Localize cortical, deep, and brainstem strokes, explain crossed findings, recognize global hypoxic injury, and connect examination patterns to urgent care.
A patient suddenly cannot name a watch and has a weak right hand. Another has a hoarse voice, reduced pin sensation on the left face, and reduced pin sensation on the right body. Both may have ischemic stroke, but their examinations point to different tissue. Begin with the affected function, then the side and level, and only then the likely arterial supply.
A syndrome predicts a location; it does not prove a particular occluded artery or determine reperfusion eligibility. Vascular territories vary, lesions can be incomplete, and several structures share a blood supply. Confirm the working localization with urgent brain and vascular imaging.
The cortical map starts with body representation and language
The internal carotid circulation supplies the ACA and MCA systems and gives rise to the ophthalmic artery. The vertebral arteries join to form the basilar artery, which supplies pontine branches and usually terminates in the PCAs. Communicating arteries and anatomical variants alter collateral flow. A diagram of a territory is therefore a useful approximation, not a rigid border for every patient. [1][2]
Medial hemisphere: ACA
Leg representation lies near the interhemispheric fissure. Medial frontal and supplementary motor regions contribute to initiation and behavior.
Lateral hemisphere: MCA
Face and arm representation, lateral language networks in the dominant hemisphere, and spatial attention networks are prominent.
Posterior and inferior hemisphere: PCA
Occipital visual cortex and inferior temporal structures are supplied here; deep branches reach thalamic and midbrain regions.
A surface-to-function map. The deep perforators need their own localization and are not represented as extra cortical areas. [3][4][5]
MCA cortical injury commonly affects the contralateral face and arm more than the leg. A dominant, usually left, superior-division infarct can produce nonfluent language with impaired expression and relatively preserved comprehension, a Broca-type pattern. Inferior-division injury can produce a Wernicke-type pattern: fluent but poorly meaningful speech with impaired comprehension and less obvious weakness. A large dominant MCA infarct may impair both expression and comprehension, producing global aphasia. Aphasia is a language disorder; dysarthria is impaired articulation. Test naming, comprehension, repetition, and fluency rather than deciding from speech volume alone. [1][4]
Upper facial muscles receive substantial bilateral corticobulbar input, so a unilateral supranuclear lesion often weakens the opposite lower face with relative forehead sparing. A pontine facial nucleus or fascicle lesion can instead weaken the whole ipsilateral face. Neither pattern replaces examination of the other cranial and limb findings. [2]
A nondominant, usually right, cortical infarct can cause left hemispatial neglect and lack of awareness of a deficit, or anosognosia. Neglect concerns attention, not simply missing one half of the visual field. Test extinction and behavior as well as confrontation fields. A destructive frontal eye-field lesion often produces an acute gaze preference toward the injured hemisphere and away from the weak limbs. This cortical rule cannot be applied unchanged to a pontine gaze-center lesion. [1]
ACA injury often gives contralateral leg-predominant weakness or sensory loss. Medial frontal involvement can add abulia, impaired initiation, grasp responses, or urinary incontinence. Bilateral medial frontal injury can produce profound loss of spontaneous activity despite wakefulness. Distribution matters: isolated foot weakness also has peripheral causes, so accompanying upper motor neuron and cortical findings strengthen localization. [3][1]
Visual fields point behind the chiasm, not automatically to one artery
A right homonymous field defect means the right half of visual space is lost in both eyes. It points to the left retrochiasmal pathway. Left occipital injury in the PCA territory is one cause, but optic tract, lateral geniculate, and optic radiation lesions can also do it. MCA infarction affecting optic radiations can combine aphasia, weakness, sensory loss, and hemianopia without requiring a second PCA infarct. [2][5]
Macular sparing means central vision within the affected hemifield is preserved. It can accompany occipital infarction when the occipital pole remains viable, often through collateral perfusion. It is variable and must not be made a mandatory or standalone diagnostic sign. Fixation and formal field testing also affect interpretation. Imaging resolves the anatomical question more reliably than a slogan about macular supply. [6]
A dominant left occipital infarct extending into the splenium can cause alexia without agraphia: the patient can write but cannot read, including the text just written. Left visual cortex is injured, and visual information from the intact right hemisphere cannot readily reach the left language network through the damaged posterior callosal connection. Bilateral occipital injury can cause cortical blindness despite preserved pupillary reflex pathways; some patients deny the visual loss, a syndrome called Anton syndrome. [5][2]
