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.