Basilar artery aneurysms account for a small percentage of intracranial aneurysms; however, they are a diverse group of lesions necessitating different treatment techniques for those that are ruptured and unruptured. Basilar apex aneurysms are the most common type and are frequently wide-necked, necessitating stent-assisted coiling or balloon remodeling. Other techniques have evolved to forego stenting in acutely ruptured wide-necked aneurysms. The prevention of delayed thromboembolic complications with dual antiplatelet therapy in patients with stents is critical. After treatment, basilar aneurysms require close follow-up to ensure complete occlusion. Basilar apex aneurysms often require delayed re-treatment, especially when previously ruptured.
Key points
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Posterior circulation aneurysms constitute 15% of intracranial aneurysms. Between 50% and 80% of these are located at the basilar artery apex.
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Most basilar apex aneurysms are wide-necked and require specialized techniques for treatment to ensure patency of the parent vessel and successful lesion embolization.
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Treated basilar artery aneurysms should be followed closely to monitor for delayed recanalization.
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In addition to basilar apex aneurysms, aneurysms of the basilar artery may involve the posterior cerebral artery, superior cerebellar artery, and the anterior inferior cerebellar artery.
Introduction
Intracranial aneurysms are found in approximately 3% to 5% of the population, with an annual rate of rupture of approximately 10 per 100,000. Of these, posterior circulation aneurysms constitute approximately 15% of all intracranial aneurysms. The most common posterior circulation aneurysm location is the basilar artery (BA) apex. Basilar apex aneurysms constitute 50% to 80% of posterior circulation aneurysms. Other locations of BA aneurysms include the basilar trunk and the junctions of the posterior cerebral arteries (PCAs), superior cerebellar arteries (SCAs), and anterior inferior cerebellar arteries (AICAs).
Traditional microsurgical approaches for the treatment of posterior circulation aneurysms often require significant brain retraction and temporary arterial occlusion that may result in significant morbidity and mortality. Specifically, third nerve palsies, perforator injury, and retraction injury are the most common. Therefore, endovascular therapy has evolved to become a mainstay in treating these lesions.
Introduction
Intracranial aneurysms are found in approximately 3% to 5% of the population, with an annual rate of rupture of approximately 10 per 100,000. Of these, posterior circulation aneurysms constitute approximately 15% of all intracranial aneurysms. The most common posterior circulation aneurysm location is the basilar artery (BA) apex. Basilar apex aneurysms constitute 50% to 80% of posterior circulation aneurysms. Other locations of BA aneurysms include the basilar trunk and the junctions of the posterior cerebral arteries (PCAs), superior cerebellar arteries (SCAs), and anterior inferior cerebellar arteries (AICAs).
Traditional microsurgical approaches for the treatment of posterior circulation aneurysms often require significant brain retraction and temporary arterial occlusion that may result in significant morbidity and mortality. Specifically, third nerve palsies, perforator injury, and retraction injury are the most common. Therefore, endovascular therapy has evolved to become a mainstay in treating these lesions.
Basilar artery anatomy
The basilar artery forms from the convergence of both vertebral arteries at the level of the pons. The BA continues to travel ventral to the pons until its termination and bifurcation into the posterior cerebral arteries in the interpeduncular cistern. During its ascent it provides numerous perforating branches to the pons and midbrain, including multiple median, circumferential, and lateral pontine branches. It emanates 3 major paired arteries: the AICAs proximally, the SCAs just proximal to its terminal bifurcation, and the PCAs as terminal branches. The posterior inferior cerebellar arteries (PICAs) most commonly arise from the vertebral artery. The AICAs and SCAs form an extensive and redundant collateral arterial system supplying the rest of the cerebellum.
In addition to the BA’s close association with the brainstem, its branches have intimate relationships with several cranial nerves. The third cranial nerve exits the mesencephalon immediately inferior to the PCA and superior to the SCA. Lateral to this is the trochlear nerve before piercing the tentorial dura. The SCA has a close relationship with the trigeminal nerve, and the distal AICA has a close relationship with the vestibulocochlear nerve.
