♦ Aneurysm Characteristics

Giant intracranial aneurysms (GIAs) are defined as aneurysms with a fundal width of 2.5 cm or more. The incidence of GIAs has been reported to be approximately 5% of all intracranial aneurysms.1 Most GIAs present in the fifth and sixth decades of life, with a slight female predominance. According to data from the International Study of Unruptured Intracranial Aneurysms (ISUIA), the 5-year cumulative rupture risk is 6.4% for cavernous giant aneurysms, 40% for anterior circulation aneurysms, and 50% for posterior circulation giant aneurysms.2 It should be noted that these data are for asymptomatic aneurysms. Symptomatic aneurysms carry a much higher risk.

Complex intracranial aneurysms are defined as intracranial aneurysms that cannot be managed with standard surgical or interventional techniques due to their anatomic characteristics or relationship to surrounding anatomy ( Table 22.1 and Fig. 22.1 ). CGIAs can require prolonged temporary occlusion time or sacrifice of a significant parent artery for definitive therapy. They may also exert significant mass effect on the surrounding brain or nervous structures, and often require complex surgical exposures to gain safe access and vascular control. Hybrid endovascular and open techniques can be used to treat some of these lesions. Previous treatment can render what was once a simple aneurysm a complex aneurysm. The incidence and characterization of CGIAs are difficult to estimate due to the subjective nature of the term as well as to the variability of their morphologic and anatomic presentations. Outcome measures are confounded by the technical skill required for successful treatment and the complexity of the required surgical or endovascular repair.

♦ Patient and Aneurysm Selection

The successful management of patients with CGIAs requires meticulous planning during every phase of treatment. Preoperatively, the patient should be evaluated and optimized with regard to medical comorbidities to avoid unanticipated complications. In addition to experienced cerebrovascular skull base neurosurgeons, the team should include medical and neurology consultants, neuroradiologists and interventionalists, neuroanesthesiologists, intraoperative neurophysiologists, and a variety of surgical partners including surgeons with cranial base/neurotology experience, and occasionally a plastic and reconstructive surgeon to help plan and execute the often complex approach and help handle reconstruction issues.

Although the ISUIA has provided some basic grounds for treatment recommendations, CGIAs can have characteristics that defy categorization based on size alone. Irregular aneurysm contour, for example, may render a lesion more prone to rupture than predicted by ISUIA data alone. Moreover, medical comorbidities and the patient’s wishes based on family history and psychological burden should be taken into account when making treatment decisions.

Surgical treatment of CGIAs can be required for lesions that have limited or no endovascular options, have failed endovascular therapy, require hunterian ligation or trapping, are at high risk for recurrence with endovascular therapy, are associated with significant intrasaccular thrombus, are associated with branch vessels which require revascularization, or are associated with significant mass effect. Some CGIAs may require a hybrid approach involving both surgical and endovascular techniques for a successful treatment. An example of this is suction-decompression of paraclinoidal or cavernous aneurysms using an endovascular balloon catheter. Alternatively, some CGIAs are best treated via endovascular means because significant surgical morbidity exists, there are confounding medical comorbidities, or because of the patient’s or family’s wishes. Finally, some CGIAs have neither surgical nor endovascular solutions, such as some fusiform or serpentine lesions, and thus are not treated.

♦ Cerebral Revascularization and Blood Flow Alteration

Challenging CGIA cases that require prolonged temporary occlusion times or involve parent artery sacrifice may require prophylactic augmentation of collateral blood flow through bypass grafting or other forms of revascularization.3 ^– 15 Options include extracranial-to-intracranial and intracranialto-intracranial bypasses. In-situ or harvested grafts can be utilized, such as the superficial temporal artery, occipital artery, radial artery, and saphenous vein. Careful patient selection has proven to be the most prominent predictor of successful revascularization strategies. Factors to be considered for revascularization include the amount of blood flow required (low- versus high-flow grafts) and the site of delivery. For CGIAs of the proximal internal carotid, bypass candidates undergo a balloon test occlusion (BTO) to assess blood flow dynamics, collateralization, and vascular reserve ( Table 22.2 ). Occasionally, BTO can be considered for more distal intracranial aneurysms (intracranial BTO).

