Multifocal motor neuropathy

Definition

Multifocal motor neuropathy is a rare condition in which the muscles in the body become progressively weaker over months to years.

Description

Multifocal motor neuropathy is often mistaken for the more catastrophic, inevitably fatal condition called amyotrophic lateral sclerosis (ALS). Unlike ALS, however, multifocal motor neuropathy can be treated; therefore, distinguishing between these two conditions is crucial.

Demographics

Multifocal motor neuropathy is a very rare condition, affecting only about 1 per 100,000 people in the population. Men are about three times as likely to be affected as women. Most patients are between the ages of thirty and fifty when symptoms are noted, with the average age of onset being 40 years.

Causes and symptoms

Multifocal motor neuropathy is thought to result from an autoimmune disorder; that is, the body's immune system accidentally misidentifies markers on the body's own nerve cells as foreign. The immune system then begins to produce cells that attack and injure or destroy either the nerve cells or the myelin sheath wrapped around the nerve cells. Because the myelin sheath allows messages to be conducted down a nerve quickly, injury to the sheath or to the nerve itself results in slowed or faulty nerve conduction.

Symptoms of multifocal motor neuropathy usually begin with gradually progressive weakness of the hands. Leg and foot weakness may follow, as well as decreased muscle volume (called muscle wasting), muscle cramps, and involuntary twitching and cramping of muscles. The weakness is asymmetric; that is, a muscle group on only one side of the body may be affected. Over time, numbness or tingling of affected areas may occur, although sensation is not lost.

Diagnosis

Diagnosis of multifocal motor neuropathy usually requires both a careful physical examination, as well as electromyographic (EMG) testing. Physical examination will reveal weakness and decreased muscle size, abnormal reflexes, muscle twitches, and totally normal sensation. EMG involves inserting a needle electrode into a muscle, and measuring the electrical activity within the muscle at rest and during use. A characteristic pattern of abnormal nerve conduction and muscle contraction will be noted on EMG.

Blood tests will usually reveal the presence of antibodies (immune cells) directed against ganglioside, a component of nerve cells.

Treatment team

Patients with multifocal motor neuropathy are usually cared for by neurologists.

Treatment

Treatment for multifocal motor neuropathy involves using intravenous immunoglobulin (IVIg) to dampen down the immune system's overactivity. If IVIg is not successful, then the immunosuppressant drug cyclophosphamide may be administered.

In very mild, early cases, treatment may not be necessary. If the condition progresses or prompts serious disability, treatment may be necessary. Treatment may then be required intermittently, if the condition progresses again.

Prognosis

Muscle strength usually begins to improve within three to six weeks of the initiation of treatment. Early treatment of multifocal motor neuropathy usually results in sufficient symptom resolution to prevent any permanent disability. Over many years, however, many patients will note a continued, slow progression of muscle weakness.

Resources

BOOKS

Asbury, Arthur K., and Stephen L. Hauser. "Guillain-Barré Syndrome and Other Immune-mediated Neuropathies." In Harrison's Principles of Internal Medicine, edited by Eugene Braunwald, et al. NY: McGraw-Hill Professional, 2001.

Griffin, John W. "Immune Mediated Neuropathies." In Cecil Textbook of Internal Medicine, edited by Lee Goldman, et al. Philadelphia: W. B. Saunders Company, 2000.

Shields, Robert W., and Asa J. Wilbourn. "Demyelinating disorders of the peripheral nervous system." In Textbook of Clinical Neurology, edited by Christopher G. Goetz. Philadelphia: W. B. Saunders Company, 2003.

WEBSITES

National Institute of Neurological Disorders and Stroke (NINDS). NINDS Multifocal Motor Neuropathy Information Page. November 1, 2003 (June 3, 2004). <http://www.ninds.nih.gov/health_and_medical/disorders/multifocal_neuropathy.htm>.

Rosalyn Carson-DeWitt, MD