MS is more common in family members of people with MS. Some say it is up to 80 times more common in first-degree relatives like brothers, sisters or children of a person with MS. Among identical twins, if one has MS, the other is 300 times more likely to get the disease than an unrelated person is.
This risk increases with the number of relatives a person has with MS, and of course with environmental factors such as smoking, which doubles the risk.
A Canadian group has examined the family risks of getting MS. Initial studies published in 1995 showed that the increased risk of MS in a family was due to genetic factors, not factors in the shared environment.1 Screening 15,000 people with MS, the researchers compared the risk of MS in genetically related versus adopted family members. Adopted relatives proved to have no higher risk of getting MS than the general population did. So it is clear that people do not “catch” MS.
More recently, the same group evaluated MS risk in 687 step-siblings of 19,746 MS index cases to determine whether any transmissible factor in a family environment might enable one to ‘catch’ MS.2 The risk of MS in these step-siblings was indistinguishable from that of the general population. Conclusion: environment influences MS risk at a population level, not at a family level.
A study of the genes involved showed that MS is not inherited as a result of a single gene, the way cystic fibrosis or muscular dystrophy are.3 Susceptibility to MS results from the interaction of several genes. At present, predicting MS in offspring is not possible.
Many wonder whether a brother or sister of someone with MS is at increased risk of getting the disease. The Canadian group has shown that the risk is higher if:
Depending on background risk in the general population (for instance, it is higher in Canada than in Australia), the combination of factors above, and such things as whether the person smokes and has a low vitamin D level, this risk may be as high as 1 in 5 or 1 in 10.
So, is MS hereditary? Well, yes to some extent, but the good news is that the risk may be very substantially reduced. For what to do, see Are My Kids or Family Members More Likely To Get MS? For more details on exact risks, see the book Overcoming Multiple Sclerosis.