Thalamic injury can produce contralateral sensory loss and later central neuropathic pain. The VPL relays body somatosensation, while the VPM relays facial somatosensation. A lesion involving both can affect face and body. Do not attribute every hemisensory presentation specifically to the VPL alone. Proximal PCA or top-of-basilar disease may extend beyond isolated vision loss to bilateral thalamic or midbrain dysfunction with altered alertness and eye findings. [2][5]
Brainstem syndromes become easier when each tract has its own crossing
A cranial nerve deficit on one side with a body deficit on the other suggests a brainstem lesion. The reason is not that every body tract has already crossed. Descending corticospinal fibers above the pyramidal decussation control the opposite body. Ascending spinothalamic fibers have already crossed in the spinal cord. Medial lemniscal fibers have crossed in the caudal medulla. Facial pain-temperature fibers in the spinal trigeminal tract and nucleus are still representing the ipsilateral face. [2]
Example: left lateral medulla. Left spinal trigeminal tract/nucleus injury gives left facial pain-temperature loss.
In the same lesion: left spinothalamic tract injury gives right body pain-temperature loss because those ascending fibers crossed lower down.
Nearby medullary functions: nucleus ambiguus injury impairs voice and swallowing; descending sympathetic disruption gives left Horner syndrome; inferior cerebellar peduncle injury gives left ataxia.
Usually outside the lateral lesion: the medial pyramid and medial lemniscus. Marked hemiparesis or vibration loss suggests extension or a different pattern.
Each line names the damaged structure before predicting the deficit. The face and body signs are crossed, but the lesion is on one side. [2][7]
Lateral medullary, or Wallenberg, infarction is associated with vertebral or PICA disease. Vertigo, nystagmus, ipsilateral limb ataxia, crossed pain-temperature loss, Horner syndrome, hoarseness, and dysphagia may occur in varying combinations. Severe swallowing dysfunction is clinically important even when arm and leg strength are preserved. Nucleus ambiguus belongs to the medulla. [7][2]
Lateral pontine AICA territory injury can combine ipsilateral lower motor neuron facial weakness, hearing or vestibular dysfunction, cerebellar findings, and crossed sensory abnormalities. Hearing loss is particularly informative because the inner-ear circulation commonly arises from the AICA system. AICA infarction is not simply every Wallenberg finding plus cranial nerves VII and VIII: medullary nucleus ambiguus injury should not be automatically assigned to a pontine lesion. PICA, AICA, and SCA cerebellar territories overlap clinically; ataxia alone seldom names the artery. [8][1]
Medial medullary injury involves the pyramid, medial lemniscus, and sometimes hypoglossal fascicles. Expect contralateral weakness and impaired vibration/position sense with ipsilateral tongue weakness; the protruded tongue points toward the weak side. Supply includes anterior spinal and paramedian vertebral branches. In the ventromedial midbrain, ipsilateral third-nerve palsy with contralateral weakness suggests Weber syndrome, involving oculomotor fascicles and the cerebral peduncle. Paramedian PCA and basilar-region branches participate. [2][7]
Bilateral ventral pontine injury can interrupt corticospinal and corticobulbar output while sparing awareness and vertical gaze. In classic locked-in syndrome, an apparently unresponsive patient may answer reliably through blinking or vertical gaze. Look for a communication channel before inferring absent awareness. Basilar occlusion can also cause coma when the arousal system is involved, so the vessel name alone does not specify consciousness. [1][14]
Horizontal gaze: cortex, pons, and connecting fibers
A left pontine horizontal gaze-center lesion prevents gaze toward the left. A left medial longitudinal fasciculus (MLF) lesion instead impairs left-eye adduction during right gaze, often with right abducting nystagmus: left internuclear ophthalmoplegia. Combine a left PPRF or abducens nucleus lesion with left MLF injury and horizontal function is reduced to right-eye abduction, the one-and-a-half syndrome. An isolated abducens nerve lesion affects abduction of one eye rather than conjugate gaze of both. [2][9]
Small deep infarcts can interrupt densely packed functions
Small-vessel disease related to hypertension and diabetes can affect deep penetrating arteries through lipohyalinosis or other arteriolar pathology. Lenticulostriate arteries supply important basal ganglia and capsular regions, but capsule supply is not exclusively MCA-derived. A small strategically located infarct can produce substantial weakness. Size is not a proxy for disability. [1][2]
Pure motor
Contralateral weakness without clear cortical findings suggests interruption of motor pathways, often in the capsule or pons. Face, arm, and leg need not be equally affected.
