Myotonia congenita--a cause of muscle weakness and stiffness.

BACKGROUND: A 56-year-old woman was referred to a neurogenetic clinic with a history of stiffness and transient weakness. A previous needle electromyogram had confirmed the presence of myotonia, but a muscle biopsy had revealed no evidence of dystrophy. INVESTIGATIONS: Neurological examination, electrophysiological studies and genetic testing. DIAGNOSIS: Recessive myotonia congenita (Becker's disease). MANAGEMENT: Explanation of the nature of the disease and treatment with mexiletine 200 mg twice daily.