Vertebrobasilar Stroke Syndromes: read the vessel, read the level
Basilar, AICA, PICA, SCA, ASA, and vertebral artery traps. Same next move every time: name the brainstem level, then decide whether the damage is medial, lateral, or top-of-basilar.
Cranial nerve sign IPSILATERAL to the lesion. Body sign (motor or sensory) CONTRALATERAL. That mismatch IS the diagnosis of a brainstem stroke. Then the vessel tells the rest: AICA adds ear and facial palsy, PICA adds nucleus ambiguus and Horner, ASA adds tongue plus vibration loss, and basilar trunk can lock the patient in.
Same-side cranial nerve sign
CN III down-and-out, CN VII full-face droop, CN IX/X hoarseness, or CN XII tongue deviation names the side of the lesion.
Opposite-side body sign
CST weakness, STT pain/temp loss, or DCML vibration loss usually lands on the body side opposite the brainstem lesion.
The vessel adds the fingerprint
Ear plus facial palsy points AICA. Dysphagia plus Horner points PICA. Tongue plus vibration loss points ASA. Awake paralysis points basilar trunk.
CT brain · first study in suspected stroke · rules out hemorrhage before tPA · tap to expand
The Opening Case
One vignette. One clot. Read the crossed pattern, name the artery.
Chart · ED · 67-year-old
A 67-year-old man with hypertension and longstanding smoking is brought in after sudden onset of severe vertigo and vomiting. On exam he is hoarse, drools when given a sip of water, and chokes. The right side of his face has decreased pain and temperature sensation. His right pupil is small, his right upper eyelid droops, and his right cheek is dry. He cannot point his finger accurately with the right hand. Limb strength is full bilaterally. He has loss of pain and temperature sensation on the LEFT side of his body, with vibration and position sense preserved everywhere.
Which artery is occluded, and which side?
Right posterior inferior cerebellar artery (PICA), lateral medullary or Wallenberg syndrome. The crossed sensory pattern is the clincher: pain and temperature gone on the RIGHT face (ipsilateral spinal trigeminal nucleus) and on the LEFT body (contralateral spinothalamic tract, already decussated below the medulla). Vibration and position sense are preserved because the medial lemniscus is in the MEDIAL medulla, not the lateral medulla where PICA feeds. Add the right Horner (descending sympathetics), the vertigo (vestibular nuclei), the dysphagia and hoarseness (nucleus ambiguus, CN IX and X), and the right-sided limb ataxia (inferior cerebellar peduncle) and you have the full Wallenberg picture. Don\'t Pick A horse that can\'t eat: Dysphagia, Pain/temp loss, Ataxia, Horner.
Brainstem Compass
Use direction words before syndrome names. Rostral means toward the top of the head. Caudal means toward the tail. Dorsal means posterior or toward the back. Ventral means anterior or toward the front. On a brainstem cross section, medial is the midline side and lateral is the edge side.
DorsalPosterior, toward the back. In the midbrain, the dorsal tectum carries the superior colliculus and pretectal region, so Parinaud lives back there.
VentralAnterior, toward the front. The ventral pons and cerebral peduncle carry descending motor fibers, so ventral lesions love weakness.
RostralToward the top of the head. Rostral brainstem points you upward from medulla to pons to midbrain.
CaudalToward the tail or spinal cord. The caudal brainstem is medulla territory, where CN XII and the pyramids make medial medullary syndrome.
The Brainstem Vessels
Lateral view. Brainstem on the left, cerebellum on the right. Tap any artery to light it up and read its syndrome.
Six arteries feed the brainstem. Each one, when it clots, leaves a signature syndrome. Tap to walk the map.
Tap any artery
ARTERY
Name
Feeds
Syndrome
Hallmark
Board Pearl
Crossed deficits live here. Ipsilateral face/cranial nerve sign + contralateral body sign tells you the lesion is in the brainstem. Cortex above the decussation cannot make a crossed pattern; only brainstem at the level of the cranial nerve nucleus can. Spot the cross, leave the cortex, walk the brainstem map.
