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
The size of the aneurysms was categorized into five groups:
1. 3-4 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. 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).
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 comparison, 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.