A stroke is caused either by interruption of blood flow to part of the brain or by bleeding from a ruptured blood vessel in the brain. The first mechanism produces an ischaemic stroke and is usually caused by a clot or other blockage in an artery supplying brain tissue. The second produces a haemorrhagic stroke, in which blood escapes from a damaged vessel. Although both are strokes, the underlying mechanisms and emergency treatments are different. The common consequence is injury to brain tissue because normal circulation is disrupted.
Ischaemic stroke can occur through thrombosis, embolism or other arterial obstruction. Thrombosis refers to a clot forming within an artery, often in association with atherosclerosis, in which fatty material and inflammatory changes cause arterial narrowing and an environment conducive to clot formation. An embolus is material that travels through the bloodstream and becomes lodged in a smaller artery. A common source is the heart, particularly in atrial fibrillation, where irregular upper-chamber contractions can permit blood to pool and form clots. The clot can then travel through the arterial circulation into a cerebral artery and obstruct blood flow.
Atrial fibrillation is particularly important because it can produce embolic strokes without causing obvious symptoms between episodes. In atrial fibrillation, the atria beat irregularly rather than contracting in a coordinated rhythm, which can allow blood to stagnate and form thrombi, particularly in the left atrial appendage. If part of such a thrombus enters the circulation, it can travel to the brain. This is why detection and appropriate treatment of atrial fibrillation can be an important component of stroke prevention. NHS guidance identifies irregular and fast heartbeats, including atrial fibrillation, as a condition that increases stroke risk.
Atherosclerosis is another major mechanism. Over time, fatty deposits, inflammatory cells and fibrous tissue can accumulate within arterial walls, forming plaques that narrow the artery. A plaque can rupture or erode, triggering platelet activation and clot formation. If the resulting thrombus substantially blocks blood flow to the brain, an ischaemic stroke can occur. Atherosclerosis can affect the carotid arteries in the neck, which supply the brain, as well as arteries within the brain itself. High cholesterol, smoking, hypertension, diabetes and other cardiovascular risk factors promote the processes that make atherosclerotic disease more likely.
Haemorrhagic stroke occurs when a blood vessel ruptures. Long-standing high blood pressure can weaken small arteries and increase the risk of intracerebral haemorrhage. Other causes include ruptured cerebral aneurysms and vascular malformations, while anticoagulant medication can increase bleeding risk in susceptible circumstances. Haemorrhage can occur within brain tissue or in the spaces surrounding the brain. The resulting blood can injure neurons directly and increase intracranial pressure. The exact cause must be established because treatments designed to dissolve clots in ischaemic stroke can be dangerous when the stroke is caused by bleeding.
Several medical and demographic factors increase the probability of stroke. High blood pressure is one of the most important modifiable risk factors, while diabetes and high cholesterol contribute to vascular damage. Smoking promotes atherosclerosis and makes blood more likely to clot. Excessive alcohol consumption can increase blood pressure and contribute to atrial fibrillation. Age is a major risk factor, although stroke can occur in younger people. Other circumstances associated with increased risk include sickle cell disease, some migraine patterns, pregnancy-related complications such as pre-eclampsia and certain medications.
A previous TIA is particularly significant because it indicates that cerebral blood flow has already been disrupted and identifies a person at increased risk of a subsequent stroke. The risk is especially important shortly after a TIA, which is why suspected TIA requires urgent assessment rather than being treated as an episode that has simply resolved. Investigations may include blood pressure measurement, blood tests, an ECG to detect atrial fibrillation, carotid ultrasound and, when appropriate, brain imaging. Identifying the underlying cause allows clinicians to target prevention through antiplatelet or anticoagulant therapy, blood-pressure treatment, cholesterol reduction, surgery or other measures when indicated.
Many strokes result from an interaction between long-term vascular risk and an immediate event such as clot formation, embolisation or vessel rupture. Some risk factors cannot be changed, including age and inherited predisposition, but many important contributors can be modified. Stopping smoking, maintaining a healthy diet, exercising, limiting alcohol and controlling blood pressure, cholesterol, diabetes and atrial fibrillation can reduce risk. Prevention does not eliminate every stroke because some occur despite careful risk-factor management, but addressing established cardiovascular risks is a major component of reducing both first and recurrent strokes.
