Aneurysm of a cerebral artery

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About 2% of all people have a balloon-like enlargement of a cerebral artery – a so-called aneurysm. These aneurysms usually do not cause any specific symptoms and are often discovered as incidental findings. However, each aneurysm can rupture and cause a severe cerebral hemorrhage. Unruptured aneurysms should therefore be checked regularly during MRI examinations. If they remain small and stable, they do not need treatment in most cases. If they grow or show signs of an increased risk of rupture, they should be closed in a microsurgical operation or via a vascular catheter. Here at Inselspital we offer the entire spectrum of aneurysm therapy by renowned experts.

Incidental aneurysm – operate or wait?

If an aneurysm has been diagnosed incidentally, in most cases we recommend a consultation with one of our neurosurgery or neuroradiology doctors who specialize in aneurysms. Usually, the images and patient history are first discussed at our weekly interdisciplinary aneurysm board. In most cases, the board decides on, regular follow-up. If there are clear risk factors for rupture, we advise treatment. It is very important to discuss and evaluate follow-up controls and treatment, as well as to inform the patient about the treatment options. In many cases, the personal attitude of the patient plays a major role. There are patients who prefer to live with the low risk of rupture and patients who prefer the one-time risk of treatment.

At Inselspital, we offer the possibility of a detailed consultation for aneurysm patients with Prof. Andreas Raabe or Dr. David Bervini. Brain aneurysms are a focus of the neuro-subjects (neurosurgery, neuroradiology and neurology) at Inselspital.

How common are aneurysms?

The incidence of undetected cerebral vascular aneurysms in the population is 2%. There are systematic analyses of angiographies that confirm this figure relatively consistently. In recent studies on the frequency of incidental findings in MRI examinations in previously healthy adults, aneurysms are also found with a frequency of 2% *. Other figures range up to 6%, but these are based on autopsy findings in which, due to the age of the persons examined and the counting of even larger vascular irregularities, figures are consistently too high and do not apply to the normal population.

How high is the risk of rupture?

If an aneurysm bursts in the head, it usually means a life-threatening condition for those affected. About 30% of patients die from the hemorrhage or its consequences. Less than 20 % of patients survive the hemorrhage without damage.

It is therefore a logical consequence to close the aneurysm before it bursts. The prerequisite, however, is that the risk of rupture is to be classified as higher than the possible complications of a prophylactic treatment. This is not always the case.

Evaluating all major international studies, small (< 7 mm) incidental (random) aneurysms of the anterior circulation have a very low risk of hemorrhage, which is about 0.2% per year. For other aneurysms, the risk of rupture is more than 10% per year. It is therefore necessary to carefully consider in which patients follow-up observation and in which patients aneurysm treatment is more beneficial.

For more information on ruptured aneurysms and brain hemorrhage, see the page Ruptured aneurysm and subarachnoid hemorrhage.

The following factors (individually or in combination) influence the risk of bleeding and can therefore help to decide whether the aneurysm should be treated. If a patient does not have any of these risk factors, the risk of bleeding from the aneurysm is less than 0.2% per year.

Risk factors +++Risk factors +
Aneurysm size > 10 mmAneurysm size > 7 mmHypertension
Size growth on MRI follow-upLocalization in the posterior circulationActive smoking
Familial aneurysm with 2 or more affected first-degree relativesIrregular shape, daughter sac, size ratio > 1Increased alcohol consumption
 Previous bleeding from another aneurysmMultiple aneurysms
 Enhancement of the aneurysm wall with contrast agent during examinationGenetic factors such as polycystic kidney disease
  Aneurysms in 2 or more second-degree relatives
  Other genetic risk

Aneurysm follow-up: how often?

We generally recommend an annual follow-up with a MRI examination without contrast agent. Even if an aneurysm is stable in size over several years, there is still a risk that it will grow. However, the intervals for follow-up examinations are adapted to the patient's age and the respective aneurysm findings. The calculation of optimal intervals for follow-up examinations is a current scientific topic of the Department of Neurosurgery and the Institute of Neuroradiology at Inselspital.

