The management of unruptured and asymptomatic intracranial aneurysms remains controversial. Since aneurysmal hemorrhage carries a high risk of mortality and morbidity it is essential to estimate the probability of aneurysm rupture in each individual case in order to decide whether treatment should be recommended. The likelihood that a given aneurysm will rupture has been the subject of many scientific studies. Tha largest studies looking at the behavior of unruptured aneurysms are ISUIA (International Study of Unruptured Intracranial Aneurysms) and UCAS (Unruptured Cerebral Aneurysm Study) - a Japanese undertaking

Below you will find two aneurysm calculators designed to make the data from these two trials easily accessible. Please read the guidance notes below the calculators for a detailed explanation of how the calculators work and their limitations.

 


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ISUIA / UCAs Aneurysm Calculators

ISUIA calculator

The ISUIA study observed the course of 4060 patients with unruptured intracranial aneurysms. 1692 patients with 2686 aneurysms were treated with conservative management; that is, they did not undergo surgical clipping or endovascular coiling. In this group the mean follow-up was 4.1 years totalling 6544 patient-years.

Aneurysm location was categorized into three groups:

1. Cavernous portion of the carotid artery.
2. Internal carotid artery, anterior cerebral artery, middle cerebral artery.
3. Posterior cerebral circulation (a heterogenous group including the vertebral arteries, basilar artery, posterior inferior cerebellar arteries, anterior inferior cerebellar arteries, superior cerebellar arteries, posterior cerebral arteries and the posterior communicating arteries).

The size of the aneurysms was categorized into four groups:

1.   < 7mm
2.   7-12 mm
3.   13-24 mm
4.    ≥25 mm

Patients were divided into two further groups - those who had previously experienced a subarachnoid haemorrhage (SAH) from a different aneurysm elsewhere in the cerebral circulation (Group 2 in the table below).  However, a significant difference in rupture rate was only found for those patients with previous SAH with small aneurysms (less than 7mm in diameter) in the anterior or posterior circulation.  For larger aneurysms in these territories, and all sizes of intracavernous aneurysms, whether the patient had had a previous SAH (Group 1 in the table below) had no significant bearing on rupture rates.

In the ISUIA study, rates of aneurysm rupture were reported as 5-year cumulative rupture rates according to size and location of the unruptured aneurysm. Below is the original table as published in the ISUIA 2003 article in The Lancet.

In the ISUIA aneurysm calculator above, the figures returned under 5 year rupture risk reflect those provided in the ISUIA table. Based on this table, the annual risk of rupture was calculated by simply dividing this number by five, giving identical figures to those reported by Da Costa et al in a 2004 Neurogurgical Focus article.  Cumulative lifetime risk based on this figure are calculated using the following equation:

         1 – (annual chance of not bleeding) x expected years of life  = risk of hemorrhage

Each time a figure for life expectancy is keyed in, the algorithm automatically updates the risk for the given aneurysm, based on its size and location.

In patients with small anterior circulation (<7mm) or intracavernous (<12mm) aneurysms, the rupture rate in the ISUIA trial was zero, i.e. no patients suffered an aneurysmal rupture. This calculator faithfully presents these data and thus returns a 0% five year rupture risk in these aneurysms, naturally equating to a 0% lifetime risk, regardless of life expectancy. This rate should be interpreted with caution as we know that small aneurysms in these locations are not entirely benign and do occasionally rupture, meaning the true rupture rate is above zero. For trhe details of the ISUIA study, please refer to the original the ISUIA 2003 article, The Lancet.

 

UCAS calculator

The UCAS study assessed 5720 patients with 6697 aneurysms. 2722 patients with 3050 aneurysms underwent surgical or endovascular repair, while the remainder received conservative treatment. The follow up period was 3 months to 9 years with a total of 11660 aneurysm-years.

Aneurysm location was categorized into seven groups:

1. Middle cerebral artery
2. Anterior communicating artery
3. Internal carotid artery
4. Internal caotid – posterior communicating artery
5. Basilar tip and basilar – superior cerebellar artery
6. Verterbal arteries, the posterior inferior cerebellar arteries and vertebro-basilar junction
7. Others

The size of the aneurysms was categorized into five groups:

1.   3-4 mm
2.   5-6 mm
3.   7-9 mm
4.   10-24 mm
5.   ≥ 25 mm

In the UCAS study, rates of aneurysm rupture were reported as annual rate of rupture according to size and location of the unruptured aneurysm. Below is the original table as published in the UCAS 2012 article in The New England Journal of Medicine.

As with the ISUIA calculator, the UCAS aneurysm calculator above calculates cumulative lifetime risk based on the annual rupture rate using the following equation:

        1 – (annual chance of not bleeding) x expected years of life  = risk of hemorrhage

Each time a figure for life expectancy is keyed in, the algorithm automatically updates the risk for the given aneurysm, based on its size and location.

As the authors of the UCAS study mention in their article, this study is limited by its exclusive inclusion of Japanese patients, and extrapolation of these results to non-Japanese populations should be done with caution.  By compatison, 90% of the patients enrolled in ISUIA were Caucasians.  Interestingly, the UCAS results generally give higher estimates of rupture risk, despite the incidence of unruptured aneurysms in the Japanese population being comparable to that of Western populations.  The reasons for this difference are unclear, but may be related to unknown genetic or environmental factors. 

In comparison to the ISUIA study, the UCAS researchers were more specific about the location of the aneurysms in their dataset, particularly in the posterior circulation.  Presumably due to small numbers of aneurysms in certain size categories in these areas, the estimated rupture rates are reported with very wide error bars, meaning the true risk of rupture of these aneurysms remains unclear.  Please refer to the UCAS table above for more information.

Disclaimer:  This tool is free to use. Results are indicative only and may not be applicable to all patient groups. The decision of whether and how to treat a particular aneurysm is a complex one best made by the patient and their family under the guidance of an experienced neurosurgeon. While we will attempt to keep this tool updated, no guarantees can be made that the data presented here is accurate or current. All rights reserved. Copyright 2013 Ralf Kockro and Tim Killeen.