A CVA (cerebrovascular accident), commonly called a stroke, is diagnosed through a combination of a rapid neurological exam, brain imaging, and blood tests. The process is designed to answer two urgent questions: Is this actually a stroke, and if so, is it caused by a blockage or by bleeding? That distinction determines everything about treatment, and hospitals aim to get the first brain scan started within 15 to 25 minutes of arrival.
The Initial Neurological Exam
The first thing that happens when a suspected stroke patient arrives at the emergency department is a structured neurological assessment. The most widely used tool is the NIH Stroke Scale (NIHSS), a roughly 15-part exam that scores how well different areas of the brain are functioning. A provider will ask you to follow simple instructions, answer questions, repeat phrases, move your eyes, lift your arms and legs, and perform basic coordination tasks like touching your finger to your nose.
The NIHSS produces a score from 0 to 42. A score of 0 means no detectable neurological deficits, while the highest scores indicate coma. The ranges break down like this:
- 0 to 5: Minor stroke
- 6 to 15: Moderate stroke
- 16 to 20: Moderate to severe stroke
- 21 to 42: Severe stroke
This score helps the medical team gauge severity and guides decisions about which treatments to pursue. It also serves as a baseline so providers can track whether you’re improving or worsening over the following hours and days.
The CT Scan: First and Most Critical Image
A non-contrast CT scan of the head is the single most important diagnostic step. It takes only a few minutes and is highly sensitive at detecting fresh bleeding inside the brain within 12 hours of symptom onset. That matters because the two main types of stroke, ischemic (caused by a clot blocking blood flow) and hemorrhagic (caused by a burst blood vessel), look identical from the outside but require opposite treatments. Giving a clot-dissolving drug to someone who is bleeding would be catastrophic.
The CT scan reliably shows hemorrhagic stroke as a bright white area on the image. Ischemic strokes, on the other hand, may not show up on CT for several hours. So in the early stages, a “normal-looking” CT in someone with clear stroke symptoms is actually useful information: it tells the team that bleeding is not the cause, which clears the path for clot-dissolving therapy. The American Heart Association’s most aggressive benchmark calls for this scan to begin within 15 minutes of walking through the door.
MRI for a More Detailed Picture
When more detail is needed, MRI with a technique called diffusion-weighted imaging can detect ischemic strokes much earlier than CT, sometimes within minutes of onset. It works by measuring changes in how water molecules move through brain tissue. When an area of the brain loses blood flow, water movement changes in a way that lights up clearly on the scan. This makes MRI far more sensitive for identifying small or very early strokes that CT would miss.
MRI isn’t always the first test, though. It takes longer, isn’t available at every hospital around the clock, and can’t be used on patients with certain metal implants or pacemakers. In most emergency settings, CT comes first to rule out bleeding, and MRI follows when the clinical picture needs further clarification.
Blood Tests Before Treatment Can Begin
While imaging is happening, the team draws blood for several key tests. These need to come back quickly, typically within 45 minutes of arrival. The most important include:
- Blood sugar: Low blood sugar can mimic a stroke almost perfectly, causing slurred speech, confusion, and weakness on one side. A simple fingerstick glucose check rules this out immediately.
- Clotting tests (INR and PTT): These measure how quickly your blood clots. If you’re on blood thinners, your clotting time may be too prolonged to safely receive clot-dissolving medication.
- Complete blood count: Checks for conditions like severely low platelet counts that would make clot-dissolving treatment dangerous.
- Heart enzyme (troponin): Strokes and heart attacks can occur together, and troponin levels reveal whether the heart muscle is under stress.
An electrocardiogram (EKG) is also run early on. Irregular heart rhythms, particularly atrial fibrillation, are a major cause of stroke, and an EKG can flag this immediately. The blood sugar check is so critical that if paramedics already measured it in the ambulance, the hospital will use that value to save time.
Vascular Imaging to Find the Source
Once the immediate emergency is addressed, the next step is figuring out where the blockage or bleeding originated. CT angiography (CTA) is the preferred method in the acute setting. It involves injecting contrast dye into a vein and then scanning the blood vessels in the neck and brain to create a detailed map. CTA can identify blocked arteries, narrowed sections, and clots that might still be treatable with a catheter-based procedure to physically retrieve them.
Doppler ultrasound of the carotid arteries in the neck is another option. It’s noninvasive, inexpensive, and can be done at the bedside, making it a useful screening tool. It works well for detecting very mild or very severe narrowing, but research shows it’s less reliable at measuring moderate stenosis in the 50 to 69 percent range. For that reason, patients whose ultrasound shows significant narrowing typically get a confirmatory CTA as well. Identifying severe carotid narrowing matters because surgical repair of the artery significantly reduces the risk of a second stroke.
Cardiac Evaluation for Clot Sources
About one in four ischemic strokes originates from a blood clot that formed in the heart and traveled to the brain. Finding that source is essential to preventing another stroke. The primary tool is echocardiography, an ultrasound of the heart.
A standard echocardiogram (performed through the chest wall) can detect obvious problems like weakened heart chambers or valve abnormalities. But a more detailed version, done by passing a small ultrasound probe into the esophagus, provides a much clearer view of structures that commonly harbor clots. This approach is considered the gold standard for spotting blood clots in the left atrial appendage, a small pouch in the heart where clots frequently form during atrial fibrillation. It’s also the best way to evaluate a patent foramen ovale (PFO), a small hole between the upper heart chambers that some people are born with, which can allow clots from the venous system to cross into the brain’s arterial supply.
Ruling Out Stroke Mimics
Not everything that looks like a stroke is one. Several conditions produce nearly identical symptoms, and part of the diagnostic process is systematically ruling them out. The most common mimics include seizures (particularly the period of temporary weakness or speech difficulty that follows a seizure), migraine with aura (which can cause one-sided weakness and visual disturbances), brain tumors, infections like encephalitis or meningitis, and metabolic problems like low blood sugar or liver failure.
This is one reason the diagnostic workup is so thorough. The CT scan catches tumors and bleeding. Blood sugar testing catches hypoglycemia. The clinical history helps distinguish migraine aura from stroke (migraine symptoms typically spread gradually over minutes, while stroke symptoms hit suddenly). In some cases, MRI is needed to distinguish a stroke from a seizure or infection that’s affecting a specific brain region.
Classifying the Stroke’s Cause
After the acute phase, doctors work to classify what caused the stroke. The most widely used system sorts ischemic strokes into five categories:
- Large artery atherosclerosis: Plaque buildup in a major artery, typically the carotid, that either blocks flow or sends debris downstream.
- Cardioembolism: A clot that formed in the heart (often due to atrial fibrillation) and traveled to the brain.
- Small vessel disease: Blockage of a tiny artery deep in the brain, usually linked to high blood pressure or diabetes.
- Other determined cause: Less common mechanisms like blood clotting disorders, arterial tears, or vasculitis.
- Undetermined cause: When testing doesn’t reveal a clear mechanism, or when multiple possible causes are found.
This classification drives long-term prevention. Someone with atrial fibrillation will likely need blood thinners indefinitely. Someone with severe carotid narrowing may need surgery. Someone with small vessel disease needs aggressive blood pressure control. Getting the diagnosis right in the days after a stroke shapes years of follow-up care.