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For further information, see CMDT Part 24-36: Myopathic Disorders

Key Features

  • A slowly progressive, dominantly inherited myopathic disorder

  • Usually manifests itself in the third or fourth decade but occasionally appears early in childhood

  • Myotonic dystrophy type 1 results from an expanded CTG repeat in a protein kinase gene on chromosome 19

  • In type 2, the defect is a CCTG repeat expansion in the gene for zinc-finger protein-9 on chromosome 3

Clinical Findings

  • Complaints of muscle stiffness

  • Marked delay occurs before affected muscles can relax after a contraction; this can often be demonstrated clinically by delayed relaxation of the hand after sustained grip or by percussion of the belly of a muscle

  • Weakness and wasting of the facial, sternocleidomastoid, and distal limb muscles

  • Cataracts

  • Frontal baldness

  • Testicular atrophy

  • Diabetes mellitus

  • Cardiac abnormalities

  • Intellectual changes


  • Electromyography of affected muscles reveals myotonic discharges in addition to changes suggestive of myopathy


  • Phenytoin, 100 mg three times daily orally; procainamide, 0.5–1.0 g four times daily orally; or mexiletine, 150–200 mg orally three times daily

    • May be helpful

    • Associated side effects, particularly for antiarrhythmic drugs, are often limiting

  • Neither the weakness nor the course of the disorder is influenced by treatment

  • Cardiac function should be monitored and pacemaker placement may be considered if there is evidence of heart block

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