The Stroke That Comes In the Back Door.
STROKE · EPISODE 02
FAST is one of the most useful things you can carry in your head. But it was built to listen at one door, and the brain has two. This is what a stroke looks like when it comes in through the back — and the one rule that matters more than any single letter.
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He Passed Every Test You Know
A man stands up from the kitchen table and the room tilts. He grabs the counter. He tries to walk to the sink and his path bends — he cannot hold a straight line. He sits back down, queasy, a hand to his stomach.
Face: fine. Arms: fine. Speech: fine. By the rules most of us are taught, this was not a stroke.
It was. It just did not come in through the front door.
What FAST Gets Right
First, FAST deserves its due. Face drooping on one side. An Arm that drifts down when both are raised. Speech that slurs or will not come. And Time — meaning call 911 now, do not wait.
FAST is simple, it is memorable, and it works. It catches most strokes. Nothing in this article is an argument for abandoning it.
Why FAST Works: The Front Door
Most strokes strike the front two-thirds of the brain — the territory fed by what is called the anterior circulation. Roughly four out of five.
That is the front door. And the front of the brain controls exactly what FAST tests: the face, the arm, the speech. So when a stroke arrives at the front, FAST hears it. The alarm goes off.
The Brain Has a Second Door
The back of the brain runs on a different system. The brainstem and cerebellum have their own arteries — the vertebral and basilar arteries, together called the posterior circulation.
This is the part of the brain that keeps you balanced, steadies your eyes, and coordinates your movement. None of that is face, arm, or speech.
So a stroke can walk in the back door, and FAST may never make a sound. The house stays quiet while the brain is in trouble.
What a Back-Door Stroke Actually Looks Like
Instead of a drooping face, it can look like this. The room suddenly spins. You feel nauseated. You cannot walk a straight line — you may look almost drunk. Your vision may split in two, or a hand may go clumsy.
Posterior circulation strokes are a minority of all strokes. FAST is less sensitive to them: in a 2024 Sydney study of 46 MRI-confirmed posterior circulation ischemic strokes, 39.1% would have been missed if FAST alone were used.
BE-FAST: An Alarm on the Back Door
Researchers wired an alarm to the second door. Two letters, added to the front of FAST.
- B — Balance: a sudden loss of balance, or trouble walking
- E — Eyes: sudden vision loss, or double vision
- F — Face: drooping on one side
- A — Arms: one arm drifts down when both are raised
- S — Speech: slurred, strange, or hard to understand
- T — Time: call 911 immediately
Balance, Eyes, Face, Arms, Speech, Time. BE-FAST.
Does Adding Two Letters Actually Help?
In the study that introduced BE-FAST, researchers reviewed the records of 736 patients admitted to a single stroke center with acute ischemic stroke. About 14% of them had none of the FAST symptoms when they arrived. Among that group, roughly 4 in 10 had problems with balance or gait, and roughly 4 in 10 had visual symptoms.
Adding those two categories reduced the share of strokes that would have been missed from about 14% to about 4%.
Two letters. A large share of the misses, gone. It is worth noting this was a retrospective review at one center, and the authors called for prospective validation — but the direction of the finding has held up in later work, and BE-FAST is now widely taught.
Why These Strokes Get Missed
I want to be honest about why these strokes are missed, sometimes even by experienced clinicians. The symptoms are slippery.
Sudden dizziness, imbalance, and nausea can also come from an inner-ear problem, a virus, or a migraine — conditions we call stroke mimics. On the surface, they can look almost identical.
This is not carelessness. These signals are genuinely hard to read.
The Rule That Matters More Than Any Letter
So here is the rule. When dizziness hits suddenly — new, severe, out of nowhere — do not try to be sure.
A fierce inner-ear attack can begin just as suddenly as a stroke. Even at the bedside, clinicians often cannot tell them apart without specific tests: a focused eye examination, and often an MRI.
So you do not have to. Sudden, new, and severe — especially if you cannot walk, or anything else feels off — means let the emergency team decide.
