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MULTIPLE SCLEROSIS — PART 1: CLINICAL TYPES & McDONALD'S CRITERIA

(Harrison's 22e · Bradley & Daroff's Neurology · Davidson's perspective)

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⚡ QUICK EXAM FRAMEWORK (What Examiners Want)

MS = Chronic inflammatory demyelinating disease of CNS characterized by lesions disseminated in SPACE and TIME. The two pillars of diagnosis: Dissemination in Space (DIS) + Dissemination in Time (DIT) → McDonald's criteria operationalize these.

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SECTION 1: CLINICAL TYPES / PHENOTYPES OF MULTIPLE SCLEROSIS

1.1 The Modern Unitary View (Harrison's 22e, 2025 — Key Conceptual Shift)

• Genetics — same HLA-DRB1*1501 risk gene across all phenotypes
• Epidemiology — similar geographic distribution
• Immunology — same autoimmune inflammatory process
• Pathology — demyelination and axonal loss occur in all forms

Exam trap: The classical subtypes are still clinically useful for assessment and management — so you must know both the old classification AND the new concept.

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1.2 Classical Clinical Subtypes (2013 Lublin Revised Classification)

TYPE 1: RELAPSING-REMITTING MS (RRMS)

1. RAW (Relapse-Associated Worsening): Disability from incomplete relapse recovery — once thought to be the primary driver
2. PIRA (Progression Independent of Relapse Activity): Disability from ongoing neurodegeneration without clinical relapses — now recognized as the dominant driver in RRMS, SPMS, and PPMS

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TYPE 2: SECONDARY PROGRESSIVE MS (SPMS)

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TYPE 3: PRIMARY PROGRESSIVE MS (PPMS)

Exam point: Despite clinical differences, PPMS represents the same underlying illness as RRMS/SPMS — genetics, pathology, and immunology are essentially identical.

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Special Categories Under the 2013 Lublin Revised Classification

• Active RRMS — relapsing (standard RRMS)
• Active SPMS — progressive + still having relapses or new MRI lesions
• Inactive SPMS — progressive disability without relapses/new MRI activity
• Active PPMS — same modifiers applied to primary progressive

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Additional Phenotypes / Stages (Critical for Exam)

CIS — CLINICALLY ISOLATED SYNDROME

• First single clinical demyelinating episode lasting ≥24 hours at normal body temperature
• Does NOT yet meet criteria for MS (only ONE event so far)
• Can represent the very first attack of MS — ~50–85% convert to MS within years
• High-risk CIS → large T2 lesion burden on MRI → almost certain conversion to MS
• Low-risk CIS → normal MRI → much lower conversion risk
• Treatment implication: Starting DMT after high-risk CIS reduces conversion to CDMS (BENEFIT, CHAMPS, ETOMS trials)

RIS — RADIOLOGICALLY ISOLATED SYNDROME

• Incidental MRI lesions consistent with MS discovered on imaging done for another reason (e.g., headache workup)
• No clinical symptoms or attacks at time of discovery
• ~1/3 will develop a first clinical attack within 5 years
• Represents the presymptomatic phase of MS

RELAPSING MS (RMS)

• Umbrella term encompassing all relapsing patients: RRMS + SPMS with ongoing relapses
• Used in clinical trial eligibility and regulatory approvals for DMTs

ACTIVE PROGRESSIVE MS

• SPMS or PPMS patients who experience relapses OR have new MRI lesions

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1.3 Disease Course — The Big Picture

• Top panel (Traditional/pre-treatment view): RIS → CIS → RRMS (disability accumulates from incomplete relapse recovery) → SPMS (when relapses stop but progression continues)
• Bottom panel (Modern/treatment era): With highly effective therapy, relapses are abolished. But "Silent Progression" (PIRA) remains. The disease continues as a continuum with both neuroinflammation AND neurodegeneration occurring together throughout

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SECTION 2: McDONALD'S CRITERIA (2017 Revision)

2.1 Historical Evolution — Must Know for Short Notes/Viva

The core principle of McDonald criteria: Diagnosis can be made earlier and more reliably using MRI biomarkers rather than waiting for a second clinical attack. This enables earlier treatment.

