Myotonia Congenita

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

Diagnosis

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

Treatment

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