The Infundibulum-Aneurysm Conundrum: A Longitudinal Single-Center Analysis

This study aims to characterize the natural history and diagnostic variability of unequivocal intracranial infundibula and diagnostically uncertain outpouchings labeled "aneurysm versus infundibulum". We retrospectively analyzed 665 intracranial outpouchings detected on CTA, MRA, or DSA at a single-center (2013-2018). Radiology reports classified lesions as unequivocal infundibula (n=321) or as conundrums with an explicit label of "aneurysm versus infundibulum" (n=344). Follow-up imaging through December 2023 was reviewed for growth, rupture, or aneurysmal transformation. Ten conundrums with later DSA follow-up also underwent blinded image re-review. A logistic model tested clinical and radiographic predictors of diagnostic ambiguity, and a literature review contextualized anatomic prevalence. Of 321 unequivocal infundibula, 146 (45%) had follow-up imaging. Among 344 conundrums, 208 (61%) were followed, yielding 1040 lesion-years of surveillance across both cohorts. No complications or morphologic changes were observed in either group. Median follow-up among followed infundibula and conundrums was 2.26 (interquartile range [IQR] 0.32-5.34) and 2.31 (IQR 0.20-5.43) years, respectively. Conundrums were more likely than unequivocal infundibula to undergo follow-up imaging (P < .001). Nonposterior communicating artery location (OR 7.4; 95% CI, 5.11-10.63) and presence of comorbid aneurysms (OR 1.9; 95% CI, 1.15-2.96) independently predicted a conundrum label. Surveillance declined steeply after 5 years, with only 13.4% of infundibula and 17.2% of conundrums still imaged at that horizon. Secondary reassessment of 10 conundrums revealed 100% concordance with DSA when morphologic criteria were rigorously applied. Literature review reaffirmed the predominance of infundibula at the posterior communicating artery but highlighted inconsistent terminology and sparse long-term outcome data. In the largest single-center cohort to date, both classic infundibula and radiologically ambiguous conundrums remained stable over medium-term follow-up. Diagnostic uncertainty appears to stem mainly from interpretive variability rather than true biologic risk. Structured reporting combined with location-informed heuristics could reduce unnecessary surveillance.