Pure sensory or sensorimotor
Thalamic injury can produce hemisensory loss; combined thalamocapsular or capsular involvement can affect both sensation and strength. The capsule is not motor-only tissue.
Ataxic hemiparesis or dysarthria with a clumsy hand
These patterns can follow small pontine or capsular lesions. Ataxia accompanies weakness in the first; disproportionate articulation and hand dexterity problems characterize the second.
Absence of aphasia, neglect, or a cortical field pattern supports a deep localization but does not prove a lacunar mechanism. Emboli and larger-vessel disease can also cause small infarcts. MRI and etiologic investigation must test the working explanation. A patient with a pure motor syndrome can have a pontine infarct even without an accompanying cranial nerve deficit. [1]
Use localization to accelerate care, not postpone it
Establish last known well, check glucose, assess airway and swallowing, and obtain urgent brain imaging to distinguish ischemia from hemorrhage. Early CT can be unrevealing in ischemic stroke, especially small posterior fossa infarction. Persistent focal findings still require the stroke pathway. Sudden monocular painless visual loss with retinal ischemic findings is an ocular vascular emergency; it differs from a homonymous defect and requires urgent stroke-centered assessment. [10][11]
The 2026 AHA/ASA ischemic stroke guidance includes alteplase or tenecteplase for eligible patients in the standard 4.5-hour window. A low NIHSS score does not make a disabling deficit harmless. Selected unknown-onset or later presentations may qualify through advanced imaging criteria. Thrombectomy eligibility depends on occlusion, imaging, timing, and clinical features; selected patients qualify up to 24 hours. The guideline specifically supports thrombectomy for appropriate basilar occlusion within 24 hours with NIHSS at least 10. These are assessment pathways, not automatic treatment for every syndrome. [10]
Headache, seizures, papilledema, or hemorrhagic lesions outside a single arterial territory can suggest cerebral venous thrombosis. Confirm with CT or MR venography. Anticoagulation is central to treatment in appropriate patients; venous hemorrhage is not managed by simply copying an arterial thrombolysis rule. Conversely, new blood within a previously documented arterial infarct supports hemorrhagic transformation. Atrial fibrillation alone does not establish that radiologic diagnosis. [12][18]
A thunderclap headache can indicate subarachnoid hemorrhage even without hemiparesis. Obtain urgent noncontrast CT. Under the 2023 AHA/ASA pathway, presentation more than six hours after onset or a new neurologic deficit calls for lumbar puncture after a negative CT when safe. A selected patient without a new deficit who is scanned within six hours on a high-quality scanner with expert interpretation may follow the early-CT pathway. Neither pathway excludes every other vascular headache cause. [17]
Global oxygen or perfusion failure creates a different map
Cardiac arrest, profound hypotension, severe hypoxemia, or inadequate oxygen content can injure many brain regions without one occluded cerebral artery. Selectively vulnerable populations include hippocampal CA1 pyramidal neurons, cortical pyramidal neurons in layers III, V, and VI, cerebellar Purkinje cells, and basal ganglia neurons. CA1 injury can contribute to impaired new memory; cerebellar injury can contribute to ataxia. Do not turn this into an invariant ranking of which neuron always dies first. [13]
Watershed infarction describes injury at border zones between arterial territories, where perfusion pressure can be particularly vulnerable. Bilateral border-zone injury after systemic hypotension is different from selective neuronal injury within a tissue region, though both can coexist. Severe anemia lowers oxygen content even when pulse oximetry appears normal. Hypoglycemia is another energy-failure mechanism; its MRI distribution can include cortex, hippocampi, or white matter and should not be assumed identical to hypoxic-ischemic injury. [13][15]
