Microsurgical Management of Giant Intracranial Aneurysms

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CHAPTER 378 Microsurgical Management of Giant Intracranial Aneurysms

Giant intracranial aneurysms have, by definition, a minimum diameter of at least 25 mm. Their dismal natural history necessitates treatment, but their size and anatomic complexity place them among the most challenging lesions confronting neurosurgeons. Neurosurgical advances have transformed giant aneurysms into primarily a surgical disease with outcomes that have improved through the decades with the advent of the operating microscope, refined microsurgical instruments, skull base surgical approaches, and bypass techniques. Treatment has evolved into a multidisciplinary approach involving neurosurgical, endovascular, and neuroanesthesia subspecialties, with improved patient selection and treatment results. Despite impressive technologic advances, endovascular therapy for giant aneurysms has clear limitations related to incomplete aneurysm occlusion, recurrence of aneurysms, ongoing angiographic surveillance, delayed retreatment, and occasional rehemorrhage. Consequently, giant aneurysms currently remain primarily a surgical disease that must be managed effectively to minimize morbidity. The challenges of a giant aneurysm are the preoperative and intraoperative decisions required to devise the best strategy and manage the treatment safely. Sound decisions depend on familiarity with the spectrum of treatment options, mastery of the techniques required in surgery, and experience. This review presents the critical elements of the surgical strategies for giant intracranial aneurysms derived from an experience with 117 surgical patients.

Historical Considerations

Cerebral aneurysms were not recognized as a pathologic entity until Biumi of Milan identified a ruptured aneurysm during an autopsy in 1765. Hutchinson described the first giant intracranial aneurysm in 1875, which was diagnosed by an audible bruit.¹ The patient refused Hutchinson’s recommendation of carotid artery ligation. The diagnosis was confirmed 11 years later when the patient died of an aortic aneurysm and, at autopsy, a partially calcified aneurysm the size of a “bantam hen’s egg” was found in the middle fossa.

In the next century, as Standard and colleagues noted,² the diagnosis of cerebral aneurysms improved significantly when Moniz developed cerebral angiography. Nonetheless, these lesions were still sometimes mistaken for brain tumors. In the early part of the 20th century, treatment included hunterian ligation of the internal carotid artery (ICA).³ According to Laws and Udvarhelyi,⁴ Dandy was the first to actually expose a cerebral aneurysm and ligate its neck by clipping.

Based on 6368 cases in a cooperative study, Locksley classified aneurysms that were 25 mm or greater as giant and observed a high rate of morbidity and mortality associated with these lesions.⁵ The first two significant studies of giant aneurysms were reported by Morley and Barr⁶ and by Bull.⁷ The size of the aneurysms in these series may well have been underestimated because the diagnosis was based solely on plain radiography and angiography. At that time, the natural history of these lesions was beginning to be understood, and surgical treatment was fraught with hazards. Morley and Barr concluded that “direct surgical attack on extracavernous giant aneurysms is seldom possible or successful except in the case of middle cerebral artery aneurysms.”⁶

During the next several decades, several large series confirmed that significant morbidity and mortality rates were associated with these lesions. Peerless and associates reported mortality rates of 68% and 85% at 2 and 5 years, respectively, for untreated giant aneurysms, and even survivors suffered marked neurological dysfunction.⁸ Michel noted a 100% mortality rate at 2 years in this patient population,⁹ and Kodama and Suzuki reported that 75% of their untreated hospitalized patients died of subarachnoid hemorrhage (SAH).¹⁰ In contrast to this dismal natural history, several large surgical series demonstrated excellent to good outcomes in 61% to 87% of all patients treated surgically, with surgical mortality ranging from 5% to 22% (Table 378-1).^8,¹⁰^–¹⁸

TABLE 378-1 Comparison of Outcomes in Surgical Series of Giant Intracranial Aneurysms

The improved outcomes associated with surgical intervention resulted from a variety of factors, including the development of microneurosurgery and skull base approaches. Alternatives to direct surgical attack, such as parent vessel ligation, aneurysm trapping, and extracranial-to-intracranial bypass procedures, have also increased surgical options for the treatment of these challenging lesions. Since the 1990s, endovascular techniques have proved to be valuable surgical adjuncts. Given the improvement in surgical outcomes and the otherwise dismal natural history of giant aneurysms, aggressive surgical treatment is warranted.

