Multiple sclerosis (MS) is a disease that commonly strikes at the prime of people’s lives, when most of us expect to be able to immerse ourselves in careers, start or add to a family, or just live life to the fullest. MS is a disease where the immune system attacks the coating around the nerve fibres – myelin – disrupting the electrical signals from the brain and spinal cord to the rest of the body. With over 37,000 people living with MS in Australia, MS is the most commonly acquired neurological condition affecting young Australians, and it is also three times more common in women than in men. The reason why this is so, is an enduring mystery of MS research.
Research has revealed several reasons why some people get MS and others don’t. Genetics is one factor, and environmental factors such as whether or not someone is a smoker or has adequate vitamin D levels is another. A prior infection with Epstein-Barr virus, which causes glandular fever, also changes the level of risk. The contributions of different factors vary for each individual with MS.
But why more women? One area of research that may reveal the secret is genetics. While much is known about the genetic risk of MS, the sex chromosomes, the X and Y, are only being looked at now. It may be that gender specific differences in genetics are driving the difference. While it will be fascinating to see what this work uncovers, it will only be part of the story.
The number of women who are affected by MS has increased over the last 50 years or so, and genetics evolve much more slowly than that, meaning that environmental factors must also be involved. Other suggested factors tie in with the risk factors of MS, changes to sun exposure or vitamin D levels over the last 50 years, particularly in women or changes in the rates of smoking among females.
One strong group of suspects are sex hormones, which in women are mainly oestrogen and progesterone. When MS occurs in children, boys and girls are affected equally, but it is after puberty that the number of females diagnosed sharply rises. After menopause, the risk of developing MS in men and women is again similar. This may be linked to the relative levels of sex hormones in women changing through the course of their lives. It is even possible that the sex hormones interact with other risk factors. Current Australian research is examining how sex hormones could change the immune response to the Epstein-Barr virus.
Pregnancy can also change the course of a woman’s MS. During the second and third trimesters, there is a 70% reduction in relapse rates compared to before pregnancy. However, there is also an increased risk of having a relapse after the baby is born, with women three times more likely to have a relapse in the 3-6 months immediately after giving birth. This is thought to be due to the dampening of the mother’s immune system that occur during pregnancy, to ensure that the mother’s immune system does not attack the growing foetus.
Other research has shown that the male and female immune systems react differently to the threat of infection and the female system may have more trouble switching off after a threat has passed – leading to autoimmunity, such as that seen in MS. Other autoimmune diseases such as lupus are also far more common in women.
Large, well-controlled research studies are still needed to understand this fundamental difference between men and women in MS. Understanding the reasons underlying this phenomenon is important, as they may lead to the discovery of new mechanisms that underpin MS and unlock new treatment avenues for everyone.