Coma lacks wakefulness; unresponsive wakefulness syndrome describes eye opening and sleep-wake cycling without behavioral evidence of awareness on appropriate assessment. Reproducible intentional responses support a different level of consciousness. Serial standardized examinations and exclusion of sedation, seizures, sensory deficits, and motor output limitations reduce misclassification. Locked-in syndrome is especially important because awareness can be intact despite profound paralysis. [14]
After cardiac arrest, do not predict a person's outcome from an estimated four-minute threshold, one examination, or one vulnerable brain region. AHA guidance supports delayed, multimodal prognostication that accounts for temperature management, sedation, and other confounders; consolidation is generally at least 72 hours after normothermia and sedative discontinuation. Explain what is known, what remains uncertain, and what serial evidence will resolve. [16]
Practice checking the side, level, and decision
Case 1
Show answer and explanations for case 1
A. Right MCA inferior division (Why this does not fit)
A right nondominant lesion more often disrupts left spatial attention than language production.
B. Left ACA cortical territory (Why this does not fit)
ACA injury more often emphasizes the contralateral leg and medial frontal functions.
C. Right PCA occipital territory (Why this does not fit)
That location would primarily affect the left visual field and does not explain the right face-arm and language pattern.
D. Left MCA superior division (Best answer)
Dominant frontal language dysfunction with contralateral face-arm weakness fits this distribution.
Takeaway: Match nonfluent aphasia and face-arm weakness to dominant lateral frontal injury.
This reverses the usual acute destructive cortical pattern.
B. No horizontal preference because the cortex does not control conjugate gaze (Why this does not fit)
The frontal eye fields influence contralateral horizontal gaze. A destructive left lesion can produce leftward preference.
C. Toward the weak side because cortical and pontine gaze lesions act identically (Why this does not fit)
Cortical and pontine gaze circuitry have different roles. Destructive left frontal eye-field injury tends to favor gaze toward the lesion, away from the weak right limbs.
D. Toward the left (Best answer)
Loss of left frontal drive toward the right often leaves a preference toward the injured hemisphere.
Takeaway: For an acute destructive cortical lesion, gaze often favors the lesion side.
A. Every PCA infarct must spare the macula through an identical arterial border (Why this does not fit)
The infarct extent and collateral anatomy are variable.
B. The result excludes any occipital lesion (Why this does not fit)
Macular sparing is compatible with occipital infarction.
C. Preservation of the entire occipital cortex, with the deficit arising in the optic tract (Why this does not fit)
The imaging and field pattern place the injury in occipital cortex. Central sparing can reflect surviving cortical macular representation rather than a different lesion site.
D. Survival of the cortical macular representation, often supported by collateral perfusion (Best answer)
Preserved occipital-pole tissue can explain macular sparing, which varies across patients.
Takeaway: Macular sparing describes preserved function, not a compulsory feature of PCA stroke.
A. New peripheral small-fiber neuropathy (Why this does not fit)
Peripheral neuropathy can cause burning pain, but the hemisensory distribution after a contralateral thalamic injury favors central poststroke pain.
B. Painful spasticity (Why this does not fit)
Spasticity can cause pain, but this stem emphasizes sensory loss and pain from light touch rather than increased tone or spasms.
C. Complex regional pain syndrome (Why this does not fit)
This can occur after stroke, but regional autonomic or trophic findings are not supplied. Hemibody sensory changes after thalamic injury favor central neuropathic pain.
