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It can occur after an activity, like reading, in which a person. Autoimmune-mediated Issacs' syndrome is typically caused by antibodies that bind to potassium channels on the motor nerve. An accommodative spasm is a condition in which the eyes focus constantly or automatically. The acquired form occasionally develops in association with peripheral neuropathies or after radiation treatment, but more often is caused by an autoimmune condition. There are hereditary and acquired (occurring from unknown causes) forms of the disorder. Onset is between ages 15 and 60, with most individuals experiencing symptoms before age 40. Speech and breathing may be affected if pharyngeal or laryngeal muscles are involved. In most people with Issacs' syndrome, stiffness is most prominent in limb and trunk muscles, although symptoms can be limited to cranial muscles. Many people also develop weakened reflexes and muscle pain, but numbness is relatively uncommon. Symptoms, which include progressive muscle stiffness, continuously contracting or twitching muscles (myokymia), cramping, increased sweating, and delayed muscle relaxation, occur even during sleep or when individuals are under general anesthesia. Issacs' syndrome (also known as neuromyotonia, Isaacs-Mertens syndrome, continuous muscle fiber activity syndrome, and quantal squander syndrome) is a rare neuromuscular disorder caused by hyperexcitability and continuous firing of the peripheral nerve axons that activate muscle fibers. Peripheral neuropathy produces symptoms such as weakness, muscle cramps, twitching, pain, numbness, burning, and tingling (often in the feet and hands).