The clinical severity was classified as Hunt and Hess grade II and World Federation of Neurosurgical Societies grade I. Non-contrast computed tomography showed acute primary intraventricular hemorrhage: blood sedimented in the dependent occipital horns of both lateral ventricles, while diffuse cisternal or cortical subarachnoid hemorrhage was conspicuously absent. The unusual combination of ventricular blood without widespread subarachnoid bleeding led investigators to look for an occult vascular source near the ventricular system.

The culprit proved to be a 3 by 3 mm saccular aneurysm arising from the highly tortuous telovelotonsillar segment of the left posterior inferior cerebellar artery, or PICA. The PICA has a complex anatomical course and supplies the medulla and inferior cerebellum, while aneurysms originating from its distal segments account for approximately 0.5% to 3% of intracranial aneurysms. Its relationship to the roof of the fourth ventricle and the foramina of Luschka and Magendie provides a possible route by which a ruptured distal aneurysm can send arterial blood directly into the ventricular system, bypassing the subarachnoid space.

This is an exceptionally rare presentation, and the authors noted that isolated intraventricular hemorrhage without a parenchymal hematoma or diffuse subarachnoid blood represents only about 1% to 3% of spontaneous intracranial hemorrhages. Multidetector computed tomography angiography of the head and neck was initially interpreted as negative for a macroscopic intracranial aneurysm. The report noted only a subtle, noncalcified hypodensity near the left foramen magnum, which was attributed to atherosclerotic plaque or a complex overlapping vascular loop.

Retrospective analysis showed that the suspicious structure occupied the same position as the aneurysm ultimately found by angiography. The misclassification illustrates a recognized limitation of CTA in the posterior fossa, where the high attenuation of the petrous temporal and occipital bones can produce beam-hardening and photon-starvation artifacts that obscure vessels and small aneurysms. The limited spatial resolution of standard clinical CTA, typically with voxel sizes of 0.5 to 0.6 mm, also makes sub-centimeter, tortuous lesions susceptible to partial-volume averaging.

For a 3 mm aneurysm, those effects can blur a saccular outpouching into an appearance resembling plaque on an axial source image. Retrospective multiformat maximum-intensity projections allowed investigators to re-examine both the aneurysm and the broader vascular anatomy. The reconstructions revealed a unilateral right-sided fetal-type posterior cerebral artery, characterized by an absent P1 segment and direct origin of the right posterior cerebral artery from the internal carotid artery.

They also showed pronounced vertebrobasilar asymmetry: the left vertebral artery was dominant and highly tortuous, whereas the right vertebral artery was hypoplastic. A fetal-type posterior cerebral artery redirects flow from the internal carotid circulation and reduces the basilar artery’s contribution to posterior-circulation flow, while unequal vertebral arteries produce asymmetric inflow. The authors proposed that this combination might create an unusual flow environment near the PICA branch point.

The report did not include patient-specific computational fluid-dynamics modeling, so the proposed hemodynamic link was presented as a hypothesis rather than as a demonstrated mechanism of aneurysm formation. Because the isolated intraventricular hemorrhage remained unexplained and the CTA findings were equivocal, a multidisciplinary neurovascular board mandated four-vessel digital subtraction angiography within 24 hours of admission. Selective microcatheterization of the left vertebral artery with three-dimensional rotational angiography definitively exposed the 3 by 3 mm saccular aneurysm.

Its dome was in the same geometric location as the structure that had been misidentified on the initial CTA, and the lesion was identified as the origin of the hemorrhage. The patient then underwent targeted endovascular coil embolization, with detachable coils used to secure the dome and reduce the risk of another rupture. Follow-up imaging at three months demonstrated complete occlusion.

Clinically, the patient made an uneventful recovery and had a modified Rankin Scale score of 0 at discharge. Isolated intraventricular hemorrhage can arise from several types of lesion, including hypertensive microangiopathy, arteriovenous malformations, aneurysms and choroid plexus abnormalities. Its distribution in this case did not provide the familiar visual cue of diffuse aneurysmal subarachnoid hemorrhage.

