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.
The size of the aneurysms was categorized into four groups:
Patients were divided into two further groups – those who had previously experienced a subarachnoid hemorrhage (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, 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 their 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. This feature should be used with caution, as it is known that aneurysm growth and rupture rate are not constant over time (see Koffijberg et al. Journal of Neurosurgery 2008).
In patients with small anterior circulation (less than 7mm) or intracavernous (less than 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 the details of the ISUIA study, please refer to the original ISUIA 2003 article, in the Lancet.