Strokes happen when blood flow to part of the brain is cut off, either by a blockage or by bleeding. About 87% of all strokes are caused by blood clots that block an artery supplying the brain. The rest are caused by blood vessels that rupture and bleed into or around brain tissue. The underlying reasons for both types range from common conditions like high blood pressure to rarer causes like inherited genetic disorders.
Ischemic Stroke: Blocked Blood Flow
The vast majority of strokes occur when a clot lodges in a blood vessel feeding the brain. This can happen in two main ways. In the first, fatty deposits gradually build up inside an artery wall, narrowing it over years until blood flow slows enough for a clot to form at the site. In the second, a clot forms somewhere else in the body, breaks loose, and travels through the bloodstream until it reaches a brain artery too small to pass through. That traveling clot can originate in the heart, the large arteries of the neck, or elsewhere in the circulatory system.
A third mechanism involves the tiny blood vessels deep inside the brain. Chronic high blood pressure or diabetes can damage these small vessels over time, thickening their walls and restricting flow. This type of damage tends to cause smaller strokes in deeper brain structures, but can still produce significant disability.
High Blood Pressure: The Leading Risk Factor
Persistently elevated blood pressure is the single most important modifiable cause of both clot-based and bleeding-based strokes. Stage 1 hypertension begins at a systolic reading of 130 or a diastolic of 80. Stage 2 hypertension, defined as systolic 140 or higher or diastolic 90 or higher, carries substantially more risk. The damage is cumulative: years of excess pressure weaken artery walls, accelerate fatty buildup, and make small vessels in the brain fragile enough to rupture.
Blood pressure during pregnancy deserves special attention. Systolic pressure at or above 160, or diastolic at or above 110, during pregnancy or in the six weeks after delivery significantly raises the risk of fatal bleeding in the brain.
Atrial Fibrillation and Heart-Related Causes
Atrial fibrillation, an irregular heart rhythm affecting millions of adults, increases stroke risk roughly fivefold. When the heart’s upper chambers quiver instead of contracting fully, blood pools and clots can form. If a clot escapes the heart and reaches the brain, it often blocks a major artery, causing a large and disabling stroke.
Other heart conditions contribute as well. Damaged or artificial heart valves, recent heart attacks, and heart failure can all create conditions where clots form more readily. Any structural problem that allows blood to stagnate inside the heart raises the chance of a clot traveling to the brain.
How Diabetes Damages Blood Vessels
Chronically high blood sugar injures blood vessels through several overlapping pathways. Excess glucose triggers the production of molecules called reactive oxygen species, which damage the inner lining of arteries, alter blood flow in the brain, and eventually kill cells. High glucose also increases the body’s production of a substance called tissue factor, which promotes clotting in small capillaries. On top of that, diabetes fuels chronic inflammation in vessel walls and impairs the energy-producing structures inside cells, leaving brain tissue more vulnerable when blood flow drops even briefly.
These effects make strokes in people with diabetes not only more likely but often more severe, because the brain tissue has less capacity to tolerate reduced blood flow.
Hemorrhagic Stroke: Bleeding in the Brain
When a blood vessel in or around the brain ruptures, the resulting bleed damages tissue both directly and by increasing pressure inside the skull. Two main types exist. Intracerebral hemorrhage occurs when a vessel inside the brain bursts, most commonly due to long-standing high blood pressure that has weakened small arteries. In younger patients, the rupture often involves a vascular malformation: a tangle of poorly formed blood vessels where arteries connect directly to veins without the normal network of tiny capillaries in between.
Subarachnoid hemorrhage occurs when bleeding happens in the space surrounding the brain, usually from a ruptured aneurysm. An aneurysm is a weak, balloon-like bulge in an artery wall. Many people carry small aneurysms without knowing it. When one ruptures, the result is sudden, severe bleeding that constitutes a medical emergency.
Smoking, Lifestyle, and Controllable Risks
Smoking accelerates the hardening and narrowing of arteries throughout the body, including those supplying the brain. It also makes blood stickier and more likely to clot. The good news is that the damage is largely reversible: five years after quitting, your stroke risk drops to the same level as someone who never smoked.
Physical inactivity, heavy alcohol use, and obesity each independently raise stroke risk. Obesity promotes high blood pressure, diabetes, and inflammation simultaneously. Heavy drinking can trigger atrial fibrillation and raise blood pressure. These factors often cluster together, compounding each other’s effects.
Strokes in Younger Adults
When someone in their 20s, 30s, or 40s has a stroke, the cause is often different from the typical risk factors seen in older adults. One of the more common culprits is carotid artery dissection, a tear in the inner wall of one of the large arteries in the neck that supply the brain. Blood seeps between the layers of the artery wall, narrowing or blocking the vessel. This can happen after obvious trauma like a car accident or sports injury, but also after something as minor as forceful coughing, aggressive nose blowing, or sudden neck rotation. In many cases, no clear trigger is ever found.
Inherited conditions also play an outsized role in younger stroke patients. CADASIL, caused by mutations in a gene called NOTCH3, progressively damages small blood vessels in the brain and is one of the most common genetic causes of stroke. Sickle cell disease, in which red blood cells become rigid and sticky, can block brain arteries in children and young adults. These genetic conditions are especially important to identify because they affect treatment decisions and have implications for family members.
Cryptogenic Stroke: When the Cause Is Hidden
About 25% of ischemic strokes are classified as cryptogenic, meaning no clear cause is found after standard testing. In a significant portion of these cases, the culprit turns out to be a patent foramen ovale, or PFO. This is a small opening between the two upper chambers of the heart that normally closes shortly after birth but remains open in roughly one in four adults.
Most people with a PFO never have problems. But in some cases, a blood clot that forms in the veins (typically in the legs) can slip through the opening directly into the arterial circulation and travel to the brain, bypassing the lungs where it would normally be filtered out. The average PFO is about 10 millimeters wide, large enough for a clot to pass through and block a major brain artery. Recent research suggests that clots may also form within the PFO itself.
Not all PFOs carry the same risk. Larger openings and those accompanied by an atrial septal aneurysm (a floppy, mobile section of the wall between the heart’s upper chambers) are more strongly linked to stroke. For patients with both a high-risk PFO and a profile suggesting paradoxical embolism, closing the PFO with a catheter-based procedure reduces the chance of another stroke by approximately 90%. For patients whose PFO appears incidental, closure may offer little benefit, making accurate risk classification essential.