Perimesencephalic Subarachnoid Hemorrhage Bleeding Patterns Are Not Always Benign: Prognostic Impact of an Aneurysmal Pathology.

Background/Objectives: Perimesencephalic subarachnoid hemorrhage (pmSAH) is generally considered to be a benign variant of spontaneous SAH. However, in rare cases, an underlying aneurysm may be present, altering both clinical management and prognosis. The aim of this study was to evaluate the prognostic impact of aneurysmal pathology in patients presenting with perimesencephalic hemorrhage, focusing on the occurrence of complications and functional outcomes. Methods: This single-center, retrospective study included 77 patients diagnosed with perimesencephalic hemorrhage between 2012 and 2022. Clinical and radiological data were extracted, including demographics, risk factors, complications (hydrocephalus, vasospasm, and delayed cerebral ischemia (DCI)), and outcome scores (Glasgow Outcome Scale (GOS) and modified Rankin scale (mRS) at discharge). Patients were divided into two groups based on the presence or absence of an aneurysm confirmed through digital subtraction angiography (DSA). Results: Of the 77 patients, 7 (9.1%) were found to have an aneurysm. While rates of complications such as hydrocephalus and DCI were higher in the aneurysm group, these differences did not reach statistical significance. However, patients with aneurysms had significantly worse functional outcomes, with higher mRS and lower GOS scores at discharge. Logistic regression confirmed the presence of aneurysms as an independent factor associated with poor outcomes (OR = 21.6; 95% CI: 1.00-467.3; p = 0.050), while other variables such as age, sex, and World Federation of Neurosurgical Societies (WFNS) score were not statistically significant. ROC analysis showed moderate discriminative power of aneurysm presence for poor outcomes (AUC = 0.72). Conclusions: The presence of an aneurysm, although rare in pmSAH, significantly worsens functional outcomes. These findings highlight the necessity of early and sensitive vascular diagnostics-particularly DSA-to reliably exclude aneurysms. Differentiating between aneurysmal and non-aneurysmal perimesencephalic bleeding is essential not only for clinical decision-making but also for optimizing resource allocation in neurocritical care.