🧠 Multiple Sclerosis (MS): Demyelinating Disease to Master for the USMLE
Multiple sclerosis is a chronic autoimmune demyelinating disorder of the central nervous system (CNS). It shows up again and again on Step 1, Step 2, and Step 3 because of its classic presentations (optic neuritis, internuclear ophthalmoplegia, sensory changes) and MRI hallmarks.
USMLE Buzzword: Multiple CNS lesions separated in time and space.
🔬 Pathophysiology
Autoimmune attack against oligodendrocytes → demyelination
Leads to impaired nerve conduction → neurological deficits
Plaques form predominantly in white matter of brain and spinal cord
Inflammatory damage and axonal loss occur over time
👀 Classic Clinical Features
| Symptom | Explanation |
|---|---|
| Optic neuritis | Painful vision loss, ↓ color vision |
| INO (internuclear ophthalmoplegia) | Impaired adduction of one eye (MLF lesion) |
| Sensory changes | Numbness, tingling |
| Motor symptoms | Weakness, spasticity |
| Bladder dysfunction | Urgency, retention |
| Fatigue | Common and disabling |
🧠 Uhthoff’s phenomenon = symptom worsening with heat exposure (e.g., hot showers)
📋 Diagnosis Strategy
| Modality | Key Finding |
|---|---|
| MRI brain/spine | Periventricular plaques ("Dawson fingers") |
| Lumbar puncture | Oligoclonal bands (IgG) in CSF |
| Visual evoked potentials | Delayed response indicating optic nerve demyelination |
⚡ MRI is the gold standard for diagnosis!
🧪 MRI Finding: Dawson Fingers
White matter lesions radiating outward from ventricles
"Dawson fingers" appearance due to perivenular demyelination
💊 Management Overview
| Goal | Treatment |
|---|---|
| Acute exacerbation | High-dose IV steroids (methylprednisolone) |
| Disease-modifying therapy | Interferon-beta, glatiramer acetate, ocrelizumab |
| Symptomatic treatment | Baclofen (spasticity), amantadine (fatigue) |
📚 Sample USMLE Vignette
A 27-year-old woman develops right eye pain and blurred vision. Two months later, she has new left arm weakness. MRI shows multiple periventricular lesions. CSF has oligoclonal bands. What is the most likely diagnosis?
✅ Answer: Multiple sclerosis
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