Myotonia Congenita

Myotonia congenita is an inherited skeletal muscle chloride channelopathy (CLCN1) causing muscle stiffness that eases with repeated movement ("warm up"). Becker type (autosomal recessive) is more common and Thomsen type (autosomal dominant) is milder. Prevalence is about 1 in 100,000 and higher in Scandinavia.

Pathophysiology. Reduced chloride conductance prevents repolarisation of the muscle membrane, causing repetitive discharges and delayed relaxation.

Stiffness after rest, worst on starting movement and in the cold, with transient weakness in Becker type, muscle hypertrophy ("athletic" appearance), grip and percussion myotonia, and eyelid lag. Onset is in childhood (Thomsen) or the first decade to adolescence (Becker). Stiffness worsens with fatigue, stress and pregnancy. There is no progressive myopathy and cardiac involvement is absent.

Electromyography shows myotonic discharges with waxing and waning ("dive bomber") sound. Short exercise test shows a transient fall in compound muscle action potential, which recovers more with cooling in Becker type. Genetic testing of CLCN1 confirms the diagnosis. Creatine kinase is normal or mildly raised.

Differential diagnosis.

• Paramyotonia congenita and sodium channel myotonia (SCN4A).
• Myotonic dystrophy types 1 and 2, and hyperkalaemic periodic paralysis.
• Neuromyotonia, Brody disease and hypothyroid myopathy.

Many mild cases need only reassurance and avoidance of triggers (cold, prolonged rest, sudden exertion).

Mexiletine. First line sodium channel blocker: 150 mg orally twice daily, increased every week to 200 mg three times daily (maximum 600 mg daily). Take with food. Do an ECG and echocardiogram before starting, and avoid in heart block, structural heart disease and severe liver disease. Monitor ECG and liver function.

Alternatives. Lamotrigine 25 mg daily, increased slowly to 100 to 200 mg daily, or carbamazepine 100 mg twice daily, increased to 200 to 400 mg twice daily. Acetazolamide 125 mg twice daily, increased as needed, helps some.

Avoid depolarising neuromuscular blockers (suxamethonium), which cause severe myotonic spasm, and anaesthetic care needs advance planning.

Lifestyle. Gentle warm up before exercise, regular exercise, and warm clothing.

Monitoring. Symptom review yearly, and ECG on mexiletine.

Referral and escalation. Refer to neurology and clinical genetics.

Special groups. In pregnancy, myotonia may worsen. Mexiletine is stopped or adjusted with specialist advice.