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

Key Features

  • Occurs both as dominantly and recessively inherited forms

  • In the dominant form, generalized myotonia without weakness is usually present from birth, but symptoms may not appear until early childhood

  • The recessive form has later onset

Clinical Findings

  • The dominant form is characterized by muscle stiffness that is enhanced by cold and inactivity and relieved by exercise, as well as muscle hypertrophy, at times pronounced

  • The recessive form is associated with slight weakness and atrophy of distal muscles


  • In the dominant form, the responsible gene on chromosome 7 encodes a voltage-gated chloride channel


  • Procainamide, tocainide, mexiletine, or phenytoin may help the myotonia, as in myotonic dystrophy

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