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aneurysm neurosurgery planning
aneurysm neurosurgery planning

Intracranial Aneurysms

Aneurysms are abnormal outpouchings of blood vessels and they appear in a variety of forms and sizes. Most aneurysms of the arteries in the brain are pouch- or berry-shaped and vary in size from a few millimetres to several centimetres in diameter.

Causes of intracranial aneurysms

Aneurysms are caused by structural changes resulting from weaknesses in the wall of the arteries of the brain and are particularly common at the junctions of arteries. Blood pressure causes these points of weakness to stretch and expand; a process that ultimately results in aneurysms and accounts for their tendency to gradually increase in size.

Symptoms of intracranial aneurysms

Patients with aneurysms can be completely symptom-free for years or even decades as long as important brain structures and nerves – such as the optic nerve to the eyes – are not compressed by the aneurysm.

In the worst case, the first symptoms can be those of a ruptured aneurysm. A ruptured aneurysm leads to a severe and acutely life-threatening bleed in the brain. Symptoms of an aneurysmal bleed range from severe headache to paralysis and unconsciousness. In these cases it is imperative that patients are immediately transferred to a specialist clinic where the aneurysm can be diagnosed and treated.

Treatment of intracranial aneurysms

Treating an aneurysm means sealing it off from its parent artery and this can be done either by applying a metal clip during a microsurgical operation or, if there is no large haematoma and the size and shape of the aneurysm allows it, using an endovascular approach to insert metal coils into the aneurysm via a catheter inserted into the artery via a small incision in the groin (coiling).

A ruptured aneurysm needs to be closed as soon as possible, since every hour delay increases the risk of re-bleeding, which is an event that carried a high risk of poor outcome. Our department is part of the nation wide stroke service and prepared for such emergencies. Our experienced team of neurosurgeons, neuroradiologists, neurologists and intensive care specialists are available around the clock.

State-of-the-art aneurysm closure technique

Whether ‘coiling’ or ‘clipping’ is advised, our hospital is equipped with the most modern equipment to treat your aneurysm. If microsurgical clipping is indicated, a 3D simulation of the exact architecture of the aneurysm will be carried out prior to the operation, allowing to plan precisely how best to expose and then definitively clip the aneurysm.

During the operation, the positioning of the clip is controlled with endoscopic micro-cameras and with intra-operative X-Ray catheter angiography, ICG (Indo Cyanine Green) fluorescence angiography and Doppler ultrasound. This ensures a completely occluded aneurysm (zero blood flow inside arrested aneurysm dome), as well as normal flow in the nearby vessels.

When is the therapy of a non-ruptured intracranial aneurysm indicated?

Unruptured aneurysms are diagnosed either as a result of symptoms, such as visual disturbance as a result of irritation of structures around the aneurysm, or by chance while investigating other problems using CT or MRI.

In some cases, an MRI or CT will be performed because of a known family tendency to form aneurysms, for example in cases of polycystic kidney disease. In all cases of unruptured aneurysms the question of whether the aneurysm should be treated, and if yes, with which method, must be considered. Aneurysms which, because of their size and compression of surrounding structures cause significant symptoms should in most cases be treated.

In cases in which aneurysms are asymptomatic and detected by pure coincidence, the decision as to whether these aneurysms should be treated is guided by calculations of the risk of rupture based on the size, position and shape of the aneurysm. In these cases, the age of the patient, their other illnesses and their individual risk profile plays an important role in the decision to treat or not.


The risk of rupture of intracranial aneurysms has been studied in a variety of multi-center trials. This calculator was developed based on the results of the PHASES meta-analysis of six major international studies  which analyzed a total of 8382 patients with 10272 unruptured aneurysms over a median time of 3 years. This is  equivalent to 29166 patient-years of follow-up. The overall five-year risk of aneurysm rupture revealed to be 3.4%. This online aneurysm risk calculator allows calculating an individual risk profile by taking into account a variety of key factors. This serves as a basis for interdisciplinary decision making regarding the best treatment option. 


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.

Clipping bei Hirnaneurysma
Clipping bei Hirnaneurysma

The likelihood that a given aneurysm will rupture has been the subject of many scientific studies. The 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. In these studies, aneurysms from many thousands of patients were categorized based on their location and size and observed over a period of years.

The result of the ISUIA and UCAS study, as well as the meta-analysis PHASES, which includes the ISUIA and UCAS data, are available in the ONLINE ANEURYSM RISK CALCULATOR. Here you can input the size and location of an aneurysm to calculate the probability, as per categorization in these studies, of it to rupture at some point in a given number of years.

In individual cases, the chances of an aneurysm rupturing can be greater than predicted in the above mentioned studies. The risk is dependent upon a number of factors such as the individual anatomy of the aneurysm (for example irregularities of the aneurysm wall) and accompanying risk factors. The risk of an aneurysm rupturing can therefore only be safely estimated in an individual consultation – this is particularly important for aneurysms which have been shown to change in size or shape on serial MRI or CT scans.

The decision as to whether an aneurysm should be treated in order to prevent a possible bleed in the future is dependent on many factors and must be carefully determined in each individual case. The risk of a rupture has to be balanced against the risks inherent in treatment and, as such, personal circumstances and considerations play as much a role in making this decision as any purely analytical risk calculation.

The treatment of an aneurysm is, as mentioned previously, performed either by applying a titanium alloy clip over the neck of the aneurysm during a microsurgical procedure or through the use of metal coils inserted via an intravascular catheter. In each case, a recommended technique will be offered after comprehensive discussion between our experienced neurosurgeons and neuroradiologists, with due consideration given the location, size and spatial contour of the aneurysm as well as a number of accompanying risk factors.

At the Center for Microneurosurgery all microsurgical aneurysm surgeries are meticulously planned on the basis patient-specific MRA, CTA and DSA (Digital Subtraction Angiography) data in a Virtual Reality simulator, which is currently the only one of its kind Switzerland. This 3D simulation allows establishing a highly accurate microsurgical concept, long before the actual operation starts. It reduces intra-operative explorative tissue dissection and facilitates safe and complete aneurysm occlusion.

Diverse Aneurysmen
Diverse Aneurysmen
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