A stroke happens when blood flow to part of the brain is cut off or when a blood vessel in the brain bursts. Without a steady supply of oxygen and glucose, brain cells start dying within minutes. Stroke is the second leading cause of death worldwide, killing roughly 7 million people each year, and it remains one of the top causes of long-term disability.
How a Stroke Damages the Brain
Your brain cells are extraordinarily hungry for oxygen and fuel. The moment blood flow drops, those cells lose the energy they need to function. This triggers a chain reaction: the cells’ internal chemistry falls apart, toxic byproducts build up, and a flood of calcium rushes in, activating enzymes that destroy the cell from the inside. Surrounding cells can be damaged by the same toxic cascade, which is why the area of brain injury often extends beyond the initial zone of blocked or disrupted blood flow.
The type and severity of damage depend on which part of the brain is affected and how long blood flow is interrupted. A stroke in the area controlling speech may leave someone unable to form words. One affecting the motor cortex can paralyze a hand, arm, or entire side of the body. Time is the critical variable: the faster blood flow is restored, the more brain tissue survives.
Types of Stroke
Ischemic Stroke
About 87% of all strokes are ischemic, meaning a blood clot blocks an artery supplying the brain. The clot can form in two ways. In one scenario, fatty deposits (plaque) build up inside a blood vessel in the brain or neck and eventually narrow it enough to trap a clot. In the other, a clot forms somewhere else in the body, often the heart, breaks loose, and travels through the bloodstream until it lodges in a brain artery.
Hemorrhagic Stroke
The remaining 13% of strokes are hemorrhagic, caused by a blood vessel in the brain rupturing and bleeding into or around brain tissue. The blood itself puts pressure on surrounding cells, and the areas downstream from the burst vessel lose their blood supply. High blood pressure is the most common driver. In cases involving bleeding around the brain’s surface, a ruptured aneurysm (a weak, ballooned-out spot on an artery wall) is responsible roughly 85% of the time. Less common causes include abnormal tangles of blood vessels and clotting disorders.
Transient Ischemic Attack (TIA)
Sometimes called a “mini-stroke,” a TIA produces stroke-like symptoms that last anywhere from a few minutes to 24 hours before resolving on their own. The blockage is temporary, and no permanent brain damage occurs. But a TIA is a serious warning sign that a full stroke may follow, and it calls for urgent medical evaluation.
What Causes Blood Clots and Bleeds
The most common source of clots is carotid artery disease, where plaque accumulates in the large arteries on either side of the neck that feed the brain. A piece of plaque or a clot that forms on its rough surface can break free and travel into the brain. Heart conditions are the other major source. Atrial fibrillation, an irregular heart rhythm affecting millions of people, allows blood to pool in the heart’s upper chambers and form clots that can then reach the brain. Heart valve disease and coronary heart disease raise clot risk as well.
Hemorrhagic strokes most often result from years of uncontrolled high blood pressure, which weakens artery walls until one gives way. Aneurysms can be present from birth or develop over time, and smoking, high blood pressure, and a family history of aneurysms all increase the chance that one will rupture.
Risk Factors You Can Control
Over 90% of the global stroke burden is linked to modifiable risk factors, meaning most strokes are, in principle, preventable. High blood pressure stands well above everything else: it accounts for an estimated 57% of the overall disability caused by stroke worldwide. After that, the biggest contributors are air pollution, smoking, high cholesterol, a high-sodium diet, high blood sugar, and kidney problems.
Sodium intake offers a concrete example of how diet shapes risk. People who consume 4,000 milligrams or more of sodium per day have roughly 2.5 times the stroke risk of those who stay at or below 1,500 milligrams. For every additional 500 milligrams per day, stroke risk climbs about 17%. The American Heart Association recommends keeping sodium under 1,500 milligrams daily for the best cardiovascular protection, though the general dietary guideline is 2,300 milligrams.
Physical inactivity, obesity, and heavy alcohol use round out the list. The encouraging takeaway is that addressing even a few of these factors, particularly blood pressure and smoking, dramatically lowers your lifetime risk.
Risk Factors You Cannot Control
Stroke incidence rises steadily with age across all groups. Sex plays a role, too: men face a higher age-adjusted stroke risk than women through most of adulthood, though that gap narrows and can reverse after about age 80. Ethnicity matters as well. Research from the Northern Manhattan Study found that Mexican Americans had higher rates of both ischemic stroke and brain hemorrhage compared with non-Hispanic white adults, particularly at younger ages. A family history of stroke or aneurysm also raises your baseline risk.
Recognizing Stroke Symptoms
The most widely taught recognition tool is the acronym BE FAST:
- Balance: sudden loss of balance or coordination
- Eyes: sudden trouble seeing in one or both eyes
- Face: one side of the face droops or feels numb
- Arm: one arm drifts downward when you try to raise both
- Speech: slurred or garbled words, or difficulty understanding others
- Time: call emergency services immediately
The older version of this acronym, FAST, omits the balance and eye symptoms. That matters because strokes affecting the back of the brain (which controls balance and vision) are missed in up to 14% of cases when only FAST criteria are used. If you notice any of these signs in yourself or someone else, even if they seem mild or come and go, treat it as an emergency.
Why Speed of Treatment Matters
For ischemic strokes, two main treatments exist: a clot-dissolving medication given through an IV, and a procedure in which doctors physically remove the clot using a catheter threaded through a blood vessel. The clot-dissolving drug works best when given within a few hours of symptom onset. The catheter-based procedure can be effective in select patients up to 16 hours after symptoms begin, and in some cases up to 24 hours, depending on how much brain tissue is still salvageable on imaging.
Hemorrhagic strokes require a different approach focused on controlling the bleeding, reducing pressure inside the skull, and stabilizing blood pressure. In some cases, surgery is needed to repair the ruptured vessel or drain accumulated blood. For both stroke types, every minute of delay translates to more brain tissue lost, which is why the phrase “time is brain” has become a guiding principle in emergency medicine.