Occurrence of myasthenia gravis in a patient with congenital myotonia.

Myasthenia gravis developed acutely in a 32-year-old woman, who incidentally was found to have myotonia congenita of the autosomal dominant mode of inheritance. The simultaneous occurrence of myasthenia gravis and myotonia congenita was established clinically, pharmacologically and electromyographically. There was no immunologic abnormality, whereas the thymus was detected by pneumomediastinographic tomograms. About 2 months after the onset of myasthenia gravis, thymectomy was performed resulting in progressive improvement of myasthenia without apparent change in the myotonia.