Epidemiology and incidence

Giant aneurysms represent 2% to 5% of all intracranial aneurysms.^6,¹⁹^–²² As with other aneurysms, giant aneurysms have a female preponderance, although Kodama and Suzuki found an equal sex distribution.¹⁰ Most patients become symptomatic in the fourth through sixth decades of life.^3,^14,^21,²³

Giant aneurysms are found in all locations throughout the intracranial vasculature (Table 378-2).^8,¹¹^–¹³ ^,16 ^,17 ^,19 In general, 34% to 67% of giant intracranial aneurysms are associated with the ICA, 11% to 40% with the anterior cerebral artery and middle cerebral artery (MCA), and 13% to 56% with the vertebral and basilar arteries.^3,^8,^{11–13,16,17,19,21}^,24 In the senior author’s (M.T.L.) experience, posterior circulation aneurysms were treated more frequently in the group of patients with giant aneurysms (23%) than in the overall group of aneurysm patients (15%), which reflects a selection bias involving these complex aneurysms that favors microsurgery over endovascular therapy. Peerless and coauthors reported that 56% of their surgical series consisted of giant aneurysms of the vertebrobasilar system,⁸ a finding reflecting a referral bias to their institution.²¹

TABLE 378-2 Distribution of Giant Aneurysms in the Cerebral Circulation

Pathophysiology

As early as 1930, Forbus proposed that aneurysms developed from defects in the medial layers of the vessel.²⁵ Pathologic evaluation of giant aneurysms often demonstrates lack of a muscular layer and degeneration of the elastic laminar layers.²⁶ Most giant aneurysms begin as saccular aneurysms located at arterial bifurcations, thus invoking hemodynamic factors in their formation. Stehbens was a proponent of acquired defects rather than congenital vessel defects contributing to the formation of cerebral aneurysms.²⁷ Vessel weaknesses could be aggravated by flow-related stress or degenerative disease such as atherosclerosis. Some authors have postulated that giant aneurysms grow from repeated intramural hemorrhage within the aneurysm wall,^20,^21,²⁸ followed by thrombus formation and neovascularization.^29,³⁰ Giant aneurysms developing de novo or from smaller aneurysms have been described.^21,^26,^31,³² They have also been associated with underlying systemic arteriopathies.^20,³³ Giant aneurysms can result directly from trauma.^20,³³ More than one aneurysm may be present in 10% to 36% of patients.^17,^24,³⁴

The morphology of giant aneurysms can be either fusiform or saccular, but their size and complexity may make it difficult to differentiate these two types. Saccular lesions most often occur at arterial bifurcations, probably the result of continuous hemodynamic stress. Fusiform or dolichoectatic lesions may result from atherosclerosis, congenital arteriopathies, or traumatic dissection.^20,^23,^35,³⁶ Numerous factors have been linked to the formation and rupture of aneurysms, including female sex, age, hypertension, connective tissue disease, and smoking.^27,^37,³⁸

As aneurysms grow, wall tension must increase to maintain vessel integrity, as calculated from Laplace’s law. This equation relates the stress over the aneurysmal wall to the radius of the lesion.³ The same hemodynamic forces that prompt an aneurysm to grow may ultimately be responsible for its rupture.

A significant proportion of giant aneurysms have associated intraluminal thrombosis—as many as 60% in some series.^3,^20,²¹ Intra-aneurysmal thrombosis may develop in areas where blood outside the turbulent flow stream stagnates, thereby promoting thrombosis. The presence of a partially thrombosed aneurysm does not appear to lower the risk for rupture of the aneurysm.^29,³⁹

Clinical Findings

Although patients with giant intracranial aneurysms may initially be evaluated for SAH, signs and symptoms related to a mass effect develop in approximately two thirds.^34,⁴⁰ Only about a third of patients have SAH as the initial symptom,^14,⁴⁰ although Onuma and Suzuki reported that 80% of the patients in their series were initially seen because of SAH.¹⁹ As many as 50% of patients with giant fusiform aneurysms have signs of a mass effect, as opposed to 20% of patients with initial findings of SAH.³⁶

