Neurology System
Neurology
High-yield neurology for USMLE Step 2/3 — brainstem localization, stroke, spinal cord syndromes, eye findings, headache, NMJ disorders, and brain tumors, extracted from Divine Intervention and organized for exam performance.
Neuroanatomy & CSF Findings
The Circle of Willis and brainstem vascular anatomy underlie almost every localization question on the USMLE — master the lateral-vs-medial rule and you can decode any brainstem syndrome. CSF findings are equally formula-driven: knowing which pattern maps to GBS vs bacterial vs viral vs MS turns LP questions into free points.
EP315
The Clutch Circle of Willis Podcast
- Lateral vs medial rule: Pain/pinprick/temperature loss or Horner's = lateral brainstem. No PPTH = medial brainstem. Use cranial nerve involved to name the level (CN9-12 = medulla; CN5-8 = pons; CN3-4 = midbrain)
- Vertebral branches: PICA → lateral medulla (Wallenberg); anterior spinal artery → medial medulla (tongue weakness CN12 + UMN signs)
- Basilar artery: AICA → lateral pons; paramedian pontine arteries → medial pons (CN6 = factor of 12 = medial). Basilar occlusion = locked-in syndrome
- Terminal basilar: Superior cerebellar artery → superior colliculus (Parinaud syndrome if stroked); PCA → occipital lobe (macular sparing because MCA collateral)
- PCOM aneurysm: CN3 courses between SCA and PCA — PCOM aneurysm compresses CN3 → blown pupil (down and out eye)
- ACommA aneurysm: Most common aneurysm location → bitemporal hemianopsia (optic chiasm compression) or worst-headache-of-life SAH
The Lateral vs Medial Rule (The Core Framework)
When you see a brainstem question, ask: Do I see PPTH (pain, pinprick, temperature, Horner's)?
YES → lateral brainstem lesion. NO → medial brainstem lesion.
| Level | Lateral (PPTH) | Lateral Blood Supply | Medial (no PPTH) | Medial Blood Supply |
|---|---|---|---|---|
| Medulla | Wallenberg (CN9, 10, 11 + PPTH) | PICA / vertebral | CN12 (tongue) + UMN contralateral | Anterior spinal artery |
| Pons | Lateral: CN5, 7, 8 + PPTH | AICA | CN6 (abduction fail) + UMN contralateral | Paramedian pontine arteries |
| Midbrain | CN3, 4 (medial midbrain) — special | SCA / PCA | CN3 + contralateral UMN | PCA/SCA branches |
Memory Hook — Factors of 12
Medially oriented cranial nerves are all factors of 12: CN3, CN4 (midbrain), CN6 (pons), CN12 (medulla). Everything else = lateral. This explains why AICA (lateral pons) affects CN5, 7, 8 — not CN6.
Complete Circle of Willis Map
| Vessel | Territory / HY Fact |
|---|---|
| Vertebral artery | Branch of subclavian; gives PICA and anterior spinal artery |
| Basilar artery | Formed by two vertebrals; gives AICA, paramedian pontine branches, SCA, then PCA |
| PCA | Primary visual cortex; occipital lobe; macular sparing (MCA collateral to macula) |
| PCommA | Connects PCA to ICA; runs alongside CN3 → PCOM aneurysm = CN3 palsy (blown pupil) |
| MCA | Face + upper extremities; dominant side = aphasia. Most common embolic vessel |
| ACA | Lower extremities (and perineum) |
| ACommA | Most common aneurysm site → bitemporal hemianopsia + thunderclap SAH |
Critical: Locked-In Syndrome
Basilar artery occlusion = complete pons infarct → quadriplegia, intact extraocular muscles only. Also caused by osmotic demyelination syndrome (overcorrect hyponatremia too fast).
EP311
The Clutch CSF Findings & Brain Imaging Podcast
- GBS: Albuminocytologic dissociation — markedly ↑ protein, 0–3 WBC (no agreement between cells and protein); post-infectious peripheral demyelination; Schwann cells; cranial nerves usually spared
- MS: Oligoclonal bands (IgG) in CSF; ONLY CN2 affected (myelinated by oligodendrocytes, not Schwann cells); demyelinating lesions separated in space and time
- Bacterial meningitis: ↑ opening pressure, neutrophils, ↑ protein, ↓ glucose. Empiric: ceftriaxone + vancomycin + steroids
- Viral meningitis: Lymphocytes, slightly ↓ glucose, roughly normal protein, normal-ish opening pressure. Supportive care
- Fungal = TB meningitis: Both — lymphocytes, ↑ opening pressure, ↓ glucose, ↑ protein. TB: cell-mediated (caseating). Fungal: India ink (Cryptococcus)
- Imaging rules: MRI preferred for posterior fossa, spinal cord, early ischemia, MS plaques. Non-contrast CT first for SAH (xanthochromia on LP if CT negative)
CSF Pattern Master Table
| Condition | Opening Pressure | WBC / Type | Protein | Glucose |
|---|---|---|---|---|
| GBS | Normal | 0–3 / — | ↑↑ (markedly) | Normal |
| MS | Normal | Normal–↑ lymphocytes | Normal–↑, IgG | Normal |
| Bacterial | ↑↑ | ↑↑ neutrophils | ↑↑ | ↓↓ |
| Viral | Normal–slightly ↑ | ↑ lymphocytes (mild) | Normal–slightly ↑ | Normal–slightly ↓ |
| Fungal / TB | ↑ | ↑ lymphocytes | ↑ | ↓ |
| SAH | ↑↑ | ↑ RBCs, xanthochromia | ↑ | Normal |
| NPH | Normal | Normal | Normal | Normal |
High-Yield Trick — NPH Opening Pressure
Normal pressure hydrocephalus has NORMAL CSF opening pressure despite ventriculomegaly. This is the opposite of pseudotumor cerebri (IIH) which has elevated pressure. Both have impaired CSF reabsorption at arachnoid granulations — but NPH stays "normal" on LP.
Why MS Only Affects CN2
All cranial nerves derive from neural crest → myelinated by Schwann cells. CN2 (optic nerve) derives from the diencephalon (neural tube) → myelinated by oligodendrocytes. MS destroys oligodendrocytes, so CN2 is the only cranial nerve vulnerable. In Guillain-Barré (Schwann cell disease), CN2 is spared but other cranial nerves can be affected.
Albuminocytologic Dissociation (GBS)
Classic LP finding: protein often 100–1000 mg/dL with WBC count of only 0–10 cells. This mismatch (albumin high, cells low) is the signature. Seen when the BBB is disrupted by inflammation but no active cellular infection or autoimmune process is attacking within the CSF compartment itself.
