Suspected subarachnoid haemorrhage
ACR AC Aneurysm/VM/SAH (2021); Perry/Backes CT-within-6-hours literatureUnenhanced CT first, and then a pathway whose next step is decided by the clock: within roughly six hours of ictus a negative modern CT reported by a radiologist is close to definitive, and beyond that window a negative CT must be completed by lumbar puncture or angiography.
Headache reaching maximum intensity within seconds to a minute, with or without vomiting, neck stiffness, collapse or a transient deficit. Most such patients do not have a subarachnoid haemorrhage, and the small proportion who do die or are disabled if it is missed.
Referenced decision support — confirm against your local protocol.
Decision support, not a directive. Protocols and timings shown are typical published ones — your local protocol takes precedence, and the vetting radiologist decides.
Accept as requested
CT Head — Unenhanced
CT Head
What we'd amend, and why
- Subarachnoid blood is dense against cerebrospinal fluid, and that contrast is at its greatest immediately after the bleed and decays over hours to days as the blood is cleared. This is why the pathway is time-dependent rather than merely urgent: the same negative scan means something very different at three hours and at three days. Thin sections through the basal cisterns matter, and intravenous contrast obscures precisely the sulcal and cisternal hyperdensity being looked for.
Nothing needs resolving before this goes ahead. Routine checks below are owned downstream.
How was this decided?
- pathwayadult — Adults
- rulerule-paeds-dose — Child-sized technique and contrast dose; checked by Radiographer at the scanner
Decision support only. Local protocol takes precedence.
Handled at the scanner(1)nothing for you to do
Settled and owned downstream. Each returns to a radiologist only on the stated trigger.
- Child-sized technique and contrast doseConfirm that a size- or weight-based protocol is selected — child-sized kV and mAs against size-based diagnostic reference ranges — and that contrast volume is calculated by weight rather than taken from an adult default. Weight-based iodinated contrast volumes of roughly 1.5–2.0 mL/kg are widely used in paediatric CT.Radiographer at the scannerAt the scannerFlags back if: No paediatric or size-based protocol exists on the scanner for the requested examination, or the requested coverage or number of phases exceeds what the clinical question needs — for example a multiphase study where a single phase answers it, or whole-body coverage for a focal question.
Worth asking the referrer (1)
None of these hold the request up. They sharpen the protocol or the plan that follows.
- How many hours have elapsed since the headache reached maximum intensity?It determines whether a negative CT ends the pathway or obliges lumbar puncture or angiography. It never delays the CT itself, which is done as soon as possible either way.
Pathways
Big forks are separate pathways; the first whose conditions match is the one used.
Within six hours of ictus — a negative CT carries its own weight
| Role | Study & protocol | Why this answers the question |
|---|---|---|
| First line | CT Head CT Head — Unenhanced usually appropriate | The density difference between subarachnoid blood and cerebrospinal fluid is at its greatest now and decays from here, which is why this is the one window in which a negative study is close to definitive. Emergency-department data support withholding lumbar puncture when a non-contrast CT performed within six hours of onset is reported as negative by an attending radiologist — conditional on a modern scanner, thin sections through the basal cisterns and a radiologist reporting it, all three of which are the vetting contribution. Contrast has no place: it obscures the cisternal hyperdensity that is the entire diagnosis. Report the elapsed hour in the study, because the negative result means something specific here and something different an hour later. |
| Second line | CT Angiogram — Intracranial (Circle of Willis) CT Angiogram — Intracranial Arteries | In this window angiography has one job rather than two: after a positive CT it localises the aneurysm and defines its neck for clipping or coiling, within hours, because rebleeding risk is front-loaded. It is not the completion of a negative early CT — adding it there converts a study that has already answered the question into a hunt for the one in fifty adults with an incidental unruptured aneurysm. |
Beyond six hours — a negative CT does not stand alone
| Role | Study & protocol | Why this answers the question |
|---|---|---|
| First line | CT Head CT Head — Unenhanced usually appropriate | Still the first study, and still done as fast as it can be — a positive scan ends the diagnostic pathway and starts the treatment one. What has changed is the meaning of a negative scan: sensitivity falls steeply once the blood begins to clear, so beyond six hours the study must be reported as not excluding the diagnosis, and the pathway is completed by lumbar puncture or, where that is refused, contraindicated or non-diagnostic, by angiography. Reporting a negative late CT without that sentence is the characteristic failure of this card. |
| Second line | CT Angiogram — Intracranial (Circle of Willis) CT Angiogram — Intracranial Arteries | Here it does the second of its two jobs — substituting for the lumbar puncture that was refused or could not be done — and the cost has to be stated when it is booked rather than discovered in the report. It finds aneurysms in roughly one in fifty adults and cannot say whether the one it finds has bled, so a positive study in a patient with a negative CT and no tap starts a difficult conversation rather than ending an easy one. |
| Problem solving | MRI Brain MRI Brain — routine unenhanced | For the patient days to weeks out, where CT has lost its sensitivity and the lumbar puncture window has closed. Susceptibility-weighted and FLAIR sequences find residual sulcal blood products and superficial siderosis long after CT has normalised, and the same study covers the alternative causes of a thunderclap headache — venous sinus thrombosis, reversible cerebral vasoconstriction, pituitary apoplexy, dissection. |
- Xanthochromia takes hours to develop, so a tap performed too early is as unreliable as a CT performed too late. The two tests fail at opposite ends of the same clock.
