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Natural History and Treatment Options


A brain aneurysm resembles a bubble or a bulge on the arteries. Typically, it arises from a weak point on the arterial wall due to wear-and-tear changes over time. Aneurysms can lead to trouble if they burst (rupture), if they increase in size (compressing surrounding brain or nerves) or if solid material from within the aneurysm sac breaks off and travels downstream to block an artery causing a stroke. The most common cause for trouble is bleeding due to aneurysm rupture.

Most aneurysms develop sporadically (i.e., for no good reasons) or are acquired from cumulative “wear-and-tear” changes in the blood vessel wall. Women have a higher tendency to develop brain aneurysms, and as a result, they have a greater frequency of bleeding. Other established risk factors for aneurysm development include excessive and active cigarette smoking, uncontrolled high blood pressure, a strong family history of brain aneurysms, older age (over 49 years), recreational drug use (especially cocaine), or in people affected by polycystic kidney disease.

Natural History 

Not all brain aneurysms are destined to bleed from the time of diagnosis. Some aneurysms may be present for only months or weeks before rupturing. Some aneurysms appear to remain stable for many years. 

The risk of bleeding is different for everyone, depending on the size of the aneurysm, where along the arterial tree it is located, and the interplay with individual patient's risk profile. The true natural history of unruptured brain aneurysm is not clear, but in general, it is accepted that there is a 1 in 100 chance of bleeding each year for the more commonly encountered brain aneurysms.

Established factors that may affect the risk of bleeding include:

  • Size - the larger the aneurysm, the greater the wall tension and therefore the higher risk of rupture

  • Shape - irregular shaped aneurysms are more likely to bleed at smaller sizes 

  • Location - certain locations along the base of the brain where aneurysms form are more risky than others due to differences in blood flow dynamics and

  • Age - older patients face a greater risk of bleeding due to cumulative "wear and tear" insults and an overall diminished ability to repair the injured aneurysm wall. 

The risk of bleeding for a given brain aneurysm is dynamic and not constant, depending on the patient's age, changes in aneurysm sizes, and changes in shape over time. Therefore, an individual assessment of the bleeding risk requires expert evaluation.

Treatment Options

Not all unruptured brain aneurysms require treatment at the time of diagnosis. Treatment recommendation needs to take into consideration the risk versus benefit ratio of pre-emptively closing down the aneurysm. These factors include patient's physiological age, aneurysm risk profile, and the treating surgeon's skillset and track record in repairing brain aneurysms. 

In general, aneurysms can be occluded by one of two ways: 1) by going inside the blood vessel and insert metallic coils (endovascular coiling) or metal stents (across the blood vessels) to occlude the sac; or 2) by repairing the aneurysm from the outside of the artery through a small skull opening and placing a strong spring clamp (clip) onto the aneurysm. In general, surgery for clipping of brain aneurysm provides the best long-term durability but requires an open skull procedure to accomplish this task. Endovascular coiling offers a less invasive treatment alternative, but lacks the robustness of repair and may not provide complete protection from the aneurysm. Up to one in four patients may require repeat treatment following endovascular intervention. It is important to discuss the pros and cons of each of these treatment options relevant to your particular brain aneurysm with your neurosurgeon. 

Some useful references 

  1. Lai LT, Morgan MK, NJ Patel. Smoking increases the risk of de novo intracranial aneurysms. World Neurosurgery 82(1-2), e195-e201.

  2. Huang H, Lai, LT. Incidence and case-fatality of aneurysmal subarachnoid hemorrhage in Australia, 2008-2018. World Neurosurgery 144, e438-e446.  

  3. Huang H, O'Neill AH, Chandra RV, Lai, LT. Asymptomatic intracranial aneurysms in the elderly: Long-term clinical and radiologic follow-up of 193 consecutive patients. World Neurosurgery 133, e600-e608.

  4. O'Neill AH, ... Lai LT. Influence of comorbidities on treatment of unruptured intracranial aneurysms in the elderly. Journal of Clinical Neuroscience 62, 38-45.

  5. O'Neill AH, Chandra RV, Lai LT. Safety and effectiveness of microsurgical clipping, endovascular coiling and stent assisted coiling for unruptured anterior communicating artery aneurysms: a systematic analysis of observational studies. Journal of neurointerventional surgery 9(8), 761-765.

  6. Asaid M, O'Neill AH, Bervini D, Chandra RV, Lai LT. Unruptured paraclinoid aneurysm treatment effects on visual function: systematic review and meta-analysis. World Neurosurgery 106, 322-330.

  7. Lai LT, Morgan MK. Outcomes for unruptured ophthalmic segment aneurysm surgery. Journal of Clinical Neuroscience 20(4), 587-592.

  8. Lai LT, Gragnaniello C, Morgan MK. Outcomes for a case series of unruptured anterior communicating artery aneurysm surgery. Journal of Clinical Neuroscience 20 (11) 1498-1502.

  9. Lai LT, Morgan MK. Use of indocyanine green videoangiography during intracranial aneurysm surgery reduces the incidence of postoperative ischaemic complications. Journal of Clinical Neuroscience 21(1), 67-72.

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