Multiple Sclerosis: Everything You Need to Know

1. What MS Is (Core Concept)

2. Epidemiology

• Prevalence: ~309 per 100,000 in US adults (~730,000 US cases, ~2.8 million worldwide - likely underestimates)
• Sex ratio: Women affected 2-2.5x more than men (consistent with autoimmune diseases broadly)
• Age of onset: Peak 3rd-4th decade; range postpubertal teens to 50s; extreme caution diagnosing before puberty or after 60
• Geography: Strong latitude gradient - prevalence approaches 1:500 in some temperate zones, much less near the Equator. Migration studies show that if you migrate before age ~15, you acquire the risk of your new country; after 15, you keep the risk of your birth country - pointing to an environmental trigger in adolescence
• Mortality: Average time from onset to death ranges 24-45 years. MS-related mortality declined 33% from 1996-2013 in Ontario. SMR ranges 1.3-2.9 vs. general population. ~50% of deaths are MS-related; infections are the most common cause of death
• Comorbidities that worsen mortality: psychiatric disease, cerebrovascular disease, cardiovascular disease, lung disease, diabetes, cancer, Parkinson disease

3. Etiology and Risk Factors

• HLA-DR15 (HLA-DRB1*1501) is the strongest genetic risk allele - confers ~3x risk
• Non-HLA loci including IL-7R, IL-2R, CD58, CLEC16A identified by GWAS
• Concordance in identical twins is ~25-30%, confirming strong but non-deterministic genetic contribution
• Siblings of affected individuals have ~2-5% lifetime risk (vs. 0.1-0.3% general population)

• Epstein-Barr virus (EBV): The strongest environmental association. Near-universal EBV seropositivity in MS patients. Military cohort data shows MS risk is ~32x higher in EBV-positive individuals. Molecular mimicry between EBNA1 and GlialCAM is a proposed mechanism (2022-2023 work - Bjornevik et al. Nat Rev Neurol 2023)
• Vitamin D deficiency: Latitude gradient partly explained by UV-B exposure and vitamin D synthesis; low vitamin D levels associated with higher MS risk and relapse rate
• Smoking: Independent risk factor for developing MS and for faster progression
• Adolescent obesity: Associated with increased risk, especially in women
• Gut microbiome: Dysbiosis identified in MS; disease-modifying therapies alter gut microbial composition

4. Pathophysiology - The Lesion

• Acute active lesion: Perivenular inflammation; CD4+ and CD8+ T cells, B cells, plasma cells, activated macrophages infiltrating the white matter. Myelin is stripped by macrophages and activated microglia. The lesion disrupts the blood-brain barrier (contrast-enhancing on MRI)
• Chronic active lesion (slowly expanding lesion): A smoldering rim of activated microglia/macrophages at the lesion edge continuing to eat away at relatively preserved axons. Identified on specialized MRI (paramagnetic rim lesions). Probably the dominant mechanism in progressive MS
• Chronic inactive lesion: A hypocellular scar of reactive astrogliosis - the "burnt out" plaque. No ongoing inflammation but irreversible axonal loss
• Cortical/grey matter lesions: Intracortical, subpial, leucocortical. MRI-invisible on standard sequences but contribute substantially to cognitive symptoms and progressive disability. Subpial demyelination is associated with meningeal B-cell follicle-like aggregates producing local inflammatory mediators

• Glutamate excitotoxicity from activated macrophages
• Reactive oxygen and nitrogen species (nitric oxide) blocking mitochondrial function
• Na+ channel redistribution along demyelinated axons → Na+/Ca2+ exchange reversal → Ca2+ overload → axon death
• Failure of energy supply to demyelinated axons (increased metabolic demand of continuous conduction vs. saltatory)

5. Clinical Disease Types

• Discrete attacks (relapses/exacerbations) of neurological dysfunction lasting >24 hours, followed by complete or partial recovery
• Between attacks: no progression (by definition)
• Pathological correlate: recurrent acute active lesions with blood-brain barrier breakdown

• Follows RRMS in ~50% of untreated patients by 10-15 years; higher rates pre-DMT era
• Gradual accumulation of disability independent of distinct relapses
• May have superimposed relapses early but eventually becomes "pure" progression
• Correlate: smoldering chronic active lesions, cortical atrophy, and diffuse axonal injury

• Progressive disability from onset, no preceding relapses
• More equal sex ratio (unlike RRMS which is female-predominant)
• Later mean age of onset (~40s)
• Predominantly spinal cord involvement → progressive myelopathy (spastic paraparesis)
• Fewer inflammatory lesions on MRI, more predominant grey matter and diffuse white matter damage

