A stroke happens when blood flow to part of your brain is suddenly cut off, either by a clot blocking an artery or by a blood vessel bursting. Without that blood supply, brain cells start dying fast: roughly 1.9 million neurons are destroyed every minute a stroke goes untreated, along with 14 billion connections between them. Understanding the different ways this can happen helps explain why strokes vary so widely in severity and why speed matters so much when one occurs.
The Two Main Types of Stroke
About 87% of strokes are ischemic, meaning a blood clot is blocking flow. The remaining cases are hemorrhagic, caused by a ruptured blood vessel that bleeds into or around the brain. The damage in both types comes from the same basic problem: brain tissue is being starved of oxygen. But the way each type starts, and what needs to happen to treat it, are very different.
How a Clot Blocks Blood Flow
Ischemic strokes begin with a blockage in one of the arteries feeding the brain. This can happen in two ways. In the first, called thrombosis, a clot forms directly inside a brain artery. This usually starts with atherosclerosis, a condition where cholesterol, fat, and other substances slowly build into a sticky plaque on the inner wall of an artery. Over years, that plaque narrows the artery. If the surface of the plaque cracks or ruptures, your body treats it like a wound and sends clotting factors to the site. The resulting clot can seal off the artery entirely.
In the second type, called embolism, a clot forms somewhere else in the body, breaks loose, and travels through the bloodstream until it lodges in a narrower brain artery. The most common source is the heart, particularly in people with atrial fibrillation, an irregular heart rhythm that affects millions of adults. When the upper chambers of the heart quiver instead of contracting fully, blood pools and moves sluggishly. That stagnant blood is far more likely to clot. Once a clot forms, a piece can break off and travel straight up the carotid arteries into the brain.
How a Blood Vessel Ruptures
Hemorrhagic strokes happen when a weakened blood vessel in the brain bursts open. The escaped blood compresses surrounding brain tissue and disrupts normal function in the area. One common cause is an aneurysm, a balloon-like bulge that forms where an artery wall has thinned. Aneurysms tend to develop at forks or branches in arteries because those spots bear more mechanical stress from blood flow. Over time, the pressure of blood pushing against a weak spot can stretch the wall further until it eventually tears.
High blood pressure is the single biggest contributor to this process. Sustained high pressure damages and weakens artery walls throughout the brain, making both aneurysm formation and rupture more likely. Cocaine use raises blood pressure sharply and unpredictably, which is why it’s a well-known trigger for hemorrhagic stroke even in younger people. Heavy alcohol use has a similar effect on blood pressure over time.
Another form of hemorrhagic stroke comes from arteriovenous malformations, tangles of abnormal blood vessels that can be present from birth. These vessels have thinner walls than normal and can leak or burst under pressure.
Strokes That Come From the Heart
Atrial fibrillation deserves special attention because it’s one of the most common and most preventable causes of stroke. The traditional explanation, dating back over a century, is straightforward: when the heart’s upper chambers fibrillate instead of beating in rhythm, blood doesn’t move through them efficiently. That stagnation creates the perfect conditions for clot formation. Once a clot reaches a certain size, a fragment can break free and travel to the brain in seconds.
What makes atrial fibrillation particularly dangerous is that many people don’t know they have it. The irregular rhythm can come and go, sometimes producing no noticeable symptoms. A person might have brief episodes for years without realizing it, all while clots are forming during those episodes.
Less Obvious Causes
About 25% of ischemic strokes are classified as cryptogenic, meaning no obvious cause is found on initial workup. In many of these cases, deeper investigation reveals a hidden source. One of the most common is a patent foramen ovale (PFO), a small hole between the upper chambers of the heart that never fully closed after birth. Roughly 25% of the general population has one, and most never know it.
The problem arises when a blood clot forms in a vein elsewhere in the body, typically the legs. Normally, that clot would travel to the lungs, where it would be filtered out. But with a PFO, the clot can pass directly through the hole into the left side of the heart and from there into the brain’s arterial system. Because the average PFO is about 10 millimeters wide, clots passing through it are often large enough to block a major brain artery. PFO and tears in the carotid artery (carotid dissection) are the two most frequent causes of stroke in younger adults.
What a “Mini-Stroke” Actually Is
A transient ischemic attack, commonly called a mini-stroke, involves the same blockage mechanism as a full ischemic stroke, but the clot dissolves or dislodges on its own before permanent damage occurs. Most symptoms disappear within an hour, though they can last up to 24 hours. The symptoms while they’re happening are identical to a full stroke: sudden numbness on one side, slurred speech, confusion, vision loss.
A TIA is a serious warning. It means the conditions for a full stroke are already present, whether that’s a narrowed artery, a clot-prone heart rhythm, or unstable plaque. Roughly 10 to 15% of people who have a TIA will have a full stroke within three months, with the highest risk in the first few days.
Why Minutes Matter
The phrase “time is brain” exists because the damage accelerates the longer a stroke goes untreated. At a rate of 1.9 million neurons per minute, even a 30-minute delay can mean the difference between a full recovery and lasting disability. Clot-dissolving treatment for ischemic stroke is most effective within 4.5 hours of symptom onset. Beyond that window, some patients may still be candidates for a procedure to physically remove the clot from the artery, which can be performed up to 24 hours after symptoms begin if brain imaging shows salvageable tissue.
The FAST acronym is the quickest way to recognize a stroke in yourself or someone else: Face drooping or numbness on one side, Arm weakness or numbness on one side, Speech that’s slurred or hard to understand, and Time to call emergency services immediately. Additional red flags include sudden loss of vision, sudden severe dizziness with loss of balance, or the worst headache of your life with no known cause.
What Raises Your Risk
High blood pressure is the leading modifiable risk factor for both types of stroke. It damages artery walls, accelerates plaque buildup, and increases the chance of a vessel rupture. Lowering blood pressure through lifestyle changes or medication directly reduces stroke risk.
Other major risk factors include diabetes, which damages blood vessels over time; smoking, which accelerates atherosclerosis and makes blood more likely to clot; high cholesterol, which feeds plaque growth; obesity; and physical inactivity. Age is a factor you can’t change: stroke risk roughly doubles every decade after age 55. A family history of stroke or heart disease also raises your baseline risk. The encouraging reality is that the majority of strokes are linked to modifiable risk factors, meaning changes in blood pressure management, diet, exercise, and smoking cessation can meaningfully shift the odds.