We present here the most up to date available map of the distribution of MS in Scotland. Our findings have shown that the latitude gradient of MS prevalence and incidence observed still exists in Scotland.
As suggested by previous work,
much of this latitude gradient is as a result of an increase in female MS risk at increasing latitudes. It must be noted, however, that, although our data does support a latitudinal gradient, the data itself could also argue for a “threshold” effect around 57°N. Until more is understood about the aetiological origin for this effect of latitude on MS prevalence, conclusions regarding this must remain tentative.
Smoking does not appear to underlie this latitudinal effect, especially given the apparent inverse
correlation of smoking prevalence with latitude. Although there is some suggestion that the sex-ratio in MS admissions may be inversely related to smoking prevalence, this is quite unlikely to be a real effect given that where MS risk is highest (NHS Orkney Islands), smoking prevalence is lowest and so, regardless of the ratio between male and female smoking behaviour, it is difficult to envision how the absence of a susceptibility factor could result in a sex-specific increase
of disease risk.
There are clear limitations in using background population measures of smoking prevalence as a surrogate for exposure in patients with MS. In particular, effects on the level of individual patients will not be detected in this sort of population-based data. Future studies should re-examine this question in relation to MS-specific smoking data.
Limitations of record linkage studies using routinely collected administrative data are well known, and include the facts that the data are limited to hospitalised patients and that information about some variables of potential interest, such as social circumstances and ethnicity, are generally unavailable. It is also worth noting that the sample size of regions primarily driving the latitudinal effect (e.g. the Orkney and Shetland Islands) and relatively small and so this effect will need to be confirmed in future work. The fact that latitudinal effects were not seen for other autoimmune and neurological diseases argues strongly against the gradient we observed being solely due to underlying trends in hospital services or background population admissions rates.
The environmental risk factor that most likely explains the latitudinal effect observed in this study is the effect of ultraviolet irradiation on vitamin D biosynthesis. Vitamin D deficiency has already been associated with risk of MS and controls the expression of HLA-DRB1*1501
, the key genetic risk factor in MS.
However, it is likely that MS prevalence will be explained only by considering a complex interplay of genetic and environmental risk factors, some of which may not yet have been identified 
The latitudinal variation in MS prevalence may have important implications for neurological service provision. For example, although the number of MS-relevant clinics per week correlates well with the total number of admissions as one would expect with the tendency of admissions to follow hospital resources (r
0.69, p<0.01), these show a trend towards inverse
correlation with MS indirect admissions rates, probably a better estimate of background disease prevalence (r
. MS disease distribution should be considered when evaluating neurology services in Scotland in the future.
In conclusion, the distribution of MS across Scotland shows nearly three-fold variation, at least as measured by hospitalisation rates. The higher rates in the North than South of Scotland persist; they are consistent with earlier findings from studies in different parts of the UK; and they are consistent with the more general finding across the world that MS prevalence increases with increasing distance from the equator in both hemispheres. The high prevalence of MS in the North of Scotland should be recognised in funding of care for MS and for studies of disease prevention.