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ANEURISM RISK CALCULATORS

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. The largest studies looking at the natural history of unruptured aneurysms are ISUIA (International Study of Unruptured Intracranial Aneurysms) and UCAS (Unruptured Cerebral Aneurysm Study) – a Japanese undertaking. More recently, a meta-analysis of these and other, smaller trials has resulted in the development of the PHASES score which aims to predict rupture risk based on patient and aneurysm factors associated with rupture.

On this website, you will find aneurysm calculators designed to make the data from these trials easily accessible. Please read the guidance notes for each calculator for a detailed explanation of how the calculators work and their limitations.

Disclaimer: These tools are 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.

Ralf A Kockro, M.D and Tim Killeen, Ph.D, MRCS

Copyright 2013-2017

PHASES CALCULATOR

Usage Guidance

Aneurysm and patient factors identified in PHASES as being associated with a significantly increased risk of aneurysmal rupture are allocated a number of points which are collated to give a final PHASES score. The risk of rupture in a particular aneurysm in a given patient can be inferred from the score. Due to its pooled nature conferring increased statistical sensitivity, the PHASES analysis identified six key risk factors for aneurysmal rupture, not all of which were predictive of rupture in UCAS and ISUIA.

Aneurysm location was categorized into three groups:

1. Middle cerebral artery
2. Internal carotid artery
3. Others (including anterior cerebral arteries, communicating arteries and the posterior circulation).

The size of the aneurysms was categorized into four groups:

1. < 7 mm
2. 7 – 9.9 mm
3. 10 – 19.9 mm
4. ≥ 20 mm

Rates of aneurysm rupture were reported as five-year rupture rates according to size and location of the unruptured aneurysm as well as ethnicity and age of the patient and the presence of high blood pressure (hypertension) or previous subarachnoid hemorrhage.

In the PHASES aneurysm calculator, the figures returned under 5 year rupture risk reflect those provided in the PHASES study. Based on this table, the annual risk of rupture was calculated by simply dividing this number by five, using the approach of da Costa et al in their 2004 Neurogurgical Focus article. As with the ISUIA and UCAS calculators, the PHASES calculator gives 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 the various patient and aneurysm factors. 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).

The risk of aneurysm rupture is known to vary across different populations. 90% of the patients enrolled in ISUIA were white Europeans or North Americans. The risk in this population appears to be significantly lower than those of the Japanese patients studied in UCAS. Small studies of patients with aneurysms in Finland suggest an especially high rupture rate for ethnic Finns. PHASES thus incorporates ethnicity (Japanese, Finnish or neither) into their algorithm. The reasons for this difference are unclear, but may be related to unknown genetic or environmental factors.

The PHASES analysis is the most complete dataset currently available for predicting the likelihood that an unruptured intracranial aneurysm will result in a subarachnoid hemorrhage. The vast majority of included patients were from UCAS and ISUIA and thus the pooled analysis suffers from the same limitations as these individual studies. Most importantly, some patients in these studies had their aneurysms treated during follow-up due to an increase in size or the development of symptoms. The removal of these high-risk patients from the cohort means that overall rupture risk may be somewhat higher than indicated in ISUIA, UCAS or PHASES.

ANEURYSM RISK CALCULATOR

The risk of rupture of intracranial aneurysms has been studied in a variety of multi-center studies. This calculator was developed based on the results of the PHASES meta-analysis of patients in ISUIA, UCAS and four smaller studies (two in Japan, one in Finland and one in the Netherlands), which analyzed a total of 8382 patients with 10272 unruptured aneurysms. In total, 220 patients experienced a subarachnoid hemorrhage during 29166 patient-years of follow-up (median 2.9 years), giving an overall five-year risk of aneurysm rupture of 3.4%. This online aneurysm risk calulator contains the PHASES data and allows calculating an individual risk profile.

ISUIA CALCULATOR

Usage Guidance

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. < 7 mm
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 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.

ISUIA Table

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.

ISUIA CALCULATOR

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. The largest studies looking at the natural history of unruptured aneurysms are ISUIA (International Study of Unruptured Intracranial Aneurysms) and UCAS (Unruptured Cerebral Aneurysm Study) – a Japanese undertaking. More recently, a meta-analysis of these and other, smaller trials has resulted in the development of the PHASES score which aims to predict rupture risk based on patient and aneurysm factors associated with rupture.

UCAS CALCULATOR

Usage Guidance

USAGE GUIDANCE
Aneurysm location was categorized into seven groups:

1. Middle cerebral artery
2. Anterior communicating artery
3. Internal carotid artery
4. Internal carotid – 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. 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.

Go to the PHASES calculator
Go to the ISUIA calculator

UCAS CALCULATOR

The PHASES score was developed based on the result of a meta-analysis of patients in ISUIA, UCAS and four smaller studies (two in Japan, one in Finland and one in the Netherlands), which analyzed a total of 8382 patients with 10272 unruptured aneurysms. In total, 220 patients experienced a subarachnoid hemorrhage during 29166 patient-years of follow-up (median 2.9 years), giving an overall five-year risk of aneurysm rupture of 3.4%.

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