Myotonia congenita with painful muscle cramps.

A sporadic Japanese case of myotonia congenita with painful muscle cramps is reported. Electromyographic examinations disclosed myotonic discharge with dive bomber sounds at insertion, and high-amplitude, high-frequency motor unit potentials during the muscle cramps. Biopsied muscle specimens and EMG findings showed non-specific mild myopathic changes. There was no abnormal expansion of CTG repeat within the myotonic dystrophy gene. This patient's disorder closely resembles Becker's myotonia congenita Type II though the family history of was non contributory.