Background CT angiography (CTA) is widely used for the detection of vascular lesions in patients with non-traumatic subarachnoid hemorrhage (ntSAH); however, digital subtraction angiography (DSA) remains the gold standard for diagnosis. Our aim was to analyze the diagnostic yield of DSA after negative high-resolution CTA findings.

Methods Records of patients with a CTA-negative ntSAH at a single institution from 2014 to 2018 were retrospectively analyzed. ntSAH patterns were categorized as cortical, perimesencephalic, or diffuse. Subsequent DSA findings were compared across the three cohorts.

Results A total of 186 patients had CTA-negative ntSAH. The ntSAH pattern was identified as cortical (n=77, 41.4%), diffuse (n=60, 32.3%), or perimesencephalic (n=49, 26.3%). In eight patients (4%), DSA revealed a vascular lesion (one cervical arteriovenous fistula and seven atypical aneurysms) after negative CTA findings. All eight patients with positive DSA findings had diffuse SAH (13% of patients with a diffuse pattern). The seven aneurysms included four blister or dissecting (two basilar artery, one superior cerebellar artery, and one dorsal wall internal carotid artery), two fusiform (one posterior communicating artery and one anterior spinal artery), and one saccular aneurysm (middle cerebral artery).

Conclusion DSA identified a causative lesion in 4% of patients with CTA-negative ntSAH, but only in patients with diffuse ntSAH. Most of the lesions detected were atypical aneurysms and were found on delayed angiograms. These results suggest that DSA can help to diagnose CTA-negative ntSAH caused by unusual aneurysms, and repeat DSA may be needed only for patients with diffuse ntSAH.