Myotonia is prominent in the hand muscles, and ptosis is common even in mild cases. Mental retardation is common. Severely affected persons die by their early 50s. 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.