An aneurysm is an abnormal, often saccular, fusiform or circumferential dilatation arising from the wall of the blood vessel. A cerebral aneurysm is an acquired lesion.
Who gets brain aneurysms?
The prevalence of cerebral aneurysms is approximately 3% based upon radiographic and autopsy studies. The mean age of presentation is at 50 years with a slight female preponderance especially in people 50 years and above, where the ratio maybe 2:1 or greater. Of all patients who have a cerebral aneurysms, approximately 20 to 30% have multiple cerebral aneurysms.
Risk factors for development of brain aneurysm
The role of genetic predisposition in development of cerebral aneurysm is demonstrated by studies showing increased prevalence of aneurysms in certain hereditary diseases and familial occurrence of cerebral aneurysms.
Some hereditary conditions associated with increased aneurysm prevalence include connective tissue diseases such as Ehlers-Danlos syndrome, pseudoxanthoma elasticum, autosomal dominant polycystic kidney disease, moyamoya syndrome.
A family history of cerebral aneurysms or subarachnoid hemorrhage as well as having a first-degree relative with an aneurysm increases relative risk of aneurysm development and subarachnoid hemorrhage. First-degree relatives of patients with subarachnoid hemorrhage have a four times higher incidence of cerebral aneurysm.
Untreated hypertension, smoking and alcohol consumption are known predisposing factors for aneurysm development and subarachnoid hemorrhage.
How brain aneurysms are discovered?
Most intracranial aneurysms are asymptomatic. Brain aneurysms are often incidentally discovered Upon brain MRI, MR angiography or CT angiography. Small cerebral aneurysms (especially less than 3 mm in diameter) may often be missed upon MRI of brain, MR angiography or CT angiography when compared to diagnostic cerebral angiography
The risk of cerebral aneurysm rupture
The natural history of unruptured intracranial aneurysms and their rupture risk has been analyzed by two large prospective studies: The International Study of Unruptured Intracranial Aneurysms (ISUIA) in the United States, Canada, and Europe and the Unruptured Cerebral Aneurysms Study (UCAS) in Japan. However the ISUIA study had significant limitations and a selection bias existed in which only patients chosen for conservative management by their treating physician were included in the study.
Both studies showed that the rupture risk in general is low for aneurysms measuring less than 7 mm located in the anterior circulation.The risk increases correspondingly with increasing aneurysm size, as well as with respect to location in the anterior versus posterior circulation.
In the ISUIA, for anterior circulation aneurysms, 5-year rates of rupture for those 7 to 12mm was 2.6 percent; for those 13 to 24 mm; 14.5 percent; and for those > 25 mm, 40 percent.
The results of the study are contrary to the finding that an overwhelming majority of ruptured aneurysms are smaller than 7 mm in diameter.
Therefore a small size is not necessarily an indication of a low rupture risk.
Multiple factors are taken into account to estimate the overall risk of aneurysm rupture, including but not limited to aneurysm location, size, prior history of subarachnoid hemorrhage, presence of multiple aneurysms, family history of aneurysms or subarachnoid hemorrhage, smoking, hypertension, etc. Aneurysm growth and behavior is difficult to predict accurately.
Consequence of a brain aneurysm rupture
The most serious consequence from brain aneurysms is their rupture leading to bleeding inside the brain called subarachnoid hemorrhage, a potentially life threatening condition with serious neurological morbidity and mortality. Patient with subarachnoid hemorrhage have have a 15% immediate mortality and a 50% mortality at 6 months.
Subarachnoid hemorrhage (SAH) from a ruptured aneurysm occurs at an estimated rate of 6 to 10 per 100,000 population. Aneurysmal subarachnoid hemorrhage is more common in Japan & Finland (22.7 per 100000 and 19.7 per 100000 Respectively). The median age of onset of SAH is between 50 to 60 years
Management of unruptured intracranial aneurysms
The management of unruptured intracranial aneurysms is controversial due to lack of prospective randomized controlled trials to guide therapy, particularly in comparing intervention with conservative management.
Currently our best information regarding management of unruptured aneurysm is based on observed rates of complications in aneurysm treatment compared to the natural history of unruptured aneurysm. There are no randomized clinical trial data that directly compare surgical clipping of aneurysm with coiling of unruptured aneurysms.
Based upon a systematic review and meta-analysis that analyzed 60 studies, 9845 patients and 10,845 aneurysms, the overall mortality associated with surgical clipping of unruptured aneurysms was 1.7 percent, while unfavorable outcomes occurred in 6.7 percent. (J Neurol Neurosurg Psychiatry. 2013 Jan;84(1):42-8. Epub 2012 Sep 25.)
Observational studies comparing endovascular treatment versus surgical clipping of unruptured cerebral aneurysms have demonstrated relatively lower morbidity and mortality and better outcomes associated with endovascular treatment in general.
High risk groups
Patients who have flow related cerebral aneurysms associated with cerebral arteriovenous malformations are at relatively higher risk of rupture due to abnormal cerebral hemodynamics and often require treatment prior to treatment of the arteriovenous malformation.
Similarly, patients who have severe carotid artery stenosis and concurrent ipsilateral cerebral aneurysms pose a particular clinical challenge as the sudden increase in cerebral blood flow and perfusion pressure following carotid endarterectomy/carotid stenting Is likely to impact aneurysm hemodynamics and shearing forces on the aneurysm wall thereby affecting rupture risk.
