paralysis or palsy (pôlˈzē) [key], complete loss or impairment of the ability to use voluntary muscles, usually as the result of a disorder of the nervous system. The nervous tissue that is injured may be in the brain, the spinal cord, or in the muscles themselves. Accordingly there may be general paralysis, involvement of only one side (hemiplegia), paralysis on both sides at one level (paraplegia or quadriplegia), or localized paralysis in a small group of nerves or muscles.
The cause of paralysis may be any injury that tears or compresses the nerves; it may be hemorrhage, tumor, infection, or substances toxic to nerve tissue. One of the most frequent causes of paralysis is stroke, in which hemorrhage, thrombosis, or obstruction of a cerebral vessel interferes with nerve function. Another disorder in which a resting tremor is one of the main symptoms, accompanied by slowness and poverty of movement, muscular rigidity, and postural instability is Parkinson's disease. Cerebral palsy is due to an injury to the brain motor tissue before or during birth. However, this disorder is nonprogressive. Partial or complete paralysis often accompanies multiple sclerosis.