Stroke occurs when blood flow to part of the brain is interrupted, causing rapid loss of oxygen and glucose delivery to neurons. Because different arteries supply different brain regions, the neurologic deficits depend strongly on which vascular territory is affected. Learning arterial territories helps students connect anatomy to bedside findings such as weakness, aphasia, neglect, or visual loss.
This is a core skill in early neurology and emergency medicine because stroke localization guides urgent diagnosis and treatment.
The major cerebral vessels include the anterior cerebral artery, middle cerebral artery, posterior cerebral artery, and the vertebrobasilar circulation. Each artery supplies characteristic cortical, subcortical, and brainstem structures, so infarction produces recognizable patterns of motor, sensory, language, visual, and cranial nerve deficits. For example, middle cerebral artery stroke often affects face and arm function more than leg function, while anterior cerebral artery stroke more often affects the leg.
Posterior circulation strokes can involve the brainstem, cerebellum, or occipital lobe and may cause vertigo, ataxia, diplopia, dysarthria, or visual field defects.
Understanding Stroke
Brain tissue has a very high energy demand and stores almost no usable fuel. When a vessel closes, nerve cells stop making enough energy to run their membrane pumps. Sodium and water then move into cells, causing swelling.
Excess glutamate is released at synapses and can overstimulate nearby neurons. This process is called excitotoxic injury. The central area with the worst loss of flow may die quickly.
Around it is the penumbra, a zone of weakly functioning tissue that can sometimes recover if circulation is restored soon enough. This is why emergency teams record the exact time a person was last known to be well.
Not every blockage causes the same amount of damage. Some brain areas receive backup flow through connections between arteries, especially near the Circle of Willis and on the cortical surface. These backup routes are called collateral circulation.
Their usefulness varies greatly between people. A gradual narrowing of a carotid artery may allow time for collateral vessels to enlarge. A sudden large clot may overwhelm those routes.
Blood pressure matters during an acute event because a fall in pressure can reduce flow to tissue that is already at risk. Swelling can raise pressure inside the skull, which further limits brain perfusion. This creates a dangerous cycle that clinicians monitor closely.
Localization depends on more than the main artery name. Motor pathways cross in the lower brainstem, so a cerebral stroke usually causes weakness on the opposite side of the body. Cortical injury often produces higher function problems such as language loss, neglect, gaze preference, or a missing part of the visual field.
In contrast, a small deep infarct can damage tightly packed fibers without affecting the cortex. This can produce a clear pattern of weakness or numbness without aphasia, neglect, or visual loss.
Brainstem lesions may cause crossed findings, with a cranial nerve problem on one side of the face and body weakness or sensory change on the other side. These patterns are especially useful because the brainstem contains many pathways in a small space.
Students often first meet stroke patterns through a focused neurologic examination. Observe facial symmetry, speech clarity, arm drift, leg strength, sensation, eye movements, coordination, and awareness of both sides of space. Sudden trouble speaking can reflect aphasia, while slurred but correctly chosen words suggest dysarthria.
These are different findings. A patient with neglect may ignore food on one side of a plate or fail to dress one side of the body. A visual field loss may cause repeated collisions with objects on the same side.
On imaging, early computed tomography is often used to look for bleeding, while vessel imaging can show an arterial blockage. Magnetic resonance imaging can detect many small early infarcts.
When learning localization, match each sign to a structure first, then connect the structure to its blood supply. This method is more reliable than memorizing long lists.
Key Facts
- Cerebral perfusion pressure: CPP = MAP - ICP
- ACA stroke classically causes contralateral leg weakness and sensory loss greater than arm and face deficits.
- MCA stroke classically causes contralateral face and arm weakness and sensory loss greater than leg deficits.
- PCA stroke often causes contralateral homonymous hemianopia due to occipital cortex ischemia.
- Left dominant hemisphere MCA stroke may cause aphasia, while right nondominant hemisphere MCA stroke may cause hemispatial neglect.
- Lacunar infarcts involve small penetrating arteries and can produce pure motor stroke, pure sensory stroke, or ataxic hemiparesis.
Vocabulary
- Arterial territory
- An arterial territory is the specific region of brain tissue supplied by a particular blood vessel.
- Aphasia
- Aphasia is an acquired disorder of language affecting speech production, comprehension, reading, or writing, usually from dominant hemisphere injury.
- Hemianopia
- Hemianopia is loss of one half of the visual field in one or both eyes, often caused by lesions behind the optic chiasm.
- Neglect
- Neglect is failure to attend to one side of space, commonly seen after injury to the nondominant parietal cortex.
- Lacunar stroke
- Lacunar stroke is a small deep infarct caused by occlusion of penetrating arteries, often related to chronic hypertension.
Common Mistakes to Avoid
- Assuming all unilateral weakness means MCA stroke, which is wrong because ACA, internal capsule, brainstem, and spinal lesions can also cause weakness and the body distribution helps localize the lesion.
- Forgetting hemisphere dominance when predicting language deficits, which is wrong because aphasia usually localizes to the dominant hemisphere, most often the left.
- Equating visual symptoms only with eye disease, which is wrong because occipital or optic radiations infarction can cause homonymous visual field loss with normal eye structures.
- Ignoring posterior circulation stroke signs, which is wrong because dizziness, diplopia, dysarthria, ataxia, and crossed findings can indicate dangerous brainstem or cerebellar ischemia.
Practice Questions
- 1 A patient has sudden right face and right arm weakness with expressive aphasia. Which arterial territory is most likely affected, and in which hemisphere?
- 2 A patient has sudden left leg weakness greater than left arm weakness and urinary incontinence. Which cerebral artery is most likely occluded?
- 3 A patient has vertigo, ataxia, dysarthria, and diplopia but no clear cortical sensory loss. Explain why this pattern suggests posterior circulation stroke rather than a typical MCA cortical stroke.