Adults
Matches your inputsDefault| Role | Study & protocol | Why this answers the question |
|---|---|---|
| First line | CT Head CT Head — Unenhanced usually appropriate | Subarachnoid blood is dense against cerebrospinal fluid, and that contrast is at its greatest immediately after the bleed and decays over hours to days as the blood is cleared. This is why the pathway is time-dependent rather than merely urgent: the same negative scan means something very different at three hours and at three days. Thin sections through the basal cisterns matter, and intravenous contrast obscures precisely the sulcal and cisternal hyperdensity being looked for. |
| Second line | CT Angiogram — Intracranial (Circle of Willis) CT Angiogram — Intracranial Arteries | Two quite different jobs, and it is worth being explicit about which one is being asked. After a positive CT, angiography localises the aneurysm and defines its neck for clipping or coiling, and it is needed within hours. After a negative CT beyond the six-hour window, it substitutes for lumbar puncture — and here it carries a real cost, because it detects the roughly one in fifty adults who harbour an incidental unruptured aneurysm and cannot say whether that aneurysm bled. |
| Problem solving | MRI Brain MRI Brain — routine unenhanced | For the patient presenting days to weeks after a thunderclap headache, CT has lost its sensitivity and lumbar puncture its window. Susceptibility-weighted and FLAIR sequences detect residual sulcal and superficial siderotic blood products long after CT has normalised, and the same study characterises the alternative causes of thunderclap headache — venous sinus thrombosis, reversible cerebral vasoconstriction, pituitary apoplexy, dissection. |
- Lumbar puncture is not an imaging study and is not scheduled here, but the imaging pathway is incomplete without knowing whether it is being done. Xanthochromia takes hours to develop, so a very early tap is as unreliable as a very late CT.
Pitfalls
- Reading a negative CT as exclusion without knowing the time since onset. Sensitivity falls steeply after the first six hours, and the request rarely states the hour.
- Applying the six-hour rule to the wrong interval or the wrong patient. The clock runs from ictus to SCAN, not from ictus to arrival, so a patient who presented at four hours and was scanned at eight is outside it. The rule was also derived in patients who were neurologically intact with a normal conscious level and a non-anaemic haematocrit; a deficit, a depressed Glasgow Coma Scale or significant anaemia takes the patient outside it whatever the clock says, and the pathway is then completed by lumbar puncture or angiography.
- Adding intravenous contrast to the initial study, which obscures the cisternal hyperdensity that is the entire diagnosis.
- Treating an aneurysm found on angiography after a negative CT as proof of rupture. Incidental aneurysms are common, and the pattern of blood — not the presence of an aneurysm — establishes which one bled.
- Anaemia lowers the density of blood and can make subarachnoid haemorrhage isodense to brain, which is a genuine cause of a falsely reassuring scan.
- Perimesencephalic blood is often non-aneurysmal, but it is not diagnosed on the unenhanced scan alone and still requires vascular imaging first.
Priors — what to pull first
- Look for prior angiography: a known, previously treated or previously observed aneurysm changes the whole interpretation of a positive study.
- A previous negative CT for an identical headache does not transfer. Each thunderclap episode restarts the clock.
What makes a good request
- The single most useful thing a request can state is the number of hours between headache onset and the scan. It changes the interpretation of a negative study more than any protocol choice does.
- Published emergency-department data support withholding lumbar puncture when a non-contrast CT performed within six hours of headache onset is reported as negative by an attending radiologist; the corollary is that outside that window a negative CT does not stand alone.
- Where lumbar puncture is refused, contraindicated or non-diagnostic, CT angiography is the usual substitute, accepting that it identifies an aneurysm rather than the haemorrhage itself and that incidental aneurysms are common.
Scoring this once it is done
The classification and risk tools this question ends in.
How these studies are acquired
Contrast, phases and timing for every study on the pathways above.
Confirm locally
- CT Head — Unenhanced: timings are typical — confirm against local protocol.
- Timings, contrast volumes and rates above are typical published values. Your department's protocol, scanner and patient population decide the actual numbers.
- Safety thresholds and premedication policy follow local policy where it differs from the cited guidance.
References
- ACR Appropriateness Criteria — Cerebrovascular Diseases: Aneurysm, Vascular Malformation, and Subarachnoid Hemorrhage (2021) · ACR Appropriateness Criteria
- Sensitivity of early brain CT to exclude aneurysmal subarachnoid haemorrhage (Stroke 2016) · Primary literature
- CT within 6 hours of headache onset to rule out SAH in non-academic hospitals (Neurology 2015) · Primary literature
- Image Gently — child-sizing the CT dose; size-based protocols and accreditation of paediatric CT dose indices · Image Gently
- Strauss KJ et al. Image Gently: Ten Steps You Can Take to Optimize Image Quality and Lower CT Dose for Pediatric Patients (AJR) · Image Gently
- AAPM Pediatric Routine Abdomen and Pelvis CT Protocol — size-based technique parameters · Other
Implemented from the cited published sources. Educational and workflow support only; confirm against current guidelines and local policy before clinical use.