An ischemic stroke occurs when blood flow to part of the brain is blocked, usually by a clot. It is the most common type of stroke, accounting for about 87% of all strokes. More than 795,000 people in the United States have a stroke each year, and the vast majority are ischemic. When brain tissue is cut off from its blood supply, cells begin to die within minutes, which is why recognizing the signs and getting treatment fast can mean the difference between full recovery and permanent disability.
How a Blockage Forms
There are two main ways an ischemic stroke happens. In the first, a clot forms directly inside an artery that supplies the brain. This typically occurs at spots where fatty plaque has built up along the artery wall, narrowing the vessel until a clot seals it off. It tends to happen at branching points in the arteries or where one artery splits off from a larger one, because these are the areas most prone to plaque buildup.
In the second, a clot forms somewhere else in the body and travels through the bloodstream until it lodges in a brain artery. This is called an embolic stroke. The clot most often originates in the heart, particularly in people with an irregular heart rhythm called atrial fibrillation. When the heart doesn’t beat in a coordinated way, blood can pool in its chambers and form clots. Those clots can then break loose and travel to the brain. Less commonly, clots can form in veins elsewhere in the body and reach the brain through a small hole in the heart wall that didn’t fully close after birth, a condition called a patent foramen ovale.
In rarer cases, the blockage isn’t a blood clot at all. It can be a fragment of calcified tissue from a heart valve, a piece of a tumor, or even an air bubble. But the overwhelming majority of ischemic strokes involve blood clots.
Recognizing the Signs
Stroke symptoms appear suddenly. The American Stroke Association uses the acronym B.E. F.A.S.T. to help people spot them:
- Balance: Sudden loss of balance or coordination, trouble walking, or dizziness
- Eyes: Sudden blurred vision, double vision, or loss of sight in one or both eyes
- Face: One side of the face droops or feels numb
- Arm: One arm is weak or numb. If you ask the person to raise both arms, one drifts downward.
- Speech: Slurred or garbled speech, or difficulty understanding what others are saying
- Time: Call 911 immediately if any of these symptoms appear
A severe headache with no known cause, sudden confusion, and numbness or weakness on one side of the body are also warning signs. These symptoms can appear in any combination, and even a single one warrants an emergency call. It’s not possible to tell from symptoms alone whether a stroke is ischemic or hemorrhagic (caused by bleeding), and the treatments are very different, so getting to a hospital fast is critical.
How Doctors Diagnose It
The first imaging test is almost always a CT scan. CT is fast and widely available, and its primary job in the first minutes is to rule out a brain bleed. If there’s no bleeding, the stroke is presumed ischemic and treatment can begin immediately.
MRI is more sensitive at detecting early ischemic damage, pinpointing exactly where the affected tissue is, and distinguishing a true stroke from conditions that mimic one (like a seizure or severe migraine). MRI can also help estimate how long ago the stroke started in patients who woke up with symptoms or can’t say when they began. However, MRI takes longer to perform, so in most emergency departments, CT remains the first-line tool.
Emergency Treatment
The primary goal is to restore blood flow as quickly as possible. Every minute that passes without treatment, more brain tissue dies.
The first option is a clot-dissolving medication given through an IV. Current guidelines recommend this treatment for eligible patients within 3 to 4.5 hours of when symptoms started. The sooner it’s administered, the better the outcome. Not everyone qualifies: people on certain blood-thinning medications or those with very high blood pressure may not be candidates.
For strokes caused by a large clot blocking a major brain artery, doctors can also perform a procedure called mechanical thrombectomy. A thin catheter is threaded through a blood vessel, typically starting in the groin, and guided up to the brain where the clot is physically removed. This procedure can be performed up to 24 hours after the last time the patient was known to be well, a much wider window than clot-dissolving medication. In many cases, both treatments are used together. The 24-hour window doesn’t mean there’s no urgency; earlier treatment still produces better results.
Transient Ischemic Attacks
A transient ischemic attack, or TIA, is sometimes called a “mini-stroke.” It involves the same type of blockage as an ischemic stroke, but the blood flow is restored on its own before permanent damage occurs. Most TIA symptoms disappear within an hour, though they can last up to 24 hours. The symptoms are identical to a full stroke while they’re happening: face drooping, arm weakness, slurred speech.
A TIA is a serious warning sign. It means the conditions that cause a stroke, such as plaque buildup or clot formation, are already present. People who experience a TIA have a significantly elevated risk of having a full ischemic stroke in the days and weeks that follow, so it should be treated as a medical emergency even if the symptoms resolve.
Major Risk Factors
The risk factors for ischemic stroke overlap heavily with those for heart disease. High blood pressure is the single largest contributor. Atrial fibrillation is another major one, because the irregular heartbeat creates ideal conditions for clots to form in the heart and travel to the brain.
Other important risk factors include:
- Diabetes: Accelerates damage to blood vessel walls
- High cholesterol: Promotes plaque buildup in arteries
- Smoking: Damages blood vessels and makes blood more likely to clot
- Obesity and physical inactivity: Contribute to nearly every other risk factor on this list
- Age: Risk doubles roughly every decade after age 55
Some risk factors can’t be changed, including age, family history, and having had a previous stroke or TIA. But the biggest drivers, including high blood pressure, smoking, and inactivity, are modifiable.
Recovery and Rehabilitation
The first three months after an ischemic stroke are the most critical period for recovery. This is when the brain is most actively rewiring itself, forming new neural pathways to compensate for damaged areas. During this window, patients sometimes experience what’s called spontaneous recovery, where a lost ability suddenly returns as the brain finds an alternative route to perform the task.
Rehabilitation typically involves a combination of physical therapy, occupational therapy, and speech therapy depending on which functions were affected. The specific deficits vary widely based on where in the brain the blockage occurred. A stroke affecting the left side of the brain often impairs speech and language. One on the right side more commonly causes problems with spatial awareness, attention, and movement on the left side of the body.
After six months, improvement is still possible but happens much more slowly. Most people reach a relatively stable baseline around this point. That doesn’t mean progress stops entirely, but gains require more intensive effort and tend to be incremental rather than dramatic.
Preventing a Second Stroke
Once you’ve had an ischemic stroke, prevention of a second one becomes a top priority. The approach depends on what caused the first stroke. For strokes related to plaque buildup in the arteries, antiplatelet medications (the same category as aspirin) are the standard preventive treatment. For strokes caused by atrial fibrillation or other heart-related clot sources, blood-thinning medications that target the clotting process more broadly are used instead.
Beyond medication, managing blood pressure, cholesterol, and blood sugar makes a measurable difference in long-term stroke risk. For people who smoke, quitting is one of the single most effective steps. Regular physical activity, even moderate walking, helps control nearly every modifiable risk factor simultaneously.