Stroke & Cerebrovascular
Rapid Review Series 98 and 99 deliver the clinical application of Circle of Willis anatomy — from recognizing bitemporal hemianopsia as an ACommA aneurysm to identifying MCA territory by the lower face + upper extremity pattern. These episodes emphasize context-reading over isolated fact recall.
RR 98EP468
Bitemporal Hemianopsia, Bacterial Meningitis & Kawasaki
- Bitemporal hemianopsia: Outer (temporal) visual field lost in both eyes → optic chiasm lesion. ACommA aneurysm rupture → bleeds onto chiasm. Also pituitary adenoma, craniopharyngioma
- ACommA aneurysm associations: Marfan syndrome, ADPKD, hypertension — can rupture (thunderclap SAH) or silently compress chiasm
- Bacterial meningitis empiric therapy (CVS): Ceftriaxone + Vancomycin + Steroids. Add ampicillin for neonates and age >50 (Listeria coverage)
- Most common cause of meningitis: Strep pneumo → ceftriaxone covers. VANC for Staph aureus. Steroids specifically when worried about Strep pneumo meningitis
Visual Field Defects — Localization
| Lesion Site | Visual Field Defect | Classic Cause |
|---|---|---|
| Before optic chiasm (optic nerve) | Complete blindness in one eye | Optic neuritis (MS), CRAO |
| Optic chiasm | Bitemporal hemianopsia (outer fields both eyes) | Pituitary adenoma, craniopharyngioma, ACommA aneurysm |
| Optic tract (after chiasm) | Homonymous hemianopsia (same visual field both eyes) | MCA or PCA stroke |
| PCA infarct (occipital cortex) | Homonymous hemianopsia WITH macular sparing | MCA collateral to macula |
Macular Sparing Mechanism
The macula receives dual blood supply from PCA AND MCA. PCA infarct destroys peripheral vision but MCA keeps the macula perfused → macular sparing. Exam clue: "cortical blindness but patient can read fine-print central text."
Meningitis Empiric Treatment
Standard (adult): Ceftriaxone + Vancomycin + IV dexamethasone (reduces CN8 hearing loss in Strep pneumo)
Neonates and >50: Add ampicillin (Listeria monocytogenes is NOT covered by cephalosporins)
Asplenic/complement deficiency: Worried about Neisseria meningitidis — ceftriaxone covers. Prophylax contacts with rifampin or ciprofloxacin.
RR 99EP473
MCA Stroke, Tuberous Sclerosis & Context-Based Diagnosis
- MCA stroke pattern: Contralateral lower face + upper extremity weakness. Left MCA = aphasia (dominant hemisphere). Most common embolic vessel — AFib, recent MI → anticoagulate
- Homunculus rule: ACA = lower extremities; MCA = face + upper extremities; PCA = vision (occipital)
- Tuberous sclerosis: Ash leaf spots (hypopigmented macules), cardiac rhabdomyomas (mitral regurg murmur), infantile spasms (West syndrome — hip arrhythmia on EEG → ACTH or vigabatrin), angiomyolipomas (↑ creatinine)
- TSC genetics: Autosomal dominant; TSC1 (hamartin, chr 9) or TSC2 (tuberin, chr 16) → loss of mTOR downregulation → unregulated cell growth
Stroke Patterns by Vessel
| Vessel | Deficits | Classic Clue |
|---|---|---|
| MCA (dominant) | Contralateral face + arm weakness, aphasia (Broca or Wernicke), visual field defect | Right-handed person with speech + right arm weakness = left MCA |
| MCA (non-dominant) | Contralateral face + arm weakness, hemispatial neglect | Person ignores left side of body/space |
| ACA | Contralateral leg weakness + urinary incontinence | Bilateral proximal leg weakness |
| PCA | Contralateral homonymous hemianopsia with macular sparing | Can't see left or right but central vision intact |
| Lacunar | Pure motor or pure sensory; no cortical signs | Hypertensive patient, small vessel disease |
Tuberous Sclerosis — Cause of Death
Neurologic disease is #1 (seizures, brain tubers). Renal disease (angiomyolipomas) is #2. Mnemonic: NR = Not Rated, alphabetically N before R.
Tuberous Sclerosis vs NF1 — Key Differentiation
| Tuberous Sclerosis | NF1 | |
|---|---|---|
| Skin | Ash leaf spots (hypopigmented) | Café au lait spots (hyperpigmented) |
| Eyes | Retinal hamartomas | Lisch nodules (iris hamartomas) |
| Brain | Cortical tubers, subependymal nodules | Optic nerve glioma (CN2) |
| Inheritance | AD — TSC1/TSC2 | AD — NF1 gene (chr 17) |
Eye, Pupils & Visual Pathways
Four dedicated episodes make the eye one of the most heavily covered topics in the Divine Intervention neurology series — and for good reason. Eye findings anchor diagnoses across neurology, oncology, infectious disease, and pharmacology simultaneously. The mechanistic approach here (ciliary ganglion, Edinger-Westphal nucleus, sympathetic chain) makes these questions pattern-recognizable rather than memorizable.
EP361
The Eye Part 1 — Anatomy, Visual Pathways, Extraocular Muscles
- CN2 is unique: Derived from diencephalon (not neural crest) → myelinated by oligodendrocytes → why MS causes optic neuritis; Guillain-Barré spares CN2
- Extraocular muscle rule: All EOM = CN3 except two: lateral rectus (CN6 abducens) and superior oblique (CN4 trochlear — SO4). SO4 depresses + intorts the eye
- Lens dislocation: Homocystinuria = down and in (AR, cystathionine β-synthase, intellectual disability, early MIs); Marfan syndrome = up and out (AD, fibrillin defect, normal intelligence)
- Macular structures: Highest visual acuity at fovea (cones); macula gets blood from PCA + MCA → macular sparing with PCA infarct; cherry red spot in Tay-Sachs (no hepatosplenomegaly) vs Niemann-Pick (with hepatosplenomegaly)
- Pupillary light reflex: Afferent = CN2; efferent = CN3 (Edinger-Westphal nucleus → ciliary ganglion → pupillary sphincter)
Extraocular Muscles — The Rule of Two Exceptions
| Muscle | Nerve | Action |
|---|---|---|
| Medial rectus | CN3 | Adducts (towards nose) |
| Superior rectus | CN3 | Elevates + adducts |
| Inferior rectus | CN3 | Depresses + adducts |
| Inferior oblique | CN3 | Elevates + abducts |
| Levator palpebrae superioris | CN3 | Elevates eyelid (also sympathetic via superior tarsal) |
| Lateral rectus | CN6 | Abducts (away from nose) |
| Superior oblique | CN4 | Depresses + intorts (SO4) |
CN4 (Trochlear) Palsy — High-Yield Facts
- Only CN to exit posteriorly from brainstem → crosses → contralateral deficit (lesion on left → right eye problem)
- Superior oblique depresses the eye → loss = eye stays elevated → vertical diplopia
- Compensation: patient tilts head away from lesion side (chin toward lesion side)
- Classic exam scenario: diplopia reading or going down stairs
Cherry Red Spot — Two Diseases
Tay-Sachs: Hex-A deficiency → GM2 ganglioside builds up in neurons only → cherry red spot, NO hepatosplenomegaly, intellectual disability.
