An unusual aneurysm arising from an extracranial lateral medullary segment of the posterior inferior cerebellar artery (PICA) is reported. The origin of the PICA was also extracranial, 10 mm below the foramen magnum. The aneurysm was not seen on three-vessel angiography. The literature is reviewed with regard to the clinical and radiological features of such aneurysms. Occipital and nuchal headache with an altered level of consciousness, intraventricular haemorrhage, and hydrocephalus are suggestive of such aneurysm. The need for four-vessel angiography is again stressed.