In 2–5% of patients who are only observed and monitored by MRI annually, the aneurysm shows an increase in size or the formation of a further bubble, a so-called daughter sac. Both findings increase the risk of hemorrhage by a factor of 10 and are an indication for treatment in most cases.

Further follow-up in the case of a proven increase in size is rarely recommended and depends on the risk factors for treatment. There is no consistent growth pattern in aneurysms and one cannot predict the risk of growth in the future even after 5 years of size stability.

What are the risks of treatment?

The risk of serious complications in the treatment of unruptured aneurysms is approximately 2% in large specialized centers for typical aneurysms of the anterior cerebral circulation and an aneurysm size of < 10 mm – for both surgery and endovascular therapy *, *, *. However, there are also rare subtypes of aneurysms, such as giant aneurysms, for which there is a significantly higher risk of treatment.

Microsurgery – Clipping

The most common serious complications of clipping in microsurgery are severe postoperative bleeding, permanent epilepsy or vascular occlusion resulting in a stroke. These risks add up to about 2%.

Endovascular therapy – coiling and/or stenting

The most common serious complications of endovascular therapy are perforation of the aneurysm with the coil or wire with subsequent bleeding, vascular occlusion resulting in a stroke, or an allergic contrast reaction. These risks also add up to about 2 %.

What are the chances of success of the treatment?

Microsurgical operation – clipping

About 97% of all aneurysms whose operation was carefully planned preoperatively can be eliminated during surgery. There are intraoperative findings, such as small but important vessels coming directly out of the aneurysm or unpredictable calcifications and thickening of the wall, which can lead to a change of treatment strategy towards endovascular therapy. In any case, the principle of the lowest treatment risk always applies in our center.

Follow-up controls after clipping are rarely necessary. Only approximately 5% of the clipped aneurysms require further observation due to an existing residue and only 1% require follow-up treatment.

Endovascular therapy – coiling and/or stenting

After endovascular therapy, about 20% of all aneurysms show a small residue or an increasing blood flow in the course. For this reason, all endovascularly treated aneurysms must usually be followed up after 6 months and after 24 months with MRI and, if necessary, repeat angiography. About 10% of all aneurysms need to be treated again after endovascular therapy. If unexpected difficulties occur during endovascular treatment, it may lead to a change of treatment strategy to surgical therapy. Here, too, the principle of the lowest treatment risk always applies.

Microsurgical and endovascular aneurysm therapy in detail

Authors: Johannes Goldberg, David Bervini, Pasquale Mordasini, Jan Gralla, Andreas Raabe.

The aim of treating a cerebral aneurysm is to cut it off as completely and permanently as possible from the blood circulation in order to prevent a rupture resulting in cerebral hemorrhage. Two different treatment techniques are available for this: Microsurgical clipping or endovascular coiling (with or without additional devices such as intracranial stents).

Clipping = microsurgical operation ⇒ treatment in neurosurgery

Coiling = endovascular intervention ⇒ treatment in interventional neuroradiology

Microsurgical clipping

The closure of an aneurysm with a clip is performed under a surgical microscope. A clip works like a clamp that separates the aneurysm from the healthy part of the vessel. This small clip, made of titanium or a special metal alloy, is placed over the neck or base of the aneurysm and then closes the aneurysm sac. This method was first used by Walter Dandy in the USA in 1937. Since then, a variety of clips with different shapes and sizes have been developed. The appropriate clip is then selected depending on the size, configuration and location of the aneurysm. A spring mechanism allows the placed clip to exclude the aneurysm from the supporting vessel, thus preventing a possible rupture.

