Posterior inferior cerebellar artery aneurysm in the fourth ventricle.

We report a patient with a ruptured aneurysm of the choroidal branch of the right posterior inferior cerebellar artery (PICA), lying in and causing an isolated haemorrhage in the fourth ventricle. MRI on the first day after bleeding revealed an abnormal vessel in the fourth ventricle, which was surrounded by a mass of intermediate signal on T1- and T2-weighted images. The aneurysm was clipped via partial splitting of the lower vermis and opening the inferior medullary velum. A postoperative angiogram confirmed complete obliteration of the aneurysm. With PICA aneurysms the rate of intraventricular haemorrhage is high and in most cases due to reflux of blood. If there is an isolated intraventricular haemorrhage, a peripheral PICA aneurysm, lying in or near the fourth ventricle, may be suspected.