Regarding endovascular treatment, the straight course of the BA and the position of apex aneurysms make them favorable to endovascular navigation and catheterization. The position of the BA does not influence the endovascular approach as it does with open vascular cases. In addition, important perforators arise from the neck and not the dome of the apex aneurysms, making endovascular approaches ideal. Approximately 60% of basilar apex aneurysms are wide-necked and may involve the PCA origins, making these more challenging to treat. Various methods have evolved to keep these critical vessels patent.
Clinical presentation and diagnosis
A large number of posterior circulation aneurysms are found incidentally on workup of unrelated symptoms. Nonruptured aneurysms may present secondary to mass effect and cranial nerve palsies. Infrequently, thrombosed and giant basilar aneurysms may present with hydrocephalus and a third ventricular mass. Because of the low incidence of posterior circulation aneurysms, their natural history is not well understood. However, the International Study of Unruptured Intracranial Aneurysms investigation revealed that unruptured posterior circulation aneurysms have a 2.6% to 50.0% chance of rupture over a 5-year period, depending on their size. Posterior circulation, and especially basilar apex aneurysms, in patients with a prior history of subarachnoid hemorrhage predict a poor outcome. This risk of rupture and poor outcome argues for aggressive treatment of these lesions, especially when the aneurysm is greater than 7 mm and life expectancy is greater than 5 years.
On rupture, presentation and prognosis are typically more severe than those of anterior circulation aneurysms, with survival rates as low as 32% within the first 48 hours of rupture and 11% at 30 days despite treatment. The dismal prognosis is related to the BA’s relationship to the brainstem and hemorrhage into the forth ventricle. Obstructive hydrocephalus secondary to rupture is particularly common among ruptured BA aneurysms.
Endovascular considerations
History
Endovascular surgery of the posterior circulation was primarily developed for the treatment of nonoperative aneurysms. Treatment most commonly involved Hunterian proximal arterial occlusion of the parent artery with detachable balloons. The advent of detachable coils in 1992 by Guglielmi began a revolution in the treatment of aneurysms. Detachable coils were originally used only for aneurysms thought to be too difficult to clip or in patients with poor prognosis. Therefore, many early series reveal frequent use with posterior circulation aneurysms. In fact, a 1997 series of 403 intracranial aneurysms found that 57% of those treated were in the posterior circulation. With the use of coiling, acutely ruptured aneurysms can be treated to significantly reduce the risk of rehemorrhage, allow aggressive therapy for the prevention of delayed cerebral ischemia, and prevent vasospasm.
General Techniques and Principles
Several basic principles and steps help ensure safe interventional procedures. Interventional procedures are usually completed under general endotracheal anesthesia. All sheaths, guides, and microcatheters are continually flushed with heparinized saline. Generally, the procedure begins with a micropuncture needle, which is ultimately upsized to a 6F short sheath unless femoral access is tortuous and a long sheath is required. Once access is obtained, 70 U/kg of heparin is administered intravenously for an activated clotting time (ACT) goal of 250 seconds. Activated clotting time point of care testing is used through the duration of the procedure to ensure adequate heparinization. Meticulous attention of heparinization and ACT checks can prevent disastrous thromboembolic complications. Protamine must be immediately available in case of intraoperative rupture. Appropriate guide and selector catheters are used to select the vessel of interest. Once selected and treatment projections are chosen, 10 mg of intra-arterial verapamil are given to prevent catheter-induced vasospasm. The guide catheter is then brought to the V2 or V3 segment of the vertebral artery as it allows. Straight microcatheters are suitable for most aneurysms; however, this is even more assured for basilar apex aneurysms, in which the trajectory is straight. After treatment, a final ACT is measured and more heparin is given as needed. For aneurysms requiring stenting or with significant coil mass exposed at the neck, a heparin infusion at 8 U/kg/hr is often used overnight to prevent delayed thromboembolic complications. If delayed thromboembolism occurs, an abciximab infusion is initiated for 24 hours and the daily clopidogrel dose is increased. A postoperative systolic blood pressure goal of less than 160 mm Hg is strictly enforced.
Basilar Apex Aneurysms
Basilar apex aneurysms are the most common posterior circulation and BA aneurysm ( Fig. 1 ). They compose approximately 50% of all posterior circulation aneurysms and 5% of all intracranial aneurysms. These aneurysms most commonly follow Rhoton’s third law of aneurysms and point superiorly in the direction blood would have flowed if not for the artery’s termination as the posterior cerebral arteries. The anatomy of the BA and basilar apex aneurysms make these ideal for endovascular technology. These anatomies dictate the treatment and ultimate outcome. They may incorporate the posterior cerebral arteries, complicating treatment, and high flow can lead to long-term coil compaction, necessitating re-treatment.