The skin incision is made as small as possible in order to achieve an optimal cosmetic result. The bony skull is then opened precisely above the lateral skull base. This creates a gentle access route under the brain or between the brain lobes. During this step, the surgical microscope provides high magnification and excellent illumination. Operating with the microscope allows the neurosurgeon to identify and protect the vessels in the vicinity of the aneurysm. With the help of modern surgical techniques such as intraoperative infrared angiography *, electrophysiological monitoring and aneurysm navigation, a significant improvement in the safety and invasiveness of clipping has been achieved. Large and complex aneurysms often require additional techniques such as bypass (blood diversion), trapping (exclusion) or wrapping techniques (repositioning). The general advantages of microsurgical clipping are a higher closure rate and, in the vast majority of cases, no further follow-up examinations.

Endovascular coiling

In the elimination of an aneurysm by means of coiling, platinum coils are placed in the aneurysm endovascularly, i.e. via the vascular pathway, using a microcatheter. The aim here is to plug the aneurysm from the inside. The coils lead to a thrombosis in the aneurysm and thus to the closure of the aneurysm.

As with surgery, endovascular treatment requires a general anesthetic. Access is usually through the femoral artery in the groin. With contrast medium and fluoroscopy, the anatomy of the vessels and the instruments can be followed and advanced into the cerebral arteries. Once the microcatheter is in the aneurysm, the coils are placed, gradually closing the aneurysm. Depending on the complexity and shape of the aneurysm, it may be necessary to use an additional balloon or a stent to ensure the correct placement of the coil in the aneurysm.

Minimally invasive endovascular treatment of intracranial aneurysms with electrolytically detachable coils was introduced in 1991 by Guido Guglielmi and colleagues as an alternative to microsurgical clipping 4. Since their introduction in the 1990s, endovascular treatment techniques have undergone rapid development – from balloon- and stent-assisted coiling to flow-diverting stents or flow diverters, which are increasingly expanding the spectrum of intracranial aneurysms that can be treated endovascularly.

Which is better: neurosurgical clipping or endovascular coiling?

In general, surgery on an unruptured aneurysm is easier than for a ruptured aneurysm with bleeding. Therefore, if the surgical or endovascular difficulty is the same – which is different for each aneurysm and must always be discussed jointly by neuroradiologists and neurosurgeons – preference should be given to the endovascular approach. In the case of incidental unruptured aneurysms, on the other hand, surgery is easier and the results of surgery and endovascular therapy are about equally good.

Today, there is no longer any competition between the two methods. Rather, they are used where they have their strengths. For example, there are localizations or forms of aneurysms that guide the therapy in one direction from the outset. The discussion about which procedure is better no longer exists.

Which factors are decisive for the choice of therapy?

In the case of an intracranial aneurysm, various factors must be considered and weighed against each other in order to be able to determine the optimal treatment strategy for the individual patient.

Aneurysm-specific factors include:

  • Size
  • Anatomical location
  • Morphology
  • Positional relationship of outgoing vessels to the aneurysm
  • Partial thrombosis
  • Wall calcifications

Patient-specific factors include:

  • Age
  • Pre-existing conditions
  • Clinical-neurological status

Size and shape of the aneurysm

The complication rate of both endovascular and surgical treatment is related to the size of the aneurysm. With clipping, the complication rate increases with size. With coiling, the success rate and closure rate are significantly lower for extremely large aneurysms. Even with very small aneurysms < 2 mm, endovascular therapy is limited, as stuffing the aneurysm sac with a coil is no longer possible below a certain size or can only be carried out with auxiliary devices such as stents and balloons. Other aneurysms are better closed endovascularly.

Anatomical location of the aneurysm

The anatomical location of the aneurysm plays a major role in the decision between clipping and coiling. The location influences the safety and effectiveness of both forms of treatment. For aneurysms of the posterior cerebral vessels, the complication rate of clipping is higher than for aneurysms of the anterior cerebral vessels 5, 10 and endovascular therapy is gentler and better. In contrast, aneurysms in the region of the bifurcation of the middle cerebral artery are clipped more frequently. This is mainly due to the usually broad-based anatomy of the aneurysms and the frequent branching off from the aneurysm neck. These morphological factors mean that endovascular treatment usually has to be performed with additional devices such as stents, which leads to a higher recanalization rate and a higher risk immediately before, during and after the procedure.

