A stroke happens when blood flow to part of the brain is cut off or when a blood vessel in the brain bursts. In both cases, brain cells begin dying within minutes from lack of oxygen. Roughly 12 million new strokes occur worldwide each year, making it the second leading cause of death globally. The causes range from preventable conditions like high blood pressure to hidden heart rhythm problems and rare genetic disorders.
The Two Main Types of Stroke
Every stroke falls into one of two broad categories, and the underlying cause depends on which type occurs. An ischemic stroke happens when a blood clot blocks an artery supplying the brain. This is by far the more common type, accounting for about 85% of all strokes. A hemorrhagic stroke happens when a weakened blood vessel ruptures and bleeds into or around the brain. Though less common, hemorrhagic strokes tend to be more deadly.
These aren’t just clinical labels. The distinction matters because the causes, warning signs, and treatments differ significantly between the two.
How Blood Clots Cause Ischemic Strokes
Blood clots reach the brain through two main routes. The first is a clot that forms directly inside an artery already narrowed by plaque buildup, a process called atherosclerosis. The carotid arteries, the two large vessels on either side of your neck, are a common trouble spot. When fatty deposits in these arteries crack or erode, the body tries to repair the damage by forming a clot at the site. That clot can grow large enough to block the artery entirely, or pieces can break off and travel deeper into the brain’s blood vessels.
A carotid artery narrowed by 50% or more is considered clinically significant, though even less severe blockages can trigger a stroke if plaque ruptures at the wrong moment. When a clot completely blocks the internal carotid artery, the resulting brain damage tends to be extensive and difficult to treat with clot-dissolving medications.
The second route is a clot that forms somewhere else in the body, usually the heart, and travels to the brain. This is called an embolism. The clot lodges in a smaller brain artery, cutting off blood flow to whatever tissue that artery feeds. The heart is the most common source of these traveling clots, particularly in people with irregular heart rhythms.
Atrial Fibrillation: A Major Hidden Cause
Atrial fibrillation, an irregular and often rapid heart rhythm, is one of the most important causes of ischemic stroke. When the upper chambers of the heart quiver instead of contracting normally, blood pools and moves sluggishly. That stagnant blood is prone to clotting. If a clot forms and gets pumped out of the heart, it can travel straight to the brain.
People with atrial fibrillation face a three- to five-fold higher risk of stroke compared to people with normal heart rhythm, even after accounting for other risk factors. What makes this especially dangerous is that atrial fibrillation often produces no obvious symptoms. Many people don’t know they have it until a stroke occurs.
High Blood Pressure Damages Arteries on Both Sides
Hypertension is the single most important modifiable risk factor for stroke, and it contributes to both ischemic and hemorrhagic types. The damage is cumulative and largely silent.
When blood pressure stays elevated over months and years, the constant mechanical force batters the walls of small arteries deep in the brain. The smooth muscle in those walls thickens, the inner lining becomes inflamed, and the vessels gradually stiffen and narrow. This process reduces blood flow and can cause small blockages called lacunar infarcts, a form of ischemic stroke that often affects areas controlling movement, sensation, or coordination.
At the same time, that sustained pressure weakens vessel walls in a different way. It breaks down the structural layers that give arteries their strength, creating tiny bulges called microaneurysms. These fragile spots are prone to rupturing, especially during sudden blood pressure spikes. Hypertension is the leading cause of hemorrhagic stroke, and the bleeding typically occurs in deep brain structures like the basal ganglia, where the smallest and most vulnerable arteries are concentrated.
What Causes a Brain Bleed
Beyond hypertension, hemorrhagic strokes can result from ruptured aneurysms, which are balloon-like weak spots in larger brain arteries. When an aneurysm bursts, blood spills into the space surrounding the brain rather than into the brain tissue itself. This is called a subarachnoid hemorrhage, and it often strikes suddenly with an explosive headache described as the worst of a person’s life.
Other causes of brain bleeding include abnormal tangles of blood vessels (arteriovenous malformations), inflammation of blood vessel walls, tears in artery linings, and blood-thinning medications that tip the balance too far toward bleeding. Some hemorrhagic strokes occur in people with no identifiable risk factors at all.
Diabetes, Cholesterol, and Other Risk Factors
Type 2 diabetes roughly doubles the risk of stroke. A large meta-analysis covering more than 775,000 people found that diabetes increased stroke risk by 83% in men and 128% in women. The reasons are multifaceted: diabetes accelerates atherosclerosis, promotes inflammation in blood vessel walls, and makes blood more prone to clotting. Women with diabetes face a particularly elevated risk compared to men with the same condition, with about 27% greater relative risk.
High cholesterol feeds the plaque deposits that narrow arteries. Smoking damages the inner lining of blood vessels, makes blood stickier, and raises blood pressure. Obesity and physical inactivity contribute to nearly every other risk factor on this list. Heavy alcohol use raises blood pressure and increases the chance of hemorrhagic stroke. These factors rarely act alone. They compound each other, and the more you have, the steeper your risk climbs.
Warning Strokes and What They Signal
A transient ischemic attack, often called a mini-stroke or TIA, produces the same symptoms as a full stroke (sudden numbness, confusion, vision loss, trouble speaking) but resolves within minutes to hours as the clot dissolves on its own. The brain recovers, but the warning is serious.
About 5% of people who have a TIA will have a full stroke within the first month. That number rises to 12% within the first year and roughly 30% within five years. A TIA means the underlying cause, whether it’s a narrowed artery, a heart rhythm problem, or something else, is still present and active.
Strokes With No Obvious Cause
About 3 in 10 strokes are classified as cryptogenic, meaning no clear cause is found after standard testing. This is frustrating for patients and physicians alike, but ongoing investigation often uncovers a hidden culprit.
One common finding is a patent foramen ovale, or PFO, a small hole between the upper chambers of the heart that normally closes shortly after birth but remains open in roughly a quarter of the population. Most people with a PFO never have problems, but the opening can allow a blood clot from the venous system to cross into the arterial side and reach the brain. Up to half of people under 60 who have a cryptogenic stroke turn out to have a PFO. Undetected atrial fibrillation is another frequent explanation, often discovered only when patients wear heart monitors for days or weeks after the initial stroke.
Genetic and Rare Causes
Some strokes, particularly those occurring in younger adults or children, stem from inherited conditions that damage blood vessels over time.
Sickle cell disease causes red blood cells to become rigid and crescent-shaped, blocking small blood vessels and damaging artery walls. About 25% of people with sickle cell disease will have a stroke before age 45. Screening children with ultrasound to measure blood flow speed in brain arteries can identify those at highest risk, and preventive blood transfusions have been shown to reduce stroke occurrence by over 90% in high-risk children.
CADASIL is a genetic condition caused by mutations that affect the smooth muscle cells in small brain arteries. It leads to recurrent small strokes and cognitive decline, typically beginning in a person’s 30s or 40s. Up to 85% of people with symptomatic CADASIL experience strokes or TIAs over their lifetime. Because it runs in families with a dominant inheritance pattern, a single affected parent gives each child a 50% chance of inheriting the condition.
Fabry disease, an inherited enzyme deficiency, causes fatty substances to build up in blood vessel walls throughout the body, increasing stroke risk even in younger patients. These genetic causes are individually rare but collectively important, especially when a stroke occurs in someone young with no traditional risk factors.