Myotonia congenita

Myotonia congenita is a congenital neuromuscular channelopathy that affects skeletal muscles, characterized by the failure of an initiated muscle contraction to terminate, which produces prolonged muscle stiffness and delayed relaxation. It is caused by mutations in the CLCN1 gene, which encodes the ClC-1 chloride channel; these loss of function mutations make muscle fiber membranes hyperexcitable and prevent normal relaxation after contraction. Patients experience delayed relaxation following voluntary contraction, muscle stiffness, muscle enlargement, and in some cases transient weakness, with complications including severe jaw muscle spasms, cramping, and an increased risk of falls. There are two main forms. Thomsen disease is autosomal dominant, clinically apparent from birth, and generally causes milder symptoms affecting the face, hands, and eyelids; it was named after Danish physician Asmus Julius Thomsen, who described it in 1876. Becker disease is autosomal recessive, appears later in childhood, and causes more severe myotonia and muscle stiffness, with patients experiencing temporary attacks of weakness that can become permanent over time.