Extensive edema in the thalamus caused by thrombosed basilar artery aneurysm.

A 69-year-old female presented with sudden onset of truncal ataxia, urinary incontinence, mental confusion, and Parinaud's sign. With conservative treatment, her ataxia and urinary incontinence resolved. Magnetic resonance (MR) imaging disclosed a round mass with laminated intramural hemorrhage in the third ventricle. Right vertebral angiography demonstrated a giant aneurysm in the distal basilar artery. Xenon-enhanced computed tomography showed that cerebral blood flow (CBF) was reduced in the thalamus bilaterally and was paradoxically decreased by acetazolamide. Two months later, MR imaging showed that the intramural hemorrhage had shrunk, and the edema in the thalamus was resolving. The CBF reduction and vascular response to acetazolamide had reversed to some extent. A partially thrombosed giant aneurysm can grow acutely as the result of fresh intramural hemorrhage. The edema is secondary to ischemia and loss of vasoresponsivity.