CT Scan for Subarachnoid Hemorrhage
Non-contrast CT head is the best initial imaging test for suspected subarachnoid hemorrhage, and should be performed immediately when SAH is clinically suspected. 1, 2
Why Non-Contrast CT is the Gold Standard
Non-contrast CT is superior to all other modalities for detecting acute subarachnoid blood because acute blood appears hyperdense (white) on CT, making it easily visible without contrast. 2
The American College of Radiology specifically recommends against using contrast in the initial CT scan because the goal is to detect blood, not characterize vessels. 2
Sensitivity is highest when performed early: CT has 98.7-99.9% sensitivity when performed within 6 hours of symptom onset, missing fewer than 1.5 in 1000 SAHs. 1
Modern high-quality CT scanners (16-slice or better) achieve sensitivities approaching 100% when images are interpreted by fellowship-trained, board-certified neuroradiologists. 1, 3
Critical Timing Considerations
The timing of CT relative to symptom onset dramatically affects diagnostic strategy:
Within 6 hours of headache onset: A negative high-quality CT scan interpreted by a neuroradiologist is likely sufficient to exclude SAH, and lumbar puncture may not be necessary. 1, 2
Beyond 6 hours from symptom onset: CT sensitivity declines (93% at 24 hours, 57-85% at 6 days), making lumbar puncture mandatory if CT is negative but clinical suspicion remains high. 1, 2
Recent data suggests modern multislice CT may maintain 99-100% sensitivity up to 24-48 hours, though this requires further validation before changing practice. 4
When to Proceed with Lumbar Puncture
Lumbar puncture for xanthochromia evaluation should be performed in these specific scenarios:
High clinical suspicion for SAH with negative or non-diagnostic CT performed >6 hours after symptom onset. 1, 2
LP should be performed >6-12 hours after symptom onset to allow time for xanthochromia development (breakdown of hemoglobin to bilirubin). 1
Spectrophotometric analysis for xanthochromia has 100% sensitivity and 95.2% specificity when properly performed. 1
Clinical Decision-Making Algorithm
Use the Ottawa SAH Rule to identify patients requiring workup:
Patients with acute severe headache reaching maximum intensity within 1 hour require investigation if they have ANY of: 1
- Age ≥40 years
- Neck pain or stiffness
- Witnessed loss of consciousness
- Onset during exertion
- Thunderclap headache (instantly peaking pain)
- Limited neck flexion on examination
Important caveat: The Ottawa SAH Rule has 100% sensitivity but only 7.6-15.3% specificity, meaning it identifies a small subset who can safely avoid testing, but most patients meeting criteria will still need imaging. 1
Atypical Presentations Require Full Workup
Do not rely on the 6-hour rule or Ottawa criteria for atypical presentations including primary neck pain, syncope, seizure, or new focal neurological deficits—these patients require complete imaging and workup regardless of timing. 1
Next Steps After Positive CT
Once SAH is confirmed on non-contrast CT, proceed immediately to vascular imaging:
CTA head is fast, non-invasive, and has >90% sensitivity for aneurysms, making it the appropriate next step. 1, 2
CTA sensitivity decreases for aneurysms <3mm (61-85% sensitivity), so **catheter-based digital subtraction angiography (DSA) remains the gold standard** with >98% sensitivity and specificity. 1, 2
For diffuse SAH patterns (basal cistern and sylvian fissure), DSA should be performed regardless of CTA results due to limitations in CTA spatial resolution for small aneurysms. 1
Common Pitfalls to Avoid
Do not order contrast-enhanced CT initially—it obscures blood detection and is inappropriate for SAH diagnosis. 2
Do not perform LP before 6 hours from symptom onset, as xanthochromia may not have developed yet. 1
Do not skip LP in high-risk patients with negative CT performed >6 hours from onset—missing SAH carries catastrophic consequences (25% mortality, severe disability in survivors). 1
Do not use MRI as the initial test—while newer sequences can detect SAH, MRI has practical limitations (availability, scan time, motion artifact, patient compliance) that make it inappropriate for acute evaluation. 1