Sections View Full Chapter Figures Tables Videos Annotate Full Chapter Figures Tables Videos Supplementary Content + Download Section PDF Listen ++ For further information, see CMDT Part 24-35: Myopathic Disorders + Key Features Download Section PDF Listen +++ ++ 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 Download Section PDF Listen +++ ++ 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 + Diagnosis Download Section PDF Listen +++ ++ In the dominant form, the responsible gene on chromosome 7 encodes a voltage-gated chloride channel + Treatment Download Section PDF Listen +++ ++ Procainamide, tocainide, mexiletine, or phenytoin may help the myotonia, as in myotonic dystrophy