Congenital myotonia

Congenital myotonia is a group of inherited neuromuscular disorders characterized by myotonia — the inability of muscles to relax quickly after voluntary contraction. This condition primarily affects skeletal muscles throughout the body, leading to muscle stiffness that is often most prominent in the legs, hands, and face. Congenital myotonia encompasses two classic forms: Thomsen disease (autosomal dominant) and Becker myotonia (autosomal recessive), both caused by mutations in the CLCN1 gene encoding the voltage-gated chloride channel ClC-1 of skeletal muscle. The condition is also sometimes referred to as myotonia congenita. Patients typically experience muscle stiffness that worsens after periods of rest and improves with repeated movement, a phenomenon known as the "warm-up" effect. Muscle hypertrophy (enlarged muscles) is common, giving patients a muscular or athletic appearance. In Becker-type myotonia, symptoms tend to be more severe and may include transient episodes of muscle weakness. Onset is usually in childhood, though the dominant Thomsen form may present earlier, sometimes in infancy. Symptoms can affect daily activities such as walking, gripping objects, and initiating movements after rest. There is no cure for congenital myotonia, but treatment focuses on symptom management. Mexiletine, a sodium channel blocker, is the most commonly used medication to reduce myotonia. Other agents such as carbamazepine, phenytoin, and acetazolamide may also be considered. Physical therapy and regular exercise can help manage stiffness. The condition is generally non-progressive or only slowly progressive, and life expectancy is typically normal. Genetic counseling is recommended for affected families to understand recurrence risks.

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