Flow Diversion in Fetal-Type Posterior Communicating Artery Aneurysms: Anatomical Challenges and Treatment Strategy

Fetal-type posterior communicating artery (PcomA) aneurysms remain a difficult subgroup in the flow-diverter era. In these lesions, the PcomA supplies a substantial portion of the posterior cerebral artery territory because the ipsilateral P1 segment is absent, hypoplastic, or functionally insufficient. The branch therefore functions as a major outflow channel, and persistent distal demand may maintain circulation through the aneurysm-neck complex after flow diversion. This review summarizes the anatomical basis of fetal-type PcomA aneurysms and the current evidence for clipping, coiling, intrasaccular devices, and flow diversion. Among PcomA aneurysms treated with flow diverters, non-fetal lesions generally show better angiographic outcomes than fetal-type lesions. In a recent meta-analysis, complete occlusion at final follow-up was 77% in non-fetal aneurysms versus 42% in fetal-type aneurysms. A multicenter comparative cohort similarly reported complete occlusion in 81.8% versus 43.7%, with a markedly shorter median time to occlusion in non-fetal lesions (6 vs. 51 months). Earlier fetal-type series often showed persistent aneurysm filling and low complete occlusion rates, whereas more recent selected cohorts have reported complete or near-complete occlusion in 60%-80% of cases, suggesting that flow diversion can still be effective in carefully selected anatomies and with tailored strategies. Overall, clipping and coiling remain important branch-preserving options, and intrasaccular treatment may be reasonable in selected lesions with favorable geometry. Flow diversion should be considered selectively, particularly for large, recurrent, broad-necked, or morphologically complex aneurysms, while recognizing that robust fetal circulation may delay or limit aneurysm occlusion. Current evidence remains constrained by small retrospective series, inconsistent definitions of fetal anatomy, and heterogeneous device strategies.