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Tuesday 15 June 2021
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What causes a Sub-Arachnoid Haemorrhage?



In approximately 75% of cases, leakage of blood occurs from a weak spot in the wall of an artery at the base of the brain that bulges outwards to form an aneurysm. This leakage of blood lasts only for a few moments and then stops spontaneously as a rule. The reasons why aneurysms develop is not fully understood but we do know that aneurysms are a relatively common finding and occur in approximately 2-4% of the population and are not usually associated with symptoms.


Approximately 20% of patients have more than one aneurysm. Ruptured aneurysms pose a much greater risk than those that have not ruptured, so there is no urgency to treat an aneurysm that has not bled. Indeed, it is not always wise to treat this kind of aneurysm. The issue will usually be discussed with you at your follow up appointment. Rupture of an aneurysm is unusual and the risk of rupture is increased by smoking, high blood pressure and excess alcohol. Once an aneurysm has bled, there is a high risk of re-bleed in the short term and so it is important to block off the aneurysm so it cannot bleed again. This is done either from within the aneurysm using special coils or by closing the neck of the aneurysm using a surgical clip during an operation.


No Vascular Abnormality Found


In approximately 15% of cases there is no vascular abnormality that is identified on cerebral angiography, which could have caused the sub-arachnoid haemorrhage. It is not known why such haemorrhages occur, but we do know that prognosis or outlook is good, and the risk of you having a future sub-arachnoid haemorrhage is very low. As no abnormality which could have caused the haemorrhage has been identified, there is no area that requires treatment, and therefore no neurosurgical or radiological intervention is necessary. This is a good result, and it is likely that whatever caused the bleed has subsequently healed itself in the process. You may go on to have a further angiogram, or MRI Scan, in order to confirm the absence of any cause. Despite the fact that intervention is not required, you will still need to allow yourself time to recover from the sub-arachnoid haemorrhage itself.


Arterio-Venous Malformation (AVM)


An arterio-venous malformation (AVM) is the cause of sub-arachnoid haemorrhage in approximately 5% of cases. An AVM is an abnormality of the vascular network, whereby arteries and veins develop in a haphazard manner, often forming a knot-like appearance of vessels. An AVM will have been present from birth. The irregular formation of the vessels gives rise to weaknesses, which can result in haemorrhage. An AVM is an area of weakness of the vessels, and there is a risk that you might have another haemorrhage in the future if it is not treated. The treatment of AVMs will depend on many factors including the size, position and exact blood vessels that are involved. The chance of another bleed from an AVM is less than from an aneurysm, and this will influence treatment options. The treatment of an AVM is very individualised and the consultant will discuss the most suitable plan with each person, but it may involve a combination of surgical and radiological procedures and sometimes a special form of radiotherapy, although it must be emphasized that AVMs are not tumours.




There are other less common causes of spontaneous sub-arachnoid haemorrhage which can include bleeding from a tumour or problems with blood clotting.