The Brainstem Syndrome Deck
Classic vascular strokes first, then the midbrain and medial-pontine add-ons that make the board stems feel mean.
Midbrain · Ventromedial
Weber Syndrome
PCA paramedian penetrators · CN III + CST
Level
Midbrain, ventromedial (the cerebral peduncle and the exiting CN III fascicles).
Artery
Paramedian penetrating branches off the PCA (the small midline twigs that dive into the midbrain from the basilar tip and proximal PCA).
CN ipsi
CN III oculomotor. The eye is fixed down-and-out, the pupil is blown (dilated and unreactive), the lid droops (ptosis). Both pupillary fibers and somatic motor fibers run through the CN III fascicle that is crushed here.
Tract contra
Corticospinal tract in the cerebral peduncle. Contralateral hemiparesis of face, arm, and leg (CST has not yet decussated; decussation is in the lower medullary pyramids).
Mnemonic
Weber = web = woven foot. Down-and-out eye on one side, weak body on the other. The eye and the leg go in opposite directions.
Pearl
Painless pupil-involving CN III palsy with contralateral hemiparesis equals Weber. (Painless rules out posterior communicating artery aneurysm, which gives painful CN III palsy without limb weakness.)
Pons · Lateral
Lateral Pontine Syndrome (AICA)
AICA · CN V + VII + VIII + STT + cerebellum
Level
Lateral pons (mid-pontine). The dorsolateral pons where CN V, VII, and VIII nuclei and fascicles ride.
Artery
Anterior Inferior Cerebellar Artery (AICA). Branches off the lower basilar artery near the pontomedullary junction and curls to the cerebellopontine angle.
CN ipsi
CN V: ipsilateral facial sensory loss, decreased corneal reflex. CN VII: ipsilateral lower motor neuron facial palsy (full hemiface including forehead) plus dry eye and dry mouth. CN VIII: vertigo, nystagmus, ipsilateral sensorineural hearing loss (cochlear nucleus dies with AICA).
Tract contra
Spinothalamic tract: contralateral body pain and temperature loss (vibration/proprioception spared, that lives medially).
AICA hits the EAR. Sensorineural hearing loss + vertigo distinguishes AICA from every other brainstem syndrome. Wallenberg is "the one where hearing is preserved." AICA is "the one where hearing is wrecked."
Pearl
Facial palsy that takes out the FOREHEAD (LMN, not central pattern) plus same-side hearing loss plus crossed body sensory loss equals AICA. Central CN VII (cortical stroke) spares the forehead; AICA does not.
Medial medulla. The pyramid (CST), the medial lemniscus, and the CN XII nucleus/fibers sit shoulder to shoulder in this tiny ventromedial wedge.
Artery
Anterior Spinal Artery (a midline branch off both vertebral arteries that runs down the ventromedial cord and feeds the medial medulla on its way).
CN ipsi
CN XII hypoglossal: the tongue deviates TOWARD the lesion when stuck out. (The healthy genioglossus pushes the tongue across the midline; with one side dead, the other side pushes unopposed.) Tongue fasciculations and atrophy follow.
Tract contra
CST (in the medullary pyramid, above or at the pyramidal decussation): contralateral hemiparesis of arm and leg, face spared (face fibers leave the CST at the level of CN VII nucleus, way above). Medial lemniscus: contralateral loss of vibration, proprioception, fine touch.
Mnemonic
Medial medulla = 3 M\'s: Motor (pyramid/CST), Medial lemniscus (DCML), and 12th nerve (Hypoglossal). Or: tongue points to the dead side, body weakness on the other.
Pearl
Crossed pattern: ipsilateral tongue deviation + contralateral arm/leg weakness + contralateral loss of vibration/position. Pain and temperature are PRESERVED (that\'s spinothalamic, which lives laterally, untouched).
