A rare nerve disorder that causes constant muscle activity that cannot be controlled, even during sleep. It often affects the muscles in the arms and legs, but may affect the whole body. Symptoms include muscle twitching, weakness, stiffness, and cramping; increased skin temperature, sweating, and heart rate; and problems with chewing, swallowing, speech, and breathing. The disorder often gets worse over time. Isaac syndrome usually occurs in people aged 15 to 60 years. It may occur with certain types of cancer and is sometimes inherited. Also called neuromyotonia.