Subarachnoid haemorrhage of unknown aetiology.

One hundred and forty consecutive subarachnoid haemorrhages (SAH) which presented either an intracranial vascular malformation (102 cases: aneurysm 80 cases, AVH 22 cases) or remained of "unknown aetiology" (38 cases) were studied. SAH caused by other factors (neoplasms, thrombo-embolisms, systemic diseases etc.) were excluded. The 38 cases with bleeding of "unknown aetiology" have been studied by complete cerebral angiography, pneumoencephalogram or CT scan, and have been followed for two years. The most important clinical factors in the three groups have been compared by a statistical method to verify the hypothesis that SAH of "unknown aetiology" is caused by vascular micromalformations which are angiographically not evident either because of their small size or because of their "spontaneous" recovery after bleeding due to thrombosis and disappearance of the malformation. From the data collected it may be concluded that SAH of "unknown aetiology" is a benign lesion, typically occurring in middle age, but with no clear characteristics that enable identification of the anatomical substratum from it originates.