Disorders of one or more of the nerves outside the brain and spinal cord, known as the peripheral nerves
Disorders of the peripheral nerves, the nerves that branch from the brain and spinal cord to the rest of the body, are called neuropathies. Depending on the nerves affected, peripheral neuropathies may affect sensation, movement, or automatic functions, such as bladder control. Rarely, a peripheral neuropathy may be life-threatening.
In developed countries, the most common cause of damage to the peripheral nerves is diabetes mellitus (see Diabetic neuropathy). Vitamin B complex deficiencies and some nutritional disorders may also result in nerve damage (see Nutritional neuropathies). In the developed world, nutritional neuropathy is often the result of a poor diet in people who abuse alcohol. Drinking too much alcohol may also damage peripheral nerves directly.
Damage to a single nerve may occur as a result of an injury (see Peripheral nerve injuries) or because of compression. For example, in carpal tunnel syndrome, the median nerve, which supplies part of the hand, is compressed at the wrist.
Neuropathy may also be associated with an infection, such as leprosy (see Hansen’s disease) or HIV infection (see HIV infection and AIDS). Guillain–Barré syndrome, a neuropathy that is rapidly progressive, is caused by an abnormal immune response that sometimes occurs after an infection.
Autoimmune disorders such as systemic lupus erythematosus, in which the immune system attacks the body’s tissues, may cause nerve damage. Occasionally, a disorder such as polyarteritis nodosa may damage nerves by causing inflammation of the blood vessels that supply them. Neuropathy may also result from certain cancers, particularly primary lung cancer, breast cancer, and lymphoma. Occasionally, neuropathy is caused by amyloidosis, in which an abnormal protein is deposited in the body.
Some drugs, such as isoniazid (see Antituberculous drugs), may cause nerve damage, as may exposure to certain toxic substances, such as lead. In some cases, the cause is unknown.
Peripheral neuropathies may affect the nerves that transmit sensory information (sensory nerves), the nerves that stimulate the muscles (motor nerves), and/or the nerves that control automatic functions (autonomic nerves).
These neuropathies usually start in the extremities (commonly the feet first, then the hands), then progress up the limbs. The symptoms may include tingling, pain, and numbness in the affected area. If the fingertips are numb, everyday tasks may become difficult. This type of neuropathy is most often caused by diabetes mellitus, nutritional disorders, or drugs.
If the motor nerves are damaged, the muscles they supply become weak, and wasting occurs eventually. In severe cases, mobility may become restricted, and, very rarely, breathing may have to be assisted by mechanical ventilation (see Intensive therapy unit). Lead poisoning may result in a neuropathy that affects the motor nerves only.
A neuropathy that is affecting one or more autonomic nerves may result in constipation, fainting due to low blood pressure (see Hypotension), diarrhoea, urinary incontinence, or erectile dysfunction. This type of neuropathy is often caused by long-standing diabetes mellitus.
Your doctor may be able to tell which nerves are affected from your symptoms and an examination. If the cause of your neuropathy is not clear, he or she will probably arrange for blood tests to look for evidence of an underlying disorder, such as diabetes, nutritional deficiencies, or an autoimmune disorder. If there is evidence of compression of a nerve root, you may also have CT scanning or MRI to assess the severity and extent of nerve damage. Special tests to assess the function of the nerves (see Nerve and muscle electrical tests) may also be carried out.
The treatment of a peripheral neuropathy depends on the cause and the type of nerve affected. For example, careful control of diabetes mellitus may keep diabetic neuropathy from worsening, and vitamin B complex injections (see Vitamins) may help a nutritional neuropathy. If motor nerves are affected, you may have physiotherapy to help to maintain muscle tone. Wearing a foot splint may assist walking. Sometimes, the underlying cause can be treated, but long-standing nerve damage may be irreversible.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.