with potassium loading. Muscle weakness never occurred. Analysis of the contraction force of the flexor digitorum muscle showed a unique type of myotonia, namely, exercise-induced delayed-onset myotonia. Microelectrode studies done on one muscle biopsy specimen revealed a normal chloride conductance of the muscle fiber membrane.