September 17, 2011

Nerves extend out from the central nervous system (spinal cord or brain) to supply muscle, skin, and other tissues. Nerves are comprised of a large number of microscopic nerve fibers. The core of each nerve fiber is the axon, which is a long, thin extension of a nerve cell located within or near the spinal cord or brain. The axon carries electrical impulses. Nerve fibers can be motor (making muscles contract), sensory, or autonomic (controlling blood vessels, sweat glands, and viscera). Large nerve fibers (which serve motor or fine sensory function) have an insulation of a fatty substance called myelin surrounding the axon. Small nerve fibers (which may be myelinated or unmyelinated) carry the sensation of pain or temperature, or are autonomic.

Neuropathy means a disorder of nerves. More specifically, it is used synonymously with polyneuropathy, implying a disorder affecting the nerves of the body diffusely. Polyneuropathy needs to be distinguished from mononeuropathy (disorder of a single nerve). The most common mononeuropathy is carpal tunnel syndrome, or median nerve entrapment in the hand, which causes fingers to go numb in sleep. Mononeuropathies of nerves arising from the brain are called cranial neuropathies. The most common cranial neuropathy is unexplained facial neuropathy, or Bell’s palsy.

Polyneuropathies are usually symmetrical. In general, the longest nerve fibers (which supply the feet) are affected first. As the disease advances, progressively shorter nerve fibers are involved and symptoms ascend up the legs. When the knees get affected, the fingers get affected too. Symptoms of polyneuropathy include numbness, tingling, burning, shooting pains, loss of sensation (leading to unnoticed injuries to feet), loss of muscle bulk in the feet, hammertoes, weakness advancing to foot drop, hand weakness and even thigh weakness, loss of balance, loss of sweating in the feet, and skin changes.

The most common cause of polyneuropathy in the developed world is diabetes. Prevalence of neuropathy in longstanding diabetics may be as high as 50%. Diabetic polyneuropathy can be mild or can be severe and disabling. Good control of diabetes decreases the risk of polyneuropathy. The list of other causes of polyneuropathy is long. A short list includes (a) heredofamilial neuropathies: Charcot-Marie-Tooth disease, others, (b) infectious: HIV, leprosy, diphtheria, (c) toxic: drugs (some drugs used for cancer and HIV, INH, amiodarone, others), metals (lead, gold, arsenic, thallium), industrial toxins, and possibly alcohol, (d) nutritional: deficiencies of thiamine, niacin, vitamin B12, vitamin E, (e) metabolic: renal failure, thyroid deficiency, (f) paraproteinemia (abnormal immunoglobulin-like protein in the blood) and paraneoplastic (distant effect of cancer), and (g) vasculitic: from autoimmune inflammation of blood vessels, either in isolation or in association with disorders like rheumatoid arthritis. Vasculitic neuropathy is often severe and could
be diffuse (spread out), asymmetrical, or affect multiple individual nerves (mononeuritis multiplex).

A specific neuropathy called Guillain-Barre syndrome (also called acute inflammatory demyelinating polyradiculoneuropathy) is caused by an autoimmune attack on myelin. It develops over days to weeks and can affect short as well as long nerves. Unlike other neuropathies, weakness of muscles predominates and can be so profound that the patient’s breathing has to be supported on a ventilator. The vast majority make an excellent recovery over weeks to months. Recovery is hastened by plasma exchange (removal of antibodies and other offending molecules from blood) or by injecting high doses of immunoglobulin intravenously. A chronic demyelinating neuropathy (CIDP) also occurs. Some forms of Charcot-Marie-Tooth disease are also demyelinating. Most other polyneuropathies, in contrast, are axonal.

In a large minority (if not the majority) of nondiabetic individuals with neuropathic symptoms, a cause of neuropathy is not found. Such cryptogenic neuropathy tends to be relatively mild and indolent.

Evaluation for neuropathy includes a careful neurological and physical examination and blood and urine tests to look for possible causes. Electrodiagnostic examination (or electronography and nerve conduction studies) is invaluable for the objective diagnosis of neuropathy, assessment of severity, and identification of demyelinating neuropathy. In rare cases a nerve biopsy is indicated.

Treatment involves reversing the cause (if identified), pain control (medications known as tricyclic antidepressants and some antiepileptic drugs are commonly used), foot care, orthotic devices (if there is foot drop), and aids for walking and hand use. Some mononeuropathies are amenable to surgery.

SEE .ALSO: Chronic pain, Pain

Suggested Reading

  • Dyck, P. J., & Thomas, P. K. (Eds.). (1999). Diabetic neuropathy. Philadelphia: W. B. Saunders.
  • Dyck, P. J., Thomas, P. K., Griffin, J. W., et al. (Eds.). (1993). Peripheral neuropathy. Philadelphia: W. B. Saunders.
  • Mendell, J. R., Kissel, J. T., & Cornblath, D. R. (Eds.). (2001). Diagnosis and management of peripheral nerve disorders (Contemporary Neurology Series, 59). New York: Oxford University Press.

Stewart, J. D. (2000). Focal peripheral neuropathies. Philadelphia: Lippincott, Williams & Wilkins.

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