myotoni

Myotonia is a neuromuscular condition characterized by delayed relaxation of skeletal muscles after voluntary contraction. It is a feature commonly associated with various inherited disorders called myotonias, which involve mutations affecting muscle ion channels, particularly chloride or sodium channels. The hallmark of myotonia is muscle stiffness or prolonged contraction experienced after activity, often described as a "locking" sensation in affected muscles.

Myotonia can be classified as either primary or secondary. Primary myotonia is inherited and typically follows an autosomal dominant pattern, with examples including myotonia congenita (Thomsen and Becker types) and paramyotonia congenita. Secondary myotonia may result from metabolic or neurological diseases, such as hypothyroidism or certain neurological disorders.

Symptoms generally include muscle stiffness, difficulty relaxing muscles after movement, muscle hypertrophy, and percussion myotonia (muscle “stunning” upon percussion). While muscle weakness is not a defining feature, some individuals may experience transient weakness during activity. Symptoms often worsen with cold or stress and improve with activity or warmth.

Diagnosis involves clinical evaluation, electromyography (EMG), and genetic testing. EMG typically reveals myotonic discharges—characteristic waxing and waning electrical activity. Treatment primarily focuses on symptom management, using medications such as mexiletine, which reduces muscle stiffness by modulating ion channel activity.

Although myotonia can significantly impact quality of life, it is generally a non-progressive condition. Advances in understanding the genetic basis of myotonias continue to inform better diagnostic and therapeutic strategies, improving patient outcomes and management.