Multiple sclerosis (MS) occurs worldwide, but its frequency varies substantially between countries and geographic regions. Contemporary global epidemiologic data estimate that approximately 2.9 million people are living with MS worldwide. The global incidence is approximately 2.1 cases per 100,000 people per year, although incidence and prevalence vary considerably between populations and regions.[1]
MS is more common in women than men, and disease onset most commonly occurs in young and middle adulthood, although MS can occur in children and older adults. Geographic, genetic, demographic, environmental, and infectious factors contribute to variation in MS risk and distribution.[2]
Epidemiology and Demographics
Incidence
The incidence of multiple sclerosis varies substantially among countries and populations. The Atlas of MS reports a global incidence of approximately 2.1 cases per 100,000 people per year among countries with available incidence data.[1]
Differences in incidence between populations should be interpreted in the context of variation in population structure, case ascertainment, access to neurological and MRI services, diagnostic criteria, and completeness of national registries and surveillance systems.[2]
Prevalence
The prevalence of [Multiple Sclerosis|[MS]] varies substantially among countries and geographic regions. Updated Atlas of MS data estimate that approximately 2.9 million people worldwide are living with MS.[1]
Reported MS prevalence has increased in many countries over time. This increase may reflect several factors, including improved case ascertainment, greater access to diagnostic services, changes in diagnostic criteria, increased survival, population growth and ageing, as well as possible changes in disease occurrence.[1][2]
Comparisons of prevalence estimates between countries should therefore consider differences in study methodology, healthcare access, diagnostic practices, registry coverage, and the quality and completeness of epidemiologic data.[2]
Case-fatality rate/Mortality rate
MS is a chronic neurological disease rather than an acutely fatal disorder. Mortality and survival among people with MS are influenced by disease-related disability, comorbidities, complications, demographic factors, and access to healthcare.
Historical mortality estimates should be interpreted cautiously because survival, diagnosis, disease classification, and management of MS have changed substantially over time.
Age
MS can occur at almost any age, but diagnosis most commonly occurs in young and middle adulthood. In the third edition of the Atlas of MS, the mean age at diagnosis across reporting countries was approximately 32 years.[2]
Pediatric-onset MS is recognized but is substantially less common than adult-onset disease. The Atlas of MS has identified at least 30,000 children and adolescents younger than 18 years living with MS worldwide, although this is likely to be an underestimate because pediatric epidemiologic data remain incomplete in many countries.[1]
MS may also present in older adults. Age should therefore not be used alone to exclude the diagnosis when the clinical and paraclinical findings are otherwise compatible with MS.[3]
Race
The epidemiology of MS is more diverse than suggested by older descriptions of MS as predominantly a disease of White populations. MS occurs across racial and ethnic groups, and contemporary population-based studies demonstrate substantial disease burden among Black, Hispanic, and other populations.[4]
Differences in reported MS prevalence between racial and ethnic groups may reflect a combination of genetic ancestry, environmental exposures, socioeconomic and healthcare factors, migration, diagnostic access, case ascertainment, and other determinants. Race and ethnicity should therefore not be used in isolation to determine whether an individual is likely to have MS.[4]
Gender
Multiple sclerosis is more common among women than men. Globally, females are approximately twice as likely to have MS as males, although the female-to-male ratio varies between countries and populations.[2]
Sex differences in MS frequency are likely to reflect complex interactions among biological, genetic, hormonal, environmental, and other factors rather than a single determinant.[5]
Region
Multiple sclerosis occurs worldwide, but prevalence varies substantially between geographic regions and individual countries. Higher prevalence has historically been reported in many populations in Europe and North America, while lower reported prevalence has been observed in several regions of Africa and Asia.[2]
Geographic differences should not be interpreted as evidence that MS is absent or necessarily rare in populations for which high-quality epidemiologic data are limited. Differences in healthcare access, neurological services, MRI availability, disease awareness, diagnostic criteria, registries, and surveillance may influence reported prevalence.[2]
Geographic variation in MS probably reflects interactions among genetic susceptibility and environmental exposures rather than a single geographic factor. Latitude, ultraviolet radiation exposure, vitamin D status, smoking, infectious exposures, migration, and other environmental factors have been investigated as potential contributors to variation in MS risk. Observational associations should not automatically be interpreted as proof of causation.
Historical migration studies have contributed to evidence that environmental exposures during childhood and adolescence may influence subsequent MS risk. However, the effect of migration varies between populations and studies and should not be expressed as a universal age-specific rule.[6]
Interpretation of older epidemiologic studies should also account for changes in MS diagnostic criteria and improved recognition of alternative inflammatory and demyelinating disorders. The revised 2024 McDonald criteria incorporate contemporary clinical, imaging, and laboratory evidence and include additional provisions intended to improve diagnostic accuracy across different populations and clinical settings.[3]
Global epidemiology and prevalence of multiple sclerosis. Source: Khan Z, Gupta GD, Mehan S (2023). Licensed under Creative Commons Attribution 4.0 (CC BY 4.0).[7]
Risk and Epidemiologic Factors
Smoking: Cigarette smoking is associated with increased risk of MS in epidemiologic studies. Among people with established MS, smoking is also associated with increased disability progression. NICE advises people with MS not to smoke because smoking increases progression of disability.[8][9]
Family history and genetic susceptibility: Having a family member with multiple sclerosis is associated with increased MS risk, supporting an important contribution of genetic susceptibility. However, MS is not inherited in a simple Mendelian pattern, and most relatives of people with MS do not develop the disease.[10][11]
Female sex: Multiple sclerosis is more common in females than males. Female sex is therefore an important demographic association with MS, although sex alone does not determine individual disease risk.[2][5]
Low vitamin D status and ultraviolet exposure: Lower vitamin D status and reduced ultraviolet radiation exposure have been associated with increased MS risk in observational studies. These associations do not establish vitamin D deficiency as a sufficient cause of MS, and epidemiologic association should be distinguished from evidence that vitamin D supplementation prevents MS.[12][13]
Epstein-Barr virus (EBV): EBV infection has a strong epidemiologic association with MS. A large longitudinal study demonstrated a marked increase in MS risk following EBV infection, providing some of the strongest epidemiologic evidence linking an infectious exposure with MS. However, EBV infection is extremely common in the general population and only a small proportion of infected individuals develop MS; EBV seropositivity alone is therefore not diagnostic of MS.[14][15]
Environmental and geographic factors: Geographic variation in MS risk probably results from interactions among genetic susceptibility and multiple environmental exposures. Latitude, ultraviolet exposure, vitamin D, smoking, EBV infection, migration, and other factors have been investigated. No single environmental factor alone adequately explains the worldwide geographic distribution of MS.[2]
Diagnostic ascertainment: Changes in diagnostic criteria, access to MRI, recognition of MS mimics, registry quality, and healthcare access can alter measured incidence and prevalence. Epidemiologic comparisons across different time periods should therefore account for changes in diagnostic ascertainment. The revised 2024 McDonald criteria represent the current international diagnostic framework.[3]
