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.
Loss or impairment of voluntary use of one or more muscles. It may be flaccid (with loss of muscle tone) or spastic (stiff). Hemiplegia (one-sided paralysis) is usually caused by stroke or brain tumour on the opposite side. Diplegia (two-sided paralysis, as in cerebral palsy) results from generalized brain disease. Spinal-cord damage (from bone or joint disease, fracture, or tumour affecting the vertebrae; inflammatory and degenerative diseases; or pernicious anemia) paralyzes the body at and below the level of the damage (paraplegia if the legs and lower body only; quadriplegia if arms and legs). Poliomyelitis and polyneuritis (neuritis of multiple nerves) result in paralysis with muscle wasting. Bell palsy (a type of neuritis) paralyzes the muscles of one side of the face. Muscular dystrophy causes paralysis by attacking muscle. Metabolic causes include myasthenia gravis. Paralysis may also have psychiatric causes (see hysteria).
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Acute infectious viral disease that can cause flaccid paralysis of muscles. Severe epidemics killed or paralyzed many people, mostly children and young adults, until the 1960s, when Jonas Salk's injectable killed vaccine and Albert B. Sabin's oral attenuated live vaccine controlled polio in the developed world. Flulike symptoms with diarrhea may progress to back and limb pain, muscle tenderness, and stiff neck. Destruction of spinal cord motor cells causes paralysis, ranging from transient weakness to complete, permanent paralysis, in fewer than 20percnt of patients. Patients may lose the ability to use their limbs, to breathe, or to swallow and speak. They may need physical medicine and rehabilitation, mechanical breathing assistance, or tracheal suction to remove secretions. A “postpolio syndrome” occurs decades later in some cases, with weakness of muscles that had recovered.
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Paralysis can be seen in breeds of dogs that are chondrodysplastic. These dogs have short legs, and may also have short muzzles. Their intervertebral disc material can calcify and become more brittle. In such cases, the disc may rupture, with disc material ending up in the spinal canal, or rupturing more laterally to press on spinal nerves. A minor rupture may only result in paresis, but a major rupture can cause enough damage to result in complete paralysis. If no signs of pain can be elicited, surgery should be performed within 24 hours of the incident, to remove the disc material and relieve pressure on the spinal cord. After 24 hours, the chance of recovery declines rapidly, since with continued pressure, the spinal cord tissue deteriorates and dies.
Another type of paralysis is caused by a fibrocartilaginous embolism. This is a microscopic piece of disc material that breaks off and becomes lodged in a spinal artery. Nerves served by the artery will die when deprived of blood. The German Shepherd is especially prone to developing degenerative myelopathy. This is a deterioration of nerves in the spinal cord, starting in the posterior part of the cord. Dogs so affected will become gradually weaker in the hind legs as nerves die off. Eventually their hind legs become useless. They often also exhibit fecal and urinary incontinence. As the disease progresses, the paresis and paralysis gradually move forward. This disease also affects other large breeds of dogs. It is suspected to be an autoimmune problem.
Cats with heart murmurs may develop blood clots which travel through arteries. If the clot is large enough to block one or both femoral arteries, there may be hind leg paralysis because the major source of blood flow to the hind leg is blocked.