What is the pathophysiology of ischemic stroke?

Pathophysiology of Ischemic Stroke

Primary Mechanism: Vascular Occlusion and Cerebral Ischemia

Ischemic stroke occurs when blocked blood vessels to the brain lead to cerebral infarction, initiating a cascade of cellular injury that progresses from oxygen deprivation to irreversible neuronal death. 1

The fundamental pathophysiologic sequence begins with arterial occlusion—either thrombotic or embolic—that reduces cerebral blood flow below the threshold needed to maintain neuronal function. 2 This deprivation of glucose and oxygen triggers the ischemic cascade, ultimately resulting in neuronal death and irreversible loss of neuronal function. 3

Mechanisms of Arterial Occlusion

Ischemic stroke arises through four distinct but non-mutually exclusive mechanisms: 1

• Perfusion failure from stenosis combined with poor collateral circulation, causing hemodynamic insufficiency 1
• In-situ thrombosis at the stenosis site due to complicated atherosclerotic plaque (rupture, hemorrhage into plaque, or occlusive plaque growth) 1
• Thromboembolic events distal to the stenosis, where plaque fragments or thrombi embolize downstream 1
• Direct occlusion of penetrating arteries at the plaque site, particularly relevant in small-vessel disease 1

The Ischemic Cascade: Cellular and Molecular Events

Energy Failure and Excitotoxicity

When cerebral blood flow drops below critical thresholds, ATP depletion occurs within minutes, causing failure of energy-dependent ion pumps. 2 This leads to:

• Ionic imbalances with massive influx of sodium, chloride, and calcium into cells 3
• Glutamate release and activation of glutamate receptors (NMDA, AMPA, kainate), triggering excitotoxicity 2, 3
• Calcium overload that activates destructive enzymes including proteases, lipases, and endonucleases 2

Mitochondrial Dysfunction and Oxidative Stress

Mitochondria serve as both victims and perpetrators in ischemic injury, generating excessive reactive oxygen species (ROS) that amplify cellular damage. 1

During ischemia and particularly during reperfusion, mitochondria become the major source of intracellular ROS. 1 Free electrons leak from the mitochondrial electron transport chain and react with molecular oxygen, generating superoxide anion (O2-). 1 This highly reactive molecule forms peroxynitrite (NO3-) when combined with nitric oxide, ultimately producing cytotoxic hydroxyl radicals that damage DNA, proteins, and lipids. 1

Mitochondrial fission precedes neuronal death after cerebral ischemia, with increased phosphorylation of dynamin-related protein 1 (Drp1) at serine 616 driving this process. 1 Inhibition of Drp1 reduces infarct volume in experimental models. 1

Inflammatory Response

Inflammation amplifies ischemic injury through glial cell activation, peripheral leukocyte infiltration, and release of damage-associated molecules. 1

The outer mitochondrial membrane serves as a platform for NLRP3 inflammasome assembly, which activates innate immune defense and pyroptosis through pro-inflammatory cytokines (particularly IL-1β) and caspase-1. 1 Acute systemic inflammatory stimuli worsen stroke outcomes, with IL-1β acting as a critical mediator. 1

Apoptosis and Cell Death

The ischemic cascade culminates in both necrotic and apoptotic cell death pathways. 3 Mitochondrial dysfunction triggers release of cytochrome c and activation of caspase cascades, leading to programmed cell death in the penumbral region. 2

The Penumbra Concept

A critical fraction of the ischemic territory exists in a "penumbral" state—tissue that is functionally impaired but structurally viable and potentially salvageable. 4

Local tissue fate depends on:

• Severity of hypoperfusion in the affected vascular territory 4
• Duration of occlusion before recanalization 4
• Collateral circulation adequacy 1

The penumbra represents the primary therapeutic target, as this tissue will either recover with reperfusion or progress to infarction without intervention. 4

Blood-Brain Barrier Disruption

Ischemia causes dysfunction of carrier-mediated nutrient and ion transport mechanisms across the blood-brain barrier. 5 Since the brain relies on continuous supply of nutrients via these transport processes, any irregularity dramatically affects neuronal function and stroke outcome. 5 BBB breakdown also permits vasogenic edema and hemorrhagic transformation. 5

Etiologic Subtypes and Their Pathophysiology

The underlying vascular pathology determines the specific ischemic mechanism: 1, 6

• Large-artery atherosclerosis (20%): Extracranial or intracranial atherosclerotic disease causing artery-to-artery embolism or hemodynamic insufficiency 1, 6
• Cardioembolism (20%): Intracardiac thrombi (predominantly from atrial fibrillation) embolize to cerebral vessels 1, 6
• Small-vessel disease (25%): Arteriolosclerosis (lipohyalinosis) of penetrating arteries causes lacunar infarcts ≤1.5 cm 1, 6
• Cryptogenic (30%): Unknown mechanism despite comprehensive evaluation 1, 6

Clinical Implications

The pathophysiologic heterogeneity from patient to patient is largely unpredictable from elapsed time or clinical deficit alone, necessitating individualized assessment with physiological imaging (DWI-PWI MRI or CT perfusion) to identify salvageable penumbra and guide therapy. 4 Variables like blood pressure, blood glucose, and oxygen saturation must be carefully managed to prevent penumbral tissue from progressing to infarction. 4

The narrow therapeutic window for interventions like thrombolysis (3-4.5 hours) and thrombectomy (6 hours) reflects the time-dependent progression of the ischemic cascade from reversible injury to irreversible infarction. 1