Epub ahead of print: Koch-Henriksen and Sorensen. Why does the north-south gradient of incidence of multiple sclerosis seem to have disappeared on the Northern hemisphere? J Neurol Sci. 2011 Oct 6.
The traditional view is that MS is particularly prevalent in temperate zones both on the northern and southern hemisphere. This uneven distribution of MS is attributed to differences in genes and environment and their interactions.
Diagnostic accuracy (making sure the daignosis of MS is correct) and case ascertainment (finding cases of MS) are sources of error and have their shares in the geographical and temporal variations, and improvements in diagnostic accuracy and case ascertainment influence incidence- and prevalence rates of MS.
MS prevalence also depends on survival; improving survival increases prevalence.
In this meta-analysis the authors have focused on the trend in the incidence and sex ratio of MS through the last five decades, and they analysed the latitudinal distribution of MS incidence, based on a recent literature search.
Their findings indicate that the prevalence and incidence rates had increased in almost all areas, but the previously reported latitudinal gradient of incidence of MS in Europe and North America could not be confirmed even when restricting the search to surveys published before 1980 or 1970. Conversely, the latitudinal gradient of prevalence rates seemed to be preserved. This apparent discrepancy can be explained by the circumstance that incidence estimates only depend on complete ascertainment for a relative short recent period of time, whereas reliable prevalence rates presuppose complete ascertainment decades back in time.
A contributory explanation for the missing latitudinal gradient for incidence may be changes in environmental factors, levelling out differences in habits of life across Europe and North America, and, not least, that the interpretation of a latitudinal gradient in Europe was based primarily on prevalence studies and reviews.
They observed in most regions studied a profound increase in female incidence of MS. The authors' suggest that last observation should prompt epidemiological studies focusing on change in female life style.
"I bet part of it is linked to smoking and vitamin D."
"Since the second world war there has been a gradual increase in the number of woman smoking. The latter is due to clever marketing on the part of the tobacco industry; particularly the manipulation of teenage girls through celebrity role models."
"Woman are now getting less good quality sun exposure due to several factors: (1) less outdoor activity (The Facebook generation), (2) the ubiquitous use of sun block and its addition to make-up products, (3) covering-up (a big problem in the Middle East and Asia), (4) a change in diet (less fish or a switch to farmed fish that has less vitamin D compared to wild fish)."
"Some experts, including me, think another factor is the greater chance of acquiring EBV infection during a early adolescence. Infectious mononucleosis is commoner in adolescent girls compared to adolescent boys."
"All this is very fascinating and if you want to propose that factor X causes MS it has to explain all these observations. Hence my cynicism about certain recent claims about causation."
"I am sure there are other factors; if you have any ideas please let me know."