In several families, the disorder has been linked to a region on chromosome 7 containing a skeletal muscle chloride channel gene. Painless muscle stiffness is most troublesome in the hands, legs, and eyelids and improves with exercise. Weakness is usually minimal. Muscles may become hypertrophied. Diagnosis is usually established by the characteristic physical appearance, by inability to release the handgrip rapidly, and by sustained muscle contraction after direct muscle percussion.
Family history is important.
Weakness does not respond to therapy, but active and passive exercises may help. Nifedipine is useful in treating myotonia unresponsive to other drugs. The hypokalemic form is due to genetic mutation in the dihydropyridine receptor-associated calcium channel gene on chromosome 1q. Symptoms and Signs Consciousness is not altered. In the hypokalemic form, attacks usually begin before the age of 16 yr.