Describe a rare case of basilar tip mycotic aneurysm (BTMA) secondary to Streptococcus mitis endocarditis and review the diagnostic challenges of intracranial mycotic aneurysms (ICMAs).

ICMAs occur in 2-10% of infective endocarditis cases, typically in distal middle cerebral artery branches. BTMAs are exceptionally rare among all ICMAs, and their natural history remains incompletely understood.

Case Report and PRISMA-based PubMed search were performed with keywords “mycotic aneurysm” and “basilar”. Two authors (H.C. and S.L.) screened publications.

23-year-old woman presented with fever and unresponsiveness. Blood cultures grew Streptococcus mitis. No dental infection identified. TEE revealed a 1-cm mitral vegetation with severe regurgitation. Brain MRI/MRA demonstrated multifocal posterior circulation infarcts and distal basilar artery (BA) occlusion without aneurysm; within 7 days, follow-up CT/CTA revealed new prepontine SAH and 3mm BTMA with associated BA vasospasm. DSA confirmed multilobulated 3.3×2.7×3.1mm fusiform aneurysm involving the distal BA and left P1 segment. Ceftriaxone was given for 10 weeks. However, three months later, she developed new left upper extremity weakness. CT/CTA revealed new right parietal SAH and 4.2×2.0mm bilobed fusiform mycotic aneurysm in the right M4 branch, which underwent embolization. Despite intervention, she died. Literature review identified 16 relevant publications, including 11 BTMAs. Two systematic reviews (Ducruet 2010; Desai 2020) demonstrated BA involvement in <2–3% of ICMAs, with 8/298 and 4/243 cases retrospectively. BTMAs have been reported in isolated case reports, with ages ranging from 8 months to 78 years; however, pooled demographic, size, or outcome data were not available.

Reported cases demonstrated variable BTMAs courses, from autopsy findings to new formation within five days. This underscores the importance of serial vascular imaging and early multidisciplinary evaluation to enable timely endovascular or surgical intervention in high-risk cases. Given their rarity, future multicenter registries may help to better define their characteristics and outcomes.