Robert J. Fox, MD: Multiple sclerosis [MS] is a disease for which the immune system gets confused and attacks the brain, spinal cord, and optic nerve. It does it in episodes lasting several days to several weeks, and does it in different pockets throughout the central nervous system, affecting the brain, the spinal cord, and the optic nerve.
After it has done that for 10, 15, 20 years, the disease seems to transform, and that active inflammation tends to settle down and become less prominent and less frequent. Instead, it is replaced by gradual little by little worsening of neurological symptoms. This often manifests as gradual worsening in walking, arm coordination, cognition, and other neurological functions. This is the part of MS we call progressive MS. We call the first part relapsing MS.
We don’t know what causes multiple sclerosis. We don’t know what triggers the immune system to attack the brain, spinal cord, and optic nerve. There are some things that we know can predispose patients to multiple sclerosis. There is genetics. Some genes predispose patients to develop MS, although the genetic influence seems to be relatively modest. A child of a mom or dad with MS will have about a 2% to 3% chance of getting MS.
That is in contrast to the background population risk of about 0.1%. So a child of someone with MS will have a 20 to 30 times higher risk of developing MS. Still, that overall risk is only 2% to 3%. To put it another way, they still have a 97% to 98% chance of not having MS. In identical twins, in the most matched genes that we see, those people only have a risk of 30% to 40% of getting MS if 1 of the twins has MS. So even in exactly identical twins, the concordance rate is only about 30% to 40%.
Environmental factors and lifestyle behaviors are also associated with MS. We know that smoking is associated with developing MS and a more aggressive course of MS over time. We also know that vitamin D is associated with MS. Those with a low vitamin D level in their teenage years going into their 20s are more likely to get MS than those with normal levels of vitamin D.