• First clinical episode of demyelination (optic neuritis, partial myelitis, brainstem syndrome)
• If MRI already shows dissemination in space (DIS), very high risk of converting to MS
• Risk of conversion with abnormal MRI: ~80% at 20 years; with normal MRI: ~20%

• MS-like MRI lesions found incidentally in a person with no symptoms
• ~50% convert to CIS/MS within 10 years

6. Presenting Symptoms - "FLAIR of Complaints"

• Paresthesias that are circumferential (not dermatomal) - suggests cord lesion
• Bandlike painful sensations around trunk or limb = myelopathic
• Vibration and proprioception loss (dorsal column) more than spinothalamic
• Lhermitte sign: electric shock sensation down the spine into limbs on neck flexion = cervical cord demyelination

• Optic neuritis - classic first presentation: painful monocular visual loss, pain worse on eye movement, central/centrocecal scotoma, desaturation to red color (best detected with a red pin). In 70% of cases, the optic disc looks normal (retrobulbar neuritis). After recovery, an afferent pupillary defect (Marcus Gunn pupil) often persists

• Upper motor neuron pattern: spastic weakness, hyperreflexia, extensor plantars
• Corticospinal tract demyelination in the cord
• Fatigue disproportionate to the degree of weakness is characteristic

• Intention tremor, limb ataxia, gait ataxia
• Charcot triad: nystagmus + intention tremor + scanning (staccato) speech - classic teaching but rarely all three present together

• Internuclear ophthalmoplegia (INO): adduction lag on the ipsilateral eye with nystagmus in the contralateral abducting eye - due to medial longitudinal fasciculus (MLF) demyelination. Bilateral INO in a young person = MS until proven otherwise
• Vertigo, diplopia, facial numbness, trigeminal neuralgia (paroxysmal)
• Dysarthria, dysphagia

• Bladder: urgency, frequency, incontinence (detrusor overactivity) OR urinary retention (detrusor-sphincter dyssynergia). ~75% of MS patients develop LUTS
• Bowel: constipation (most common) or fecal urgency/incontinence
• Sexual dysfunction: erectile dysfunction in men; orgasmic dysfunction and loss of libido in both sexes

• Short-term memory loss, word-finding difficulty, processing speed slowing, cognitive fatigue

• 50% have moderate-to-severe depression

• Pseudobulbar affect (pathological laughing/crying) in advanced disease
• Euphoria (classically described but actually rare)
• Dementia is uncommon but cognitive impairment is a major cause of disability/job loss

• Uhthoff phenomenon: worsening of existing symptoms with heat (exercise, hot bath, fever) - due to conduction failure in demyelinated fibers, which worsens at higher temperatures. Key: not a true relapse, resolves with cooling
• Fatigue: one of the most disabling and underdiscussed symptoms; can manifest as "unplugged" in the afternoon even after good sleep
• Pain (50%): from dysesthesias, trigeminal neuralgia, painful tonic spasms, musculoskeletal from abnormal gait

7. Diagnosis - McDonald 2017 Criteria

• Periventricular (≥3 lesions, juxtaventricular - touching the ventricle)
• Juxtacortical/cortical
• Infratentorial (brainstem, cerebellum, spinal cord)
• Spinal cord

• New T2 or gadolinium-enhancing lesion on a follow-up MRI, OR
• Simultaneous presence of gadolinium-enhancing AND non-enhancing lesions on a single scan (lesions of different ages = different times), OR
• New clinical relapse

• Ovoid, periventricular, perpendicular to ventricles = "Dawson fingers" (on sagittal FLAIR)
• T2/FLAIR hyperintense in white matter, periventricular, corpus callosum, brainstem, cerebellum, spinal cord
• Acute lesions: gadolinium-enhancing (ring or homogeneous) reflecting BBB breakdown, lasting 4-6 weeks
• Chronic lesions: T1 hypointense "black holes" = irreversible axonal loss
• Spinal cord lesions: typically ≤2 vertebral segments, peripheral, incomplete cross-section (distinguishes from NMO/NMOSD which is ≥3 segments, central cord, complete cross-section)

• Oligoclonal bands (OCBs): positive in >95% of established MS; type 2 pattern (bands in CSF not serum) is MS-specific
• IgG index elevated
• Mild lymphocytic pleocytosis in active disease (<50 cells)
• No rise in total protein usually

• Visual evoked potentials (VEPs): prolonged P100 latency (slowed conduction through optic nerve) even after clinical recovery from optic neuritis - demonstrates a subclinical lesion

8. Key Differential Diagnoses

• NMO: bilateral optic neuritis, area postrema syndrome (intractable hiccups/vomiting), longitudinally extensive transverse myelitis (≥3 vertebral segments), LETM
• Anti-AQP4 IgG seropositivity is diagnostic
• MOG-IgG disease: cortical encephalitis pattern, bilateral optic neuritis, often better prognosis