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2.2 The 2017 McDonald Criteria — Complete Breakdown

PREREQUISITE (Non-negotiable):

A "clinical attack" = subjective report/objective finding of neurological symptom typical of MS, lasting ≥24 hours, at normal body temperature, without fever/infection.

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PILLAR 1: DISSEMINATION IN SPACE (DIS)

Key 2017 change: Cortical lesions now count as a distinct location AND can be used interchangeably with juxtacortical lesions.

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PILLAR 2: DISSEMINATION IN TIME (DIT)

Key 2017 change: The 2010 criteria required that symptomatic brainstem/spinal cord lesions NOT be used for DIS or DIT. The 2017 criteria removed this restriction — any typical MS location now counts.

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THE GAME-CHANGING 2017 ADDITION: CSF Oligoclonal Bands (OCBs)

• Multiple studies showed OCBs are an independent risk factor for further clinical activity
• OCBs found in >90% of confirmed MS patients
• Absence of OCBs does NOT exclude MS (10–20% of confirmed MS lack OCBs at any given time)
• The IgG index (CSF IgG/Albumin ÷ Serum IgG/Albumin) is elevated in >90% of MS patients

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PPMS Diagnosis — Special Criteria

1. 1 year of progressive disability independent of relapse
2. PLUS two of the following three MRI criteria:
   • ≥1 DIS T2 lesion in the brain (periventricular, cortical, juxtacortical, or infratentorial)
   • ≥2 T2 lesions in the spinal cord
   • Positive CSF OCBs

Note: For PPMS, if CSF OCBs are absent, intrathecal IgG synthesis documentation can also support diagnosis.

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2.3 McDonald Criteria Summary Table (2017) — Exam-Ready

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2.4 Key MRI Concepts for McDonald Criteria

• Lesions oriented perpendicular to the lateral ventricles on sagittal MRI
• Represent perivenular inflammation along medullary veins
• Located in periventricular white matter
• Classic and highly characteristic of MS

• Gd crosses disrupted blood-brain barrier (BBB) at sites of acute inflammation
• Marks active/new inflammatory lesions
• Enhancement typically lasts <1 month
• Active Gd+ lesion = acute MS plaque

• Remain as permanent hyperintense foci after inflammation resolves
• Represent the cumulative "burden of disease"
• Found in virtually all MS patients (>95%)

• ~1/3 of T2 lesions appear as T1 black holes
• Indicate irreversible demyelination and axonal loss
• Progress over time in chronic lesions

• A vein visible at the center of an MS lesion on susceptibility-weighted imaging
• Helps distinguish MS from other white matter diseases
• Now mentioned in Harrison's 22e as diagnostically useful

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2.5 CSF in McDonald Criteria Context

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2.6 Evoked Potentials (Supporting Evidence)

Key point: EPs detect subclinical lesions at sites remote from the clinical lesion — thus providing objective evidence of spatial dissemination.

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SECTION 3: RELATED CONCEPTS FOR COMPLETE UNDERSTANDING

3.1 Pre-MS Stages — The Disease Continuum

Genetic susceptibility (HLA-DRB1*1501)
        ↓
Environmental triggers (EBV infection, low Vitamin D, smoking)
        ↓
RIS (Radiologically Isolated Syndrome) — subclinical MRI lesions, no symptoms
        ↓
CIS (Clinically Isolated Syndrome) — first clinical attack
        ↓
RRMS (Relapsing-Remitting MS) — relapses + remissions + silent progression
        ↓
SPMS (Secondary Progressive) — relapse-independent disability accumulation

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3.2 EDSS — Expanded Disability Status Scale

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3.3 Practical Diagnostic Algorithm (How to Apply McDonald Criteria Clinically)

• ≥24 hours duration, at normal temperature, no fever/infection
• Typical symptoms: optic neuritis, transverse myelitis, brainstem/cerebellar syndrome, hemispheric syndrome

