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Most forms of multiple sclerosis (MS) strike women twice as often as men. Primary progressive MS, though, affects men and women in nearly equal numbers. Researchers don’t know why it happens, but here's what scientists understand about this type:

The 'Equal Opportunity' MS

For people with primary progressive MS, there are no attacks followed by later improvement, as in the more common forms of MS. Symptoms steadily get worse from the time they’re diagnosed.

The gender differences in this type of the disease seem to vary by age group. In the largest studies done so far, scientists kept tabs on hundreds of men and women with primary progressive MS for decades. They found:

  • Under age 30, equal numbers of men and women had the condition.
  • There were more women than men who got it after age 45.
  • Almost two women for each man were diagnosed with primary progressive MS after age 50 -- still short of the rates in other forms of MS.

This type of MS is also unique in how severe its symptoms are. MS in men is usually worse than in women. But large studies of primary progressive MS show:

  • Early on, the symptoms were just as severe and got worse just as fast for men as for women.
  • After about 20 years of living with primary progressive MS, men's disease finally began to "outrun" the women's in terms of how bad their symptoms were.

Why the differences? At this point, there are more questions than answers. Sex hormones may be one reason. Scientists are also studying clues in the body chemistry and brains of people with the condition. But research into this form of MS is only beginning.

Men vs. Women With All Types of Multiple Sclerosis

In terms of all types of MS, at least twice as many women as men have the condition. But the ratio may be rising even higher. Some recent estimates place the number at 4 to 1 -- and suggest it is still going up.

And ladies are more likely to have their first symptoms at younger ages. Three women for every man got MS before age 20.

Sex Hormones and Multiple Sclerosis

Multiple sclerosis doesn't hit all women at the same time in life. But it almost always begins before menopause. Symptoms often get worse after childbirth or at the end of a menstrual cycle.

When men get MS, it's usually in their 30s or 40s -- just about the time their testosterone levels start to decline.

These clues suggest that sex hormones, like estrogen and testosterone, play a role in the disease. It may be the balance of hormones, rather than their actual levels, that's important.