Narrow-necked aneurysms can be primarily coiled; however, up to 60% of basilar apex aneurysms are wide-necked (>4 mm). Therefore, a variety of techniques have developed to prevent coil prolapse into the BA lumen or occlusion of the PCAs. The most common techniques include balloon remodeling, stent-assisted coiling, and multiple microcatheter techniques.
Balloon remodeling involves using a nondetachable balloon to segregate the aneurysm neck from the parent vessel during occlusion. After aneurysm catheterization, the balloon is inflated across the aneurysm neck, allowing coil deployment and preventing prolapse. The balloon should go across the aneurysm neck into the dominant P1 to protect the vessel. Highly formable balloons, such as the HyperForm (Covidien, Plymouth, MN, USA) or TransForm (Stryker, Kalamazoo, MI, USA), can be inflated and formed to protect bilateral P1 segments and the aneurysm neck. Deflation before coil detachment helps ensure prolapse will not occur. Large framing coils will further help prevent prolapse, whereas the balloon prevents coil loops from prolapsing during deployment. Intermittent balloon inflation can be performed throughout coiling. In a small series of 52 aneurysms, this technique allowed for complete occlusion in 40 patients (77%), subtotal in 9 (17%), and incomplete occlusion in 3 (6%). Balloon remodeling is particularly beneficial in acutely ruptured aneurysms when stenting and dual antiplatelet therapy is less than optimal. It does, however, involve temporary flow arrest and no guarantee of coil stability after balloon deflation.
The use of dual balloon remodeling has been described but is infrequently required, because newer balloons are now highly formable. This technique involves balloons in each PCA that are intermittently inflated during embolization. Unfortunately, this technique frequently requires bilateral femoral access and is associated with a higher risk of thromboembolic complications.
Stent-assisted coiling can also be used to prevent coil prolapse into the BA and obstruction of the PCAs (see Fig. 1 ). The stent configuration depends on the aneurysm’s neck incorporation of the P1 segments. If a single P1 is incorporated at the base, frequently a single stent is required. However, if the neck is broad enough, stenting with a Y-configuration may be required, which involves 2 stents arising from the top portion of the BA into each P1 segment. Afterward, the microcatheter can be guided into the aneurysm and coiled (ie, the “coil through” technique). Alternatively, the microcatheter can be “jailed” into the aneurysm dome before embolization. A multicenter study of 45 aneurysms at 7 institutions were treated with Y stenting with good results: 92% of patients had Raymond I or II occlusion, 83% of Raymond III occluded aneurysms had better occlusion grade on delayed angiographic imaging, and 10% of patients recanalized, requiring repeat procedures. Thromboembolic complications were lower in this series, and typically range from 4% to 8% in multiple series. In-stent stenosis was infrequent and asymptomatic in all patients.
Unfortunately, placement of stents necessitates the need for dual antiplatelet therapy, which is less appealing after acute subarachnoid hemorrhage. The need for aggressive medical therapy and external ventricular drainage, and the possible future need for ventriculoperitoneal shunting for ruptured aneurysms, is associated with increased risk for hemorrhage during dual antiplatelet therapy. In these cases that may require stent-assisted coiling, the authors attempt to secure the dome to prevent acute rehemorrhage, knowing that re-treatment will be required in a delayed fashion.
For elective aneurysm treatment, patients are treated with aspirin and clopidogrel for a minimum of 7 days before the procedure. Immediately postprocedure, all patients are kept on low-rate heparin infusions (8 U/kg/hr) overnight to prevent delayed presentation of thromboembolic events. Dual antiplatelet therapy is continued for a minimum of 6 weeks, after which the patient will remain on aspirin. Up to 47% of patients may be clopidogrel-nonresponders and should be evaluated preoperatively if possible to reduce thromboembolic complications. The authors favor the use of open-cell stents (NeuroForm stent systems, Stryker) as opposed to closed-cell stents because of the decreased risk of kinking and the possibility of poor vessel apposition requiring in-stent angioplasty after delivery. However, clinically there seems to be little difference in outcome between these stent types.
