Epidemiology & Pathophysiology of Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune demyelinating disease of the central nervous system (CNS) with both inflammatory and neurodegenerative components. It is the most common nontraumatic disabling neurologic condition in young adults. Complex interactions between genetic, environmental, and lifestyle risk factors affect both disease susceptibility and clinical course. Although initial descriptions date to the Middle Ages, therapeutic options remained limited until the past several decades, with the first disease-modifying therapy (DMT) approved more than 120 years after Charcot's initial pathologic descriptions in the 1860s.

Epidemiology

Prevalence and Demographics

The prevalence of MS in the adult US population was estimated at 309 per 100,000 in 2010, with an extrapolated prevalence of 337–362 per 100,000 in 2017 — translating to more than 900,000 individuals. For comparison, estimated prevalence in 1990 was only 58 per 100,000 (approximately 300,000 patients). Several factors likely contribute to this apparent increase, including widespread access to MRI, evolving diagnostic criteria allowing earlier diagnosis, and population aging with longer survival.

Female predominance has been consistent, with a female-to-male ratio of approximately 3:1 persisting across racial populations. The 2019 estimated total US economic burden of MS was $85 billion, with prescription medications (DMT and non-DMT) accounting for nearly $40 billion of direct costs. Costs of DMTs have risen dramatically and far outpace the rate of inflation. MS ranks second only to congestive heart failure in direct cost of care among chronic medical conditions.

Pathophysiology

Immune Dysregulation: Central and Peripheral Tolerance

Normally, CNS-autoreactive immune cells are deleted during development through central tolerance in the thymus (T cells) or bone marrow (B cells). Peripheral tolerance mechanisms typically prevent any escaped autoreactive cells from causing disease. MS pathogenesis involves failure of these mechanisms, influenced by a complex interplay of genetic and environmental factors that promote activation of autoreactive cells.

T Cells

MS was historically considered a primarily T cell–driven disease. Primary T cell subsets implicated include:

• CD8+ T cells — activated microglia and CD8+ T cells cooperate in myelin destruction
• CD4+ T helper 1 (TH1) cells — produce interferon gamma
• CD4+ TH17 cells — produce interleukin (IL)-17

Autoreactive T cells also produce granulocyte-macrophage colony-stimulating factor (GM-CSF). Some DMTs work at least partly by shifting T cell differentiation from proinflammatory TH1/TH17 toward less inflammatory TH2 phenotypes.

B Cells

The role of B cells in MS pathophysiology has been increasingly recognized. B cells contribute through multiple mechanisms:

In MS, the normal anti-inflammatory B cell function (producing IL-10, IL-35, transforming growth factor β1) may be impaired. B cell–depleting agents (anti-CD20 monoclonal antibodies: rituximab, ocrelizumab, ofatumumab) have emerged as highly effective DMTs. Ocrelizumab is the only DMT currently FDA-approved for primary progressive MS (PPMS).

Microglia

Microglia are resident CNS immune cells that shift between pro- and anti-inflammatory phenotypes. They contribute to both active and chronic lesion formation, and on the opposite spectrum, can facilitate remyelination and neuronal repair. In early active MS lesions, approximately 40% of phagocytic cells are proinflammatory microglia.

Microglia mediate neurodegeneration through several mechanisms:

• Cytokine release (TNF-α, IL-1β) → cytokine-induced cell death, inhibition of astrocytic glutamate reuptake
• Glutamate release → excitotoxicity
• Reactive oxygen/nitrogen species release → oxidative stress, mitochondrial injury
• Induction of dysfunctional RNA-binding proteins

Activated microglia are found at the periphery of mixed active/inactive lesions (also called chronic, slowly expanding, or "smoldering" lesions). Bruton tyrosine kinase (BTK) is expressed in both B cells and CNS microglia, making BTK inhibitors a promising therapeutic class currently in clinical trials for both relapsing and progressive MS.

Relapsing MS Pathology

Relapsing MS is primarily characterized by acute inflammatory activity with blood-brain barrier disruption, evidenced by gadolinium-enhancing lesions on MRI. Classic acute lesions feature:

• Infiltrates of inflammatory B, T, and plasma cells and macrophages surrounding a central vein
• Proinflammatory microglia
• Demyelination with concurrent axonal and neuronal injury
• Variable remyelination as lesions evolve (partially mediated by anti-inflammatory microglia)

Previously considered a predominantly white matter disease, cortical and deep gray matter involvement are now well characterized across all MS subtypes and are associated with disability progression.

Progressive MS Pathology

Additional mechanisms contributing to neurodegeneration in progressive MS include chronic microglial activation (even in normal-appearing white matter), impaired ion homeostasis, mitochondrial injury, and meningeal inflammation.

Risk Factors

Ultraviolet Radiation and Vitamin D

MS prevalence increases in a geographic gradient, with higher prevalence in northern regions. Living in areas with high ambient UVB radiation and increased summer sun exposure during childhood is associated with decreased MS risk. UVB exposure may also influence MS risk independently of vitamin D.

