How common are brain aneurysms?
Unruptured brain aneurysms are present in approximately 3.2% of the population. Put another way a little more than one in fifty persons will have a brain aneurysm. In most cases an aneurysm will have developed over the course of a persons' life. Brain aneurysms do occur in children but are extremely rare. Less than five per-cent of aneurysms are diagnosed before the age of eighteen years.
Rupture of aneurysms has been identified as occurring more commonly in the Finnish and Japaneese populations but the prevalence of unruptured aneurysms does not seem to differ from the figure above suggesting that there are environmental factors that render those populations more susceptible to bleeding. Most people who harbour a brain aneurysm a unaware of its presence.
There are diseases which increase the likelihood that cerebral aneurysms will develop. Patients with Adult Polycystic Kidney Disease (APCKD) are more than twice as likely to develop an aneurysm possibly becuase they are more likely to be exposed to period of prolonged high blood pressure. These aneurysms do not seem to be at increased risk of rupture per-se. Other conditions assosciated with the development of aneurysms include:
- Ehler-Danlos syndrome
- Marfans's syndrome
- Loeyes-Dietz syndrome
- Coarctation of the aorta
- Alpha 1 anti-trypsin deficiency
- Neurofibromatosis type 1
- Fibromuscular dysplasia
- Tuberous sclerosis
- Alpha-glucocidase deficency
What is the risk that an aneurysm would bleed?
A great deal of effort has gone into efforts to understand the natural history of a cerebral aneurysm- that is; what occurs if the aneurysm is left without treatment. The truth is that we cannot predict the future reliably and your specialist will only be able to offer an estimate of the risk of rupture over time. The best quality research we can presently refer to comes from The International Study of Unruptured intracranial Aneurysms (ISUIA2). and a large Japaneese clinical trial called the Unruptured Cerebral Aneurysm Study (UCAS). From these and similar studies it has been learned that two factors are assosciated with a risk of bleeding:
- The size of the aneurysm
- The exact location of the aneurysm within the brain
The smallest aneurysms appeared to have relatively small risks of bleeding in the short term. The results of ISUIA2 are frequently, but erronously, cited as indicating small anterior circulation aneurysms below 7mm in diameter have no risk of bleeding in the short term. It is more correct to say their risk of bleeding may be less than the risks of treatment in the short term. We also see that the risk of treatment increase as the aneurysms grow larger.
A careful balance must therefore be struck between the risk of intervening and what could be gained from treating the aneurysm. As one ages the number of life-years one might gain by treating an unruptured aneurysm reduces. Inevitably the risks assosciated wiith treatment tend to rise later in life and the risk-to-benefit relationship grows less favorable.
Furthermore the above is likely to be rather an oversimplification of what promotes aneurysmal bleeding. The shape and size of the affected blood vessels may have a haemodynamic affect which increases the stresses on the wall of the aneurysm. Female sex appears assosciated with higher rates of ruptured aneurysms. It is increasingly clear that there individual may harbour genetic vunerabilities that puts them at increased risk both of aneurysm development and subsequent bleeding. It is not yet possible however to synthesise these data and provide a precise, individualised risk.
For some the discovery of an unruptured aneurysm, even if believed to be of relatively low-risk, may become a psychological burden. The management of each aneurysm must therefore be considered on a case-by-case basis. There is usually not pressure to reach a final decision quickly and it is important to take time to arrive at the right decision for the individual.
Can the risk of rupture be reduced?
Yes.There are factors which we understand increase the chances that an aneurysm will bleed in the future. Some have a greater effect than others but it stands to reason that where we can modify our behaviour or lifestyle to remove or control such factors we should be able to reduce the risk of an aneurysm rupturing. These risk factors include:
- Cigarette Smoking
- Uncontrolled High Blood Pressure
- Excessive Alcohol Intake
- Cocaine Use
Some of these factors probably act in concert to damage blood vessels and the benefits of controlling them are likely to take time to be realised. It is clear that cessation of cigarette smoking should be strongly encouraged. In countries where cigarette smoking has demonstrably reduced the rates of aneurysm rupture have been seen to reduce as well.