An ischemic stroke happens when a blood vessel supplying the brain becomes blocked, cutting off oxygen and nutrients to brain tissue. About 65% of all strokes worldwide are ischemic, making it by far the most common type. The blockage is almost always caused by a blood clot, but where that clot forms and why it forms varies significantly from person to person.
How a Blocked Artery Damages the Brain
When blood flow to part of the brain drops sharply, the affected neurons begin to die within minutes. These cells at the core of the blockage are essentially beyond rescue. But surrounding that dead core is a larger zone of brain tissue called the ischemic penumbra, where blood flow is reduced but not completely cut off. This tissue can survive for several hours on marginal blood supply, and it’s the primary target of emergency stroke treatment.
At the cellular level, the damage unfolds in a chain reaction. Without oxygen, brain cells can no longer produce energy, and the pumps that regulate sodium, calcium, and water across cell membranes start to fail. Sodium floods in, dragging water with it and causing the cells to swell. Calcium follows, triggering the release of massive amounts of a signaling chemical called glutamate. That glutamate overstimulates neighboring neurons, which then flood with calcium themselves, amplifying the damage outward from the original blockage site. This cascade also generates free radicals that tear apart cell membranes and other critical structures.
Within four to six hours, the protective barrier between the bloodstream and the brain starts to break down. Fluid leaks into surrounding tissue, producing swelling that peaks around three to five days after the stroke and gradually resolves over several weeks. In the hours and days following the initial event, inflammatory genes activate, further compromising tiny blood vessels in the area.
Blood Clots From Atherosclerosis
The single most common pathway to ischemic stroke involves atherosclerosis: the slow buildup of fatty, cholesterol-rich deposits (plaques) inside artery walls. These plaques typically develop over decades in the carotid arteries of the neck or in major arteries inside the skull. The stroke itself usually isn’t caused by the plaque gradually narrowing the artery. Instead, the plaque ruptures. When it does, the body treats the exposed plaque like an open wound, rapidly forming a blood clot at the site.
That clot can block the artery right where it formed, or pieces of it can break off and travel upstream into smaller brain arteries, lodging wherever the vessel becomes too narrow to pass through. In some cases, fragments of the plaque itself break free and embolize to the brain without a clot forming first. Thromboembolism from ruptured plaque is the primary stroke mechanism in most patients with large artery atherosclerotic disease.
Clots That Form in the Heart
The second major source of brain-blocking clots is the heart itself. Atrial fibrillation, an irregular heart rhythm that affects millions of people, is the classic culprit. When the upper chambers of the heart quiver instead of contracting fully, blood pools and stagnates, particularly in a small pouch called the left atrial appendage. Stagnant blood clots easily, and when a piece of that clot breaks free, it travels directly up through the carotid arteries into the brain.
Clots originating from the heart tend to be larger than those from carotid plaque, which is why cardioembolic strokes are often more severe. They’re more likely to block a major intracranial artery, destroying a larger area of brain tissue. Other heart conditions that can send clots to the brain include recent heart attacks (where clots form on damaged heart wall tissue), prosthetic heart valves, and structural heart defects like a patent foramen ovale, a small hole between the upper chambers that most people never know they have.
Small Vessel Disease and Lacunar Strokes
Not all ischemic strokes involve large arteries or traveling clots. Deep inside the brain, tiny penetrating arteries branch off major vessels to supply structures critical for movement, sensation, and coordination. These vessels are so small that disease in their walls alone can seal them shut.
The most common process here is called lipohyalinosis: years of high blood pressure cause the walls of these tiny arteries to thicken, stiffen, and eventually close. Hypertension drives the muscle layer of these vessels to overgrow, while fatty deposits accumulate beneath the inner lining and gradually choke off the opening. The resulting strokes, called lacunar infarcts, are small (less than 20 mm across) and occur in deep brain regions. They may cause very specific symptoms, like weakness on one side of the body or numbness, without affecting language or vision. Despite their small size, they can be disabling, and having one significantly raises the risk of having more.
The Role of High Blood Pressure
Hypertension is the single largest modifiable risk factor for ischemic stroke, and it contributes to every mechanism described above. Chronically elevated blood pressure accelerates atherosclerosis in large arteries, damages the tiny vessels deep in the brain, and promotes the kind of heart remodeling that leads to atrial fibrillation. Data from large population studies show that for every 10-point increase in systolic blood pressure, stroke risk rises by about 9%. That relationship is continuous: there’s no sharp cutoff below which blood pressure stops mattering.
Diabetes and High Cholesterol
Chronically elevated blood sugar damages blood vessels through several overlapping mechanisms. High glucose increases the production of clotting factors in local capillaries, making small clots more likely. It also triggers inflammation along vessel walls, attracting immune cells that release enzymes capable of breaking down the blood-brain barrier. On top of that, excess glucose generates reactive oxygen species, aggressive molecules that damage proteins, enzymes, and DNA inside cells. Over time, diabetes also impairs the energy-producing structures within cells (mitochondria), weakening their built-in defenses against oxidative damage.
Abnormal cholesterol levels feed directly into atherosclerosis. Elevated LDL cholesterol infiltrates artery walls and forms the fatty core of plaques. These plaques narrow arteries, attract inflammatory cells, and eventually rupture to trigger the clot-and-embolism sequence that causes most large-artery strokes. The combination of diabetes, high blood pressure, and abnormal lipids is especially dangerous because these conditions reinforce each other at every step of the process.
Causes in Younger Adults
Ischemic stroke in people under 50 often has a different profile. While atherosclerosis and heart disease still play a role, a much larger share of strokes in this age group result from arterial dissection. Cervical artery dissection accounts for about 2% of all ischemic strokes overall but up to 25% in adults under 50.
A dissection occurs when the inner lining of an artery in the neck tears, allowing blood to seep into the vessel wall. The trapped blood forms a bulge that narrows or blocks the artery, and clots that form at the tear site can break off and travel to the brain. The causes are often a combination of genetic predisposition (including connective tissue disorders), anatomic quirks like unusual artery curvature, and a triggering event. That trigger can be surprisingly minor: a chiropractic neck manipulation, a roller coaster ride, vigorous sports, or even forceful coughing or sneezing.
Inherited Clotting Disorders
Some people carry genetic mutations that make their blood clot more readily than normal. A meta-analysis published in the Journal of the American Heart Association found that deficiencies in protein C and protein S, natural anticoagulants the body produces, roughly doubled the odds of ischemic stroke. A common genetic variant called Factor V Leiden raised stroke odds by about 23% in people carrying one copy of the mutation, while a mutation in the prothrombin gene increased odds by roughly 41%. These inherited conditions are individually uncommon, but they’re worth investigating in younger stroke patients or those with a family history of abnormal clotting.
When No Cause Is Found
Despite thorough testing, doctors cannot identify the cause of about 17% of ischemic strokes on average. These are classified as cryptogenic strokes. A subset of these, called embolic strokes of undetermined source (ESUS), show a pattern on brain imaging that strongly suggests a clot traveled from somewhere, but the source can’t be pinpointed. To qualify as ESUS, the stroke must not be a small deep-brain lacunar type, there must be no significant artery narrowing feeding the affected brain area, and no obvious heart condition to explain the clot. Cardiac monitoring for at least 24 hours is required as part of the workup.
Many cryptogenic strokes are eventually attributed to undetected atrial fibrillation. The irregular rhythm can be intermittent, appearing for only minutes or hours at a time, which makes it easy to miss on standard testing. Extended heart monitoring over weeks or months catches many of these cases. Other hidden sources include small holes in the heart, aortic plaques, and cancer-related clotting states that weren’t initially suspected.