Niemann-Pick: Sphingomyelinase deficiency → builds up in neurons AND reticuloendothelial system (liver + spleen) → cherry red spot + hepatosplenomegaly.
EP365
The Eye Part 2 — Accommodation, MLF, INO, Horner's Intro
- Accommodation triad: (1) Lens thickens (ciliary muscle — parasympathetic, Edinger-Westphal → ciliary ganglion); (2) both eyes converge (medial recti via CN3); (3) pupil constricts (sphincter pupillae via CN3)
- INO vs CN3 palsy: MLF lesion → adduction fails on horizontal gaze but accommodation is NORMAL (MLF not used for accommodation). CN3 lesion → both gaze adduction AND accommodation impaired
- INO + MS: MLF myelinated by oligodendrocytes → MS demyelinates MLF → ipsilateral adduction nystagmus on horizontal conjugate gaze. Contralateral eye abducts with nystagmus
- CN4 (trochlear) — key extra fact: Crosses brainstem → contralateral eye deficit. Test: accommodate to see if medial rectus works independently of gaze; if yes → MLF, not CN3
- Horner's syndrome pathway: 3-neuron pathway: hypothalamus → lateral brainstem (first neuron) → C8-T2 → superior cervical ganglion → eye (third neuron)
Internuclear Ophthalmoplegia (INO) — The Classic MS Eye Finding
The medial longitudinal fasciculus (MLF) connects the CN6 nucleus (abducens, lateral pons) to the CN3 nucleus (oculomotor, midbrain) — making coordinated horizontal eye movements possible.
| MLF Lesion (INO) | CN3 Palsy | |
|---|---|---|
| Gaze adduction | FAILS — nystagmus on adducting eye | FAILS — eye stuck laterally (down and out) |
| Accommodation | NORMAL — medial rectus works for near | IMPAIRED — ciliary + medial rectus both fail |
| Pupil | Normal size | Blown (mydriasis) if compressive cause |
| Classic cause | Multiple sclerosis | PCOM aneurysm, uncal herniation |
Testing the Difference — Key Exam Strategy
If a patient can't adduct their eye on horizontal conjugate gaze, test accommodation. Bring an object close → if both pupils adduct and constrict normally → the medial rectus works fine → it's an INO (MLF problem), not a CN3 problem.
EP436
Eye Findings and The USMLEs — Floridly High-Yield
- Pupil miosis: Opioids (+ respiratory depression) or cholinergic toxidrome (+ diarrhea, bradycardia). Mydriasis = sympathomimetics (cocaine) or anticholinergics (diphenhydramine, TCAs — dry, hot, tachycardic)
- Parinaud syndrome: Upward gaze palsy. Pinealoma (compresses superior colliculus) or SCA stroke. Sundown sign (eyes look down)
- CRAO: Fundal pallor (white retina) — embolus from AFib/MI. CRVO: blood and thunder fundus, cotton wool spots, flame hemorrhages — hypercoagulability
- Diabetic retinopathy: Cotton wool spots (both types); proliferative = neovascularization → treat with laser photocoagulation. Proliferative → retinal detachment from tractional forces
- CMV retinitis: CD4 <50 — white fluffy/necrotic lesions; treat with ganciclovir. Herpes keratitis: dendritic ulcers → acyclovir
- Drug toxicities: Digoxin → yellow-green vision; ethambutol → optic nerve damage; hydroxychloroquine → retinopathy (annual eye exam); methanol → optic nerve (treat with fomepizole)
HY Eye Findings — Rapid Association Table
| Finding | Diagnosis | Key Detail |
|---|---|---|
| Blue sclera | Osteogenesis imperfecta | Type 1 collagen deficiency → choroidal vessels visible |
| Lisch nodules (iris) | NF1 | Brown hamartomas on iris; also optic glioma |
| Lenticonus (oil droplet) | Alport syndrome | Type 4 collagen defect (COL4A5); hematuria, sensorineural hearing loss |
| White reflex (leukocoria) | Retinoblastoma | RB gene mutation → risk of osteosarcoma |
| Corneal clouding (newborn) | Galactosemia / galactokinase deficiency | Galactitol accumulates in lens |
| Hollenhorst plaques | Cholesterol embolization | After cardiac catheterization; refractile bodies in retina |
| Drusen (yellow deposits) | Age-related macular degeneration | Wet AMD: VEGF inhibitors (bevacizumab); Dry: antioxidants |
| AV nicking | Severe hypertension | Copper/silver wire changes; see in renal artery stenosis, FMD, Conn's |
| Papilledema + child abuse | Shaken baby syndrome | Retinal hemorrhages + papilledema in non-verbal child |
| Anterior uveitis | JRA oligoarticular, sarcoidosis | JRA → must have slit-lamp exams |
Afferent vs Efferent Pupillary Defect
Afferent (CN2) defect: Shine light in affected eye → BOTH pupils don't constrict. Shine in unaffected eye → BOTH constrict. (All-or-nothing per eye tested.)
Efferent (CN3) defect: One pupil NEVER constricts regardless of which eye you shine light in. The affected pupil stays dilated always.
EP458
Horner's Syndrome — A USMLE Perspective
- Classic triad: Ptosis (superior tarsal muscles), miosis (dilator failure → parasympathetics unopposed), anhydrosis (sweat gland denervation) — all from sympathetic failure to eye
- 3-neuron pathway: (1) Hypothalamus → lateral brainstem → C8-T2 spinal cord; (2) C8-T2 → brachial plexus → apex of lung → superior cervical ganglion; (3) superior cervical ganglion → travels around internal carotid → eye
- 1st neuron lesion: Horner's + PPTH (pain/temp) on face + contralateral body = Wallenberg syndrome (PICA/lateral brainstem stroke)
- 2nd neuron lesion: Horner's + brachial plexus signs + hand weakness = Pancoast tumor (apex squamous cell lung cancer)
- 3rd neuron lesion: Horner's + multiple extraocular muscle deficits = carotid dissection or cavernous sinus thrombosis (fibers travel around internal carotid in carotid sinus)
- Cluster headache: Partial Horner's — ptosis + miosis but NO anhydrosis (postganglionic fibers to sweat glands unaffected)
Horner's — Localizing by Companion Symptoms
| Neuron | Path | Companion Findings | Classic Cause |
|---|---|---|---|
| 1st (central) | Hypothalamus → lateral brainstem → C8-T2 | PPTH + crossed cranial nerve deficits | PICA stroke (Wallenberg), lateral cord lesion |
| 2nd (preganglionic) | C8-T2 → brachial plexus → apex lung | Hand weakness, brachial plexopathy, Pancoast mass on CXR | Pancoast (superior sulcus) tumor — squamous cell |
| 3rd (postganglionic) | Superior cervical ganglion → ICA → carotid sinus | Multiple extraocular deficits (CN3, 4, 6) | Carotid dissection, cavernous sinus thrombosis |
Cluster Headache — Partial Horner's
In cluster headache, the postganglionic fibers to the sweat glands diverge after the superior cervical ganglion along the external carotid. The inflamed cavernous sinus affects only fibers to the eye, not to the forehead sweat glands. Result: ptosis + miosis, NO anhydrosis. This is "partial" Horner's.
