Neuropathies

Neuropathy is damage to one or more nerves.

Neuropathies can cause abnormal sensations (paresthesias), pain, loss of sensation, and weakness. Sometimes body functions are impaired. These neuropathies are called autonomic neuropathies because body functions are controlled by nerves that operate without conscious effort (autonomic nerves).

As many as one fifth of older people have some sort of neuropathy. The effects of a neuropathy range from slight to disabling. They may be temporary or permanent.

Causes

Physical injury can damage a nerve. Often, the injury is caused by prolonged pressure on a nerve that runs close to the surface of the body near a prominent bone. Examples are nerves in the knees, elbows, shoulders, or wrists. Pressure on a nerve during a long, sound sleep or after sitting too long with the legs crossed may be enough to cause damage. Such damage is usually only temporary. Repeated pressure on the nerve that supplies the fingers (due to habitually using the hands in the same way) can result in carpal tunnel syndrome.

See the figure Carpal Tunnel Syndrome: Numb Hands.

Diabetes and excessive use of alcohol are common causes of neuropathies. Other common causes are nutritional deficiencies (particularly of vitamin B₁₂), kidney disorders, cancer (such as multiple myeloma), and the use of certain drugs. These drugs include amiodarone (used to treat abnormal heart rhythms), colchicine (used to treat gout), and vincristine (used to treat cancer).

Less commonly, neuropathies result from infections (such as Lyme disease) and radiation therapy for cancer. Rare causes include exposure to toxic substances (such as lead, mercury, arsenic, or gold) and amyloidosis, which is a little known disorder.

Symptoms

Neuropathies may cause a pins-and-needles feeling, burning pain, numbness, or weakness in the affected part. If the neuropathy is severe, muscles may waste away (atrophy). Muscles are affected (even though they are not directly damaged) because healthy nerves are needed to keep muscles healthy.

Some people lose the abilities to feel vibrations (vibratory sense) and to know where their arms and legs are (position sense). If position sense is lost, walking and even standing may become unsteady. Consequently, muscles may not be used and may eventually weaken and waste away.

Other people lose the ability to feel temperature and pain. Consequently, they often burn themselves. Open sores may develop because these people are unaware of prolonged pressure or other injuries. Without pain as a warning of too much stress, joints are susceptible to injuries. This type of injury is called Charcot's joint. The affected joint becomes stiff, painful, and swollen. Without treatment, the joint may be permanently damaged.

Diabetes or a kidney disorder can damage the nerves that control body functions, such as salivation, digestion, urination, heart rate, blood pressure, and sweating (causing an autonomic neuropathy). Such damage can result in constipation, loss of bowel or bladder control (leading to fecal or urinary incontinence), sexual dysfunction, and changes in blood pressure. Blood pressure may suddenly fall when a person stands up (a disorder called orthostatic hypotension). The skin may become pale and dry, and less sweat may be produced.

Diagnosis

Doctors can often identify the cause of a neuropathy on the basis of symptoms and the results of a physical examination. Certain tests can help locate the damaged nerve and provide additional information. They include electromyography (which records electrical activity in muscles) and nerve conduction velocity studies (which measure how quickly nerve signals travel along nerves).

Doctors try to identify the disorder causing symptoms so that it can be treated. Blood and urine tests may be done to check for diabetes or a kidney disorder. Occasionally, a nerve biopsy is necessary. For this procedure, a small piece of a nerve, usually one near the surface, is removed through a small incision and examined under a microscope.

Treatment and Outlook

Specific treatment depends on the cause. If the cause is a disorder, it is treated. For example, vitamin B₁₂ deficiency may be treated with vitamin B₁₂ supplements. If a drug is the cause, it is discontinued if possible.

If the cause is exposure to a toxic substance, exposure to the substance is stopped. Doctors may give the person a drug that binds with the substance, causing it to pass out of the body in the urine. This treatment is called chelation therapy. Chelation drugs remove the substance slowly. These drugs can have serious side effects.

If pressure is causing the problem, avoiding the pressure may relieve symptoms. For example, sitting without crossing the legs may help. Moving the affected part can relieve the pressure. Then nerves can function normally again, and the symptoms may stop.

For pain, anticonvulsants, such as gabapentin, phenytoin, or carbamazepine, are sometimes useful. Other drugs that may help include some antidepressants, such as tricyclic antidepressants and selective serotonin reuptake inhibitors (SSRIs), and creams containing capsaicin (a substance in hot red peppers).

Physical therapy sometimes reduces muscle weakness.

The outlook for people with a neuropathy depends on the cause. Some neuropathies, such as carpal tunnel syndrome, resolve completely with treatment. Other neuropathies become progressively worse. An example is the neuropathy that causes numbness in the hands and feet of people with diabetes.