Isaacs' syndrome

Isaacs' syndrome is a rare nerve and muscle (neuromuscular) disorder. Doctors may also refer to it as neuromyotonia, Isaacs-Mertens syndrome, or continuous muscle fiber activity syndrome. It's characterized by abnormal nerve impulses from motor neurons of peripheral nerves. These impulses cause continuous activity in muscle fibers. Causes may include:

• An autoimmune disorder in which antibodies bind to the potassium channels on peripheral nerves
• Exposure to toxins, such as to gold or mercury
• Certain tumors, such as of the lung and thymus gland (thymoma) and lymphoma
• Genetic predisposition

Signs and symptoms of Isaacs' syndrome include:

• Continuous or intermittent twitching
• Progressive stiffness
• Persistent undulating movement in the skin that lies above the affected muscles (myokymia)
• Muscle cramps
• Slowed movement
• Increased sweating
• Slurred speech
• Difficulty chewing and swallowing

A doctor may diagnose Isaacs' syndrome by electromyography, which measures the tiny electrical discharges produced by muscles. There's no specific treatment for this disorder. Treatment is directed at managing the signs and symptoms, and may include:

• Immunosuppressive medications
• Anti-seizure medications, such as phenytoin or carbamazepine. In December 2007, the Food and Drug Administration announced that people of Asian ancestry should be tested for a genetic mutation associated with serious skin reactions before using carbamazepine.