Headache Syndromes
The Clutch Headache Podcast distills headache management into two axes: acute treatment vs. prophylaxis, and primary vs. red-flag secondary headaches. The vignette-based approach maps directly to exam question structure — every headache type has a signature presentation and a specific treatment that cannot be swapped with another type.
EP372
The Clutch Headache Podcast
- Migraine: Pulsatile, unilateral, nausea/photophobia/phonophobia; acute = sumatriptan (triptan) or ergotamine; avoid in coronary vasospasm, prior stroke/MI. Prophylaxis if ≥8/month = beta-blocker, TCA, or topiramate
- Cluster: Unilateral periorbital, autonomic features (tearing, rhinorrhea, partial Horner's). Acute = 100% O2 or triptan; prophylaxis = verapamil (1st line); also lithium, steroids
- Tension: Bilateral band-like, no nausea/aura; treat with NSAIDs and relaxation
- Trigeminal neuralgia: Lancinating unilateral facial pain triggered by wind/touch; treat with carbamazepine (1st line)
- SAH: Thunderclap ("worst headache of life"); non-contrast CT first → LP for xanthochromia if CT negative; nimodipine prevents post-bleed vasospasm; RFs: Marfan, ADPKD, hypertension
- Temporal arteritis (GCA): Age >50, unilateral temporal headache, jaw claudication, proximal muscle ache, ESR/CRP ↑; treat immediately with IV steroids; biopsy within 3 days (skip-lesions → can get false negative)
Headache Treatment Summary
| Type | Acute Treatment | Prophylaxis | Key Side Effects |
|---|---|---|---|
| Migraine | Triptans (5-HT agonists), ergotamine; NSAIDs for mild | Beta-blockers, TCAs, topiramate (≥8/month) | Triptans: CI in vasospastic angina, prior stroke; serotonin syndrome with SSRIs |
| Cluster | 100% O2, intranasal sumatriptan | Verapamil (1st), lithium, steroids | Verapamil: bradycardia, constipation, hyperprolactinemia |
| Tension | NSAIDs, relaxation | TCAs if chronic | — |
| Trigeminal neuralgia | Carbamazepine | Carbamazepine ongoing | Agranulocytosis, SIADH, neural tube defects (teratogenic), AV block |
Topiramate Side Effects — Classic Exam Trap
Started on migraine prophylaxis → acute flank pain = topiramate crystalline nephropathy (kidney stones). Also causes cognitive slowing ("dopamax"). Teratogenic — avoid in pregnancy.
Red-Flag Headache Vignettes
| Presentation | Diagnosis | Next Step |
|---|---|---|
| Thunderclap + worst headache of life | Subarachnoid hemorrhage | Non-contrast CT → LP for xanthochromia → nimodipine |
| Chronic worsening, morning vomiting, position change | Brain tumor (ICP ↑) | MRI brain with contrast |
| Obese woman + OCP + blurry vision + papilledema | Pseudotumor cerebri (IIH) | LP (↑ opening pressure >200), acetazolamide |
| Elderly + jaw claudication + scalp tenderness | Temporal arteritis | IV steroids IMMEDIATELY, then temporal artery biopsy |
| Acute onset + fixed mid-dilated pupil | Acute angle closure glaucoma | Pilocarpine drops, timolol, laser iridotomy |
| Post-procedural (LP or cath) positional headache | Post-dural puncture headache | Caffeine, blood patch |
Spinal Cord
Three episodes build an integrated understanding of spinal cord anatomy, classic lesion syndromes, and the rare-but-tested disorders that appear as "weird" questions. The key insight: every spinal cord syndrome can be decoded by asking which tracts are and aren't affected. The corticospinal already crossed; the dorsal columns haven't yet; the spinothalamic crosses at entry level — these three rules unlock all spinal cord localization.
EP371
The Spinal Cord — Tracts, Pathways, and Rules
- Dorsal columns: Vibration, proprioception, fine touch. Ascend ipsilateral in spinal cord → synapse at gracile/cuneate nuclei (medulla) → cross → thalamus (VPL/VPM)
- Spinothalamic tract: Pain, temperature, crude touch. Cross in spinal cord (anterior white commissure) immediately → ascend contralateral; travel LATERALLY in brainstem
- Corticospinal tract: UMN from cortex → corona radiata → posterior limb of internal capsule (lenticulostriate artery — Charcot-Bouchard aneurysms in hypertension!) → cerebral peduncles → cross in medullary pyramids → MEDIALLY in brainstem
- Brainstem laterality rule: Dorsal columns = medial; corticospinal = medial; spinothalamic = LATERAL. Horner's pathway = lateral. Explains why PICA strokes cause PPTH + Horner's (lateral) but spare vibration (medial)
- Anterior spinal artery syndrome: Everything lost EXCEPT dorsal columns. Artery of Adamkiewicz supplies anterior cord → risk in AAA repair
Spinal Cord Tract Summary
| Tract | Carries | Decussates Where | In Brainstem | Cord Lesion = Ipsi or Contra? |
|---|---|---|---|---|
| Dorsal columns | Vibration, proprioception, fine touch | Medulla (gracile/cuneate nuclei) | Medial | Ipsilateral |
| Spinothalamic | Pain, temp, crude touch | Spinal cord (at entry level) | Lateral | Contralateral |
| Corticospinal | Motor (UMN) | Medullary pyramids | Medial | Ipsilateral (already crossed) |
The Posterior Limb of Internal Capsule
Blood supply: lenticulostriate artery (from MCA). In severe HTN → Charcot-Bouchard aneurysms form → rupture = hemorrhagic stroke → dense contralateral hemiplegia (the "dense paralysis" of the USMLE). The most common location of hypertensive hemorrhagic stroke.
