Magnetic resonance angiography demonstration of an angiographically occult anterior communicating artery aneurysm.

In almost one in six patients with spontaneous subarachnoid haemorrhage (SAH) no lesion responsible for the bleed will be found by cerebral angiography. Current management strategies include repeat cerebral angiography after a period of 1-8 weeks during which a number of these patients will rebleed with considerable morbidity and even mortality. We report the case of a young female patient with spontaneous subarachnoid haemorrhage. Brain computed tomography (CT) demonstrated blood mainly in the basal frontal interhemispheric fissure suggesting the presence of an anterior communicating artery (ACoA) aneurysm. Cerebral angiography was negative. Magnetic resonance (MR) angiography performed the same day showed a small aneurysm arising from the posterior wall of the ACoA. This was confirmed at surgery. It is suggested that MR angiography, guided by the CT findings, probably has a place in the management of patients with SAH of "unknown aetiology" before repeat catheter angiography is undertaken.