The symptoms related to aneurysmal mass effect depend on the aneurysm’s location. In the anterior circulation, mass effect can be manifested as pain, visual field and acuity defects, and extraocular dysfunction. Dementia and mental disturbances, as well as hemiparesis and epilepsy, have also been described.²⁰ Lownie and coworkers reported that an aneurysm had to be 3.5 cm to produce dementia and that those between 2.7 and 3.2 cm compressed the optic apparatus.⁴¹ In the posterior circulation, multiple cranial nerve dysfunctions may be present. If compression on the brainstem is significant, bulbar palsies and hemiparesis can also occur.²⁰ The remainder of patients may have headache, syncope, sinusitis, epistaxis, confusion, cerebrovascular accidents, or symptoms related to head trauma.^20,⁴⁰

The risk for thromboembolism to distal vascular territories from a partially thrombosed giant aneurysm must be emphasized. In the senior author’s experience, 7% of patients with giant aneurysms initially had symptoms of distal thromboembolism, including transient ischemic attacks and stroke. Sano and associates reported that five conservatively treated patients all died of infarction.⁴² Naturally, only complete isolation of an aneurysm from the cerebral circulation eliminates the risk for rupture, continued enlargement, and thromboembolic phenomena.

Although SAH is less commonly associated with giant intracranial aneurysms than with small ones, it remains devastating. The annual rate of rupture of giant intracranial aneurysms is higher than that of smaller aneurysms. In several studies the annual risk for rupture correlated with increasing aneurysm size.⁴³^–⁴⁵ Recent natural history studies have demonstrated an annual rupture rate for giant intracranial aneurysms of around 6%,^46,⁴⁷ higher than the 1% to 3% rate quoted for smaller aneurysms.¹²

SAH from giant aneurysms may be associated with more severe neurological deficits. According to Laplace’s law, the higher stress over the giant aneurysmal wall could result in a greater amount of SAH during rupture.^2,²¹ As with smaller intracranial aneurysms, once SAH has occurred, more than 50% of patients die or experience significant complications.^24,⁴⁸ The goal is to diagnose aneurysms before they rupture or produce irreversible deficits.

Diagnosis

High-quality four-vessel cerebral angiography has long been considered the “gold standard” for the diagnosis of cerebral aneurysms; it provides information about an aneurysm’s location, anatomy, adjacent branch vessels, collateral circulation, and distal cerebral perfusion.⁴⁰ It also reveals other intracranial aneurysms that might be treated simultaneously. Anteroposterior, lateral, and oblique views define the relationships between inflow and outflow vessels and the aneurysm neck. Superselective injections and manual compression techniques may clarify the anatomy or collateral blood flow. One caveat with traditional angiography is that it shows luminal filling but may fail to reveal the aneurysm’s true size if the aneurysm is thrombotic.

Computed tomography (CT) is useful in defining the outer dimensions of an aneurysm. Frequently, the aneurysm wall is thickened and calcified, which can influence the treatment strategy.²⁰ When contrast material is administered, this eggshell border may appear as a “target sign,” with contrast material filling the center and the calcified rim appearing hyperdense.³ CT is essential in patients with SAH because it visualizes acute blood. CT also helps define the relationships between the aneurysm and skull base. CT angiography (CTA) avoids some of the risks of stroke associated with cannulation of the head and neck vessels during traditional angiography. Three-dimensional reconstructions demonstrate the aneurysm’s anatomy in exquisite detail but are not as helpful as angiography in visualizing the hemodynamics of the aneurysm. Still, angiography and CTA complement each other well and together give the neurosurgeon a clear picture of the anatomy of the aneurysm before surgery.⁴⁹

Magnetic resonance imaging (MRI) offers another noninvasive technique for identifying and defining giant aneurysms. The breakdown products of blood, including deoxyhemoglobin, methemoglobin, and hemosiderin, have distinct signal characteristics on T1- and T2-weighted imaging that can be used to identify intraluminal clot. Signal flow voids, which indicate active blood flow within aneurysms, help define the filling component of an aneurysm. Magnetic resonance angiography (MRA) uses the flow void phenomenon to reconstruct an anatomic model of the vasculature.⁵⁰ Even though MRI does provide information about the actual and luminal anatomy of the aneurysm, it is particularly helpful in evaluating the aneurysm’s compressive effects on adjacent brain and neural structures.⁵¹ Postoperative follow-up may be difficult with MRA, however, because of clip or coil metal artifacts.⁵²

Management

Given the dismal natural history of giant cerebral aneurysms, a conservative or expectant approach should be considered only in patients with mitigating factors (e.g., poor medical grade, refusal of intervention). Patients with SAH should be managed according to accepted principles and protocols. In particular, we recommend aggressive control of hypertension, prompt institution of calcium channel blockers (e.g., nimodipine), and ventriculostomy if indicated.⁴⁰ We recommend definitive treatment within 48 hours for ruptured aneurysms.⁵³

