A stroke happens when blood flow to part of your brain gets cut off or when a blood vessel in your brain ruptures. Without a steady supply of oxygen-rich blood, brain cells start dying fast, roughly 1.9 million every minute the stroke goes untreated. There are two main types, and they happen through very different mechanisms.
Blocked Blood Flow: Ischemic Stroke
About 87% of all strokes are ischemic, meaning a clot or blockage stops blood from reaching part of the brain. This can happen in two ways.
The first is a clot that forms right inside a brain artery. Over years, cholesterol, fat, and blood cells build up along the inner walls of your arteries in a process called atherosclerosis. These deposits, called plaques, narrow the artery and can eventually trigger a clot at the site. This is especially common in the carotid arteries, the two large arteries on either side of your neck that supply blood to your brain. A carotid artery clogged with plaque can become so narrow that blood flow to the brain slows to a trickle, or a piece of plaque can break off and travel deeper into the brain where it lodges in a smaller artery.
The second mechanism is an embolism, where a clot forms somewhere else in the body (usually the heart) and travels through the bloodstream until it gets stuck in a brain artery. The most common cardiac source is atrial fibrillation, an irregular heart rhythm that lets blood pool and clot inside the heart’s upper chambers. Other high-risk conditions include recent heart attacks, heart valve problems, and infections of the heart lining.
A third, less common pattern involves the tiny arteries deep inside the brain. Chronic high blood pressure damages these small vessels over time, thickening and weakening their walls until they close off entirely. These “small vessel” strokes tend to be smaller but can still cause significant damage.
Burst Blood Vessels: Hemorrhagic Stroke
Hemorrhagic strokes account for the remaining cases and happen when a blood vessel in or around the brain breaks open. Blood spills into the surrounding tissue, creating pressure that damages brain cells both directly and by cutting off normal circulation.
The most common type is intracerebral hemorrhage, where bleeding occurs directly inside the brain tissue. High blood pressure is the leading cause. Years of elevated pressure weaken artery walls until they eventually give way. The second type, subarachnoid hemorrhage, involves bleeding in the space between the brain and the thin membranes covering it. This is often caused by a ruptured aneurysm, a balloon-like bulge in a weakened artery wall that can burst without warning.
Arteriovenous malformations (AVMs), which are tangles of abnormal blood vessels present from birth, can also rupture inside the brain. Head trauma is another cause, particularly in older adults or people on blood-thinning medications.
The Major Risk Factors
High blood pressure is the single biggest risk factor for both types of stroke. Current guidelines set 130/80 mm Hg as the threshold for treatment to prevent a first stroke. Every point above that range adds incremental risk because sustained pressure damages artery walls throughout the body, accelerating plaque buildup in some vessels and weakening walls in others.
Diabetes significantly raises ischemic stroke risk through several overlapping pathways. High blood sugar damages the inner lining of blood vessels, promotes inflammation, and makes the blood more prone to clotting. People with diabetes also tend to have higher rates of high blood pressure and abnormal cholesterol, which compound the damage.
Other well-established risk factors include smoking, which accelerates atherosclerosis and makes blood stickier; high cholesterol, which fuels plaque growth; obesity; physical inactivity; and heavy alcohol use. Atrial fibrillation deserves special mention because it increases stroke risk roughly fivefold, and many people have it without knowing.
Strokes in Younger Adults
Strokes aren’t limited to older people. When someone under 50 has an ischemic stroke and doctors can’t find an obvious cause, one common culprit is a patent foramen ovale (PFO), a small hole between the upper chambers of the heart that most people are born with but that normally closes in infancy. In about half of young adults with unexplained strokes, a PFO is present. Roughly 60% of these otherwise unexplained strokes can be attributed to the opening, which allows a clot from the venous system to cross into the arterial side and travel to the brain. For people under 60 whose stroke is linked to a PFO, current guidelines recommend a procedure to close the hole.
Cervical artery dissection is another cause seen more often in younger people. A tear in the lining of an artery in the neck (sometimes from trauma, sometimes spontaneous) creates a flap that disrupts blood flow and can generate clots.
The Warning: Transient Ischemic Attack
A transient ischemic attack (TIA), sometimes called a mini-stroke, produces stroke-like symptoms that resolve within minutes to hours. The temporary blockage clears on its own, but a TIA is a serious warning signal. In the first 90 days after a TIA, the risk of a full ischemic stroke is nearly 20 times higher than in the general population, with estimates ranging from 3% to 20% depending on other risk factors. Many of those strokes happen within the first few days.
Recognizing a Stroke
Speed matters more than almost anything else. The acronym BE-FAST covers the major warning signs:
- Balance: Sudden loss of balance or coordination, trouble walking
- Eyes: Sudden vision loss in one or both eyes, or double vision
- Face: One side of the face droops when you try to smile
- Arm: One arm drifts downward when you raise both arms
- Speech: Slurred or garbled speech, difficulty repeating a simple sentence
- Time: Call emergency services immediately if any of these signs appear
The older FAST acronym (Face, Arm, Speech, Time) misses strokes whose primary symptoms are balance problems or vision changes. Adding the B and E catches a larger share of cases. Symptoms typically appear suddenly and without pain, which is part of what makes them easy to dismiss or explain away. Any combination of these signs, even if they seem to improve, warrants emergency evaluation.