Being unsure is not a reason to wait. It is the reason to go.
Sudden Rarely Travels Alone
Look for the company it keeps. Not just dizziness, but dizziness plus:
- Double vision
- Slurred words
- A numb or clumsy hand
- A stagger you cannot correct
- Eyes that drift, or will not hold still
One of these, arriving suddenly, alongside others — that is not a head cold. That is the brain asking for help.
And the dangerous move is the one most of us would make: lie down, and wait to see if it passes.
Won't BE-FAST Cry Wolf?
A fair question. Sometimes, yes — it will occasionally flag something harmless.
But that is the right trade. A false alarm costs you an afternoon in the emergency department. A missed stroke can cost you the rest of your life.
The Bottom Line
Most vertigo is not a stroke — but a stroke can look exactly like vertigo.
Keep FAST. It is still your front door, and it is a good one. BE-FAST simply adds the back — the balance and the vision FAST was never built to hear.
You do not have to diagnose anything. You only have to know the pattern: sudden, new, and severe. Call 911, and let the experts sort it out.
And if it came on suddenly and then faded away, do not relax. That can be the only warning the brain gives — and it is its own emergency.
Frequently asked questions
Can dizziness alone be a stroke?
It can be, though most dizziness is not. Isolated sudden dizziness is far more often caused by an inner-ear condition or a migraine. What raises concern is dizziness that is sudden, new, and severe, that comes with an inability to walk steadily, or that arrives alongside another neurological symptom such as double vision or slurred speech. Because these presentations can be difficult to separate at the bedside, sudden and severe dizziness should be evaluated in an emergency department rather than at home.
Can you have a stroke without facial drooping or slurred speech?
Yes. Face, arm, and speech symptoms come mainly from strokes in the front of the brain. A stroke in the brainstem or cerebellum can instead cause sudden imbalance, difficulty walking, double vision, clumsiness of a hand, or abnormal eye movements, while leaving face, arm, and speech testing normal. This is the reason Balance and Eyes were added to the FAST mnemonic.
What causes a stroke in the back of the brain?
The causes overlap with strokes elsewhere in the brain: high blood pressure, atrial fibrillation, atherosclerosis, diabetes, high cholesterol, and smoking. Posterior circulation strokes can also be caused by a tear in the wall of a vertebral artery, known as a dissection, which is a comparatively more common cause in younger adults and can follow neck injury or, sometimes, occur without any clear trigger.
Can younger adults have this kind of stroke?
Yes. Stroke is more common with age, but it occurs at every age, and posterior circulation strokes in particular are worth knowing about in younger people. Younger patients presenting with sudden dizziness and imbalance are also more likely to have their symptoms attributed to something benign, which can delay evaluation. Age does not rule out stroke.
Sources and references
- Aroor S, Singh R, Goldstein LB. BE-FAST (Balance, Eyes, Face, Arm, Speech, Time): Reducing the Proportion of Strokes Missed Using the FAST Mnemonic. Stroke. 2017;48(2):479–481. DOI: 10.1161/STROKEAHA.116.015169.
- Edlow JA, Carpenter C, Akhter M, et al. Guidelines for Reasonable and Appropriate Care in the Emergency Department 3 (GRACE-3): Acute dizziness and vertigo in the emergency department. Academic Emergency Medicine. 2023;30(5):442–486. DOI: 10.1111/acem.14728.
- Tanglay O, Cappelen-Smith C, Parsons MW, Cordato DJ. Enhancing Stroke Recognition: A Comparative Analysis of Balance and Eyes-Face, Arms, Speech, Time (BE-FAST) and Face, Arms, Speech, Time (FAST) in Identifying Posterior Circulation Strokes. Journal of Clinical Medicine. 2024;13(19):5912. DOI: 10.3390/jcm13195912.
- American Stroke Association. Patient education materials on stroke warning signs and BE-FAST.
- Mayo Clinic. Dizziness and vertigo: when to seek emergency care.