Anatomy, Physiology, and Pathophysiology of Stroke
Two circulations, one rule
Anterior circulation comes from the internal carotid artery, which gives off the ophthalmic, posterior communicating, and anterior choroidal branches before dividing into the middle cerebral artery (MCA) and anterior cerebral artery (ACA). Posterior circulation runs from the vertebral arteries into the basilar artery, supplying the brainstem, cerebellum, and posterior cerebral arteries (PCA). The circle of Willis links the two but is complete in only a minority of people, so collateral supply varies widely from patient to patient.
Territory-to-symptom mapping
- MCA: contralateral face and arm weakness/sensory loss, gaze preference toward the lesion, aphasia if dominant hemisphere, neglect if nondominant.
- ACA: contralateral leg weaker than arm, abulia, urinary incontinence.
- PCA: contralateral homonymous hemianopia, visual agnosia, memory impairment.
- Basilar/vertebral: cranial nerve deficits crossed with long-tract signs, diplopia, dysarthria, ataxia, or locked-in syndrome.
- Lacunar (small vessel): pure motor, pure sensory, or sensorimotor deficits without cortical signs.
Core, penumbra, and the clock
Normal cerebral blood flow is about 50 mL/100 g/min. Below roughly 20 mL/100 g/min tissue stops functioning but remains viable (penumbra); below about 10 mL/100 g/min it infarcts within minutes. Reperfusion rescues penumbra, not core. Roughly 1.9 million neurons are lost per minute of untreated large-vessel occlusion. Hemorrhagic stroke adds hematoma expansion, mass effect, edema, and possible herniation within hours.
Traps: cortical signs (aphasia, neglect, hemianopia) indicate large-vessel rather than lacunar stroke. Gaze deviation points toward the hemispheric lesion. Posterior circulation strokes score low on the NIHSS and are easily missed.
Rule of thumb: right hemiparesis + global aphasia + rightward gaze = left MCA occlusion.