EP374
Spinal Cord Lesions — Syndromes and Differentials
- Brown-Séquard (hemisection): Ipsilateral dorsal column loss + ipsilateral UMN signs; contralateral pain/temp loss (spinothalamic crosses at cord level)
- Central cord (syringomyelia): Bilateral cape-like pain/temp loss (anterior white commissure disrupted); Chiari 1 association (cerebellar tonsils herniate through foramen magnum)
- B12 deficiency (subacute combined degeneration): Dorsal columns + corticospinal = proprioception loss + UMN signs + positive Romberg; check methylmalonic acid + homocysteine (both ↑ in B12 deficiency; only homocysteine ↑ in folate deficiency)
- Tabes dorsalis (tertiary syphilis): Dorsal columns ONLY — positive Romberg, no UMN signs. IV drug user / sex worker context
- ALS: UMN + LMN combined; NO sensory deficits. Riluzole (NMDA antagonist — glutamate excitotoxicity). Qualifies for Medicare at any age
- Conus medullaris vs cauda equina: Conus = UMN predominant, bilateral; cauda equina = LMN predominant, unilateral (peripheral nervous system → better prognosis)
Spinal Cord Syndrome Master Table
| Syndrome | Tracts Affected | Clinical Pattern | Classic Cause |
|---|---|---|---|
| Anterior cord (ASA) | Spinothalamic + corticospinal (anterior 2/3); dorsal columns SPARED | Bilateral pain/temp loss + paralysis; vibration/proprioception intact | AAA repair, aortic surgery (artery of Adamkiewicz) |
| Central cord | Anterior white commissure → bilateral spinothalamic | Cape-like bilateral pain/temp loss; saddle anesthesia | Syringomyelia (Chiari 1), hyperextension injury in elderly |
| Posterior cord (dorsal column) | Dorsal columns only | Romberg (+), vibration/proprioception lost; no motor; no pain/temp | B12 deficiency (partial), tabes dorsalis, Friedreich ataxia |
| Brown-Séquard | Ipsilateral: dorsal column + corticospinal. Contralateral: spinothalamic | Ipsi: vibration/proprioception loss + UMN. Contra: pain/temp loss | Gunshot wound, knife, MS |
| Subacute combined degen | Dorsal columns + corticospinal (+ spinocerebellar) | Vibration/proprioception loss + UMN + sensory ataxia; macrocytic anemia | B12 deficiency (pernicious anemia, vegans, Crohn's) |
| ALS | Corticospinal (UMN) + anterior horn (LMN); NO sensory tracts | Spasticity + fasciculations; no sensory deficits | Idiopathic; riluzole slows progression |
B12 vs Folate vs Tabes Dorsalis
B12 deficiency: Methylmalonic acid ↑ + homocysteine ↑. Subacute combined degeneration (dorsal + corticospinal).
Folate deficiency: Only homocysteine ↑ (no methylmalonic acid elevation). No neurological syndrome.
Tabes dorsalis: Syphilis. Dorsal columns only. Romberg (+) but no UMN signs.
EP506
Weird But HY Spinal Cord Disorders
- MS vs NMO vs transverse myelitis: MS = relapsing/remitting + brain lesions + oligoclonal bands; NMO (Devic) = anti-NMO IgG antibodies, episodic but long attacks, optic neuritis, NO brain lesions; transverse myelitis = post-viral, one-time, no oligoclonal bands, no anti-NMO
- Copper deficiency: Mimics B12 subacute combined degeneration but methylmalonic acid and homocysteine are NORMAL; causes = zinc excess (common cold supplements) or bariatric surgery (copper absorbed in duodenum)
- Spinal cord infarction: Post-cardiac surgery, post-AAA, AFib emboli → anterior spinal artery syndrome pattern; no dorsal column involvement
- Spinal cord compression workup: MRI first → IV steroids + surgical decompression + radiotherapy. Liquid tumors (lymphoma, leukemia, myeloma) → radiotherapy preferred
- Spinal epidural abscess vs hematoma: Abscess = fever + cord compression; hematoma = anticoagulation context + sudden onset. Both need urgent MRI
MS vs NMO vs Transverse Myelitis
| MS | NMO (Devic) | Transverse Myelitis | |
|---|---|---|---|
| Pattern | Relapsing/remitting | Episodic (long attacks) | Usually single episode |
| Brain lesions on MRI | Yes (white matter plaques) | No | No |
| CSF oligoclonal bands | Yes | No | No |
| Antibody | None specific | Anti-AQP4 (NMO-IgG) | None |
| Optic neuritis | Yes (CN2) | Yes (severe) | No |
| Trigger | Unknown | Unknown | Recent viral infection |
Copper Deficiency — The Sneaky B12 Mimic
Question looks exactly like B12 deficiency but methylmalonic acid and homocysteine are normal. High-yield clues: bariatric surgery (Roux-en-Y bypasses duodenum) or zinc supplementation (zinc competitively inhibits copper absorption). No macrocytic anemia either.
Neuromuscular Junction & Pharmacology
The NMJ cluster links clinical diagnosis (fatigable weakness, incremental vs decremental EMG) to mechanistic pharmacology (AChE inhibitors, presynaptic vs postsynaptic targets). EP435 and EP443 together cover myasthenia gravis, Lambert-Eaton, botulism, and the entire AChE inhibitor framework — while RR 105 wraps in SCLC paraneoplastic syndromes including Lambert-Eaton and ectopic ACTH.
EP435
Diseases of the Neuromuscular Junction
- Myasthenia gravis: Anti-AChR antibodies (postsynaptic). Fatigable weakness (worse with use — opposite of Lambert-Eaton). Decremental EMG. Normal reflexes. CT chest for thymoma. Treat: pyridostigmine → steroids → thymectomy; crisis = plasmapheresis or IVIG
- Lambert-Eaton: Anti-presynaptic voltage-gated calcium channel antibodies. Muscle weakness improves with use (↑ Ca2+ recruitment). Incremental EMG. Hyporeflexia + autonomic symptoms (dry mouth, constipation, impotence). Associated with SCLC
- Botulism: Toxin cleaves SNAP-25 (SNARE protein) → ↓ ACh release → descending flaccid paralysis. Honey (infants <1yr), home-canned goods (adults ingest preformed toxin), wound (black tar heroin injection). Treat: antitoxin
- Tetanus: Toxin cleaves VAMP (synaptobrevin) → blocks GABA/glycine from Renshaw cells → spastic paralysis, trismus, risus sardonicus. Treat: antitoxin + wound debridement + benzos
- EMG key: Botulism initial incremental then fails; MG = decremental; Lambert-Eaton = incremental
MG vs Lambert-Eaton — Complete Comparison
| Myasthenia Gravis | Lambert-Eaton | |
|---|---|---|
| Target | Postsynaptic nicotinic AChR | Presynaptic voltage-gated Ca²⁺ channel |
| Weakness pattern | Fatigable — WORSE with use (bulbar, proximal) | Improves with use (proximal limb girdle) |
| EMG response | Decremental | Incremental |
| Reflexes | Normal | Hyporeflexia / areflexia |
| Autonomic symptoms | Absent | Present (dry mouth, constipation, impotence) |
| Associated tumor | Thymoma (CT chest) | Small cell lung cancer (SCLC) |
| Ice pack test | Positive (cools AChE → ↑ ACh) | Not used |
| Edrophonium test | Positive (short-acting AChEI) | Not used |
Myasthenic Crisis vs Cholinergic Crisis
Myasthenic crisis: Worsening weakness, respiratory failure → pyridostigmine insufficient → plasmapheresis or IVIG (removes antibodies).
