The signs of a stroke: what FAST catches, and the one in seven it misses

If you remember one thing about spotting a stroke, make it FAST: Face drooping on one side, Arm weakness with one arm drifting down, Speech that is slurred or strange, and Time to call 911 [s5]. It is the most useful public-health message in stroke care because it is short, it maps onto the commonest signs, and it ends with the only correct action. But it is a filter, not a full net — and knowing what it lets through can be the difference between calling for help and waiting.

Why speed is the whole point

Stroke is a race against tissue loss. A 2006 analysis in Stroke put numbers on the cliché "time is brain": in a typical large-vessel ischaemic stroke, an estimated 1.9 million neurons, 14 billion synapses and 12 km of myelinated fibres are destroyed each minute [s2]. Scaled up, that is 120 million neurons an hour, and the untreated ischaemic brain ages roughly 3.6 years for every hour without treatment [s2]. Those are modelled averages, not a readout from any one patient — but the direction is not in doubt, and it is why every guideline treats stroke as a dial-999 emergency rather than a wait-and-see.

Treatment windows are real and they are narrow. Clot-dissolving alteplase was shown to help when given 3 to 4.5 hours after symptom onset in the ECASS III trial, where the median time to treatment was 3 hours 59 minutes [s3]. Mechanical clot retrieval can work later in selected patients: the DAWN trial randomised people treated 6 to 24 hours after they were last known well, when imaging showed a mismatch between a severe deficit and a still-small infarct [s4]. Later is possible for some, but earlier is always better, and no one can know at home which window they are in.

What FAST misses

FAST is deliberately simple, and simplicity has a cost. When researchers at the University of Kentucky reviewed 736 patients admitted with acute ischaemic stroke in 2014, 14.1% had no FAST symptom at all when they presented [s1]. Among those missed by FAST, 42% had gait imbalance or leg weakness, 40% had visual symptoms, and 70% had one or the other [s1]. Adding those two categories cut the proportion of strokes not identified from 14.1% to 4.4% [s1] — the reasoning behind the expanded mnemonic BE-FAST, which prefixes Balance and Eyes.

That is why the standard symptom list is broader than the four FAST letters. The US National Library of Medicine describes the warning signs of a stroke as sudden numbness or weakness of the face, arm or leg — especially on one side; sudden confusion or trouble speaking or understanding speech; sudden trouble seeing in one or both eyes; sudden difficulty walking, dizziness, or loss of balance or coordination; and a sudden severe headache with no known cause [s5]. The recurring word is sudden. A symptom that appears out of nowhere, particularly on one side of the body, is the pattern that should trigger a call.

What to do

The action is the same whichever symptom shows up first. If you think you or someone else is having a stroke, call 911 right away [s5]. Every minute counts during a stroke [s5]. Note the time symptoms started, if you can — it directly shapes which treatments are on the table — but do not delay the call to work it out, and do not drive the person yourself. Symptoms that ease within minutes still warrant emergency assessment: a transient episode can be the warning before a larger stroke.

The limits

The 14.1% missed figure comes from a single centre's records for one year and has not been confirmed in a large prospective study; the BE-FAST authors say as much and call for validation before public education is rewritten [s1]. The "time is brain" figures are population averages built from imaging and neuroanatomy estimates, not measurements from an individual [s2]. And the treatment-window trials describe carefully selected patients under specialist care — they are the reason to get to hospital fast, not a promise about any one outcome.

None of this is a substitute for professional assessment. The single evidence-based instruction that applies to everyone is unglamorous and unchanging: if a stroke is possible, call emergency services immediately rather than waiting to see whether it passes.