myotonia

Myotonia is a neuromuscular phenomenon characterized by delayed relaxation of skeletal muscles after voluntary contraction or percussion, producing stiffness that can persist for seconds. It is most commonly a feature of certain genetic channelopathies and muscular dystrophies, rather than a single disease.

Causes and classification include inherited and acquired forms. The most common inherited form is myotonia congenita, caused by mutations in the CLCN1 gene, which encodes the skeletal muscle chloride channel. There are two classic subtypes, Thomsen and Becker myotonia, with Becker typically more severe. Other inherited forms arise from mutations in the SCN4A gene, leading to paramyotonia congenita or other myotonic phenotypes. Myotonia also features prominently in the muscular dystrophy diseases myotonic dystrophy type 1 (DM1) and type 2 (DM2), caused by nucleotide repeat expansions in DMPK and CNBP genes, respectively, and these conditions involve multisystem involvement beyond myotonia.

Clinical features vary by form. Patients may have grip myotonia (difficulty releasing a hand grip), facial stiffness, and generalized or focal muscle stiffness. A warm-up phenomenon, where repeated movements reduce stiffness, is common in some non-dystrophic myotonias, while paradoxical or cold-induced myotonia can occur in others.

Diagnosis relies on clinical assessment and electromyography, which shows characteristic myotonic discharges. Genetic testing confirms the underlying mutation. Management is individualized and may include physical therapy to improve strength and range of motion; pharmacologic treatment with sodium channel blockers such as mexiletine can lessen myotonia. Other antimyotonic agents, lifestyle adjustments to avoid triggers, and monitoring for associated conditions (especially in DM1/DM2) are important aspects of care.