From the Guidelines
The head CT scan may appear normal in cases of subarachnoid hemorrhage (SAH) after 6 hours due to the rapid clearance of blood from the subarachnoid space.
Key Factors
- The sensitivity of non-contrast head CT scans for detecting SAH decreases significantly after 6 hours, as noted in a 2016 meta-analysis that studied 8907 patients and found a sensitivity of 98.7% and specificity of 99.9% for head CT performed within 6 hours of symptom onset 1.
- The Ottawa SAH Rule can be used to identify patients who require additional investigation for SAH, including those with age ≥40 y, neck pain or stiffness, witnessed loss of consciousness, onset during exertion, thunderclap headache, or limited neck flexion on examination 1.
- For patients presenting > 6 hours from ictus with high suspicion for SAH, lumbar puncture (LP) for xanthochromia evaluation should be performed to confirm the diagnosis 1.
Diagnostic Considerations
- High-quality CT scanners can detect SAH with high sensitivity, especially when images are interpreted by fellowship-trained, board-certified neuroradiologists 1.
- CT angiography (CTA) and digital subtraction angiography (DSA) may be used as additional diagnostic modalities to evaluate cerebrovascular anatomy and aneurysm geometry 1.
- The choice of diagnostic modality depends on the clinical presentation and the suspected underlying cause of SAH.
From the Research
Sensitivity of Head CT in Subarachnoid Hemorrhage
- The sensitivity of head CT in detecting subarachnoid hemorrhage (SAH) decreases over time, with studies suggesting that CT performed within 6 hours of headache onset is highly sensitive for the detection of SAH 2, 3, 4.
- Beyond 6 hours, the sensitivity of CT decreases, and lumbar puncture may be necessary to rule out SAH 2, 5, 4.
- Modern CT scanners have improved sensitivity and specificity in the diagnosis of SAH, especially when performed within 24 hours of symptom onset 5.
Time-Dependent Test Characteristics of Head CT
- The sensitivity of head CT performed ≤6 hours after ictus is high, ranging from 98.5% to 100% 3, 4.
- However, the sensitivity of head CT performed >6 hours after ictus decreases, ranging from 90.0% to 99.0% 5, 4.
- The negative predictive value of head CT performed within 6 hours of headache onset is also high, making it a reliable test for ruling out SAH in this time frame 3, 4.
False-Negative Interpretations of Cranial CT
- False-negative interpretations of cranial CT can occur, especially if the CT is performed within 6 hours of headache onset and is not interpreted by a board-certified neuroradiologist 6.
- Blinded independent review of CT studies can reveal concordant agreement for either definite or probable evidence of SAH in cases of presumed CT-negative SAH 6.
- This highlights the importance of careful interpretation of CT scans and the potential need for repeat imaging or lumbar puncture in cases of suspected SAH with a negative CT scan 6.