Cholinergic crisis (from too much pyridostigmine): Miosis, sweating, bradycardia, diarrhea, excess secretions. Treat: stop pyridostigmine + atropine.
Botulism — Three Scenarios
| Type | Mechanism | Context |
|---|---|---|
| Infant botulism | Spores ingested → germinate in GI tract → produce toxin | Honey to infant <1 yr (no intestinal flora) |
| Food-borne (adult) | Preformed toxin ingested | Home-canned goods, improperly preserved foods |
| Wound botulism | Spores in wound → germinate → toxin | Black tar heroin injection — always on boards |
EP443
The Clutch Acetylcholinesterase Inhibitor Podcast
- 4 groups of AChE inhibitors: (1) Organophosphates — nerve agents/pesticides (irreversible); (2) Alzheimer's agents — donepezil, rivastigmine, galantamine (cross BBB, central); (3) Quaternary — pyridostigmine, neostigmine (do NOT cross BBB, peripheral only); (4) Physostigmine — crosses BBB → reverses atropine overdose
- Organophosphate poisoning: Cholinergic toxidrome (SLUDGE + miosis + bradycardia). Treat: atropine (muscarinic reversal) + pralidoxime (reactivates AChE within reversible window)
- Alzheimer's mechanism: Basal nucleus of Meynert dysfunction → low ACh → give donepezil/rivastigmine/galantamine. Memantine (NMDA antagonist, blocks glutamate excitotoxicity) = 2nd line
- Physostigmine: Only AChEI that crosses BBB → use for atropine or anticholinergic overdose (reverses both central and peripheral effects)
- Contraindications: Mechanical bowel/bladder obstruction (rupture risk), COPD/asthma (bronchospasm), heart block, WPW with AFib, peptic ulcer disease (↑ gastric acid)
AChE Inhibitor — USMLE Classification
| Group | Drugs | Crosses BBB? | Use |
|---|---|---|---|
| Organophosphates | Sarin, VX, parathion | Yes (nerve agents) | Toxin only; treat with atropine + pralidoxime |
| Alzheimer's agents | Donepezil, rivastigmine, galantamine | Yes | Alzheimer's disease exclusively (do NOT use pyridostigmine for Alzheimer's) |
| Quaternary (peripheral) | Pyridostigmine, neostigmine | No | MG (pyridostigmine), reverse neuromuscular blockade (neostigmine), urinary retention, post-op ileus |
| BBB-crossing reversal agent | Physostigmine | Yes | Atropine overdose / anticholinergic toxidrome with central effects |
Caffeine = Weak AChE Inhibitor
Caffeine is a non-competitive AChE inhibitor → raises ACh → activates muscarinic receptors in GI tract → GI motility ↑ → loose stools. This explains the classic "I always poop after coffee" phenomenon.
RR 105EP488
Lambert-Eaton, SCLC Paraneoplastics & ADH Physiology
- SCLC paraneoplastics: (1) SIADH → hyponatremia + ↑ urine osmolality; (2) Ectopic ACTH → hypokalemia + metabolic alkalosis (cortisol acts like aldosterone) — does NOT suppress with high-dose dexamethasone; (3) Lambert-Eaton (see EP435)
- Lambert-Eaton diagnosis tip: Antibodies can be described as anti-calcium channel, anti-voltage-gated channel, or anti-presynaptic channel — all mean the same thing. Incremental EMG = "improved neuromuscular transmission with muscle challenge"
- ADH functions: (1) Free water reabsorption at principal cell; (2) Urea reabsorption (builds medullary concentrating gradient); (3) Vasoconstriction (vasopressin); (4) Von Willebrand factor release from Weibel-Palade bodies (treats von Willebrand disease with desmopressin)
- Central DI vs nephrogenic DI: Water deprivation test → urine osmolality doesn't rise appropriately. Give desmopressin: if urine osmolality normalizes = central DI; if no change = nephrogenic DI (lithium, demeclocycline, hypercalcemia)
SCLC Paraneoplastic Syndromes
| Syndrome | Mechanism | Lab Pattern | High-Yield Clue |
|---|---|---|---|
| SIADH | Ectopic ADH | ↓ serum Na, ↑ urine osmolality, euvolemic | Seizing elderly smoker with Na of 120 |
| Ectopic ACTH | SCLC secretes ACTH → ↑ cortisol → acts like aldosterone | ↓ K, metabolic alkalosis, hyperpigmentation possible | Lung mass + ↓K + alkalosis = SCLC (not Cushing's — no dex suppression) |
| Lambert-Eaton | Anti-voltage-gated Ca²⁺ channel → ↓ ACh release | Incremental EMG | Proximal weakness that improves with repeated activity |
Ectopic ACTH vs Cushing's Disease
Ectopic ACTH (SCLC): Does NOT suppress with high-dose dexamethasone. Very high cortisol, very high ACTH.
Cushing's disease (pituitary): Suppresses with high-dose dexamethasone. Bilateral adrenal hyperplasia.
Cushing's syndrome (adrenal adenoma): Low ACTH (feedback suppressed). Suppresses with neither dose.
Brain Tumors & Mass Lesions
The key to brain tumor questions is the combo of location + histology + associated syndrome. EP516 systematically maps tumor types to their age group (supra vs infratentorial), risk factors, and histologic markers. RR 99 adds the contextual diagnosis framework — recognizing medulloblastoma by its drop metastases or tuberous sclerosis by its rhabdomyoma on echo without needing to memorize the echo appearance itself.
