Doctors diagnose a stroke using a combination of physical examination, brain imaging, and blood tests, often within minutes of your arrival at the hospital. The most critical step is a CT scan of the brain, which current guidelines say should happen within 25 minutes of walking through the door. Everything that follows, from advanced imaging to heart and blood vessel tests, builds on that initial picture to confirm the diagnosis, identify the type of stroke, and guide treatment.
The Physical Exam in the First Minutes
Before any scan, the medical team runs through a structured neurological exam. The standardized version used in most stroke centers checks 11 categories of brain and body function: your level of consciousness, ability to answer basic questions (what month is it, how old are you), ability to follow simple commands (open and close your eyes, grip and release a hand), horizontal eye movements, visual fields, facial symmetry, arm strength, leg strength, coordination, sensation, language, speech clarity, and whether you’re aware of both sides of your body and surroundings.
Each category gets a score, and the total gives the team a quick, standardized measure of how severe the neurological damage is. That score also serves as a baseline. If your numbers improve or worsen over the next hours, it tells doctors whether treatment is working or the stroke is progressing.
Why a CT Scan Comes First
A non-contrast CT scan is the most widely used first imaging step for stroke worldwide. Its main job is not to confirm an ischemic stroke (the kind caused by a blood clot) but to rule out a hemorrhagic stroke (bleeding in the brain). This distinction matters enormously because the treatments are opposite: clot-dissolving medication can save brain tissue after a clot-based stroke but would be dangerous if the brain is actively bleeding.
CT is fast, available in virtually every emergency department, and excellent at detecting fresh bleeding. Its weakness is that signs of a clot-based stroke can be subtle or completely invisible in the first several hours. A normal-looking CT scan does not mean you haven’t had a stroke. It means there’s no bleeding, which clears the path for clot-dissolving treatment if the clinical picture fits.
MRI for a Closer Look
When doctors need more detail, an MRI with a specialized sequence called diffusion-weighted imaging can detect damaged brain tissue within minutes of a stroke’s onset. It works by measuring how water moves through cells. When brain cells die, water gets trapped inside them, and this shows up as a bright spot on the scan. Reported sensitivity ranges from 88% to 100%, with specificity in the same range, making it far more accurate than CT for identifying early ischemic strokes.
MRI isn’t always the first choice because it takes longer, isn’t available around the clock at every hospital, and can’t be used for patients with certain metal implants or who can’t hold still. But when the CT looks normal and the clinical suspicion for stroke is high, or when doctors need to distinguish a stroke from something else entirely, MRI is the strongest tool available.
Perfusion Imaging to Guide Treatment
One of the most important questions in acute stroke care isn’t just “did a stroke happen?” but “is there brain tissue we can still save?” Perfusion imaging, usually done with CT, answers this by mapping blood flow throughout the brain in real time. It reveals two zones: a core of tissue that’s already dead, and a surrounding area that’s starved of blood but not yet permanently damaged.
That surrounding area is the target. If the volume of salvageable tissue is large relative to the dead core, aggressive treatment like clot retrieval or clot-dissolving medication is more likely to help. Studies have shown that patients treated within three hours have measurably more tissue salvaged compared to those who receive no treatment. If the core is already massive, covering more than a third of the affected brain territory, the risks of treatment may outweigh the benefits. Perfusion imaging gives doctors the information to make that call.
Blood Tests and Ruling Out Mimics
Blood work during a stroke evaluation serves two purposes: checking for conditions that mimic stroke, and establishing a baseline for treatment decisions. A bedside blood sugar test is one of the first things done because low blood sugar can cause sudden weakness, confusion, and slurred speech that looks identical to a stroke. A complete blood count, clotting tests, and basic metabolic panels round out the picture.
Stroke mimics are surprisingly common. In one study of 950 patients brought in by ambulance with suspected stroke, nearly 43% turned out to have something else. Among neurological mimics, seizures (about 20%), migraines (about 19%), and peripheral nerve problems (about 11%) were the most frequent. Non-neurological mimics included cardiovascular conditions, psychiatric episodes, and infections. Migraine aura without headache is a particularly tricky one. It can cause numbness, visual disturbances, and even speech problems, but symptoms typically spread gradually across different areas rather than hitting all at once, and brain imaging comes back normal.
Finding the Cause After the Diagnosis
Once a stroke is confirmed, the next round of testing focuses on why it happened, because preventing a second stroke depends on identifying and treating the underlying cause. Two key tests are an ultrasound of the carotid arteries in the neck and an ultrasound of the heart.
Carotid ultrasound looks for narrowing caused by plaque buildup in the arteries that supply blood to the brain. In one study, over 61% of stroke patients had abnormal findings on carotid imaging, with atherosclerosis (plaque buildup) being the most common. Heart ultrasound searches for problems that could send clots to the brain: enlarged chambers, weak pumping function, valve disease, or a small hole between the heart’s upper chambers. Nearly 70% of stroke patients in the same study had abnormal heart findings, even though only about 11% had a known history of heart disease before the stroke. These results directly change treatment. A stroke caused by a narrowed carotid artery may call for surgery to open the vessel, while a stroke from a heart rhythm problem like atrial fibrillation typically means long-term blood-thinning medication.
When Symptoms Have Already Resolved
If your symptoms disappeared before you reached the hospital, doctors must determine whether you had a transient ischemic attack, sometimes called a mini-stroke. The challenge is that by the time you’re being examined, the neurological exam may be completely normal. Diagnosis relies heavily on your description of what happened: what symptoms you experienced, how quickly they started, how long they lasted, and whether they included things like one-sided weakness, speech difficulty, or sudden vision loss in one eye (which can point to a problem in the carotid artery).
Doctors use a scoring system called ABCD2 to estimate your short-term stroke risk after a TIA. It factors in your age (over 60 adds a point), blood pressure (140/90 or higher adds a point), whether you had weakness (2 points) or speech problems alone (1 point), how long symptoms lasted (over 60 minutes is 2 points, 10 to 59 minutes is 1 point), and whether you have diabetes (1 point). A score of 6 or 7 carries an 8% risk of a full stroke within 48 hours. A score below 4 drops that risk to about 1%. Most stroke centers will admit patients scoring 4 or higher for observation and fast-tracked testing, but even lower scores warrant urgent evaluation because a low score doesn’t guarantee safety, particularly if there’s significant carotid narrowing.
Discovering a Stroke You Never Knew About
Sometimes a stroke shows up on a brain scan done for a completely unrelated reason, like a scan for headaches, dizziness, or a head injury. These are called silent brain infarcts. They cause no obvious symptoms at the time but leave visible scars on imaging. In screened populations, silent strokes are associated with a two-to-threefold increased risk of a future symptomatic stroke and dementia, independent of other risk factors.
Vascular neurologists generally treat the discovery of a silent stroke with the same seriousness as a symptomatic one. As one specialist put it plainly: “A stroke is a stroke.” Most doctors who encounter these findings believe they signal elevated risk for both future strokes and cognitive decline, and they typically respond by optimizing blood pressure control, cholesterol management, and other preventive measures, just as they would after a stroke you felt happen.