By Will Dunham

WASHINGTON, May 14 (Reuters) - A testosterone gel slowed brain deterioration and boosted thinking ability in men with multiple sclerosis, according to a small study showing a possible new way to treat the incurable disease.

Writing on Monday in Archives of Neurology, University of California, Los Angeles researchers said the study was based on the fact that men develop MS less frequently than women and the idea that the male sex hormone testosterone may be protective.

MS is thought to be an autoimmune disease -- in which the immune system attacks rather than protects the body -- that affects the central nervous system. The men in the study had a common form of MS in which periods of symptoms alternate with periods of remission.

The men applied a testosterone gel to their shoulders daily for a year. Tests of cognitive function -- the ability to think, learn and judge -- improved, and brain atrophy diminished to the level of normal aging, the study found.

The treated men also had increased muscle mass.

But the study involved only 10 men, and bigger studies are needed, the researchers cautioned. The average age of the men was 46.

"The reason it's important is there are no neuroprotective drugs for MS, and so this would be a candidate," said Dr. Rhonda Voskuhl, a professor of neurology at UCLA and senior author of the study, in a telephone interview.

There is no cure for MS, which affects more than 1 million people worldwide, including an estimated 300,000 people in the United States. It is twice as common in women than men. Symptoms often first appear between the ages 20 and 40.

Multiple sclerosis can be a mild illness in some people while causing permanent disability in others. Symptoms may include numbness or weakness in one or more limbs, partial or complete loss of vision, tingling or pain, electric-shock sensations with certain head movements, tremors and an unsteady gait.

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