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Genetic and environmental risk factors are distinct in NMOSD and MS
The prevalence of multiple sclerosis (MS) has been reported from many countries in the world, and prominent geographical differences among regions are well known: over 100 per 100 000 in European and North American populations vs 0–20 per 100 000 in Asian populations. These differences suggest that genetic factors are important in the development of MS. Moreover, it is also known that higher latitude is associated with higher prevalence of MS. The latitude gradient for MS risk is thought to be related to low sun exposure, possibly because this is associated with low serum levels of vitamin D, which has an immunomodulatory effect.
In contrast, data on the prevalence of neuromyelitis optica (NMO) and its wider disease spectrum, NMO spectrum disorders (NMOSD), are scant, although few reports suggest that,1 2 in contrast to MS, the prevalence of NMO is comparatively similar globally and rarely exceeds 5/100 000.
A previous Japanese nationwide survey revealed that the estimated crude prevalence of MS in Japan is 7.7/100 000, while another recent study reported a crude NMOSD prevalence of 4.1/100 000 in a local area of Hokkaido located in Northern Japan.3 In their JNNP paper, Miyamoto and colleagues first showed the results of a nationwide survey conducted in Japan based on a well-designed epidemiological methodology and the 2006 Wingerchuk criteria for NMO; the estimated prevalence was 1.64 per 100 000 for NMO and 3.42 for NMOSD.4 To compare the data with those from other countries, we have searched publications using search terms ‘neuromyelitis optica’, ‘prevalence’ and ‘100,000’ on the PubMed database, and citations in the papers. Moreover, we selected publications calculating the prevalence of patients with NMOSD who had fulfilled the 2006 or 2015 diagnostic criteria for NMO or NMOSD. Figure 1 shows the reported prevalence of NMO/NMOSD by country or area; the prevalence is relatively similar, which is apparently different from the uneven global distribution of MS prevalence, and 5 or less per 100 000 habitants except for Martinique (10 per 100 000). Although lower prevalence with increasing latitude was reported in a study from Australia and New Zealand,5 a latitude gradient for NMO/NMOSD prevalence is not proven (figure 2). Meanwhile, low vitamin D levels were reported in NMO/NMOSD in a few reports.6
The similar global prevalence suggests that ethnic or genetic backgrounds do not play a major role in the development of NMO/NMOSD, and the lack of a clear latitude gradient of NMO/NMOSD prevalence argues against a strong role for sun exposure and vitamin D levels in conferring the individual disease risk. Thus epidemiology and risk factors are considerably different in MS and NMO/NMOSD, emphasising the distinct immunopathogenesis of both conditions. Further epidemiological studies will be required to elucidate the risk factors for NMO/NMOSD.
Contributors MM drafted this article, and SK and FP made critical revision of the article for important intellectual content.
Funding This work was supported in part by the Health and Labour Sciences Research Grant on Intractable Diseases (Neuroimmunological Diseases) from the Ministry of Health, Labour and Welfare of Japan, and by Deutsche Forschungsgemeinschaft (DFG Exc 257 to FP).
Competing interests None declared.
Provenance and peer review Commissioned; internally peer reviewed.