Myotonia Congenita

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

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