Subarachnoid haemorrhage and a normal angiogram: Distinction between a perimesencephalic and an aneurysmal pattern of haemorrhage on CT

In 15 to 20 percent of patients with a spontaneous subarachnoid haemorrhage no aneurysm is found on the initial angiogram. This group of patients is not homogeneous. Two thirds of these patients have a non-aneurysmal perimesencephalic pattern of haemorrhage on CT and these patients invariably have a good prognosis, which obviates the need for a second angiogram. Most of the remaining one third of patients have an occult aneurysm, and these patients are at risk of rebleeding, other complications and a poor outcome. This article describes the clinical features, pattern of haemorrhage on CT and management of these two different subsets of patients with a subarachnoid haemorrhage and without an aneurysm on the initial angiogram.