D. Central poststroke neuropathic pain (Best answer)
A prior sensory pathway lesion can produce delayed pain and allodynia on the contralateral side.
Takeaway: Sensory pathway injury can later produce abnormal pain, not only numbness.
A. Facial and body pain-temperature pathways both cross in the medulla (Why this does not fit)
Body spinothalamic fibers have already crossed in the spinal cord. Lateral medullary injury can affect ipsilateral facial input and contralateral body input.
B. The medullary sensory decussation carries both facial and body pain-temperature input (Why this does not fit)
The medial lemniscal decussation carries dorsal-column modalities from the body. This pain-temperature pattern instead reflects spinal trigeminal and spinothalamic anatomy.
C. The sensory pattern requires separate peripheral lesions on the two sides (Why this does not fit)
A single lateral medullary lesion accounts for the pattern because face and body pathways are affected at different stages relative to their crossings.
D. Facial pain-temperature fibers have not crossed in the brainstem; spinothalamic fibers crossed in the cord (Best answer)
The two systems represent different sides at this medullary level.
Takeaway: Track the crossing of each sensory system separately.
A. Isolated pontine AICA injury need not involve the medullary nucleus ambiguus (Best answer)
Pontine and medullary syndromes share some pathways but not every cranial nucleus.
B. Dysphagia in any AICA infarct establishes direct nucleus ambiguus destruction (Why this does not fit)
Nucleus ambiguus is medullary. Swallowing impairment requires assessment but does not prove this nucleus is included in an isolated pontine infarct.
C. Facial weakness and hearing loss together localize an isolated lesion to the medulla (Why this does not fit)
This pair supports lateral caudal pontine or associated auditory-system involvement. It is not the defining pair for an isolated lateral medullary lesion.
D. PICA and AICA syndromes can be distinguished solely by ataxia (Why this does not fit)
Both territories can cause ataxia. Associated cranial nerve findings and imaging provide more useful localization.
Takeaway: Compare shared tracts without relocating cranial nuclei.
A. Test reliable responses to commands via vertical gaze or blinking (Best answer)
A preserved eye-based response may reveal awareness despite severe paralysis.
B. Assume absence of limb withdrawal proves absent awareness (Why this does not fit)
Motor output failure can mask awareness.
C. Classify awareness from spontaneous eye opening alone (Why this does not fit)
Eye opening establishes arousal, not awareness. Reliable command-following through a preserved channel is important in severe motor impairment.
D. Use only spoken commands requiring a hand squeeze (Why this does not fit)
A paralyzed patient cannot demonstrate awareness through a hand response. A preserved response channel such as gaze or blinking helps avoid misclassification.
Takeaway: Behavioral testing must account for the motor and sensory pathways still available.
A. Hemorrhagic transformation of the ischemic infarct (Best answer)
The prior infarct and new blood in the same tissue establish the temporal and anatomical relationship.
B. Primary hypertensive intracerebral hemorrhage unrelated to the preceding infarct (Why this does not fit)
Primary hemorrhage is a differential, but petechial blood within documented infarcted tissue after several days supports hemorrhagic transformation.
C. Infarct extension without hemorrhagic transformation (Why this does not fit)
New blood establishes a hemorrhagic component. Pure ischemic extension cannot account for that CT finding.
D. Cerebral amyloid angiopathy as the established cause (Why this does not fit)
Amyloid angiopathy can cause lobar hemorrhage, but this imaging sequence supports blood appearing within a recent ischemic infarct. It does not establish amyloid disease.
Takeaway: Use prior imaging and the location of new blood to identify hemorrhagic transformation.
A. Bilateral MCA main-trunk embolic infarction as the only possible mechanism (Why this does not fit)
Emboli may contribute to some border-zone infarcts, but this junctional pattern after profound hypotension supports compromised perfusion.
B. Superior sagittal sinus thrombosis (Why this does not fit)
Venous thrombosis can cause bilateral lesions, but the stated arterial border-zone distribution after hypotension favors an arterial perfusion mechanism.
