Risk Factors for MS in Children—Do Environmental Conditions Have a Causal Role?

However, in another study conducted in Sweden, Montgomery et al. found that maternal smoking during pregnancy was not associated with an increased risk of MS in offspring diagnosed with MS before age 16.

Vitamin D Exposure
No studies of vitamin D exposure in pediatric MS have been published, stated Dr. Munger. However, several prospective studies in adults have looked at vitamin D by measuring sun exposure, dietary intake, and levels of serum 25-hydroxyvitamin D [25(OH)D] levels, which are an integrated measure of both sun and dietary sources. Dr. Munger noted that several case-control studies have examined vitamin D exposure in childhood and risk of developing MS as an adult. In one study conducted in Tasmania, Australia, researchers found a 50% to 60% reduced risk of MS with more than four hours of sun exposure between ages 6 and 10. In another study conducted in northern Norway, greater time spent outdoors in the summer between ages 16 and 20 was associated with a 50% reduced risk of MS. In addition, consuming fish—a primary dietary source of vitamin D—more than three times per week was associated with a 50% reduced risk of MS in this cohort. Also, in a study conducted in North America, time spent outdoors in the summer or time spent sun tanning in childhood was associated with a 60% reduced risk of MS.

Because these are case-control studies, “we have to consider that these associations could be explained by recall bias or selection bias,” said Dr. Munger. She also pointed out that another limitation of these studies is that sun exposure is not a direct measure of vitamin D exposure.

The strongest evidence to date that vitamin D may reduce MS risk comes from two prospective studies. The first, among women in the Nurses’ Health Study cohorts, found that women with a dietary intake of at least 400 IU/day of vitamin D had a 40% reduced risk of MS. The main limitation of this study, however, is that diet contributes little to the overall vitamin D nutritional status. Therefore, in the second study, vitamin D exposure was measured by blood levels of 25(OH)D in healthy young adults in the US military. Among whites, levels of 25(OH)D greater than 100 nmol/L were associated with a 50% reduced risk of MS, compared with those with levels less than 75 nmol/L. Whether adequate vitamin D nutrition is associated with a reduced risk of pediatric MS is an important question and one that is currently being studied.

Other Associations With MS Risk
“There have been some other factors in pediatric-onset MS that have been looked at,” said Dr. Munger. In a study conducted in France, researchers assessed the association between hepatitis B vaccination and pediatric-onset MS. An assessment of 143 MS cases with onset before age 16 and 1,122 matched controls showed that neither the timing nor the number of hepatitis B vaccinations was associated with MS risk. However, in a follow-up study, one specific brand of hepatitis B vaccine was associated with a threefold increased risk for MS three years after the last vaccine dose.

In the same population, researchers found that chicken pox appeared to be protective against MS, with 77% of MS cases reporting a history of clinically observed chicken pox, compared with 85% of controls.

Current evidence supports a link between EBV infection and pediatric-onset MS. Studies examining exposure to cigarette smoke and pediatric MS risk are conflicting and more studies are warranted. While there is growing evidence that vitamin D may decrease MS risk among adults, there are currently no studies in pediatric MS. A history of clinically observed chicken pox appears to be protective, and the association between hepatitis B vaccination and pediatric-onset MS remains unclear. However, confirmation of all these findings is needed in larger studies, concluded Dr. Munger.