Lateral medulla. Nucleus ambiguus, spinal trigeminal nucleus, vestibular nuclei, inferior cerebellar peduncle, descending sympathetics, and the spinothalamic tract all live in this dorsolateral wedge.
Artery
Posterior Inferior Cerebellar Artery (PICA), branch of the vertebral. Vertebral artery occlusion alone often produces the same syndrome through downstream PICA territory loss.
CN ipsi
CN IX, X (nucleus ambiguus): dysphagia, dysphonia (hoarseness), absent gag. CN V (spinal trigeminal nucleus): ipsilateral face pain and temperature loss. Vestibular nuclei: vertigo, nystagmus, vomiting. Sympathetics: ipsilateral Horner (ptosis, miosis, anhidrosis).
Tract contra
Spinothalamic tract: contralateral body pain and temperature loss. Vibration, proprioception, and motor are SPARED (those live medially).
Plus
Ipsilateral cerebellar ataxia (inferior cerebellar peduncle). NO hearing loss (cochlear nucleus is fed by AICA, not PICA).
Mnemonic
"Don\'t Pick A horse that can\'t eat": Dysphagia, Pain/temp loss, Ataxia, Horner. PICA = the artery that wrecks all four.
Pearl
Crossed sensory: same side face pain/temp loss, OPPOSITE side body pain/temp loss. Vibration/position preserved everywhere. No limb weakness. The pure "sensory cross" with hoarseness, vertigo, and Horner = Wallenberg.
Midbrain · Paramedian Tegmentum
Benedikt Syndrome
CN III + red nucleus + medial lemniscus
Level
Rostral midbrain tegmentum, near the red nucleus and oculomotor fascicles.
CN ipsi
Ipsilateral CN III palsy: ptosis, dilated pupil, and a down-and-out eye.
Tract contra
Contralateral vibration and proprioception loss from medial lemniscus involvement. Contralateral weakness can join if the cerebral peduncle is clipped.
Red nucleus
Contralateral tremor, ataxia, or choreoathetoid movement. This is the movement-disorder flavor that separates Benedikt from plain Weber.
Pearl
CN III palsy plus contralateral tremor or ataxia means the lesion is deeper and more dorsal than Weber. Think red nucleus.
Midbrain · Dorsal
Parinaud Syndrome
Superior colliculus + pretectal area + cerebral aqueduct
Level
Posterior or dorsal midbrain. The vertical gaze machinery sits behind the aqueduct.
Eye clue
Impaired upgaze, convergence-retraction nystagmus, and light-near dissociation. The packet phrase is pseudo Argyll Robertson pupil.
Mass clue
Pineal-region germinoma or pinealoma can compress the dorsal midbrain and obstruct the cerebral aqueduct.
Ventricle
Aqueduct compression blocks CSF flow from third to fourth ventricle, causing noncommunicating hydrocephalus.
Pearl
Kid or young adult with headache, vomiting, hydrocephalus, and cannot look up: dorsal midbrain from pineal-region compression.
Medial caudal pons. This is where the horizontal gaze center, abducens nucleus, facial fibers, and corticospinal tract crowd together.
Gaze
A PPRF or CN VI nucleus lesion prevents both eyes from looking toward the affected side. Fascicular CN VI damage gives ipsilateral abduction failure.
Face
Ipsilateral facial weakness can appear because CN VII fibers loop through the caudal pons around the abducens nucleus.
Body
Contralateral hemiparesis comes from corticospinal tract damage before the medullary decussation.
Pearl
Cannot look toward the lesion plus contralateral weakness points to medial pons, not lateral pons. Hearing loss moves you back to AICA.
Cranial Nerve Nuclei: which level owns which nerve
Midbrain owns 3 and 4. Pons owns 5, 6, 7, 8. Medulla owns 9, 10, 11, 12. Tap a level to light up its nerves.