C. Selective hippocampal neuronal injury without territorial ischemia (Why this does not fit)
These lesions occupy arterial junctions rather than an isolated hippocampal pattern. Severe hypoperfusion can produce border-zone infarction.
D. Border-zone ischemia related to compromised perfusion (Best answer)
Distal junctional territories can be vulnerable when systemic perfusion falls.
Takeaway: A watershed pattern is a perfusion map, not a ranking of cell fragility.
CA3 participates in hippocampal circuitry, but CA1 has greater classic vulnerability to brief global ischemia.
B. Dentate granule cells (Why this does not fit)
These are hippocampal neurons, but the canonical selectively vulnerable population tested here is CA1 pyramidal cells.
C. CA1 pyramidal neurons (Best answer)
These neurons are selectively vulnerable to hypoxic-ischemic injury and contribute to hippocampal memory function.
D. CA2 pyramidal neurons (Why this does not fit)
CA1 pyramidal neurons are particularly vulnerable to hypoxic-ischemic injury. CA2 is not the characteristic selectively vulnerable hippocampal population tested here.
Takeaway: Use regional anatomy to connect post-hypoxic deficits to vulnerable neurons.
A. Cerebellar granule cells (Why this does not fit)
Granule cells participate in cerebellar processing, but Purkinje cells are the classic selectively vulnerable cortical population in hypoxic injury.
B. Purkinje cells (Best answer)
These cerebellar cortical neurons are vulnerable to hypoxic injury and are important for coordination.
C. Cerebellar basket cells (Why this does not fit)
These inhibitory interneurons are anatomically relevant, but they are not the characteristic selectively vulnerable output neurons emphasized in this setting.
D. Cerebellar Golgi cells (Why this does not fit)
Golgi interneurons regulate local input processing; Purkinje cell injury is the more characteristic hypoxic cortical association.
Takeaway: Cerebellar dysfunction after global injury has a different anatomical substrate from hippocampal amnesia.
A. Pulse oximetry directly measures cerebral metabolic demand (Why this does not fit)
Oximetry estimates arterial hemoglobin saturation. It does not quantify tissue demand, hemoglobin concentration or cerebral blood flow.
B. Saturation measures neither available hemoglobin nor adequacy of perfusion (Best answer)
Severe anemia and low blood flow can reduce oxygen delivery despite high saturation.
C. A saturation of 99% guarantees normal cerebral oxygen content and flow (Why this does not fit)
Saturation is only one component of oxygen delivery.
D. Normal saturation excludes an oxygen-delivery problem when lung function is intact (Why this does not fit)
Adequate saturation of the remaining hemoglobin does not fully compensate for profound anemia and hypotension. Oxygen content and flow can both be inadequate.
Takeaway: Oxygen delivery depends on content and blood flow, not saturation alone.
A. Subarachnoid hemorrhage with assessment for an aneurysmal source (Best answer)
Explosive onset and bilirubin-related xanthochromia support subarachnoid bleeding despite a delayed negative CT. Vascular evaluation is needed to identify an aneurysm or another source; the fluid finding alone does not prove an aneurysm.
B. Primary thunderclap headache without a secondary cause (Why this does not fit)
A primary diagnosis requires exclusion of secondary causes. Evidence of blood breakdown in CSF makes an unexplained benign thunderclap diagnosis inappropriate.
C. Uncomplicated RCVS with no associated hemorrhage (Why this does not fit)
RCVS can produce thunderclap headache and can be complicated by subarachnoid bleeding. It cannot explain away evidence of bleeding as an uncomplicated nonhemorrhagic episode; vascular imaging must assess the cause.
D. Migraine without aura causing the headache episode (Why this does not fit)
A first explosive headache with bilirubin-related xanthochromia is not explained by uncomplicated migraine. The CSF finding requires a hemorrhage evaluation.
Takeaway: Delayed negative CT does not exclude SAH. Evidence of subarachnoid bleeding requires urgent evaluation of its source.