EP516
The Floridly HY Brain Tumor Podcast
- Location rule: Adults = supratentorial (above tentorium cerebelli); children = infratentorial (cerebellum). Exception: craniopharyngioma is supratentorial in kids
- GBM: Most common primary adult brain tumor; GFAP marker; "butterfly" across corpus callosum; pseudopalisading necrosis; poor prognosis → discuss hospice/goals of care
- Meningioma: Most common benign; estrogen + progesterone receptors → women; dural tail on MRI; APC gene (FAP) + brain tumor = Turcot syndrome
- Ependymoma: Children → 4th ventricle (obstructive hydrocephalus); adults → cauda equina (LMN symptoms). Perivascular pseudorosettes. NF2 association
- Pinealoma: Compresses superior colliculus → Parinaud (upward gaze palsy); compresses aqueduct of Sylvius → ↑ lateral + 3rd ventricles dilated (not 4th); produces HCG → precocious puberty
- Hemangioblastoma: Posterior fossa; polycythemia (ectopic EPO); calcifications on imaging; VHL (von Hippel-Lindau) association
Brain Tumor Master Table
| Tumor | Age | Location | Histology Key | HY Association |
|---|---|---|---|---|
| GBM | Adults | Cerebral hemispheres (supratentorial) | GFAP; pseudopalisading necrosis; butterfly pattern | Most common primary adult brain tumor; discuss hospice |
| Meningioma | Adults (women) | Dural surface (extraaxial) | Psammoma bodies; dural tail on MRI | Estrogen/progesterone receptors; FAP/APC (Turcot); NF2 |
| Pyelocytic astrocytoma | Children | Cerebellum (infratentorial) | Rosenthal fibers (pink corkscrew); GFAP marker | Most common childhood brain tumor overall; good prognosis |
| Medulloblastoma | Children | Cerebellum (infratentorial) | Homer-Wright rosettes; small blue cells | Drop metastases via CSF to spinal cord |
| Ependymoma | Kids (4th V) / Adults (cauda equina) | 4th ventricle / Filum terminale | Perivascular pseudorosettes | NF2; obstructive hydrocephalus |
| Craniopharyngioma | Children | Supratentorial (Rathke's pouch) | "Motor oil" cholesterol cysts; calcifications | Bitemporal hemianopsia; compresses pituitary |
| Pinealoma | Any age | Pineal gland (above superior colliculus) | Variable | Parinaud syndrome; precocious puberty (HCG); compresses aqueduct |
| Hemangioblastoma | Adults | Posterior fossa / cerebellum | Lipid-laden stromal cells; abundant blood vessels | VHL; polycythemia (EPO); calcifications |
| CNS lymphoma | Immunocompromised | Periventricular white matter | Diffuse large B-cell lymphoma | HIV + EBV; CD4 <50; treat with methotrexate |
Obstructive Hydrocephalus — Localize by What's Dilated
Medulloblastoma / ependymoma (4th ventricle blocked): Lateral + 3rd + cerebral aqueduct + 4th ventricle all dilated.
Pinealoma (aqueduct of Sylvius blocked): Only lateral and 3rd ventricles dilated. 4th ventricle normal.
Aqueductal stenosis (newborn): Same pattern as pinealoma — most common cause of neonatal non-communicating hydrocephalus.
RR 99EP473
Medulloblastoma Drop Mets, GBM & Context-Based Brain Tumor Diagnosis
- Medulloblastoma drop metastases: Cerebellar mass + multiple spinal cord lesions on imaging = medulloblastoma (seeding via CSF pathways) — NOT MS (MS has no cerebellar mass)
- GBM vs astrocytoma terminology: USMLE may call GBM simply "astrocytoma" — both use GFAP as tumor marker. Don't be thrown off by the name
- Tuberous sclerosis — cardiac rhabdomyoma: Ash leaf spots (hypopigmented macules) → if echocardiogram shows intracardiac mass = rhabdomyoma. TSC1 (hamartin, chr 9) / TSC2 (tuberin, chr 16) → ↑ mTOR → unregulated proliferation
- Rhabdomyoma vs left atrial myxoma: Tuberous sclerosis rhabdomyoma → murmur of MR. Left atrial myxoma (sporadic) → murmur of mitral stenosis (obstructs mitral valve). Myxoma bits break off → embolic stroke
Medulloblastoma vs Pilocytic Astrocytoma vs MS — Differentiation
| Medulloblastoma | Pyelocytic Astrocytoma | MS | |
|---|---|---|---|
| Cerebellar mass | Yes | Yes | No (never) |
| Spinal cord lesions | Yes — drop mets | No | Yes — but no cerebellar mass |
| Calcifications | Rare | Possible | No |
| Histology | Homer-Wright rosettes | Rosenthal fibers | Demyelination, not mass |
Dementia & Degenerative Disease
EP599 approaches dementia as a pattern-recognition and clinical management challenge, not a fact-recall exercise. The key moves: distinguish dementia from MCI (ADL impairment), identify which dementias are treatable vs irreversible, and know the dangerous drug combinations — especially antipsychotics in Lewy body disease and anticholinergics in any dementia.
EP599
The Clutch Dementia Podcast
- Alzheimer's: Hippocampal atrophy + hydrocephalus ex vacuo; extracellular amyloid plaques; intraneuronal neurofibrillary tangles; basal nucleus of Meynert (ACh). ApoE4 = risk; ApoE2 = protective; Down's syndrome → Alzheimer's by age 40. First line: AChEI; second line: memantine
- Lewy body dementia: #2 most common; visual hallucinations (classic!), parkinsonism, orthostatic hypotension, falls, REM sleep behavior disorder. Alpha-synuclein inclusions. Avoid antipsychotics (↑↑↑ NMS risk)
- NPH ("wacky, wet, wobbly"): Cognitive decline + gait apraxia + urinary incontinence. Ventriculomegaly. Normal CSF opening pressure (not elevated!). LP → dramatic symptom improvement. VP shunt curative
- Frontotemporal (FTD): Disinhibition, OCD behaviors, inappropriate social behavior. Pick bodies. No AChEI benefit. Early age onset
- CJD: Rapidly progressive dementia + myoclonus + hyperreflexia → death in months. CSF 14-3-3 protein ↑. Prion disease
- Dangerous drugs in dementia: Anticholinergics (worsen cognition, trigger delirium) and antipsychotics (NMS in Lewy body) — both black-boxed. Avoid first-generation antipsychotics especially
Dementia Type Comparison
| Type | Prevalence | Key Features | Pathology | Treatment |
|---|---|---|---|---|
| Alzheimer's | #1 | Memory first, then global; MCI → ADL impairment | Amyloid plaques (extracellular) + tau tangles (intraneuronal); hippocampal atrophy | AChEI (donepezil/rivastigmine/galantamine) → memantine |
| Lewy body | #2 | Visual hallucinations early, parkinsonism, fluctuating cognition | Alpha-synuclein Lewy bodies | AChEI; NO antipsychotics (NMS) |
| Vascular | #3 | Stepwise decline; localizable deficits; CVD risk factors | Multiple small infarcts | Vascular risk factor control; aspirin |
| NPH | Reversible | Wacky + wet + wobbly; ventriculomegaly; normal LP pressure | Impaired CSF reabsorption (arachnoid granulations) | VP shunt (curative); NO AChEI |
| FTD | Early onset | Behavioral/personality changes; socially inappropriate | Pick bodies (tau inclusions) | No effective pharmacotherapy; NO AChEI |
| CJD | Rare/rapid | Rapid dementia + myoclonus; death in months | Prion disease; spongiform change; 14-3-3 protein in CSF | Supportive only |
NPH vs IIH — Critical Distinction
Both caused by impaired CSF reabsorption at arachnoid granulations. But:
NPH: Normal CSF opening pressure, ventriculomegaly, cognitive + gait + bladder symptoms. VP shunt curative.