After completion of a diagnostic angiogram, a balloon is placed just proximal to the aneurysm in question and inflated. Temporary flow occlusion is confirmed, and the reserve supply from leptomeningeal and circle of Willis vessels is qualitatively analyzed as are flow dynamics including venous washout. Although an angiographic study of the collateral circulation is helpful, it does not assess the impact on the patient’s clinical examination. As such, clinical changes are assessed by monitoring the patient’s clinical examination during a hypotensive challenge as well as by electroencephalogram (EEG) and single-pass single photon emission computed tomography (SPECT) imaging. Typically, clinical exams are performed at baseline and every 5 minutes after balloon inflation. If the patient tolerates all forms of provocative testing, then the parent artery in question can be sacrificed with minimal risk of postoperative complications. However, in some cases the need for a prophylactic bypass procedure may be necessary regardless of the results of a BTO, especially when prolonged temporary occlusion times are anticipated. Furthermore, in younger patients, a bypass may be considered to prevent a possible de novo aneurysm formation that may result from altered hemodynamics (e.g., anterior communicating artery [ACoA] aneurysm formation years after carotid artery occlusion). The BTO is also useful in determining the most adequate type of graft (high flow versus low flow). If the patient tolerates the clinical exams but not the hypotensive challenge, or demonstrates asymmetric cerebral blood flow on SPECT, then the patient would benefit from a low-flow revascularization procedure. However, if the patient develops significant deficits at normotension during the BTO, then a high-flow bypass, usually using a harvested radial artery or saphenous vein graft, may be a better option. Patients who cannot tolerate any temporary occlusion or in cases requiring the need for a jump revascularization graft to avoid interruption in distal blood flow, the excimer laser-assisted nonocclusive anastomosis (ELANA) procedure has been shown to be safe and effective in experienced hands.16 ^, 17 Finally, maintenance of in-situ vessels can be accomplished in innovative ways. Efferent vessels emanating from the aneurysmal fundus or neck may be reimplanted (end-to-side) or attached through a side-to-side anastomosis to either the proximal parent artery or to another surrounding vessel. This strategy ensures adequate size matching and flow requirements.

Intraoperative graft patency monitoring can be assessed with indocyanine green (ICG) videoangiography, micro-Doppler ultrasound, or intraoperative angiography.3 ICG is a noninvasive means to assess flow patency whereby a fluorescent dye is injected intravenously and visualized during the surgical procedure using a filter attached to the operating microscope. Patients undergoing accompanying revascularization procedures are placed on aspirin therapy for at least a week before the revascularization procedure. However, if the procedure is planned within a week, the patient can be loaded with aspirin at the time of the procedure. In cases of hypercholesterolemia, a statin can be administered pre- and postoperatively as it has been proven to affect long-term graft patency.18

One of the primary concerns during complex vascular lesions surgery is the acute and delayed ischemic complications that might result from temporary occlusion. Temporary flow arrest or permanent parent artery sacrifice may be necessary for the treatment of complex and giant aneurysms. For patients who can tolerate little if any disruption in cerebral perfusion as determined by a BTO, cerebral revascularization can be helpful. In certain cases, bypass may be required solely to provide necessary cerebral blood flow during extended temporary occlusion times during aneurysmorrhaphy. Alternatively, when a parent artery sacrifice is necessary and sufficient collateral supply does not exist, permanent revascularization is necessary. Furthermore, some efferent vessels distal to the lesion may not be salvageable without bypass techniques. In cases in which efferent branches emanate from the fundus or very close to the neck and sacrifice of these branches would have a significant effect on the patient’s neurologic status, reimplantation or regional reanastomosis of these efferent branches into other parent arteries can sometimes substitute for a bypass procedure. However, if several efferent branches exist, a bypass to one branch may be necessary while reimplanting the remaining branches to other local arteries.

Alteration of cerebral blood flow can also facilitate the manipulation and treatment of GIAs. Minimizing or completely stopping inflow to these lesions can convert them into soft, pliable sacs that can be more easily manipulated. Several techniques have been developed over the last several years to render these giant lesions safer and easier to treat. One option is deep hypothermic circulatory arrest. This technique has seen a recent resurgence due to the wider availability of advanced intraoperative critical care. Disadvantages include the additional personnel required, as well as the potential added complications of cerebral edema, thromboembolism, and coagulopathy.19 ^, 20 Alternatively, a hypothermic low-flow cardiopulmonary bypass technique has been used to sustain a reduced cardiac output of approximately 500 mL/min.21 This technique renders the aneurysmal sac more pliable without the added risk of complete circulatory arrest. In recent years, a simpler pharmacologic blood flow reducing technique has been utilized. Adenosine, a fast-acting nucleoside, has been used to induce bradycardia and asystole for several seconds, thereby shutting down cardiac output temporarily. This simple technique can allow the prepared surgeon enough time to manipulate the aneurysm without the need for more invasive measures. This is especially beneficial in those patients who cannot tolerate temporary occlusion or those with severe atheromatous disease. Finally, endoluminal balloons can be used for temporary occlusion of the afferent vessel to the aneurysm. This endovascular approach provides access to segments of the parent artery, obviating the need of a complex open approach to the petrous carotid or proximal vertebral artery.