The Hemodynamic Trap: Fetal-Type Posterior Cerebral Artery and Occult PICA Aneurysms.

Isolated intraventricular hemorrhage (IVH) is an uncommon presentation of ruptured aneurysms, often misdiagnosed as hypertensive hemorrhage. Distal posterior inferior cerebellar artery (PICA) aneurysms can easily be missed on computed tomography angiography (CTA) due to their small size and skull-base artifacts. A 48-year-old female presented with a thunderclap headache. Initial imaging showed isolated IVH without subarachnoid hemorrhage. A primary CTA was read as negative, noting only a suspected plaque near the foramen magnum. However, multiformat reconstructions revealed a right-sided fetal-type posterior cerebral artery (fPCA) and a dominant left vertebral artery. High clinical suspicion prompted digital subtraction angiography (DSA), which confirmed a 3 × 3 mm distal PICA aneurysm exactly where the "plaque" was suspected on the CTA. This case highlights the limitations of CTA in the posterior fossa. It also suggests a hemodynamic mechanism for aneurysm formation: the combination of an fPCA and asymmetric vertebral inflow alters local blood flow. We hypothesize that this architectural mismatch may cause turbulence and increased wall shear stress, predisposing the distal PICA to aneurysm formation. In patients with isolated IVH and an fPCA variant, a negative CTA cannot definitively rule out a vascular lesion. Early DSA is essential to detect occult aneurysms and prevent subsequent rebleeding.