a rare disorder resulting from abnormal, spontaneous muscle activity of neural origin, manifested as continuous muscle stiffness and delayed relaxation after exercise, often accompanied by pain, cramps, fasciculations, hyperhydrosis, and muscle hypertrophy (on EMG, manifests as myokymia). Isacc syndrome usually begins in the lower extremities but can affect abdominal, upper extremity, vocal, and respiratory muscles; it is most often sporadic, although autosomal dominant inheritance has been reported. Probably an autoimmune disease, with antibodies against the potassium channels of peripheral nerves. SYN: Isaac-Merton syndrome.