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Intracranial Aneurysm
October  2019

A 49-year-old man, asymptomatic and a runner, applied for life insurance. His mother died of a ruptured cerebral aneurysm (age at death not provided). Two years prior to the application, he was having some headaches and a magnetic resonance angiography (MRA) was performed. It showed a 4-mm aneurysmal dilatation of the middle cerebral artery. No further treatment or follow-up was reported.

What is the significance of a 4-mm intracranial aneurysm and the family history of rupture?

Intracranial or Cerebral Aneurysms (ICA) were found in 1.8% if asymptomatic individuals in the population-based Rotterdam study. There is a slight female preponderance for having ICA. The prevalence increases with age and with certain conditions like adult polycystic kidney disease. The prevalence ratio was reported as 3.4 when there was a family history of aneurysm or subarachnoid hemorrhage. Another study found a prevalence of ICA in 8.7% of first-degree relatives of patients with ICA.

While some connective tissue diseases such as Ehlers-Danlos and pseudoxanthoma elasticum do show an association with ICA, only a small fraction of familial cases have an identifiable heritable syndrome. It has been suggested that aneurysms occur in similar locations in families and that they tend to rupture at a smaller size. Besides family history, other factors that increase the risk of aneurysms and subarachnoid hemorrhage are cigarette smoking, hypertension, estrogen deficiency and coarctation of the aorta.

Approximately 85% of ICAs are in the anterior circulation of the Circle of Willis. Sites at the junction of two arteries are the most common. Intracranial aneurysms may be described as saccular or fusiform. Saccular aneurysms are thin-walled and more prone to rupture. Most ICAs are asymptomatic, although rarely they can place pressure on a nerve causing a cranial neuropathy.

There is also an increased risk (2x or more) of ICA rupture in the posterior arterial system (vertebrobasilar, posterior cerebral arterial system or posterior communicating arteries) as compared to the anterior system (anterior communicating, anterior cerebral or internal carotid arteries). When there was a family history of subarachnoid hemorrhage and an ICA, there was an observed rupture rate of 1.2% per year which was 17 times higher than the rate observed in the International Study of Unruptured Intracranial Aneurysms (ISUIA).

​Aneurysm Size
​Frequency of enlargement over 47 months
​< 8mm
​8mm - 12mm
​> 12mm

Aneurysm Size
​Frequency of rupture 
over 5 years

​7mm - 12mm
13mm - 24mm
​> 24mm
Figure 1 - Aneurysm growth and rupture is more common in larger aneurysms

Treatment of ICA is not without risk. One meta-analysis of reports found a 1.7% mortality risk of aneurysm clipping and 6.7% rate of unfavorable outcomes (morbidity). There are reports that endovascular repair, as opposed to surgical repair, results in better mortality and morbidity outcomes. Indications are that older age patients (>70) do not fare as well as younger patients. Smaller aneurysms (<7 mm) are generally not repaired due to the lower risk of rupture. Age and location of the aneurysm as well as risk factors and patient preferences all play a role in determining treatment.

Returning to the case
A 4-mm intracranial aneurysm is considered small and at low risk of rupture. However, the family history increases the risk of both growth and rupture. At a minimum it would be prudent to have at least one follow-up evaluation by a neurologist to determine if there is any growth or development of additional findings.

  • Vernooij MW et al, “Incidental findings on brain MRI in the general population,” N Engl J Med. 2007;357(18):1821.
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  • Morita A et al, “The Natural Course of Unruptured Cerebral Aneurysms in a Japanese Cohort,” N Engl J Med 2012;366:2474-82.
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  • Broderick JP et al, “Greater Rupture Risk for Familial as Compared to Sporadic Unruptured Intracranial Aneurysms,” Stroke. 2009 June ; 40(6): 1952–1957.
  • UpToDate last accessed Aug 2019.
  • Backes D et al, “Patient- and Aneurysm-Specific Risk Factors for Intracranial Aneurysm Growth A Systematic Review and Meta-Analysis,” Stroke. 2016;47:951-957.