Thrombosed Large Distal Posterior Inferior Cerebellar Artery Aneurysm Mimicking an Infratentorial Ependymoma

Large or giant intracranial aneurysms can simulate brain tumors clinically and radiologically by virtue of their progressive mass effect. Unlike aneurysms from alternative locations, those arising from the distal posterior inferior cerebellar artery (PICA) are uncommon. We report a patient who experienced progressive hemiparesis with magnetic resonance imaging findings suggestive of an infratentorial ependymoma. Intraoperatively, a thrombosed large aneurysm of the distal PICA was unexpectedly encountered. The aneurysm was clipped and the patient did not develop any permanent neurological deficit. This case illustrates the radiological nuances of large aneurysms and infratentorial ependymomas. Three-dimensional contrast-enhanced magnetic resonance angiography can be falsely negative and the importance of the “target” sign is emphasized. One should be cognizant of this possible diagnosis for patients with midline fourth ventricular lesions in order to reduce surgical risk. 1. Introduction Aneurysms arising from the posterior inferior cerebellar artery (PICA) are uncommon and constitute 0.5 to 3.0% of all intracranial aneurysms [1]. While most arise at its origin from the vertebral artery, distal PICA aneurysms are less frequently encountered with an incidence of 0.3 to 1.4% [1–3]. Large aneurysms, defined as a diameter equal to or greater than 15？mm, arising from the distal PICA are rare [2, 4]. Half of these lesions are thrombosed on presentation and can imitate a posterior fossa neoplasm [5]. Occasionally the neurosurgeon is confronted with this unexpected finding intraoperatively and is required to immediately alter surgical strategies. Although posterior circulation aneurysms have been described to mimic tumors on computed tomography (CT) imaging since the 1970s, the occurrence of such has diminished with the advent of magnetic resonance imaging (MRI) and MR angiography (MRA) [5–12]. We report a patient with a partially thrombosed large distal PICA aneurysm that simulated an infratentorial ependymoma which was only discovered intraoperatively. The radiological features of these pseudotumor aneurysms are discussed. 2. Clinical Presentation A 79-year-old woman, with a history of hypertension, presented with dizziness and progressive left side weakness for two months. There was neither headache nor symptoms to suggest raised intracranial pressure. She found it increasingly difficult to walk and required assistance. Physical examination revealed left hemiparesis of medical research council (MRC) power grade 4/5. Deep tendon reflexes were equivocal and