BRAINSTEM STACK + CORTICAL MAP

One rule governs all 3 levels. Then pivot to cortex.
THE ONE RULE THAT WORKS AT EVERY LEVEL:

🟢 MEDIAL syndrome = Motor (CST) + that level's Medial CN + Medial lemniscus
🔴 LATERAL syndrome = Sensory (spinothalamic) + cerebellar + autonomic + lateral CNs

Every brainstem syndrome is just: "Which level? Which side?"

LEVEL 3: MIDBRAIN — CN III, IV

CEREBRAL PEDUNCLE CST + CN III fibers exit here TECTUM Superior colliculus • Pretectal area TEGMENTUM Red nucleus • Substantia nigra CN III exits ventrally between peduncles DORSAL (Tectum) VENTRAL (Peduncle)

🟢 WEBER (Medial Midbrain)

Vessel: PCA branches

Ipsilateral CN III palsy
→ Ptosis + "down and out" eye + dilated pupil

+ Contralateral hemiplegia
→ Cerebral peduncle (CST)

💡 "CN III + crossed body" = Weber. The classic.

🔴 BENEDIKT (Tegmentum)

Vessel: PCA branches

Ipsilateral CN III palsy (same)

+ Contralateral tremor/chorea
→ Red nucleus hit

💡 Weber + tremor = Benedikt. Red nucleus is the difference.

🟣 PARINAUD (Dorsal)

Cause: Pinealoma compressing tectum

Upgaze palsy (can't look up)

Light-near dissociation

Convergence-retraction nystagmus

💡 Young patient + can't look up = pinealoma until proven otherwise

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LEVEL 2: PONS — CN V, VI, VII, VIII

MEDIAL (Basis Pontis) CST + CN VI + CN VII loop + PPRF Basilar artery branches LATERAL CN V motor CN VIII Mid/Sup Cer Ped DORSAL (4th ventricle floor) CN VI & VII nuclei on floor VENTRAL

🟢 MILLARD-GUBLER (Ventromedial Pons)

Vessel: Basilar paramedian branches

Ipsilateral CN VI palsy (can't abduct eye)
Ipsilateral CN VII palsy (LMN facial droop)

+ Contralateral hemiplegia (CST)

💡 Same formula: medial CN + crossed body. Just at pons level.

🔴 LOCKED-IN (Bilateral Ventral Pons)

Vessel: Basilar artery occlusion

Quadriplegia (bilateral CST gone)

Anarthria (can't speak)

CONSCIOUS — tegmentum/reticular formation spared

Vertical eye movement PRESERVED (midbrain spared)

💡 Can only blink and look up. Cognition completely intact.

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LEVEL 1: MEDULLA — CN IX, X, XI, XII

🟢 DEJERINE (Medial)

ASA → 4 M's: Motor + Med Lemniscus + MLF + Motor of tongue (XII)

Tongue TO lesion. Body AWAY.

🔴 WALLENBERG (Lateral)

PICA → Everything except motor. IX,X → dysphagia. Horner's. Ataxia. Crossed sensory.

NO hemiplegia ever.

Already built in detail → see medulla_crosssection.html

THE MASTER PATTERN

Level Medial Syndrome Medial CN Lateral Syndrome Vessel
Midbrain Weber (III + hemiplegia) III Benedikt (III + tremor) PCA
Pons Millard-Gubler (VI,VII + hemiplegia) VI, VII Locked-in (bilateral) Basilar
Medulla Dejerine (XII + hemiplegia) XII Wallenberg (everything else) ASA / PICA
THE FORMULA:
Every medial syndrome = ipsilateral CN palsy + contralateral hemiplegia
Just change the CN per level: III → VI,VII → XII

EXAM PIVOT: Given clinical features → identify which CN is hit → tells you the level → name the syndrome

CORTICAL LOCALIZATION — THE PIVOT MAP

If the Q asks "cortical" instead of "brainstem" — use this.

Central Sulcus Lateral Sulcus FRONTAL Motor (precentral) Broca's (L inf frontal) Personality (prefrontal) Micturition center PARIETAL Sensory (postcentral) Neglect (R non-dominant) Gerstmann's (L dominant) TEMPORAL Wernicke's (L post sup) Memory (hippocampus) OCCIPITAL Vision PCA territory Motor strip Sensory strip

FRONTAL

Motor strip (precentral gyrus) — contralateral weakness

Broca's area — non-fluent aphasia ("Broken Broca")

Prefrontal — personality change, disinhibition

Supplementary motor — alien hand

Dementia link: FTD = personality + behavior first

PARIETAL

Sensory strip (postcentral gyrus) — contralateral numbness

R parietal — hemispatial neglect (ignores left side)

L parietal — Gerstmann's (acalculia, agraphia, finger agnosia, R-L confusion)

Epilepsy link: parietal seizures = tingling/numbness

TEMPORAL

Wernicke's area — fluent but meaningless speech ("Wordy Wernicke")

Hippocampus — memory (Alzheimer's starts here)

Uncus — olfactory hallucinations (uncal herniation)

Epilepsy link: temporal lobe = déjà vu, lip smacking, automatisms

OCCIPITAL

Primary visual cortex — homonymous hemianopia

PCA stroke = contralateral visual field loss

Bilateral = cortical blindness (Anton's = denies blindness)

Epilepsy link: occipital seizures = flashing lights/colors

DEMENTIA PIVOT:
Alzheimer's → temporal (memory first) → parietal (visuospatial)
FTD → frontal (personality first) → temporal (language variant)
Lewy Body → occipital (visual hallucinations) + fluctuating

EPILEPSY PIVOT:
Temporal = most common focal. Frontal = nocturnal, brief, bilateral motor.
Parietal = sensory aura. Occipital = visual aura.

CRANIAL NERVES — EXAM-WEIGHT ONLY

CN Name Type Test High-yield lesion
III Oculomotor Motor Eye movements, pupil Weber / Uncal herniation (down-out + dilated pupil)
V Trigeminal Mixed Face sensation, jaw Wallenberg (Sp V nucleus → ipsi face loss)
VI Abducens Motor Lateral gaze Raised ICP (long course, first to go). Millard-Gubler.
VII Facial Mixed Face movement, taste UMN vs LMN: forehead sparing = UMN. Bell's palsy = LMN.
IX, X Glossopharyngeal, Vagus Mixed Gag, swallowing, voice Wallenberg → dysphagia, hoarseness. Jugular foramen syndrome.
XII Hypoglossal Motor Tongue protrusion Dejerine → tongue deviates TO lesion (LMN). Away = UMN.
UMN vs LMN face (CN VII):
UMN = forehead spared (bilateral cortical supply to upper face)
LMN = entire half of face paralyzed including forehead

Tongue rule: LMN lesion → tongue deviates TO the lesion (muscle weak, pushed away by good side)

5 RETENTION ANCHORS

  1. Every medial syndrome = ipsi CN + contra hemiplegia. Weber (III), Millard-Gubler (VI,VII), Dejerine (XII). One formula, three levels.
  2. Wallenberg is the odd one — lateral, PICA, NO motor weakness. If there's hemiplegia, it's not Wallenberg.
  3. Locked-in = bilateral ventral pons. Conscious but can only blink and look up. Basilar artery.
  4. Cortex: 4 lobes = FPTO. Frontal=motor/personality, Parietal=sensory/neglect, Temporal=language/memory, Occipital=vision. Dementia type maps to lobe.
  5. CN VII UMN vs LMN. Forehead spared = UMN (cortical). Entire face = LMN (Bell's). This is the most commonly tested CN distinction in medicine.