Patients with nondystrophic myotonias typically have myotonia as an isolated symptom, without muscular wasting, although the patient's myotonia may be associated with muscle weakness and fatigue or transient attacks of paralysis. Episodes of myotonia may be triggered by cold (paramyotonia congenita), potassium (potassium aggravated myotonia), or exercise (Thompson and Becker myotonia). Most cases of nondystrophic myotonia are caused by mutations in 2 skeletal muscle ion channels: the voltage-sensitive sodium channel responsible for carrying the action potential (SCN4A) and the chloride channel responsible for maintaining the resting membrane potential (CLCN1).1
Hoffman EP, Kaminski HJ. Mexiletine for Treatment of MyotoniaA Trial Triumph for Rare Disease Networks. JAMA. 2012;308(13):1377–1378. doi:10.1001/jama.2012.12906
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