Multiple Sclerosis: A High-Yield USMLE Review
Multiple sclerosis (MS) is an immune-mediated demyelinating disease of the central nervous system whose hallmark is dissemination in space and time — multiple CNS lesions occurring in multiple episodes. It classically strikes young adults (often women) and is a favorite exam topic through its signature presentation of optic neuritis and internuclear ophthalmoplegia.
Pathophysiology
Autoreactive T cells cross the blood-brain barrier and trigger local inflammation involving microglia, macrophages, and B cells, leading to destruction of myelin. Demyelination produces conduction block along affected axons, which manifests as the acute neurological deficit. As inflammation subsides, partial remyelination and sodium channel reorganization on bare axons allow symptoms to improve, though often incompletely. With repeated attacks, cumulative axonal loss becomes irreversible, causing brain and cord atrophy and progressive disability.
Presentation
- Optic neuritis: eye pain with vision loss and an afferent pupillary defect (APD/Marcus Gunn pupil); because inflammation is retrobulbar, the optic disc looks normal initially — 'the patient sees nothing, the doctor sees nothing' — with disc pallor developing weeks to months later
- Prior neurological episode in a different CNS location (e.g., limb weakness or sensory symptoms) that resolved spontaneously over weeks, reflecting a relapsing pattern
- Bilateral internuclear ophthalmoplegia (INO) in a young patient — failure of adduction on lateral gaze with nystagmus in the abducting eye and intact convergence, from a medial longitudinal fasciculus lesion
- Recovery of deficits is often incomplete, and repeated attacks can lead to progressive disability
Diagnosis
- Brain MRI: multiple T2-hyperintense lesions in periventricular white matter; the simultaneous presence of gadolinium-enhancing (new) and non-enhancing (old) lesions demonstrates dissemination in time, while lesions in multiple CNS locations demonstrate dissemination in space (McDonald criteria)
- CSF analysis: oligoclonal bands present in CSF but not in serum (intrathecal IgG synthesis), an elevated IgG index, and mild lymphocyte-predominant pleocytosis with normal or slightly elevated protein
- Afferent pupillary defect (APD): the most objective sign of optic neuropathy, helping distinguish optic neuritis from other causes of vision loss
Management
- Acute relapse: high-dose IV methylprednisolone (e.g., 1 g/day for 3–5 days)
- Long-term: initiate disease-modifying therapy (DMT) — options include injectable agents (interferon-beta, glatiramer), oral agents (fingolimod, dimethyl fumarate, teriflunomide), or infusion agents (natalizumab, ocrelizumab)
High-yield
- Dissemination in space AND time is the defining hallmark — multiple lesions, multiple episodes
- Bilateral INO in a young patient should make you think MS
- Oligoclonal bands in CSF (but not serum) indicate intrathecal IgG synthesis
- Optic neuritis is retrobulbar, so the disc appears normal early: 'the patient sees nothing, the doctor sees nothing'
- APD (Marcus Gunn pupil) is the most objective sign of optic neuropathy
Pitfalls
- NMOSD is not MS: it features severe, poorly-recovering (often bilateral) optic neuritis, longitudinally extensive transverse myelitis (≥3 segments), area postrema syndrome, and AQP4 antibodies against astrocytes — and MS disease-modifying treatments may worsen it
- A normal brain MRI does not rule out MS; early disease may have a normal scan, so clinical history and follow-up imaging remain essential
- Do not assume vision loss is optic neuritis without checking for an APD, which distinguishes optic neuropathy from other causes
Don't just memorize Multiple Sclerosis — practice reasoning through it on branching cases where your decisions shape the patient.