Why this works
The cranial nerve nuclei are stacked rostral-to-caudal in numerical order down the brainstem. So once you know the nerve, you know the level; once you know the level, you know which arteries are in play. CN III? Midbrain, think Weber. CN VII + VIII? Pons, think AICA. CN IX/X/XII? Medulla, think Wallenberg or medial medullary.
The medial brainstem has four M targets.
Motor pathwayCorticospinal or corticobulbar tract damage gives contralateral UMN weakness.
Motor nucleusThe midline motor cranial nerves are III, IV, VI, and XII. Factors of 12, except I and II.
MLFThe medial longitudinal fasciculus links the gaze nuclei. Damage gives internuclear ophthalmoplegia or gaze palsy patterns.
Medial lemniscusContralateral vibration and proprioception loss means the dorsal-column pathway is hit in the medial brainstem.
The lateral brainstem has four S targets.
SympatheticDescending sympathetics give ipsilateral Horner syndrome when the lateral brainstem is hit.
SpinothalamicContralateral body pain and temperature loss is the lateral long-tract clue.
Sensory CN VSpinal trigeminal nucleus or tract gives ipsilateral face pain and temperature loss.
SpinocerebellarCerebellar peduncle involvement gives ipsilateral ataxia and dysmetria.
Use the cranial nerve to choose the floor.
MidbrainCN III and IV. CN III palsy with contralateral weakness is Weber. CN III plus tremor or ataxia is Benedikt.
PonsCN V, VI, VII, VIII. Hearing loss or LMN facial palsy points hard at AICA lateral pons.
MedullaCN IX, X, XI, XII. Hoarseness and dysphagia mean nucleus ambiguus, while tongue deviation means CN XII.
The traps that stop the rule from lying to you.
CN V trapFacial pain and temperature loss uses spinal trigeminal anatomy. It can ride down into lateral medulla, so it does not localize pons by itself.
Vestibular trapVertigo, nystagmus, nausea, and vomiting can happen in pons or medulla. Hearing loss is the clue that pushes you to AICA/lateral pons.
CPA trapSlow tinnitus, hearing loss, facial numbness, lost corneal reflex, and facial spasm can be cerebellopontine angle compression, often vestibular schwannoma, not an acute stroke.
Master Synthesis
All four tracts run vertically down the brainstem. All cranial nerve nuclei stack rostrocaudally beside them. All vessels feed in from the front. One diagram, the whole exam.
Synthesis schematic · vessels red · tracts blue tones
The Brainstem Master Diagram
Tracts run head-to-toe. CN nuclei stack by level. Arteries strike sideways. The whole brainstem on one canvas.
Vessels (red)DCMLCSTSTTSCT
Reading the master diagram
TractsDCML is medial vibration and position. CST is medial motor. STT is lateral pain and temperature. SCT is lateral-dorsal cerebellar input.
VesselsBasilar rises up the middle. AICA swings to lower lateral pons. PICA loops to lateral medulla. ASA runs down the medial medulla.
CN StackCN III and IV live in midbrain. CN V to VIII live in pons. CN IX to XII live in medulla.
Exam MoveA stroke is a vessel crossing a tract or nucleus at one level. That intersection is the syndrome.
Side by Side
Four syndromes, six rows. Read across.
