Isolated intraventricular hemorrhage from an anterior communicating artery aneurysm via the lamina terminalis: illustrative case.

BACKGROUND: Rupture of an anterior communicating artery (ACoA) aneurysm causing isolated intraventricular hemorrhage (IVH) without accompanying subarachnoid hemorrhage (SAH) is exceptionally rare. Although an anatomical mechanism has been proposed, direct supporting evidence remains scarce. OBSERVATIONS: A 78-year-old male presented with isolated IVH initially attributed to hypertensive hemorrhage. Retrospective review of MR angiograms, prompted by a persistent hyperdense focus in the third ventricle on follow-up CT, revealed a small aneurysm at the right A1-A2 junction. Three-dimensional CT angiography and intraoperative findings demonstrated aneurysmal dome engagement with the lamina terminalis, suggesting protrusion into the third ventricle. The patient underwent surgical clipping and was discharged without neurological deficits. LESSONS: This case offers radiological and intraoperative evidence that an ACoA aneurysm may extend toward the third ventricle through the lamina terminalis and present as isolated IVH. This atypical presentation poses diagnostic challenges, including a tendency to initially attribute the hemorrhage to hypertensive causes. Clinicians should recognize that a single negative or nondiagnostic MR angiography study does not exclude an aneurysmal source, particularly when small lesions are not specifically sought. When alternative etiologies are excluded and findings support an aneurysmal origin, prompt treatment should be considered even without SAH. https://thejns.org/doi/10.3171/CASE26603.