Marlow, ChristineCuoco, Joshua A.Ravina, KristineSloboda, Cole A.Entwistle, John J.2023-12-042023-12-042023-05-222694-1902PMC10550645CASE23150 (PII)https://hdl.handle.net/10919/116717BACKGROUND Pure arterial malformations are characterized as unique cerebrovascular lesions with a dilated, coil-like appearance and tortuous arteries without early venous drainage. Historically, these lesions have been described as incidental findings with a benign natural history. However, pure arterial malformations can rarely demonstrate radiographic progression and develop associated focal aneurysms with an unclear risk of rupture. Whether radiographic progression of these lesions or the presence of an associated aneurysm warrants treatment remains controversial. OBSERVATIONS A 58-year-old male presented with sudden-onset left hemiparesis. Computed tomography revealed a large, acute, right frontotemporoparietal intraparenchymal hemorrhage with underlying irregular curvilinear calcifications. Diagnostic cerebral angiography revealed a dysplastic right middle cerebral artery dissecting aneurysm along the M2 segment associated with a pure arterial malformation, which was treated with endovascular flow diversion in a delayed fashion. LESSONS Pure arterial malformations with associated focal aneurysms may not exhibit a benign natural history as once thought. Intervention should be considered for ruptured pure arterial malformations to mitigate the risk of rerupture. Asymptomatic patients with a pure arterial malformation with an associated aneurysm should at least be followed closely with interval radiographic imaging to evaluate for malformation progression or changes in aneurysmal morphology.Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 Internationalaneurysmcerebrovascularendovascularpure arterial malformationvascular lesionvascular malformationArticle - Refereedhttps://doi.org/10.3171/CASE23150521372187312694-1902