The Nervous System · Part Four · How It Fails and Recovers
Lesson 53 / 61
Stroke and Vascular Disease
The abrupt loss of brain perfusion that turns minutes of ischemia into a map of lost function.
Stroke is the vascular catastrophe in which an occluded or ruptured cerebral artery starves neurons of oxygen, and the territory of the failing vessel writes the clinical syndrome onto the body. Roughly 85 percent of events are ischemic, arising from atherothrombosis, embolic occlusion, or lacunar small vessel disease, while the remaining 15 percent are hemorrhagic. Because perfused tissue ceases to function within 60 to 90 seconds of oxygen loss, the geography of the Circle of Willis and its collaterals decides what survives.
Ischemic share
85 percent of all strokes
Functional failure
Neurons silence after 60 to 90 seconds of anoxia
Berry aneurysm SAH
80 percent of subarachnoid bleeds, mortality up to 50 percent
Thrombolysis window
Tissue plasminogen activator within 3 to 4.5 hours
01TERRITORY
Vessel Writes Syndrome
Stroke localizes because each cerebral artery owns a defined territory, and losing that vessel subtracts exactly its tissue. The forebrain draws from the two internal carotid arteries and the basilar artery, knit together at the Circle of Willis where the anterior communicating and posterior communicating arteries bridge the anterior and posterior circulations. The internal carotid gives rise to the ophthalmic, posterior communicating, anterior choroidal, and the anterior and middle cerebral arteries. The basilar yields the paired posterior cerebral arteries. This architecture is not decoration. It is a redundancy circuit, and the completeness of an individual ring of Willis predicts whether an occlusion becomes a deficit or a near miss.
The middle cerebral artery dominates the convexity, and its branches sort the syndrome by division. An anterosuperior occlusion brings contralateral face and arm weakness greater than leg, with dysarthria from injury to supranuclear speech pathways, and Broca aphasia when the lesion sits on the left. A posteroinferior occlusion produces incongruent hemianopia, an agitated confused state from limbic temporal involvement, and Wernicke aphasia on the left or hemineglect on the right. The anterior cerebral artery, by contrast, sacrifices the leg. Reading the body backward to the vessel is the oldest skill in vascular neurology.
02PENUMBRA
The Salvageable Rim
The ischemic penumbra is the tissue caught between dead core and healthy brain, electrically silent but still structurally alive. Brain tissue stops functioning after 60 to 90 seconds without oxygen, yet irreversible infarction takes hours, and within the ischemic field a gradient of severity emerges. Part of the territory dies at once while a surrounding crescent survives on collateral trickle, suspended in a state where membranes still hold but signaling has stopped. This rim is the entire premise of acute intervention. Every restored vessel and every recruited collateral is an attempt to convert penumbra back into living, computing cortex before the gradient collapses inward.
Collateral flow is the brain own hedge against occlusion. The contralateral internal carotid can perfuse both anterior and middle cerebral pairs through the Circle of Willis, and the external carotid can route around a blockage through anastomotic branches. At the boundaries between major arterial territories lies the watershed area, a crescent of cortex fed by sub half millimeter anastomoses in the depths of cortical sulci, its richness varying widely between individuals. When systemic pressure falls, these border zones starve first. The penumbra concept reframes stroke from a fixed lesion into a contest between failing supply, recruited reserve, and the clock.
03LACUNES
Small Vessel Strokes
Lacunar infarcts are tiny deep strokes that produce outsized, stereotyped deficits because of where the small vessels run. Occlusion of a lenticulostriate branch of the middle cerebral artery, or a perforating branch of the basilar artery, classically yields pure motor hemiplegia. The mechanism is precise. Loss of perfusion strikes the corticospinal tract as it funnels through the posterior limb of the internal capsule, the corona radiata, or the basis pontis, paralyzing the contralateral arm, leg, and lower face. A single threadlike artery, failing, mimics a far larger event by hitting a bottleneck of descending fibers.
The deep territory holds other signature lesions. A pure sensory stroke localizes to the ventral posterolateral and ventral posteromedial thalamic nuclei, fed by lenticulostriate and thalamogeniculate vessels, producing contralateral hemisensory loss alone. The recurrent artery of Heubner, the medial striate branch arching above and lateral to the optic chiasm, supplies the head of the caudate and the anterior limb of the internal capsule, and its loss brings contralateral face-and-arm weakness with frontal-type behavioral change. The anterior choroidal artery, feeding the posterior limb of the internal capsule, can fell hemiparesis, hemianesthesia, and a visual field cut together. Small vessels, deep targets, and the body learns which fibers were passing through.
04REGULATION
Flow Under Control
Cerebral blood flow is actively governed, and its three regulators form the substrate that stroke overwhelms. Autoregulation is the fastest, the capacity of tissue to defend its own supply through a direct myogenic response to any rise in pressure. Hydrogen ion concentration in the extracellular fluid around the arterioles tunes caliber chemically, so hypercapnia, an excess of plasma carbon dioxide, drives vasodilation, while hypocapnia constricts. Rising extracellular potassium also dilates. This trio normally holds perfusion steady across swings in systemic pressure, and it is the integrative balance of vascular supply and neural demand that anchors a stable central integrative state.
Stroke is what happens when that governance fails locally and catastrophically. Once flow drops below the threshold the regulators can defend, the blood brain barrier itself begins to break down, favoring cerebral edema and hemorrhagic transformation, and thrombolytic tissue plasminogen activator further raises that risk. The immune response acutely disturbs extracellular homeostasis and neuronal excitability through shifts in astrocytic glutamate and potassium buffering, driving toxicity and cell death. The same buffering machinery that steadies the resting brain becomes, in ischemia, an engine of secondary injury. Regulation and ruin run on one circuit.
The territory of a failing vessel is not a region on a map but a catalog of behaviors waiting to be erased, and the collateral that survives decides which ones return.
05HEMORRHAGE
When Vessels Burst
Hemorrhagic stroke is the 15 percent in which a vessel ruptures rather than clogs, and its presentation is unmistakable. Subarachnoid hemorrhage, most often 80 percent of the time, results from rupture of an intracranial berry aneurysm, announcing itself as the sudden worst headache of a lifetime, with altered mental status and meningeal signs of neck stiffness from blood irritating the meninges. Morbidity is heavy and mortality reaches up to 50 percent. Unlike the slow recruitment of collaterals that can soften an occlusion, a bleed is immediate, mechanical, and pressure driven, and the spilled blood injures tissue both by mass effect and by chemistry.
The posterior circulation carries its own vivid lesions. A unilateral posterior cerebral artery occlusion produces homonymous hemianopia, with macular vision often spared by dual middle cerebral supply. Bilateral occlusion can yield cortical blindness, named Anton syndrome, in which patients lose sight on confrontation yet remain unaware of the deficit, an anosognosia. Prosopagnosia, the inability to recognize faces, follows bilateral fusiform damage in the inferior occipitotemporal cortex. From the brainstem and cerebellum fed by the vertebral and basilar arteries to the visual cortex behind, the vascular tree maps directly onto the catalog of what a stroke can take.