Spring babies appear to be at higher risks for multiple sclerosis (MS) according to a recent analysis of several published studies involving northern populations.
Researchers from Queen Mary University of London reported a significantly increased incidence of MS among infants born in April and a 5 to 7% lower than expected rate among those born in October and November, months following summer in the Northern hemisphere.
In the UK, around 100,000 individuals suffer from MS, a disease for which there is no cure yet. MS arises when the body’s own immune system attacks the nervous system, resulting in damage to the myelin sheath, the protective layer that insulates nerve cells. When this happens, nerve impulses slow or stop, leading to numbness, visual disturbances, difficulty in maintaining balance, bowel disorders and psychological symptoms.
Vitamin D and season of birth
During winter months, people residing at elevated latitudes do not get adequate skin exposure to ultraviolet light of the correct wavelength (290 to 315 nm). Therefore, their bodies may be unable to synthesise sufficient vitamin D.
The scientists believe that a pregnant woman’s vitamin D levels influence the immune status of the growing foetus, and thus determine later susceptibility to MS.
The authors reviewed research published after 2000 which recorded month or season of birth for both the MS group and the healthy control group. 10 studies met the inclusion criteria and data on 151,978 MS births were compared to investigate the link between latitudes and MS risk.
Reviewing all the studies together, the scientists reported a greater observed-to-expected MS birth ratio in April and a decreased ratio in October and November.
A “population conservative” analysis was performed by eliminating studies with partially or completely overlapping patient data: a clear connection with a decreased risk of MS was seen only for November. The authors explained that this observation was due to the exclusion from the analysis of countries above 52 degrees north.
A “geographically conservative” analysis including populations above latitude 52 degrees north — populations which see little sun from October to March — showed considerably more MS births in April and May and considerably fewer incidences in October and November.
An overall conservative analysis excluded all studies from the two previous conservative analyses. Although this analysis included only 78,488 subjects, it again showed a considerably increased risk of MS in April and May and a much lower risk in October and November.
Due to a lack of available studies, the Southern hemisphere was excluded.
This study, published in the Journal of Neurology Neurosurgery and Psychiatry, is the first to show that the season of birth effect is significantly stronger at higher altitudes, with fluctuating maternal vitamin D levels likely to be the most probable cause.
“To me, the month of birth is just a marker for maternal vitamin D status; that’s the key driver behind the risk. We believe as a team of researchers that vitamin D supplementation during pregnancy will help to reduce MS incidence,” study author Dr. Ramagopalan told Medscape Medical News.
Dobson R, Givannoni G and Ramagopalan S (2013) The month of birth effect in multiple sclerosis: systematic review, meta-analysis and effect of latitude. J Neurol Neurosurg Psychiatry. 84(4):427-432.