Weber
AICA
Med. Medullary
Wallenberg
Level
Ventromedial midbrain
Lateral pons
Medial medulla
Lateral medulla
Artery
PCA paramedian
AICA
Anterior spinal
PICA (or vertebral)
CN ipsi
III (down-and-out, dilated pupil, ptosis)
V + VII LMN + VIII (hearing loss, vertigo)
XII (tongue toward lesion)
IX + X (dysphagia, hoarseness) + spinal V (face pain/temp loss)
Tract contra
CST (hemiparesis face/arm/leg)
STT (body pain/temp loss)
CST (arm/leg, face spared) + DCML (vibration/position loss)
STT (body pain/temp loss)
Hallmark
Down-and-out eye + contra hemiparesis
Hearing loss + facial palsy + vertigo
Tongue deviation + contra weakness, vibration loss
Crossed sensory + dysphagia + Horner + ataxia
Pearl
Painless CN III palsy with limb weakness
Forehead-involving facial palsy (LMN) plus same-side hearing loss
3 M\'s: Motor, Medial lemniscus, 12th nerve
Don\'t Pick A horse that can\'t eat (Dysphagia, Pain/temp, Ataxia, Horner)
One-line rule
Weber = midbrain + CN III + contra weakness. AICA = pons + ear + facial droop + crossed sensory. Medial medulla = tongue toward + contra motor + contra vibration. Wallenberg = crossed sensory + dysphagia + Horner + ataxia.
BenediktCN III palsy plus contralateral tremor or ataxia means red nucleus involvement, not plain Weber alone.
ParinaudCannot look up plus light-near dissociation points to dorsal midbrain, often from pineal-region compression with aqueduct obstruction.
Medial PonsHorizontal gaze palsy toward the lesion plus contralateral weakness points to PPRF or CN VI nucleus with basilar paramedian territory.
CPAProgressive tinnitus, hearing loss, facial numbness, lost corneal reflex, and facial spasm points to cerebellopontine angle compression.
These are the posterior-circulation patterns that look similar until you ask one clean question: is the disaster ventral pons, top of basilar, cerebellum, osmotic, or spinal cord?
Locked-in syndrome
The clue: quadriplegia plus an awake patient who can blink or move vertically.
The chain: basilar trunk occludes ventral pons -> corticospinal and corticobulbar tracts die -> limbs, face, speech, and swallow shut down -> upper brainstem remains awake enough for blink communication.
VesselBasilar trunk or ventral pontine perforators.
Do not call itVegetative state. Cortex and awareness are not the problem.
Basilar versus vegetative
Locked-in is a motor output failure. The cortex can still understand you. Vegetative state is cortical or thalamocortical dysfunction with preserved basic brainstem reflexes. On boards, a blink-code patient is locked-in until proven otherwise.
Ipsilateral or Contralateral: the master concept
Why brainstem strokes are crossed, every time.
The crossed-deficit rule
Cranial nerve nuclei sit AT the level of the lesion. A clot there damages the nerve on its own side, so the cranial nerve sign is ipsilateral.
Long tracts carry body information that crossed below the brainstem or will cross in the lower medulla. Above that crossing, the body finding lands opposite the lesion. That mismatch is the brainstem fingerprint.
IPSILATERAL (same side)
Cranial nerve signs. Nucleus is right there. CN III down-and-out, CN VII facial droop, CN XII tongue toward, CN IX/X dysphagia. Also: ipsilateral cerebellar ataxia (peduncles are local), ipsilateral Horner (descending sympathetics are local).
CONTRALATERAL (other side)
Long tract signs. CST hemiparesis. STT pain/temp loss. DCML vibration/position loss. These tracts have already crossed (or will cross) below the lesion, so the body deficit lands on the opposite side.
The one exception worth knowing
The SPINAL trigeminal nucleus sits in the lateral medulla, so a Wallenberg stroke gives IPSILATERAL face pain and temperature loss (from spinal V) PLUS CONTRALATERAL body pain and temperature loss (from STT). That double-cross is a Wallenberg signature: same side face, opposite side body. Don\'t let the trigeminal "cranial nerve" label fool you, it follows the same rule (nucleus is right there = ipsilateral).
Quiz
One vignette at a time. Shuffle, cross-out, highlight, commit, then read the chain.
Medically reviewed by Kaitlyn Cocuzzo, MD and Fatima Ali, DO · Last updated July 1, 2026 at 10:03 PM ET
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