• Perimesencephalic subarachnoid haemorrhage
• posterior cerebral artery
• intracranial aneurysm
• subarachnoid haemorrhage

Perimesencephalic subarachnoid haemorrhage is a distinct radiographic and clinical entity of uncertain aetiology. This syndrome is diagnosed by the characteristic pattern of extravasated blood on brain CT in combination with a negative cerebral angiography.1 Current practice requires digital subtraction angiography (DSA) to exclude an aneurysm in patients with a perimesencephalic pattern of subarachnoid haemorrhage (SAH). Although up to 96% of patients with this pattern of SAH do not have an aneurysm, and will undergo an uneventful clinical course with invariably good outcome,1 about 4% of patients might have a vertebrobasilar aneurysm requiring emergent treatment.1

We present a patient with a perimesencephalic pattern of SAH caused by rupture of dissecting posterior cerebral artery (PCA) aneurysm. To our knowledge, this is the first reported case of perimesencephalic pattern of SAH caused by a dissecting aneurysm in such a location.