Another technique that allows for the treatment of wide-necked aneurysm is the dual microcatheter technique. This technique avoids the need for balloon remodeling when the risk of temporary occlusion is too high or in acutely ruptured aneurysms in which avoidance of stent-assisted coiling is preferred. This technique involves the use of 2 microcatheters within the aneurysm dome. The first catheter deploys a framing coil without detaching it. The second catheter is then used to deploy a second framing coil. The successive deployment of 2 coils helps interlock their position and provide coil stability in the aneurysm dome to prevent prolapse. They can subsequently be detached and coiling can continue, with filling coils alternating between both microcatheters. Tandem coil placement can also be performed with dual microcatheter access; through alternately partially deploying a coil from each of the microcatheters, a complex coil mass can be formed and prevent prolapse. When using this technique, special attention to microcatheter removal at the end of coiling is critical to prevent shifting of the coil mass.
Finally, less common practices for the treatment of basilar apex aneurysms include transcirculation techniques via a posterior communicating artery and use of flow diversion technology. Transcirculation approaches require guide catheters in the internal carotid artery to access a large posterior communicating artery and in a vertebral artery to access the basilar apex aneurysm. As such, there is an increased risk of thromboembolic complications. Several case reports discuss the use of flow-diverting stents, such as the Pipeline Embolization Device (PED; Micro Therapeutics Inc, Irvine, CA, USA). The use of this technology requires the use of dual antiplatelet therapy and may place small pontine perforator branches at risk. The long-term outcomes of PEDs for the treatment of BA aneurysms are not well-known, currently limiting this application.
Basilar Trunk Aneurysms
Basilar trunk aneurysms constitute fewer than 1% of intracranial aneurysms and approximately 8% of vertebrobasilar artery aneurysms. Dissecting aneurysms of the basilar trunk are more common than saccular aneurysms. Saccular aneurysms of the basilar trunk most commonly occur in the distal third of the BA and often are categorized with AICA aneurysms. In a series of 8 saccular basilar trunk aneurysms, 7 presented with a subarachnoid hemorrhage. All 8 were treated with primary coiling with complete or near-complete occlusion; 1 patient died secondary to vasospasm, and 2 patients ultimately required additional treatments for coil compaction.
Dissecting and fusiform BA trunk aneurysms are associated with a high rate of morbidity and mortality. They can lead to progressive brainstem compression and subarachnoid hemorrhage, with 2-year survival rates as low as 20%. These lesions often require vessel remodeling and a combination of stenting and stent-assisted coiling. The use of flow diversion embolization has been reported with varying success; however, this should be used with caution, because the long-term risks of perforating arteries or subsequent basilar artery thrombosis are not well-known.
PCA Aneurysms
Posterior cerebral artery aneurysms are uncommon and constitute approximately 1% of all intracranial aneurysms. Typically they are large and a mixture of saccular and fusiform in nature, involving the P1 or P2 segment, often at the junction of P2 and the posterior communicating artery ( Fig. 2 ). Most frequently, these aneurysms present with subarachnoid hemorrhage, visual deficits, or third nerve dysfunction. In a small series by Ciceri and colleagues, 66% of PCA aneurysms were treated without parent vessel occlusion, with an associated 10% morbidity. The remaining third required parent vessel sacrifice. One of these cases presented with memory loss from a giant P3 aneurysm compressing the hippocampus. Vessel preservation must be attempted when treating P1 segment aneurysms, because this segment provides end-artery thalamoperforators and possibly the artery of Percheron. In another small series of 10 P2 aneurysms, distal vessel occlusion was found to be tolerated. This finding is thought to be caused by the hemodynamic balance between anterior and posterior choroidal arteries, pericallosal arteries, and middle cerebral artery branches with distal PCA branches.
Related posts:
General Technical Considerations for the Endovascular Management of Cerebral Aneurysms
Pitfalls and Complications Management in the Endovascular Treatment of Aneurysms
Cerebral Vasospasm
Vertebrobasilar Fusiform Aneurysms
Endovascular Management and Treatment of Acute Ischemic Stroke
Endovascular Management of Intracranial Atherosclerosis
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