Incidence, prevalence and characteristics of MS in São Miguel, an island in the Atlantic

Multiple sclerosis (MS) is a chronic inflammatory-demyelinating disease of the central nervous system (CNS). With approximately 2.3 million people estimated to live with MS globally, incidence and prevalence of the disease vary significantly across regions. Generically, the global distribution of MS varies with geographic latitude. However, MS epidemiological data in Southern Europe and Mediterranean countries revealed areas of unexpectedly high prevalence and incidence of MS, thereby demonstrating that clear exceptions to the variation of MS with latitude exist. Epidemiological evidence on MS in Portugal is limited to studies concerning specific regions of the country and therefore not representative of the entire country. More studies form different regions of the country are needed in order to effectively establish MS prevalence and incidence in Portugal. The aim of this study is to determine MS incidence and prevalence, as well as gather information on other clinical data in São Miguel, the biggest island of the Azores, a Portuguese archipelago situated in the North Atlantic.

This is a cross-sectional study of MS patients from the only hospital of São Miguel, an island with a population of 137,150 habitants. Only patients from São Miguel were included in the study. 1/07/2019 was established as the prevalence day and demographic characteristics (sex, age at diagnosis and year of diagnosis) and clinical outcome (type of the disease, treatment, clinical severity score [EDSS]) were reported. 5-year Incidence of MS was studied from 1/07/2014 to 01/07/2019.

47 patients were enrolled in the study, 27 (57.4%) were female and 20 (42.6%) were male. 43 patients were classified as having RRMS and 4 patients classified as having SPMS. Prevalence of MS in São Miguel island was 34.3/100.000. The prevalence for females was 38.5/100.000 and for males was 29.9/100.000. Age-adjusted and sex-adjusted (to the population of the European Union in 2019) MS prevalence were 31.1/100.000 and 34.3/100.000 respectively. Incidence was 2.04/100,000 person-years for the overall population, 2.56/100,000 person-years for females and 1.49/100,000 person-years for males. Age-adjusted and sex-adjusted MS incidence were 1.74/100.000 person-years and 2.04/100.000 person-years respectively. Regarding clinical characteristics, the mean EDSS was 2.93 ± 1.85, the mean age at the time of the diagnosis was 31.77 ± 8.47 years and the mean time of evolution of the disease (period between diagnosis and prevalence date) was 9.19 ± 6.55 years. Treatment options were diverse and included first and second-line treatments as well as Immune reconstitution therapies. Interferon-beta was the most prescribed medication. 2 patients were not making any DMT.

The incidence and prevalence of MS reported by our study is lower than what was registered by other studies. Several reasons may explain these results such as the ancestry of the population of São Miguel, migratory flows and limitations in detecting all MS patients. Incidence and prevalence were higher in females. A marked heterogeneity in clinical characteristics was seen, with great variability in degree of disability, age at diagnosis and time of MS evolution. Treatment options were quite varied in the population, with Interferon-beta being the most prescribed DMT.