Novel CLCN1 Mutation in Carbamazepine-Responsive Myotonia Congenita

Myotonia congenita is a nondystrophic muscle disorder characterized by muscle stiffness and muscle hypertrophy. The disorder can be inherited in an autosomal-dominant (Thomsen disease) or autosomal-recessive (Becker disease) manner. Both forms of myotonia congenita are attributable to mutations in the CLCN1 gene. Treatment with a variety of medications has led to long-term improvement in the clinical course of affected individuals. We describe a Honduran boy with myotonia congenita and a novel p.L287I mutation in the CLCN1 gene. The patient's unaffected father carries the same mutation, most likely reflecting autosomal-recessive myotonia congenita, with an inability to find a second mutation. The patient received carbamazepine treatment for 1 year, resulting in decreased muscle stiffness, increased strength, and improved quality of life in school and with peers.