Special circumstances

Although most aneurysms can be treated with microsurgical or endovascular therapy, there are rare aneurysms that require a special treatment approach. These are mainly huge (> 2.5 cm), calcified or partially thrombosed aneurysms. Depending on the clinical indication, these can be treated microsurgically or endovascularly. Under certain circumstances, a bypass may also be necessary. The treatment of partially thrombosed intracranial aneurysms – especially if they are also large – can be challenging for both therapeutic methods. The disadvantage of endovascular therapy is that tight and stable closure of the aneurysm is difficult, especially in the case of partially thrombosed aneurysms. In these cases, microsurgery is a good alternative if the aneurysm neck is easily accessible for clip application. Wall calcifications in the neck area are often a contraindication for clipping, as correct placement of the clip at the aneurysm neck is difficult here due to the calcification. So-called flow diverters have opened up new possibilities in endovascular therapy, especially in the large proximal vessels.

Patient-specific factors

Several studies have shown that patient age is an important factor influencing the success of aneurysm treatment6, with older patients more likely to benefit from less invasive endovascular treatment. A subgroup analysis of the ISAT study shows an increased rate of disability or death in older patients with ruptured intracranial aneurysms (43.9% vs. 39.9%) when directly comparing the microsurgical treatment group with the endovascular treatment group *. On the other hand, microsurgical therapy seems to offer advantages in younger patients, especially those with incidentally diagnosed unruptured aneurysms, due to the better long-term closure rate and the lower rate of complications related to the procedure compared to older patients.

Patients in poor clinical-neurological condition after severe subarachnoid hemorrhage and patients with vasospasm are preferential candidates for endovascular treatment. If a patient develops a space-occupying hematoma after an aneurysm rupture, it is usually useful to clip the ruptured aneurysm immediately during emergency surgical hematoma removal. In patients with both ruptured and unruptured aneurysms with severe pre-existing conditions, endovascular therapy is preferred because of the lower perioperative risks.

Other factors

Other decisive factors in the treatment of aneurysms are the experience and the number of treatments performed by the responsible neurosurgeons and interventional neuroradiologists. Several studies have shown a correlation between low complication rates and so-called "high-volume" centers with experienced surgeons for both the microsurgical as well as the endovascular treatment of ruptured and unruptured aneurysms *. Therefore, if possible, aneurysm treatment should be performed by experienced and specialized neurosurgeons and interventional neuroradiologists at a supra-regional neurovascular center in order to achieve the best possible result for the patient.

Our experience at Inselspital – international cutting-edge medicine in aneurysm treatment

At Inselspital, every patient with an aneurysm is assessed individually. After thorough consideration of all relevant factors, neurosurgeons and interventional neuroradiologists jointly decide which procedure has the greatest chance of success for the patient while at the same time posing the lowest risk. The factors and the consideration are then discussed with the patient and the best course of action is determined together.

The Aneurysm Center at Inselspital offers the entire spectrum of aneurysm therapy, state-of-the-art technologies and the necessary experience. With more than 1200 annual therapy decisions, always made jointly by neurosurgeons and interventional neuroradiologists, we are a specialized center of international renown. Thus, experts from Inselspital are often invited speakers at international congresses and are invited to participate in expert groups and in the development of Europe-wide recommendations.

The optimization of aneurysm surgery is also a research focus of the Department of Neurosurgery. In recent years, we have been particularly concerned with three safety strategies that allow us to perform diagnostics and monitoring during surgery that go beyond the normal standard. You can find out more about this under vascular navigation, functional monitoring & safety through intraoperative neurophysiology und ICG infrared angiography.


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