LONDON (Reuters) - Drugs given to patients with multiple sclerosis can reduce brain tissue loss and slow down the progression of the illness, scientists said on Friday.

Multiple sclerosis (MS), a central nervous system disorder, can cause the brain to shrink very early in the disease. But Italian researchers have found that prompt treatment with interferon beta-1a drugs lessens the shrinkage.

Dr Massino Fillippi, of the University Ospedale San Raffaele in Milan, said his research shows that "interferon beta-1a, given subcutaneously once weekly, can alter this process significantly."

Avonex, made by U.S. pharmaceutical company Biogen and Swiss drug manufacturer Serono's Rebif are interferon beta-1a drugs.

MS, which affects about a million people worldwide, occurs when immune system cells attack and destroy the myelin sheath that protects the nerve cells in the brain and spinal cord.

Symptoms such as tingling, fatigue, loss of balance and slurred speaking can be intermittent which can make it difficult to diagnose. The illness is twice as common in women as in men.

The Italian researchers said that after two years of follow-up the disease progressed in about a third of patients given the drug, compared to half who had taken a placebo.

Brain imaging showed that brain-tissue loss was also greater in the placebo patients.

"The report by Filippi and colleagues is encouraging and commends investigators to include brain atrophy as an outcome measure in future trials of potential disease-modifying treatments in multiple sclerosis," David Miller, of University College London, said in a commentary about the research.

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