• Fever, systemic illness during attack → suspect ADEM
• CSF protein >100 mg/dL, >75 WBCs, PMNs → not typical MS
• Symmetric lesions → metabolic/genetic disorder
• Basal ganglia involvement → ADEM, vasculitis, CADASIL
• Anti-AQP4 antibody positive → Neuromyelitis Optica (NMOSD)
• Anti-MOG antibody positive → MOGAD
• No OCBs with atypical course → reconsider diagnosis

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3.4 Differential Diagnosis of MS (For McDonald Criteria — "Exclusion" Step)

• Neuromyelitis optica spectrum disorder (NMOSD) — anti-AQP4 Ab+, area postrema syndrome, severe attacks
• MOG antibody-associated disease (MOGAD) — anti-MOG Ab+, ADEM phenotype
• ADEM (Acute Disseminated Encephalomyelitis) — monophasic, post-infectious/vaccine
• SLE, Sjögren's syndrome, Behçet's disease, vasculitis, sarcoidosis

• Neuroborreliosis (Lyme disease)
• HTLV-1/2 (tropical spastic paraparesis)
• HIV encephalopathy
• Progressive Multifocal Leukoencephalopathy (PML — JC virus)
• Neurosyphilis

• CADASIL (subcortical infarcts, family history, NOTCH3 mutation)
• Antiphospholipid antibody syndrome
• CNS vasculitis
• Spinal dural AV fistula

• Vitamin B12 deficiency (subacute combined degeneration)
• Copper deficiency myelopathy

• Metachromatic leukodystrophy (adults)
• Adrenoleukodystrophy/adrenomyeloneuropathy

• Arnold-Chiari malformation
• Cervical spondylotic myelopathy

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3.5 High-Yield Exam Facts — "The Points That Win Marks"

🎯 These are the facts that separate toppers from average scorers:

1. RRMS = 85–90%, PPMS = 10–15% — SPMS always starts as RRMS
2. PPMS: more equal sex ratio (1:1), later onset (~40 yrs), more spinal cord dominant
3. McDonald 2017 key changes: (a) Cortical lesions added as DIS location, (b) OCBs substitute for DIT in CIS+DIS, (c) Symptomatic brainstem/spinal cord lesions now CAN be used for DIS/DIT
4. MRI lesions occur 9–10× more often than clinical relapses — most MS activity is "clinically silent"
5. Dawson's fingers = periventricular lesions perpendicular to ventricles on sagittal MRI
6. Gd enhancement persists <1 month — marks acute active lesion
7. OCBs in >90% MS — but 10–20% can be negative; OCBs must be absent from paired serum
8. EDSS 6.0 = unilateral walking assistance; EDSS 6.5 = bilateral assistance
9. PIRA (Progression Independent of Relapse Activity) is now recognized as the dominant driver of disability even in RRMS
10. CSF red flags (NOT typical MS): >75 cells/μL, protein >100 mg/dL, PMN predominance
11. "Active" modifier in progressive MS = still having relapses or new MRI lesions
12. CIS → high-risk if large T2 lesion burden on MRI; treat early to delay CDMS
13. VEP prolonged P100 latency = optic nerve demyelination (most common EP abnormality in MS)
14. For PPMS diagnosis: need 1 year progression + 2 of 3 MRI/CSF criteria
15. Anti-AQP4 antibody = NMOSD (NOT MS); if positive, McDonald criteria do NOT apply

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Sources Used

• Harrison's Principles of Internal Medicine, 22nd Edition (2025) — Chapter 455: Multiple Sclerosis
• Bradley and Daroff's Neurology in Clinical Practice — Chapter 80: Multiple Sclerosis (McDonald Criteria, MRI criteria, CSF)
• Thompson AJ et al. Diagnosis of multiple sclerosis: 2017 revisions of the McDonald criteria. Lancet Neurol 17:162, 2018 (cited in Harrison's)
• Lublin FD et al. 2013 revised clinical course descriptions

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📚 What's Coming Next

• Part 2: Pathogenesis, Epidemiology, Clinical Features (Symptoms & Signs), Investigations
• Part 3: Treatment (Acute attacks, DMTs, Symptomatic therapy), Prognosis, Special Situations (Pregnancy, Fatigue, Uhthoff's phenomenon)