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Peripheral neuropathy (peripheral neuritis) is fairly common. The incidence varies with the specific type of neuropathy.

Peripheral neuropathy is not a specific, separate disease. It is a manifestation of many conditions that can cause damage to the peripheral nerves. The peripheral nervous system includes all nerves not in the central nervous system (CNS), which includes the brain and spinal cord. The peripheral nervous system is the nerve system used by the CNS to communicate with the rest of the body, including the cranial nerves and the nerves supplying the internal organs, muscles, skin, and other areas. Damage to the nerves of the peripheral nervous system impairs communication between the CNS and the rest of the body.

Symptoms depend on whether the disorder affects the sensory nerve fibers (the fibers that transmit sensory information from the affected area to the brain and spinal cord) or motor nerve fibers (the fibers that transmit impulses from the brain and spinal cord to the area) or both. Damage to sensory fibers results in changes in sensation ranging from perception of abnormal sensations, pain, decreased sensation, lack of sensation, or an inability to determine joint position sense in the area. Sensation changes usually begin in the feet or hands and progress toward the center of the body with peripheral neuropathies that include degeneration of the axon portion of the nerve cell or loss of myelin surrounding the axon.

Damage to the motor fibers impairs movement or function of the area supplied by the nerve, because impulses to the area are blocked. Impaired nervous stimulation to a muscle group results in weakness, decreased movement or control of movement. Loss of nerve function causes structural changes in muscle, bone, skin, hair, nails, and body organs. Structural changes are caused by lack of nervous stimulation, not using the affected area, immobility, and lack of weight bearing. There is muscle weakness and muscle wasting (atrophy, loss of muscle mass). Recurrent, unnoticed injury to the area may occur due to lack of ability to feel, causing injury which may result in infection or structural damage. Changes include ulcer formation, poor healing, loss of tissue mass, scarring, and deformity.

The autonomic nervous system is the part of the peripheral nervous system that controls involuntary or semi-voluntary functions, such as control of internal organs. Damage to nerves of the autonomic nervous system causes changes in functioning of these organs, resulting in symptoms such as blurred vision, decreased ability to sweat (anhidrosis), dizziness or fainting associated with a fall in blood pressure, decreased ability to regulate body temperature, disturbances in function of the stomach or bowel, disturbances of bladder function, and sexual dysfunction.

Peripheral neuropathy may involve damage to a single nerve or nerve group (mononeuropathy) or may multiple nerves (polyneuropathy). Neuropathy involves damage to the axon of the nerve cell or the myelin sheath. Causes include nerve destruction from various disorders and nerve destruction caused by pressure. Degeneration of the axon slows or blocks conduction at the point of the degeneration. Demyelination (destruction of the myelin sheath around the nerve cell) greatly decreases the speed of impulse conduction through the nerve.

In some cases, no detectable cause of the neuropathy can be identified. Pressure injury may be caused by direct injury or compression of the nerve by other nearby body structures. Compression can be caused by peripheral nerve tumors (rare), tumors that press on nerve tissue (rare), abnormal bone growth, or cysts or collections of fluid or tissue that press on nerves. Compression can also be caused by pressure from casts, splints, braces, crutches, or other appliances. Prolonged periods in cramped positions may cause nerve injury. Nerve damage may occur after surgery due to being in one position for a prolonged period of time. Entrapment neuropathy includes compression of a nerve where it passes through a narrow space. The mechanical factors may be complicated by ischemia (lack of oxygen that results from decreased blood flow) in the area. Systemic causes of neuropathy include disorders affecting the connective tissues of the nerves, disorders affecting the blood supply to the nerve cells, hereditary conditions, metabolic or chemical disorders, and other disorders that damage nerve tissue.

Some of the specific conditions associated with neuropathy include: