Myotonic Disorders

Myotonic Disorders

Myotonia Congenita (Thomsen’s disease):
Autosomal dominant – possible a defect in chloride channel in muscle membrane  impaired repolarisation
prevalence of 2/100 000
myotonia present at birth  peculiar cry, difficulty feeding
no muscle weakness or wasting
cold aggravate muscle stiffness; symptoms become less severe in adults
muscles later hypertrophy, movements stiff and slow. Hypertrophy due to continual involuntary isometric exercise
treatment – responds wells to procainamide, phenytoin, quinidine

Myotonic Dystrophy (Steinert’s disease):
• Autosomal dominant
• prevalence of 13/100 000; M=F; most common adult muscular dystrophy
• onset around ages 20-30, but earlier if mother is carrier
• onset is with myotonia and/or weakness – often with a difficulty in releasing grip on an object. May have leg weakness (difficulty kicking a ball) or foot drop (if severe); proximal muscles remain strong
• other features include early frontal baldness, facial muscle weakness, cataracts, depressed or absent reflexes, lung and cardiac muscle involvement, gonadal atrophy
• premature death due to cardiac failure or chest infection
• treatment often not warranted

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