A transient ischemic attack (TIA) happens when blood flow to part of the brain is temporarily blocked, usually by a blood clot. The blockage resolves on its own, typically within minutes to hours, and doesn’t cause permanent brain damage. But the causes behind that temporary blockage vary, and understanding them matters because roughly 3% of people who have a TIA will have a full stroke within 48 hours, and up to 9% within 90 days.
The Three Main Mechanisms
About half of all TIAs are caused by atherosclerosis, the buildup of fatty plaque inside the arteries that supply the brain. These plaques narrow the vessel over time, and they can also rupture, triggering a clot that briefly blocks blood flow before dissolving or dislodging. This is the same process behind most heart attacks, just happening in the arteries leading to or inside the brain.
Small vessel disease in the brain accounts for roughly 25% of cases. The tiny arteries deep inside the brain narrow and stiffen over years, especially in people with high blood pressure. When these vessels can’t dilate properly to meet the brain’s oxygen demands, pockets of tissue lose blood flow temporarily. On MRI scans, this kind of damage shows up as white matter lesions and small areas of dead tissue called lacunar infarcts, which are common in older adults and linked to higher stroke risk over time.
Heart-related clots cause about 20% of TIAs. A clot forms in the heart, breaks loose, travels through the bloodstream, and temporarily lodges in a brain artery. The most common heart condition behind this is atrial fibrillation.
Atrial Fibrillation and Heart-Related Clots
Atrial fibrillation (AFib) is an irregular heart rhythm that allows blood to pool in a small pouch of the heart called the left atrial appendage. Pooled blood clots more easily, and those clots can travel to the brain. Among people with AFib who’ve already had a TIA, the rate of full ischemic stroke runs about 7% per year even with aspirin therapy. Blood thinners cut that risk by more than half compared to aspirin alone.
Other heart conditions that can send clots to the brain include recent heart attacks (where clots form on damaged heart tissue), heart valve disease, and a patent foramen ovale, which is a small hole between the upper chambers of the heart that never fully closed after birth. Any condition that creates turbulent or sluggish blood flow inside the heart raises the chance of clot formation.
High Blood Pressure, Diabetes, and Cholesterol
These three conditions are the most common modifiable risk factors, and many TIA patients have at least one of them. High blood pressure damages artery walls throughout the body, including the small vessels deep in the brain and the larger carotid arteries in the neck. Over time, this damage accelerates both plaque buildup and small vessel stiffening.
Diabetes roughly doubles the odds of having an ischemic stroke or TIA. High blood sugar injures the lining of blood vessels, promotes inflammation, and makes blood more prone to clotting. In studies of TIA and stroke patients, about one in four had diabetes. The combination of diabetes with other metabolic problems like high blood pressure and abnormal cholesterol raises risk even further. In one large study, hypertension was present in 40% of patients who also had diabetes, compared to 25% of those without metabolic risk factors.
High cholesterol contributes by feeding the growth of arterial plaques. When LDL cholesterol accumulates in artery walls, plaques become larger and more unstable, making them more likely to crack open and trigger a clot.
Blood Disorders That Increase Clotting
Less commonly, problems with the blood itself can cause a TIA. One well-documented example is polycythemia vera, a condition where the bone marrow produces too many red blood cells. The excess cells thicken the blood, slow its flow, and activate platelets in ways that promote clotting. Thrombosis is the first symptom in about 20% of people with polycythemia vera, and roughly 70% of those clotting events are TIAs or strokes.
Other clotting disorders can also be responsible. Conditions that make the blood “hypercoagulable,” meaning it clots too easily, include antiphospholipid syndrome, factor V Leiden mutation, and elevated levels of certain clotting proteins. These are particularly worth investigating when a TIA occurs in a younger person without the typical risk factors like high blood pressure or atherosclerosis.
Sickle cell disease is another blood condition that raises TIA and stroke risk, especially in children and young adults. The misshapen red blood cells can clump together and block small arteries in the brain.
Carotid Artery Disease
The carotid arteries run along each side of your neck and are the brain’s main blood supply. When plaque builds up in these arteries, it can narrow them significantly or break apart, sending debris and clots upstream into the brain. Carotid artery disease is one of the most treatable causes of TIA because the narrowing can be detected with ultrasound and addressed with surgery or stenting in severe cases. A TIA caused by carotid disease often produces symptoms on one side of the body: weakness in one arm, drooping on one side of the face, or vision loss in one eye.
Less Common Causes
Arterial dissection, a tear in the wall of an artery leading to the brain, can cause TIAs in younger adults. This sometimes happens after neck trauma, chiropractic manipulation, or even sudden head movements, though it can also occur spontaneously. The tear creates a flap of tissue that disrupts blood flow or triggers clotting at the site.
Vasculitis, or inflammation of blood vessel walls, narrows arteries and can cause temporary ischemia. This is rare but can occur with autoimmune diseases like lupus or giant cell arteritis. Severe dehydration, infections causing systemic inflammation, and certain medications that affect clotting can also contribute in some cases.
Why a TIA Is a Warning
The combined risk of a full stroke after a TIA ranges from 10% to 20% in the first three months if the underlying cause goes untreated. The highest danger period is the first two days, when the stroke risk sits around 3%. After that, risk stays elevated at about 5% by one week and 8% by one month.
Doctors use a scoring system called ABCD2 to gauge how urgent the situation is. It assigns points based on five factors: age 60 or older, blood pressure above 140/90, whether symptoms included weakness or speech problems, how long the symptoms lasted, and whether you have diabetes. Higher scores signal a greater short-term stroke risk.
The modern definition of TIA no longer relies only on whether symptoms resolve within 24 hours. Instead, it hinges on whether brain imaging shows actual tissue damage. MRI with a specialized technique called diffusion-weighted imaging detects signs of ischemia in roughly half of all TIA cases. When imaging reveals damage, the event is reclassified as a minor stroke regardless of how quickly symptoms cleared. This distinction matters because it changes treatment decisions and risk predictions going forward.
Identifying the specific cause of a TIA drives what happens next. Someone with AFib needs blood thinners. Someone with severe carotid narrowing may need a procedure. Someone with uncontrolled blood pressure or diabetes needs aggressive management of those conditions. The TIA itself is temporary, but the underlying cause persists until it’s addressed.