STROKE: Acute Disruption of Cerebral Blood Flow
Stroke is an acute neurological event caused by a sudden interruption of blood flow to a region of the brain, resulting in neuronal injury and loss of function. Because neurons are highly metabolically active and have minimal energy reserves, even brief periods of reduced perfusion can lead to irreversible damage. Stroke remains a leading cause of death and long-term disability worldwide.
Stroke is divided into ischaemic and haemorrhagic types. Although their initiating mechanisms differ, both result in impaired oxygen and glucose delivery, disruption of cellular homeostasis and secondary injury processes that extend damage beyond the initial insult.
What You Need to Know
Stroke occurs when cerebral blood flow is suddenly disrupted, depriving brain tissue of the oxygen and glucose required for normal cellular function. In ischaemic stroke, blood flow is obstructed, most commonly by thrombosis or embolism, causing rapid failure of aerobic metabolism. Neurons have minimal energy reserves, so depletion of adenosine triphosphate occurs within minutes, leading to loss of membrane ion gradients, impaired electrical signalling, and early neuronal dysfunction.
Haemorrhagic stroke arises from rupture of a cerebral blood vessel, allowing blood to escape into brain tissue or surrounding spaces. This produces injury through several mechanisms acting at the same time. Accumulating blood compresses adjacent tissue, raises intracranial pressure, and disrupts normal cerebrospinal fluid dynamics, while the sudden loss of vascular integrity compromises perfusion to surrounding brain regions. Neuronal signalling becomes disorganised both from reduced blood supply and from mechanical distortion of neural tissue.
An ischaemic stroke occurs when cerebral blood flow is abruptly reduced or interrupted, most commonly due to arterial thrombosis or embolic occlusion. This leads to insufficient delivery of oxygen and glucose, resulting in failure of cellular metabolism, loss of ATP, and disruption of ion gradients. Subsequent excitotoxicity, cytotoxic oedema, and activation of inflammatory pathways contribute to neuronal injury and cell death within the affected vascular territory.
Across both stroke types, several shared physiological processes drive acute injury:
Abrupt interruption of oxygen and glucose delivery leading to energy failure
Loss of ionic homeostasis causing neuronal depolarisation and dysfunction
Secondary injury from oedema, raised intracranial pressure, and impaired perfusion
The extent of neurological injury depends on how severely and how long cerebral blood flow is reduced, as well as the availability of collateral circulation. Early restoration of perfusion can preserve vulnerable but still viable tissue, whereas prolonged disruption leads to irreversible neuronal death and permanent neurological deficit.
Beyond the Basics
Cerebral perfusion and ischaemic injury
Normal brain function depends on tightly regulated cerebral blood flow to deliver oxygen and glucose required for energy production. When perfusion falls below a critical threshold, neurons are forced to rely on anaerobic metabolism, which generates far less adenosine triphosphate. As ATP levels fall, energy-dependent ion pumps fail, allowing sodium and calcium to accumulate inside neurons. This leads to cellular swelling, loss of membrane stability, and abnormal depolarisation.
Within the affected vascular territory, injury is not uniform. The ischaemic core represents the region of most severe hypoperfusion, where energy failure is profound and neuronal death occurs rapidly and irreversibly. Surrounding this core is the ischaemic penumbra, an area with reduced but not absent blood flow. Neurons in the penumbra are electrically silent and functionally impaired but remain structurally viable for a limited time. Preserving this tissue through timely reperfusion is central to limiting stroke-related disability.
Excitotoxicity and cellular injury
Energy failure during cerebral ischaemia disrupts normal neurotransmitter regulation. Large amounts of glutamate, the main excitatory neurotransmitter, are released into the synaptic cleft and are not effectively cleared. Excessive stimulation of glutamate receptors allows uncontrolled calcium entry into neurons.
Elevated intracellular calcium activates enzymes that degrade cell membranes, damage mitochondria, and fragment DNA. This process, known as excitotoxicity, causes neuronal injury that extends beyond the initially ischaemic region. As a result, the infarct may expand over time, even after blood flow has been partially restored.
Inflammation and secondary brain injury
Following the initial vascular insult, inflammatory pathways become activated. Microglia, the resident immune cells of the central nervous system, release cytokines and other inflammatory mediators. These substances increase permeability of the blood–brain barrier, allowing plasma proteins and fluid to leak into brain tissue.
This inflammatory response contributes to vasogenic oedema, meaning swelling caused by extracellular fluid accumulation. Oedema increases intracranial volume and further compromises local blood flow. Secondary injury processes driven by inflammation may continue for hours to days after the initial stroke, contributing to delayed neurological deterioration.
Haemorrhagic stroke and mass effect
In haemorrhagic stroke, rupture of a cerebral blood vessel leads to accumulation of blood within the brain parenchyma or surrounding spaces. The expanding haematoma exerts mass effect, compressing adjacent tissue and disrupting normal neural connections. This mechanical compression impairs neuronal function independently of blood flow interruption.
Blood breakdown products are directly toxic to neurons and trigger an intense inflammatory response. As intracranial pressure rises, cerebral perfusion is reduced not only locally but across wider brain regions. In severe cases, this combination of compression, inflammation, and reduced perfusion can lead to widespread secondary ischaemia and brain herniation.
Cerebral oedema and raised intracranial pressure
Both ischaemic and haemorrhagic strokes can result in cerebral oedema through multiple mechanisms. Cytotoxic oedema occurs early when neurons and glial cells swell due to ion pump failure and intracellular water accumulation. Vasogenic oedema develops later as blood–brain barrier disruption allows fluid and proteins to enter the extracellular space.
As oedema progresses, intracranial pressure rises within the fixed volume of the skull. Increased pressure reduces cerebral perfusion pressure, limiting blood flow to already vulnerable tissue. This creates a self-perpetuating cycle in which reduced perfusion worsens ischaemia, leading to further swelling and expansion of injury.
Clinical Connections
Stroke commonly presents with sudden onset focal neurological deficits, including unilateral weakness, speech disturbance, visual loss, or altered level of consciousness. The pattern of symptoms depends on which cerebral vessel is affected and which brain regions experience acute hypoperfusion or haemorrhage. Because neurons fail rapidly when deprived of oxygen and glucose, clinical deficits often evolve quickly in the early phase, with progression indicating expanding injury, cerebral oedema, or rising intracranial pressure.
Early assessment focuses on identifying evolving neurological change and physiological instability.
Clinical features that signal increased risk include:
Worsening weakness, aphasia, or visual disturbance suggesting infarct expansion or haemorrhage progression
Declining level of consciousness indicating raised intracranial pressure or brainstem involvement
Abnormal vital signs, such as hypertension, hypoxia, or hyperglycaemia, which exacerbate secondary brain injury
Neuroimaging is essential for distinguishing ischaemic from haemorrhagic stroke and directing time-critical management decisions. Ongoing monitoring centres on preserving cerebral perfusion, maintaining airway protection and oxygenation, and reducing secondary complications such as aspiration, infection, venous thromboembolism, and pressure injury. Interpreting neurological and physiological changes through an understanding of cerebral blood flow, oedema, and secondary injury mechanisms supports timely escalation and improves the chance of preserving salvageable brain tissue.
Concept Check
Why are neurons particularly vulnerable to interruptions in blood flow?
What distinguishes the ischaemic core from the penumbra?
How does excitotoxicity amplify neuronal injury after ischaemia?
Why does haemorrhagic stroke increase intracranial pressure?
How can cerebral oedema worsen neurological damage after stroke?