There has been a sharp increase in the occurrence of multiple sclerosis (MS) among women over time, but rates among men appear to have remained stable.
Data from a voluntary MS registry shows that while in 1940, twice as many women as men in the U.S. had multiple sclerosis, by 2000, four out of five cases were occurring among women, University of Alabama professor of biostatistics Gary Cutter says.
That represents an increase in the ratio of women to men of nearly 50 per cent per decade, and it mirrors recent findings from other countries with more comprehensive MS registries, including Canada, Norway, and Denmark.
An unpredictable disease of the central nervous system, multiple sclerosis (MS) can range from relatively benign to somewhat disabling to devastating, as communication between the brain and other parts of the body is disrupted.
Multiple sclerosis is widely believed to be an autoimmune disease, a condition in which the immune system attacks components of the body as if they're foreign.
In multiple sclerosis, the body mistakenly directs antibodies and white blood cells against proteins in the myelin sheath, a fatty substance that insulates nerve fibers in your brain and spinal cord. This results in inflammation and injury to the sheath and ultimately to the nerves that it surrounds. The result may be multiple areas of scarring (sclerosis). Eventually, this damage can slow or block the nerve signals that control muscle coordination, strength, sensation and vision.
Multiple sclerosis affects an estimated 300,000 people in the United States and probably more than 1 million people around the world — including twice as many women as men. Most people experience their first signs or symptoms between ages 20 and 40.
It is not clear why the disease rates seem to be increasing only among women, but the observation could help researchers searching for the cause or causes of the disease, Cutter says.
Some believe that environmental or viral influences early in life trigger the disease in people who are genetically predisposed toward getting it. Though there are many theories about what these triggers are, there is no proof that any of them cause multiple sclerosis.
"We have to ask ourselves, 'What has been going on over the last 50 years or so that would affect women more than men?'" Cutter says.
During that time, obesity and smoking rates have increased among women, the oral contraceptive was introduced, and there has been a trend toward earlier menstruation and later childbirth.
These factors influence levels of the sex hormones, and there is some evidence that sex hormones play a role in MS by suppressing the immune system.
Most women with MS have fewer symptoms of the disease during pregnancy. After delivery, symptoms often return.
The sex hormone connection is just one avenue that needs to be explored, Cutter says.
"We also need to ask the general questions about what women do differently than men, such as use of hair dye and use of cosmetics that may block vitamin D absorption," he says in a news release. "At this point we are just speculating on avenues of research that could be pursued."
MS rates are highest among people living farthest from the equator, leading to speculation that vitamin D deficiency due to low sun exposure contributes to the disease.
Gary Birnbaum, director of the Multiple Sclerosis Treatment and Research Center at the Minneapolis Clinic of Neurology in Golden Valley, feels that there probably isn't any single "smoking gun" that can explain all cases of MS.
"If it were that simple we probably would have been able to figure it out by now," he says. "MS may not be a single disease. It may be a syndrome. The pathway may actually be very different for different people."