The three main causes of strokes are blood clots that form in the brain’s arteries, blood clots that travel to the brain from elsewhere in the body, and bleeding from a ruptured blood vessel in the brain. The first two are types of ischemic stroke, which accounts for about 65% of all strokes worldwide. The third is hemorrhagic stroke, responsible for roughly 35%. Together, these mechanisms cause nearly 12 million new strokes each year globally.
Blood Clots That Form in the Brain (Thrombotic Stroke)
The most common type of stroke happens when a clot builds up directly inside an artery that supplies the brain. This is called a thrombotic stroke, and it almost always starts with atherosclerosis, the gradual buildup of fatty plaque along artery walls. Over years, cholesterol, calcium, and other substances accumulate and narrow the artery. Eventually, a clot forms at the site of that plaque and blocks blood flow completely.
The carotid arteries in the neck are a frequent trouble spot. These two large arteries carry blood from the heart to the brain, and plaque tends to collect where they branch. When the narrowing reaches 70% or more, the five-year stroke risk climbs significantly. People with 80 to 99% blockage face roughly 2.5 times the stroke risk of those with moderate narrowing. Below 70%, the five-year risk on modern medical treatment stays under 5%, but severe narrowing pushes it to around 15%.
High blood pressure is the single biggest driver of this process. Chronically elevated pressure damages artery walls, making them stiffer and more prone to plaque buildup. Diabetes roughly doubles the risk of ischemic stroke. Smoking nearly doubles it as well, and having both diabetes and a smoking habit together raises stroke risk by about 2.6 times compared to someone with neither.
Blood Clots That Travel to the Brain (Embolic Stroke)
In an embolic stroke, the clot forms somewhere else in the body and then travels through the bloodstream until it lodges in a brain artery too narrow for it to pass through. The most common origin is the heart.
Atrial fibrillation, an irregular heartbeat affecting millions of people, is the leading cardiac cause. When the heart’s upper chambers quiver instead of contracting fully, blood pools and moves sluggishly. That stagnant blood is prone to clotting. Once a clot breaks loose, it can travel up to the brain in seconds. People with atrial fibrillation have three to five times the stroke risk of people with a normal heart rhythm, even after accounting for other risk factors.
Clots can also originate from plaque in the carotid arteries. A piece of plaque or a small clot breaks off and gets carried upstream into smaller brain arteries. Less commonly, clots form in leg veins and reach the brain through a small hole between the heart’s upper chambers called a patent foramen ovale, which about one in four people have without knowing it. Rare causes include fat or air bubbles entering the bloodstream.
Bleeding in the Brain (Hemorrhagic Stroke)
Hemorrhagic stroke occurs when a blood vessel in the brain bursts, spilling blood into surrounding tissue. The bleeding itself damages brain cells, and the pooling blood creates pressure that damages even more. Though hemorrhagic strokes make up about 35% of cases, they carry the highest death rate of any stroke type.
There are two subtypes. Intracerebral hemorrhage, which accounts for roughly 29% of all strokes, happens when a vessel deep in the brain ruptures. Subarachnoid hemorrhage, about 6% of strokes, involves bleeding on the brain’s surface, typically from a burst aneurysm (a balloon-like weak spot in an artery wall).
Hypertension is the leading cause of hemorrhagic stroke by a wide margin. Years of high blood pressure gradually weaken the walls of small arteries in the brain. The muscle layer of these tiny vessels degenerates and is replaced by scar-like tissue. Microscopic bulges called microaneurysms form along the weakened walls. When pressure spikes or the wall deteriorates enough, these microaneurysms rupture. The symptoms tend to be sudden and severe: an explosive headache, vomiting, neck stiffness, and rapidly worsening neurological problems.
Other causes include vascular malformations (tangles of abnormal blood vessels present from birth), blood-thinning medications, and a condition called cerebral amyloid angiopathy, where a protein deposits along brain artery walls and weakens them over time. This last cause is more common in older adults and is a frequent reason for hemorrhagic stroke in people over 65.
Warning Signs Before a Full Stroke
A transient ischemic attack, often called a mini-stroke, produces the same symptoms as a full stroke but resolves on its own, usually within five minutes. The clot dissolves or dislodges before it causes permanent damage. Symptoms can include sudden numbness on one side, confusion, trouble speaking, vision loss, or loss of balance.
A TIA is not a minor event. Nearly 1 in 5 people who have a suspected TIA will have a full stroke within 90 days. Even more striking, 2 in 5 people diagnosed with a TIA turn out to have already had an actual stroke when they receive proper brain imaging. The brief duration of symptoms makes TIAs easy to dismiss, but they are one of the clearest warning signals the body produces.
Why Treatment Timing Matters
For ischemic strokes, the standard clot-dissolving treatment works best when given within four hours of symptom onset. A mechanical procedure to physically remove the clot can be effective up to six hours after symptoms start, and in select patients whose brain imaging shows salvageable tissue, this window can extend to 24 hours. Every minute of delay costs brain cells, which is why stroke symptoms always warrant an immediate call to emergency services.
Hemorrhagic strokes require a different approach. The priority is reducing blood pressure quickly but carefully toward a target of about 140 mmHg systolic, while avoiding drops that are too steep or too fast. In some cases, surgery is needed to relieve pressure from accumulated blood.
The Risk Factors You Can Change
All three stroke types share overlapping risk factors, and many of them are modifiable. High blood pressure is the dominant one, contributing to both clot formation and vessel rupture. Keeping blood pressure in a healthy range does more to prevent stroke than any single intervention.
Diabetes increases the risk of ischemic stroke by about 2.3 times and hemorrhagic stroke by about 1.6 times. Smoking raises ischemic stroke risk nearly twofold on its own. When smoking and diabetes are present together, the risks don’t just add up; they multiply, pushing overall stroke risk to more than 2.6 times normal. Atrial fibrillation, high cholesterol, physical inactivity, and heavy alcohol use round out the major modifiable risks. Identifying and managing even one of these factors meaningfully lowers the chance of all three stroke types.