IIH (pseudotumor cerebri): ↑ CSF opening pressure (>200 mm H₂O), papilledema, obese woman / OCP / tetracycline. Treat: acetazolamide, weight loss.
Dementia Mimics — Reversible Causes
- B12 deficiency: Check serum B12, methylmalonic acid, homocysteine. Neuropsychiatric symptoms before anemia in some patients
- Hypothyroidism: Check TSH. "Myxedema madness" — cognitive impairment reverses with levothyroxine
- Subdural hematoma: Rapidly progressive dementia in elderly or alcoholic (bridging vein laceration from cerebral atrophy). Crescent-shaped hyperdensity on CT
- Hepatic encephalopathy: Acute "dementia" in end-stage liver disease. Asterixis (flapping tremor). Treat: lactulose, rifaximin
Cranial Nerves & Facial Weakness
EP543 unravels why upper and lower facial weakness distinguish central from peripheral CN7 lesions — an insight that requires understanding the bilateral innervation of the upper face from both hemispheres. Once this mechanism is clear, Bell's palsy vs MCA stroke vs AICA stroke become straightforward discriminations based on accompanying findings rather than isolated facts.
EP543
Facial Muscle Weakness and The USMLEs
- Upper face has bilateral cortical innervation: Both hemispheres send UMN fibers to BOTH CN7 nuclei for the upper face. Lower face fibers cross to only the contralateral CN7
- MCA stroke (UMN, central): Contralateral LOWER face weakness only. Patient can close eye and wrinkle forehead normally. Also: contralateral arm weakness ± aphasia
- Bell's palsy / CN7 lesion (LMN, peripheral): Ipsilateral UPPER + LOWER face weakness. Cannot close eye, cannot wrinkle forehead. CN7 in lateral pons — AICA territory
- Bell's palsy causes: HSV-1 reactivation (most common, isolated), AICA stroke (+ contralateral pain/temp loss from spinothalamic), acoustic neuroma (gradual + hearing loss), Lyme disease (bilateral Bell's palsy)
- AICA stroke differentiation: Bell's palsy + contralateral body pain/temperature loss = AICA stroke (not just infection). Spinothalamic tract is lateral in pons — affected in lateral pontine stroke
- Treatment: Bell's palsy from HSV-1 = oral steroids + acyclovir within 72 hours. Eye drops to protect cornea (can't close eye)
Central vs Peripheral Facial Weakness
| Central (UMN) — MCA Stroke | Peripheral (LMN) — CN7 Lesion | |
|---|---|---|
| Forehead wrinkling | NORMAL (bilateral UMN innervation) | IMPAIRED (all upper face gone) |
| Eye closure | NORMAL | IMPAIRED (levator/orbicularis) |
| Lower face | Contralateral weakness | Ipsilateral weakness |
| Associated findings | Contralateral arm weakness ± aphasia ± visual field cut | Varies by cause (see table below) |
Bell's Palsy — Distinguishing the Cause
| Cause | Additional Findings | Key Clue |
|---|---|---|
| HSV-1 reactivation | Isolated facial palsy, no other deficits | Most common; treat with steroids + acyclovir |
| AICA stroke | Contralateral pain/temp loss (spinothalamic) + ipsilateral hearing loss (CN8) | Lateral pontine syndrome signs present |
| Acoustic neuroma (CN8 tumor) | Progressive unilateral hearing loss + tinnitus; gradual onset facial weakness | Age 30-60; NF2 if bilateral |
| Lyme disease | BILATERAL Bell's palsy; tick exposure; erythema migrans history | Bilateral = think Lyme until proven otherwise |
| Ramsay Hunt syndrome | Painful vesicles in ear canal (herpes zoster reactivation in geniculate ganglion) | CN7 + CN8; treat with valacyclovir |
Neural Tube Defects & Congenital Neurologic Disorders
EP636 covers neural tube defects with a biostatistics angle (AFP vs acetylcholinesterase as screening vs confirmatory tests) alongside the clinical and anatomical hierarchy of spina bifida variants. The holoprosencephaly + sonic hedgehog connection and the trisomy 13 / fetal alcohol syndrome associations are the integrative additions that distinguish a good score from an excellent one.
EP636
Neural Tube Defects and The USMLEs
- Risk factors: Maternal folate deficiency (before AND during pregnancy) + maternal diabetes → supplement folate BEFORE conception
- Screening: Maternal serum AFP ↑ + amniotic fluid AFP ↑. Most common cause of ↑ maternal serum AFP = incorrect dating
- Confirmatory: Amniotic fluid acetylcholinesterase (neural tissue leaks AChE into amniotic fluid) — higher PPV than AFP alone
- Spina bifida occulta (closed NTD): Caudal neural pore fails to close but skin intact; AFP normal; skin dimple or tuft of hair on back
- Myelomeningocele (open NTD): Spinal cord + meninges herniate; bowel/bladder dysfunction, bilateral leg paralysis. Associated with Chiari 2 malformation (tonsils + cerebellar vermis herniate through foramen magnum)
- Anencephaly: Rostral neural pore fails → no forebrain/swallowing center → polyhydramnios (fetus can't swallow amniotic fluid)
Neural Tube Defect Hierarchy
| Type | USMLE Name | What Herniates | AFP | Key Finding |
|---|---|---|---|---|
| Spina bifida occulta | Closed NTD | Nothing (skin intact) | Normal | Skin dimple / hair tuft at lumbar |
| Meningocele | Open NTD (cystic spina bifida) | Meninges only | ↑ | Fluid-filled sac; less neurologic deficit |
| Myelomeningocele | Open NTD (more common than meningocele) | Spinal cord + meninges | ↑↑ | Paraplegia + bladder/bowel dysfunction; Chiari 2 |
| Anencephaly | Open NTD (most severe) | Brain absent (rostral) | ↑↑ | Polyhydramnios; incompatible with life |
Chiari Malformations — Quick Comparison
Chiari 1: Cerebellar tonsils herniate through foramen magnum. Often asymptomatic until adulthood → headaches, ataxia. Associated with syringomyelia.
Chiari 2: Tonsils + cerebellar vermis herniate. Associated with myelomeningocele. Seen in newborns. More severe.
Holoprosencephaly
Cerebral hemispheres fail to separate (sonic hedgehog mutation). Causes: fetal alcohol syndrome or trisomy 13 (Patau syndrome). Features: cyclopia, single monoventricle on imaging, cleft lip/palate, polyhydramnios (absent swallowing center). Imaging: fused basal ganglia + monoventricle.