Ring-like (Donut-Shaped) Intracranial Aneurysms: A Warning Morphology of Mural Jet Flow and Pre-Rupture Instability

Background/Objectives: "Ring-like" intracranial aneurysms-historically described as "doughnut-like" or "donut sign"-represent a rare configuration in which a central thrombus coexists with a circumferential mural flow ring. Traditionally considered a radiologic curiosity, this morphology likely reflects a shear-driven hemodynamic state rather than a stable organized thrombus. We aimed to summarize all PubMed-documented cases of ring-like aneurysms, define their morphologic and clinical spectrum, and assess their hemodynamic significance, rupture risk, and treatment outcomes. An additional aim is to formalize the use of the term "ring-like aneurysm" as a distinct morphologic subtype and to clearly differentiate it from the neuroradiologic "donut sign," which represents an imaging appearance rather than a specific anatomic configuration. Methods: A systematic PubMed search (1996-2024) was conducted using the following combinations of keywords and Boolean operators: ("ring-like aneurysm" OR "donut aneurysm" OR "doughnut aneurysm" OR "ring-shaped aneurysm" OR "circumferential lumen" OR "central thrombus") AND ("intracranial" OR "cerebral" OR "basilar" OR "aneurysm"). Only English-language, PubMed-indexed reports describing true ring-like (donut-shaped) aneurysms were included. Non-indexed, non-English, and serpentine or fusiform aneurysms mimicking ring-like morphology were excluded. Extracted data included aneurysm location, size, presentation (ruptured, symptomatic, or incidental), treatment strategy, and clinical outcome. Statistical proportions were analyzed using descriptive methods, Wilson 95% confidence intervals, and a binomial test to compare the observed subarachnoid hemorrhage (SAH) rate against the expected conservative rupture proportion. Results: The search identified 16 individual patients reported in 10 publications. All aneurysms were large or giant (14-36 mm) displaying characteristic thrombosed pattern. Ruptured presentation occurred in 6 out of 16 cases (37.5%) and symptomatic unruptured in 10 (62.5%). No incidental cases were reported. Posterior circulation involvement was present in 44%, with a female predominance of 69%. Conclusions: Ring-like aneurysms constitute a distinct, shear-maintained hemodynamic entity combining mural jet flow with central thrombosis. Their frequent symptomatic or ruptured presentation supports the concept that this morphology represents a pre-ruptural configuration rather than a chronic thrombotic residue. Early recognition and targeted endovascular exclusion of the inflow zone are essential to prevent delayed rupture.