Myotonia

Myotonia is defined at an electrical level as repetitive discharge of the muscle fibre membrane after initial activation, which occurs due to dysfunction of the membrane’s ion channels, most commonly the chloride channel, less commonly the sodium channel. This manifests clinically as stiffness of the muscle and delayed relaxation after voluntary contraction (e.g. difficulty relaxing the grip after clenching the fingers, and stiffness in the thigh muscles and difficulty walking on first moving after rest). Disabling myotonia may respond to carbamazepine, phenytoin or, often most effectively, mexiletine, although supplies are now limited. This chapter takes a closer look at the condition, its symptoms, presentations, diagnosis, and treatment.