Vitamin D exerts immunomodulatory effects through multiple pathways:

• Inducing differentiation to regulatory T and B cells
• Decreasing production of proinflammatory cytokines
• Increasing secretion of anti-inflammatory cytokines
• Promoting oligodendrocyte maturation (direct neural effect)

Low vitamin D levels have been associated with increased MS risk, increased risk of conversion from clinically isolated syndrome (CIS) to clinically definite MS, and increased relapse risk. Despite this, data on the efficacy of vitamin D supplementation for clinical and radiographic outcomes remain mixed. Patients with MS also show a diminished serologic response to vitamin D supplementation compared with controls, suggesting altered vitamin D metabolism.

Genetics

The human leukocyte antigen (HLA) complex contains multiple genes related to immune system function. HLA-DRB1*15:01 is the strongest known genetic risk factor for MS, present in up to 30% of the US and Northern European population. HLA-A*02 is associated with decreased MS risk. The combination of HLA-DRB1*15:01 presence and HLA-A*02 absence confers higher risk than HLA-DRB1*15:01 alone.

Many other single nucleotide polymorphisms (typically near immune-related genes outside the HLA complex) have been identified. Gene–environment interactions that contribute to MS pathogenesis include vitamin D levels, childhood obesity, EBV infection, and smoking. However, genetic testing for MS risk variants is not currently recommended in clinical practice, even in patients with a family history of MS.

Epstein-Barr Virus

Although EBV seropositivity is very common in the general population, its role in MS pathogenesis is likely multifactorial, involving interactions between EBV, predisposing genetic factors, and environmental risk factors. Other viruses (HHV-6, CMV, HSV, human endogenous retroviruses) have been explored but have less well-established associations with MS.

Smoking

Smoking has been associated with increased MS risk, conversion from CIS to clinically definite MS, and conversion from relapsing to secondary progressive MS. Smoking may also reduce efficacy of some DMTs, including interferon and natalizumab. Passive smoke exposure also increases MS risk. Interestingly, oral tobacco use is associated with decreased MS risk, suggesting that lung inflammation (rather than nicotine itself) may be the driver.

Gene–environment interactions are relevant: among Swedish patients with HLA-DRB1*15:01 who lacked HLA-A*02, smoking contributed to MS development in 41% of cases. Clinician education regarding MS-specific smoking risks and referral to cessation support services may increase the likelihood of cessation.

Obesity

Childhood and adolescent obesity have been associated with increased risk of both pediatric and adult-onset MS, with a seemingly stronger association in females. In pediatric patients, obesity may precede MS diagnosis by several years. Abdominal obesity has also been associated with more severe disability. Proposed mechanisms include promotion of a proinflammatory milieu, decreased vitamin D bioavailability, and interactions with predisposing genetic factors and EBV infection.

Microbiome and Diet

Gut microbiome composition differs between patients with MS and controls. Species of Bacteroides and Firmicutes produce short-chain fatty acids from indigestible fiber, ultimately increasing production of regulatory T cells that suppress autoreactive immune cells. Bacteria also produce cytokines influencing both pro- and anti-inflammatory responses.

A unifying "MS diet" has not been established. Studies evaluating diets high in vegetables, fruits, and whole grains have not demonstrated consistent associations with relapse rate or disability progression, though higher diet quality may improve symptomatic issues such as fatigue and depression.

Comorbidities

Comorbid conditions are common in MS, increase with age, and can contribute to diagnostic and treatment delays. At the time of MS diagnosis, common comorbidities include depression, anxiety, hypertension, chronic lung disease, and hyperlipidemia.

Populations of Special Interest

Race-Specific Considerations

MS has traditionally been considered to predominantly affect White people of Northern European ancestry, but recent data challenge this assumption. Studies suggest a higher MS incidence in the US Black population than previously recognized, with up to a 47% higher risk of MS in Black women compared with White women.

Disparities in MS care persist: Black and Hispanic patients are less likely to see a neurologist, and patients of color have been disproportionately underrepresented in clinical trials of DMTs, limiting ability to draw race-specific conclusions about treatment efficacy.

Socioeconomic Status

Lower socioeconomic status has been associated with worse disability, worse MS-related symptoms (fatigue, depression, anxiety), and reduced treatment adherence. Nearly three-fourths of patients at one MS center experienced financial worry, and cost of care contributed to nonadherence to clinic visits, imaging, or medications in more than one-third of patients.

References

1. Ward M, Goldman MD. Epidemiology and pathophysiology of multiple sclerosis. Continuum (Minneap Minn). 2022;28(4):988-1005.
2. Wallin M, Culpepper W, Campbell J, et al. The prevalence of MS in the United States. Neurology. 2019;92(10):e1029-e1040.
3. Bjornevik K, Cortese M, Healy BC, et al. Longitudinal analysis reveals high prevalence of Epstein-Barr virus associated with multiple sclerosis. Science. 2022;375:296-301.
4. Lanz T, Brewer RC, Ho PP, et al. Clonally expanded B cells in multiple sclerosis bind EBV EBNA1 and GlialCAM. Nature. 2022;603:321-327.
5. Kamma E, Lasisi W, Libner C, et al. Central nervous system macrophages in progressive multiple sclerosis. J Neuroinflammation. 2022;19:45.
6. Bebo B, Cintina I, LaRocca N, et al. Economic burden of multiple sclerosis in the United States. Neurology. 2022;98:e1810-e1817.
7. Langer-Gould A, Gonzales E, Smith J, Li B, Nelson L. Racial and ethnic disparities in multiple sclerosis prevalence. Neurology. 2022;99(7):e685-e692.