"95% of cases of perimesencephalic subarachnoid haemorrhage have a normal cerebral angiogram and the source of bleeding is not identified; the cause is thought to be a venous bleed. This is referred to as non-aneurysmal perimesencephalic SAH. Transient elevation of venous pressure, especially in individuals with anomalous venous drainage patterns, may be a risk factor 6. Recurrent haemorrhage is rare."
"95% of cases of perimesencephalic subarachnoid haemorrhage have a normal cerebral angiogram and the source of bleeding is not identified; the cause is thought to be a venous bleed. This is referred to as non-aneurysmal perimesencephalic SAH. Transient elevation of venous pressure, especially in individuals with anomalous venous drainage patterns, may be a risk factor 6. Recurrent haemorrhage is rare."
"The other 5% of cases are due to a vertebrobasilar aneurysm and the prognosis is worse 1,2. Rare causes include arteriovenous malformation, dural arteriovenous fistula, trauma and vascular tumours 4."
"There are specific criteria for non-aneurysmal perimesencephalic SAH, which, some have argued, negate the need for DSA if fulfilled with a negative CTA 5:"