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Multifocal motor neuropathy is a very rare progressive muscle disorder that causes progressive weakening of the sufferer's muscles over a period of years. It is an autoimmune disorder in which a person's immune system begins misidentifying nerve cells as foreign organisms, causing it to begin attacking the person's motor nerve fibers and the myelin sheath surrounding the nerves. The damage begins to interfere with nerve transmissions to the muscles, resulting in gradually diminishing strength. For reasons still not understood, the damage is concentrated in the motor nerve fibers; sensory nerves are spared, and so multifocal motor neuropathy does not cause significant sensory impairment.
The earliest signs of multifocal motor neuropathy are usually seen in the hands in the form of wrist drop, reduced dexterity, or reduced grip strength. Symptoms are often seen in muscles connected to the ulnar, radial, and median nerves in the arms and the common peroneal nerves in the legs. As it progresses, multifocal motor neuropathy causes increasing muscle weakness and decreased muscle mass. The disease does not progress symmetrically, and a person may have symptoms in different muscles on different sides of the body.
The disorder is accompanied by cramps and muscle twitches, called fasciculations, which are not necessarily limited to the same areas areas of the body suffering from muscle weakness. Sensation in the affected areas remains intact, although some sufferers experience tingling or numbness. The condition is rarely fatal, but if left untreated can eventually cause significant disability.
Multifocal motor neuropathy is sometimes mistaken for amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease. However, multifocal motor neuropathy does not progress as quickly, and unlike ALS, it is treatable. It is a rare condition, affecting approximately one person in 100,000, and is approximately three times more common in males than in females. The ultimate cause of the immune system malfunction that results in the symptoms of multifocal motor neuropathy is unknown. Women with this condition sometimes experience a worsening of symptoms during pregnancy.
Multifocal motor neuropathy is treated with immunomodulatory therapy intended to stop the immune system's attacks on the sufferer's motor nerves. Intravenous immunoglobulin, a blood product containing densely concentrated antibodies from blood donors, is the most common treatment used for this purpose. The immunosuppressant cyclophosphamide is also sometimes used, given intravenously, although it can have serious side effects and is usually used only in cases where treatment with intravenous immunoglobulin has proven unsuccessful.