Imaging for Subarachnoid Hemorrhage Diagnosis
Non-contrast head CT is the primary imaging modality for diagnosing subarachnoid hemorrhage, with sensitivity approaching 100% within the first 6 hours and 98-100% within the first 12 hours after symptom onset. 1, 2, 3
Initial Diagnostic Imaging
- Non-contrast CT head should be performed immediately when SAH is clinically suspected, as it is the cornerstone of diagnosis 1, 2, 3
- Contrast should NOT be used in the initial CT scan because the goal is to detect blood, not characterize vessels 3
- CT sensitivity is time-dependent: 98.7-99.9% within 6 hours, declining to 93% at 24 hours, and 57-85% by day 6 1, 2, 3
When CT Alone Is Insufficient
If CT is negative but clinical suspicion remains high, lumbar puncture for xanthochromia evaluation is mandatory, particularly in patients presenting more than 6 hours from symptom onset. 1, 2, 3
Critical Timing Algorithm:
- 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 3
- Beyond 6 hours from symptom onset: Lumbar puncture is mandatory if CT is negative but clinical suspicion persists, as CT sensitivity declines significantly 1, 3
Vascular Imaging After Positive CT
Once SAH is confirmed on non-contrast CT, proceed to vascular imaging to identify the bleeding source:
- CT angiography (CTA) is the appropriate next step as it is rapid, readily available, and has >90% sensitivity for aneurysms 1, 3
- CTA sensitivity decreases for aneurysms <3-5 mm (56-85% sensitivity) 1, 3
- Digital subtraction angiography (DSA) remains the gold standard with >98% sensitivity and specificity, particularly for small aneurysms 1, 2, 3
Role of MRI
MRI has limited utility in the acute emergency setting for SAH diagnosis due to practical constraints:
- MRI using FLAIR or gradient-echo T2*-weighted sequences can detect SAH with high sensitivity 1, 4
- Practical limitations include: routine availability, logistics with acutely ill patients, sensitivity to motion artifact, longer study time, and cost 1
- MRI is most useful for subacute/chronic SAH detection (where CT sensitivity drops significantly) or when evaluating patients with negative catheter angiography 1, 4
Common Pitfalls
- Misdiagnosis occurs in up to 12% of cases, with the most common error being failure to obtain a non-contrast cranial CT 2
- Do not skip lumbar puncture in patients presenting beyond 6 hours with negative CT, as this can miss up to 7% of SAH cases 1, 5
- Failure to recognize a sentinel bleed can lead to catastrophic outcomes, as 15-37% of patients with major SAH had a warning headache beforehand 1, 2
- Proper CSF specimen handling and interpretation are critical—look for xanthochromia and elevated bilirubin, not just red blood cell counts 1