The disease is a rare autoimmune disorder characterized by progressive weakness and impaired sensory function in the legs and arms. The federal agency said it designated Gamunex as an "orphan" drug. The orphan drug designation provides manufacturers with financial incentives to develop treatments for diseases that affect fewer than 200,000 people in the United States.

Chronic inflammatory demyelinating polyneuropathy, which affects about 25,000 people annually in the United States, is caused by an immune system attack on the body's peripheral nervous system. The effects of it -- progressive muscle weakness, loss of deep tendon reflexes, tingling, and numbness -- are caused by damage to the fatty material that covers and protects the nerve fibers, called the myelin sheath.

Researchers said it is likely Gamunex helps improve muscle function in patients with chronic inflammatory demyelinating polyneuropathy by modulating the immune system's response to the inflammation that damages the myelin sheaths but the exact mechanism is not known.

Gamunex is manufactured by Talecris Biotherapeutics Inc. of Research Triangle Park, N.C.