Federal health regulators have approved an Acorda Therapeutics drug to improve walking in patients with multiple sclerosis, an often disabling disease that affects the nervous system.

The Food and Drug Administration says Ampyra is the first drug approved to help multiple sclerosis patients walk. The agency says 400,000 patients in the U.S. have the disease.

Multiple sclerosis affects the brain and nervous system, causing loss of balance, muscle spasms and other movement problems.

Acorda, based in Hawthorne, N.Y., conducted two trials tracking patients' ability to complete a walking exercise. Both studies showed significant improvement in patients' walking speed.