9. Disease-Modifying Therapies (DMTs)

Platform (Moderate Efficacy) Therapies

High-Efficacy Therapies

10. Acute Relapse Treatment

• High-dose methylprednisolone (IVMP) 1g/day IV x 3-5 days - first-line
• Accelerates recovery from relapses but does NOT change long-term outcome or disability accumulation
• Oral high-dose methylprednisolone (500-2000 mg/day) has equivalent efficacy to IV in several trials
• Plasma exchange (plasmapheresis): used for severe relapses unresponsive to steroids (e.g., devastating optic neuritis, transverse myelitis); removes circulating antibodies

11. Symptom Management

12. Pregnancy and MS

• Disease activity typically decreases during pregnancy, especially in 3rd trimester (exacerbations reduced by ~2/3) - thought due to immunomodulation by α-fetoprotein and steroid hormones
• Postpartum period (first 6 months): rebound in relapse rate
• Pregnancy does NOT change the long-term natural history
• Most DMTs are contraindicated in pregnancy:
  • Glatiramer acetate and interferon-β: generally considered safest if benefit outweighs risk; some guidelines allow GA throughout
  • Natalizumab: can continue to ~34 weeks, then stop (infant born with neonatal thrombocytopenia risk)
  • Teriflunomide and cladribine: absolutely contraindicated (teratogenic)
  • Breastfeeding: contraindicated while on most DMTs
• Women with stable MS should be counseled that pregnancy is generally safe

13. Progressive MS - The Hard Problem

• Chronically activated microglia/macrophages (slowly expanding/smoldering lesions)
• Meningeal B-cell aggregates releasing cytokines causing subpial cortical demyelination
• Mitochondrial failure in demyelinated axons
• Energy-starved axons that simply cannot sustain conduction over time

• Ocrelizumab: modestly slows disability in PPMS (NNT ~10 over 2 years for any disability reduction)
• Siponimod: slows progression in active SPMS (those with recent relapses or active MRI)
• BTK inhibitors (tolebrutinib, evobrutinib, fenebrutinib): in phase 3 trials targeting microglial activation; represent the next frontier

14. Monitoring and Biomarkers

• MRI: scan at 3-6 months after starting therapy (baseline), then at 12 months, then annually or when clinically indicated. New T2 lesions or enhancing lesions = sub-optimal disease control
• Neurofilament light chain (NfL): serum NfL is a biomarker of axonal injury. Elevated in active disease, correlates with disability worsening, brain atrophy, and relapse rate. Emerging as a treatment-response biomarker
• OCT (Optical Coherence Tomography): measures retinal nerve fiber layer (RNFL) thickness - a surrogate for neuro-axonal loss; each episode of optic neuritis leads to measurable RNFL thinning
• EDSS (Expanded Disability Status Scale): most widely used disability scale; 0-10 scale. Steps 0-5: ambulatory; 6: requires aid; 7: wheelchair; 8: bed-bound; 9: helpless; 10: death. Criticized for being heavily weighted on ambulation
• MSFC (MS Functional Composite): 25-foot walk, 9-hole peg test, PASAT - more sensitive than EDSS

15. Natural History Without Treatment

• Average time to EDSS 6 (needs walking aid): ~15-20 years from onset in RRMS
• ~50% of untreated RRMS converts to SPMS within 10-15 years
• Favorable prognostic factors: female sex, younger age at onset, sensory or optic nerve relapse onset, low lesion burden on initial MRI, complete recovery from first relapse, low relapse frequency early
• Adverse factors: male sex, older onset, motor or cerebellar relapse onset, high early relapse frequency, incomplete recovery, high T2 lesion burden and early T1 black holes, early brain atrophy

16. Emerging Concepts (2025)

• Silent progression (PIRA - Progression Independent of Relapse Activity): disability worsening occurs even in patients with NEDA (no relapses, no new MRI lesions). ~50% of "RRMS" patients show PIRA. This challenges the binary relapsing vs. progressive model and suggests MS is a single disease with two concurrent processes - acute inflammation AND chronic neurodegeneration - from the start
• BTK inhibitors block Bruton's tyrosine kinase in B cells AND microglia (CNS-penetrant), directly addressing smoldering neuroinflammation. Phase 3 trial results are highly anticipated
• Remyelination therapies (opicinumab, bexarotene, clemastine, LINGO-1 antibody) have shown partial effects in trials; none yet practice-changing
• EBV vaccine is being developed as potential MS prevention strategy following the landmark military cohort data