It is debatable whether these aneurysm require treatment prior to carotid endarterectomy or carotid stenting
Who should be treated?
Patients who are considered to be symptomatic from cerebral aneurysms for example,thromboembolism from aneurysm leading to stroke, cranial nerve palsies, sentinel hemorrhage should be promptly treated.
Asymptomatic aneurysms ≥7 to 10 mm in diameter should be strongly considered for treatment, after a careful assessment of the patient and aneurysm specific risks and benefits
Patients whose aneurysms have demonstrated an increase in size/morphology/development of blebs/daughter sacs on follow-up MR/CT angiography/conventional cerebral angiography should be considered for treatment, as a changing aneurysm morphology indicates a change in behavior and therefore rupture risk
Treatment modality: Microsurgical clipping versus endovascular treatment of cerebral aneurysms
The choice of intervention, whether surgical clipping versus endovascular coiling of an aneurysm depends upon multiple patient and aneurysm specific factors such as age, comorbidities,rupture status, size,morphology, location of aneurysm and patient preference.
Data from the International subarachnoid aneurysm trial (ISAT) Which compared outcomes of patients with ruptured aneurysm treated with microsurgical clipping versus endovascular coiling demonstrated better outcomes with endovascular coiling compared with neurosurgical clipping.
Aneurysms with a narrow neck, and with location in posterior circulation are preferentially treated with endovascular means
Patient selection for endovascular treatment
A very careful assessment of the patient's comorbid condition and aneurysm characteristic are taken into account, for selection of the best modality of aneurysm treatment.
Patients with poorly controlled diabetes, severe cardiac disease, chronic obstructive pulmonary disease, thrombotic disorders requiring chronic anticoagulation, are generally considered for endovascular management.
Endovascular treatment of cerebral aneurysms has been shown to be safe and effective in people who are 70 years and older
Factors favoring microsurgical clipping are aneurysms with unfavorable neck configuration, branching vessels out of the aneurysm sack, middle cerebral artery aneurysms or patients with intracerebral hematoma
Endovascular treatment of cerebral aneurysms
The treatment of intracranial aneurysms has evolved over the years from early application of the Hunterian carotid ligation to aneurysm clipping to currently available minimally invasive endovascular tools and technologies.
Since the invention of detachable coils by Guido Guglielmi in 1989 and It's Food and Drug Administration (FDA) approval in 1995, rapid technological advances continues to drive innovations in cerebral aneurysm treatment from use of balloons, stents, liquid embolic agents such as Onyx, flow diverting stents (Pipeline) to endosaccular devices such as WEB.
Guglielmi Detachable Coil System
Guglielmi detachable coils (GDCs) are made of a soft platinum alloy attached to a stainless steel delivery wire. A small micro catheter is carefully introduced within the aneurysm and coils are delivered within the aneurysm under live fluoroscopy. The coils induce thrombosis and obliteration of the aneurysm from the cerebral circulation.
Stent-assisted Coiling Technique
The complex or wide-neck aneurysms can be treated by placing a specially designed intracranial sent across the neck of aneurysm that serves to bridge the aneurysm neck as well as support the coils and prevent them from collapsing into the patient vessel.
Balloon-assisted Coiling Technique
Similar to the use of an intracranial stent, specially designed balloons can be inflated across the neck of the aneurysms to aid in the treatment of complex or wide-neck aneurysms. This technique is especially useful in patients with subarachnoid hemorrhage from ruptured aneurysms who cannot be treated with stent-assisted coiling
Onyx HD 500
Onyx HD 500 (Micro Therapeutics, Irvine, CA) is a liquid embolic agent that has been used in selected patients with aneurysms that are unsuitable for coil treatment or in whom previous treatment has failed to occlude the aneurysm.
Parent artery sacrifice is used in selected patients where other options have either failed or not available. A temporary test balloon occlusion is first performed to assess collaterals and the patient's tolerance for parent vessel sacrifice. If the patient successfully passes temporary balloon occlusion test, permanent occlusion is performed. However, if the patient shows neurologic changes during any aspect of the test, a surgical bypass is done before permanent vessel occlusion.
Flow diversion for the treatment of intracranial aneurysms
Endovascular reconstruction of the diseased parent vessel using Pipeline Embolization Device (PED) (ev3-Covidien, Irvine, CA, USA) as a flow diverter stent has made it possible to treat fusiform or circumferential cerebral aneurysms that are technically challenging for conventional microsurgical clipping or endovascular therapies.
The U.S. Food and Drug Administration has approved the utilization of the PED for the treatment of large or giant wide-necked aneurysms from the petrous to the superior hypophyseal segments of the internal carotid artery (ICA).
The PED is a self-expanding stent that provides 30-35% metal surface area coverage across the neck of aneurysm far exceeding the 6.5-9.5% coverage for regular intracranial stents designed for treatment of intracranial aneurysms. This high metal density allows the PED to reduce the blood flow going into the aneurysm and thereby decrease the aneurysm wall shear stress inducing aneurysm thrombosis and occlusion. The PED device promotes neo endothelialization across the neck of the aneurysm and curative endovascular reconstruction of the patent vessel and the aneurysm obliteration overtime
Currently available endovascular devices have made it possible to treat some of the most challenging aneurysms which were previously considered to be too dangerous to be treated via any means.
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