ABSTRACT Cerebral arterial aneurysms are a significant source of morbidity and mortality throughout the world. Cerebral aneurysms are well documented, with characteristic imaging findings both pre- and postrupture and typical morphologies and locations well described in the literature. Our case illustrates a rare entity of an intraparenchymal cerebral arterial aneurysm; an entity which was previously described rarely and only within the anterior circulation. We present a very rare case of an incidentally discovered, partially thrombosed posterior circulation (PICA) aneurysm fully invested within the parenchyma of the medulla. A 41-year-old man with no known past medical history presented as an unrestrained passenger status post-motor vehicle collision, seated in the rear of a prisoner transport van near the point of impact. On presentation he was alert and oriented, with a reported Glasgow Coma Scale of 15. The patient was able to provide history and reported loss of consciousness initially after the collision and, on presentation to the emergency department, complained of headache and "blurry vision." A CT scan of the brain was ordered by the emergency physician. Noncontrast-enhanced CT scan of the brain demonstrated a focus of hyper-attenuation in the region of the obex, on the inferior most sections. No other focal abnormalities were identified. Given the history of trauma, the differential diagnosis included a brain stem or fourth ventricle hemorrhage versus an incidental mass lesion, such as a cavernous malformation, hemangioblastoma, or possibly meningi-oma. A neurosurgery consult was obtained and the patient, who remained clinically stable, went on to MR imaging of the brain. On MR imaging, the lesion was noted to be well circumscribed, with a smooth semilunar hemosiderin rim and central T1 hyperintensity, and was fully invested within the parenchyma of the medulla (Fig 1). Given the hemosiderin rim and intrinsic T1 hyper intensity representing various stages of blood products, cavernous malformation remained a differential diagnostic consideration (Fig 2).1 There was minimal mass effect, making other mass lesions much less likely. However, despite the signal intensity characteristics, which were typical of cavernous malformation, the smooth nature of the rim versus the classic "popcorn" appearance expected in cavernous malformation led to continued inclusion of atypical aneurysm on the differential. CTA and then conventional angiogram were subsequently performed and confirmed the diagnosis of PICA branch aneurysm (Figs. 3 and 4). The various stages of blood products seen on MR imaging suggested prior partial thrombosis. The patient was treated with dexamethasone, aspirin, and plavix before undergoing endovascular stent placement and coiling. He complained of intermittent diplopia immediately postprocedure, which corresponded with a right homonymous hemianopsia on neuro-opthalmologic testing. His symptoms gradually decreased in frequency before discharge.
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