Instead, the CT showed blood concentrated in the ventricular system, with little or no visible blood in the basal cisterns or cortical surface. The source report warns that this atypical pattern can be misattributed in emergency care to hypertension or an occult bleeding disorder, creating false diagnostic confidence while a treatable aneurysm remains undetected. Previous reports have documented isolated intraventricular hemorrhage caused by distal PICA aneurysms, but the authors described this case as the first to report the combination of that bleeding pattern, a unilateral fetal-type posterior cerebral artery and marked vertebrobasilar asymmetry.

That combination was presented as a possible morphological predisposition, not as a population-level risk estimate. The imaging sequence provides the clearest practical lesson from the case. The aneurysm was present on the original CTA dataset, but its small size, tortuous location and resemblance to plaque led to an incorrect interpretation.

Retrospective multiplanar maximum-intensity projections helped reveal its saccular form, while digital subtraction angiography established the diagnosis and defined its relationship to the left PICA. The report therefore emphasizes the need for a low threshold for catheter angiography when an isolated ventricular hemorrhage has no identified source, particularly when CTA is negative or uncertain in the artifact-prone skull-base region. This is a diagnostic observation from one patient, not a comparison of imaging strategies or proof that DSA improves outcomes across a broader population.

The case also does not establish how frequently small distal PICA aneurysms are missed on routine CTA. The proposed hemodynamic explanation is the most provocative yet least settled aspect of the report. The basilar artery normally serves as a major downstream recipient of vertebral inflow.

A unilateral fetal-type posterior cerebral artery reduces downstream basilar demand, and the dominant, highly tortuous left vertebral artery supplies more inflow than the hypoplastic right vessel. The authors infer that this mismatch could direct high-momentum flow toward the origin of the left PICA, encouraging flow separation, local turbulence and elevated wall shear stress. Such shear forces have been linked in biomechanical research to endothelial degradation and saccular aneurysm formation.

The aneurysm’s location in the distal loop of the dominant vertebral system is spatially consistent with that idea, but spatial correspondence alone cannot establish causation. Without patient-specific fluid simulation, pressure measurements or longitudinal observations, the case cannot determine whether the vascular configuration caused the aneurysm, promoted its growth or simply coexisted with it. Taken together, the case shows how a tiny aneurysm, an unusual route of bleeding and a difficult imaging environment can combine to create a deceptive emergency presentation.

The distal PICA aneurysm was not entirely absent from the first CTA; it was obscured by posterior-fossa artifacts and partial-volume effects and then misread as plaque. Its rupture produced a blood pattern that lacked the diffuse subarachnoid component often associated with aneurysmal hemorrhage, while the unusual posterior-circulation anatomy supplied a plausible but unvalidated biomechanical context. The reported clinical course was favorable after DSA localized the lesion and coils secured it, with complete occlusion visible at three-month follow-up.

Broader conclusions must remain limited: this single case cannot show that fetal-type posterior cerebral arteries or vertebrobasilar asymmetry routinely cause PICA aneurysms, nor can it quantify the diagnostic accuracy of CTA for these lesions. Its strongest contribution is to document a missed distal PICA aneurysm presenting as isolated intraventricular hemorrhage and to show how careful image reconstruction, followed by catheter angiography, can reveal a small lesion that was visible but overlooked.

Subject of Research: Single-patient case report of a missed distal PICA aneurysm with isolated intraventricular hemorrhage and unusual posterior-circulation anatomy

Article Title: The Hemodynamic Trap: Fetal‐Type Posterior Cerebral Artery and Occult PICA Aneurysms

Article References: Khadem, S. H., Ghari Saadati, A., Fathabadi, A., & Khoroushi, F. (2026). The Hemodynamic Trap: Fetal‐Type Posterior Cerebral Artery and Occult PICA Aneurysms. Clinical Case Reports, 14(10), Article e73711. https://doi.org/10.1002/ccr3.73711

Image Credits: AI Generated

DOI: 10.1002/ccr3.73711

Keywords: PICA aneurysm, intraventricular hemorrhage, computed tomography angiography, digital subtraction angiography, fetal-type PCA, vertebrobasilar asymmetry, posterior fossa, endovascular coiling, case report, brain hemorrhage, Hemodynamic, Trap