Prehospital stroke assessment: last known well, LVO screening and choosing the right hospital

Ask an emergency physician what they most want from a paramedic on a stroke call and the answer is rarely a perfect neuro exam. It is a reliable time. Prehospital stroke assessment is a set of small, careful decisions, and most of the downstream treatment options depend on whether the crew got the first one right: when was this person last known to be normal?

This article walks through that question, how stroke and large-vessel-occlusion (LVO) screening fit in, and how destination decisions follow. The specifics always belong to your service's medical directives, your provincial standards and your regional stroke protocol.

#Prehospital stroke assessment starts with the clock

Time last known well (LKW) is not the same as time of onset, and it is definitely not the time someone noticed a problem. It is the last moment the patient was witnessed, or reliably known, to be at their neurological baseline.

That distinction matters because reperfusion treatments are time-limited. Canadian and international guidelines describe defined windows for thrombolysis and, in selected patients, longer windows for endovascular thrombectomy. If the timeline is wrong in either direction, a patient may be denied a treatment they could have had, or exposed to one that is unsafe.

Common traps:

• Wake-up strokes. The patient went to bed well and woke with symptoms. LKW is when they went to sleep, or the last time someone confirmed they were normal overnight, not when they woke.
• Found down. If no one saw the onset, LKW is the last confirmed normal contact, which may be the previous evening.
• Fluctuating symptoms. If symptoms fully resolved and then returned, your directives and the receiving team will want to know both times. Record them; do not average them.
• Vague witnesses. "This morning" is not a time. Anchor it: a phone call, a TV program, a coffee order, a text message timestamp.

Write LKW down early and say it during handover exactly as you documented it, including how you established it.

#Stroke scales: what they do and do not do

Prehospital stroke scales are screening tools. Simple facial, arm and speech checks are designed to be fast and sensitive enough to flag a possible stroke for activation, not to diagnose one.

Things to keep in mind:

1. Use the scale your service uses, the way it is written. Different services in Canada use different tools. Consistency matters more than personal preference.
2. A negative screen does not rule out stroke. Posterior circulation strokes can present with dizziness, imbalance, visual disturbance or vomiting and can be missed by simple tools.
3. Check glucose. Hypoglycemia is a well-known stroke mimic. Your directives will say when and how to check it; do it before you commit to a stroke pathway.
4. Consider other mimics. Seizure with post-ictal weakness, migraine and sepsis in an older adult can all look like stroke. Note what you found; the hospital will sort it out, but your observations help.

#Large-vessel-occlusion screening in general terms

Not every stroke hospital can perform endovascular thrombectomy. That is why many regions add an LVO screening step: a short severity-based tool that looks for signs suggesting a large artery is blocked, such as gaze deviation, marked weakness on one side, or problems with language or awareness of one side of the body.

In general terms:

• A positive LVO screen raises the likelihood that the patient might benefit from thrombectomy.
• It is not perfect. Some patients with LVO screen negative, and some who screen positive turn out to have something else.
• What a positive screen triggers, such as bypass to a comprehensive centre, depends entirely on your regional protocol, transport times and the time since LKW.

Your job is to apply the tool accurately and report the result clearly, not to make the diagnosis.

#Destination decisions

Once you have a time and a screen, destination is the next decision. Regional stroke systems in Canada set rules about which patients go to the nearest stroke-capable hospital and which bypass to an endovascular centre. Those rules weigh transport time, the patient's timeline and screening results.

A few principles hold across most systems:

• Decide early. Destination should be clear before you leave the scene, not on the highway.
• Follow the protocol, document deviations. If you go somewhere the protocol did not direct, record why.
• Pre-alert. A concise pre-alert with age, LKW, key findings, screen results, glucose and anticoagulant use lets the stroke team meet you ready to move.
• Bring the witness, or their number. The person who can confirm LKW is valuable at the door. If they cannot ride along, get a reliable phone number.

#For training leads

Stroke calls are frequent enough that crews feel comfortable with them, which is exactly why the subtle errors persist. Areas worth targeting:

• LKW drills. Build scenarios around wake-up strokes, found-down patients and fluctuating symptoms. Score the crew on the time they report, not just the scale they performed.
• Posterior and atypical presentations. Include cases where a simple screen is negative but the history should raise concern.
• Destination reasoning. Give crews the regional map and transport times and ask them to justify their destination aloud.
• Handover quality. Practise a short, structured pre-alert and handover that leads with LKW.

Scenario practice, whether on a manikin or in a decision-training tool such as Imedica, is a good place to rehearse the history-taking that a real scene often rushes.

#The takeaway

A good stroke call is built on a correct clock. Pin down time last known well, apply your service's stroke and LVO screens exactly as written, check for mimics, choose a destination early under your regional protocol, and pre-alert with the information the stroke team needs most.

Drafted for Imedica Field Notes. Physician review of this article is pending.