Sometimes nerves repeatedly send electrical impulses to muscles, resulting in overstimulation. Overstimulation is thought to be a factor in stiff-person syndrome and Isaacs syndrome.

Stiff-person syndrome: This syndrome is more common among women and often occurs in people with diabetes or certain kinds of cancer, including Hodgkin lymphoma (Hodgkin disease).

The cause may be an autoimmune reaction—when the body produces antibodies that attack its own tissues. Antibodies to an enzyme called glutamic acid decarboxylase are present, but whether they cause the symptoms is unknown.

Muscles of the trunk, abdomen, and legs gradually become stiffer and enlarge. Muscles of the arms, head, and neck are affected less.

The sedative diazepam can consistently relieve the muscle stiffness. Plasma exchange, in which toxic substances are filtered from the blood, is sometimes tried but often without success. Immune globulin, given intravenously, may help relieve symptoms for weeks or months. Usually, the disorder progresses, leading to disability and stiffness throughout the body.

Isaacs syndrome: This rare disorder has no known cause. It often occurs in people with cancer.

Muscles, particularly those in the arms and legs, continually twitch, moving like a bag of worms. This symptom is called myokymia. Spasms and cramps may intermittently occur in the hands and feet. Muscle stiffness is common. Sweating may be increased.

Symptoms can be relieved by the anticonvulsants carbamazepine or phenytoin.