Myotonic Dystrophy

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

Diagnosis

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

Treatment

• 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