Once the decision for intervention has been made, several treatment options are available in the neurosurgeon’s arsenal, including direct and indirect surgical attack, as well as endovascular procedures. Other considerations include the use of intraoperative electrophysiologic monitoring, intraoperative angiography, and specialized anesthesia techniques. Because of the potential for blood loss, all patients must have their blood typed and crossmatched, and central venous access should be available. If hypothermic circulatory arrest is being considered, a Swan-Ganz catheter should be placed for cardiac monitoring. Cerebral protective anesthetics such as isoflurane should be used. Cerebral protective agents such as barbiturates can be used during temporary vessel occlusion, and their efficacy can be monitored with electroencephalographic techniques.^54,⁵⁵ Intraoperative angiography may help determine whether residual luminal filling is present after the aneurysm has been clipped and can be useful in assessing the patency of bypass procedures. Furthermore, any stenosis or kinking associated with a clipped or reconstructed vessel can be confirmed immediately. Microvascular Doppler ultrasonography may be used to determine hemodynamically significant vessel stenosis. Videography with indocyanine green dye has been useful in assessing complete closure of aneurysms and the patency of parent and branch arteries, and it is faster and easier than catheter-based angiography.

The optimal skull base approach to a giant aneurysm should offer a wide degree of exposure with full visualization of the aneurysm’s origin, outflow, associated perforators, adjacent vessels, and nearby neural structures (Fig. 378-1). The location of the aneurysm dictates the most appropriate skull base approach (Table 378-3). The basic principles of minimal brain retraction with maximal bone exposure apply. Increasingly, endovascular techniques are being combined with operative techniques to improve outcomes.⁵⁶^–⁶¹

FIGURE 378-1 Specific skull base approaches to aneurysms are shown by shaded areas. Selection of the proper approach can provide access to almost any skull base lesion.

(Courtesy of Barrow Neurological Institute, Phoenix, AZ.)

TABLE 378-3 Selection of Surgical Approach Based on Location of the Giant Aneurysm

SITE OF ANEURYSM	SKULL BASE APPROACH
Proximal internal carotid artery	Pterional, orbitozygomatic
Bifurcation of the internal carotid artery	Pterional, orbitozygomatic
Proximal anterior cerebral artery	Pterional, orbitozygomatic
Distal anterior cerebral artery	Pterional, orbitozygomatic, interhemispheric
Middle cerebral artery	Pterional, orbitozygomatic
Vertebral artery	Far lateral
Vertebrobasilar junction	Far lateral
Midbasilar artery	Petrosal, far lateral, orbitozygomatic
High basilar artery	Orbitozygomatic
Posterior inferior cerebellar artery	Far lateral, suboccipital
Anterior inferior cerebellar artery	Petrosal, far-lateral, orbitozygomatic
Superior cerebellar artery	Orbitozygomatic

From Lemole GM Jr, Henn J, Spetzler RF, et al. Surgical management of giant aneurysms. Oper Tech Neurosurg. 2000;3:239-254.

Anterior Circulation Approaches

Orbitozygomatic-Pterional Approach

The pterional transsylvian approach is one of the most commonly used approaches to aneurysms of the anterior circulation. It provides access to the entire circle of Willis and its constituent branches. Exposure can be further enhanced by drilling the pterion and bony ridges over the floor of the frontal fossa.⁴⁰ When combined with a transsylvian pterional approach, orbitozygomatic osteotomy can greatly increase the extent of exposure.⁶²^–⁶⁵ Removal of the orbital rim, orbital roof, lateral wall, and zygomatic arch significantly enhances access to anterior skull base and upper clival lesions. Removing the orbitozygomatic bar provides the surgeon’s hands a wider corridor of access to the lesion and a shallower depth of field. Deep bypass procedures at the skull base are often technically difficult, and the added exposure can be essential for suturing vessels. This is particularly the case with giant MCA aneurysms that must be excised and the parent vessel reanastomosed. In addition, the orbitozygomatic approach provides a lower trajectory along the skull base, which reduces the need for cerebral retraction (Fig. 378-2).

FIGURE 378-2 Orbitozygomatic approach. A, Standard pterional craniotomy. B, Removing the orbitozygomatic bar provides a wider degree of access and a more shallow surgical field. Furthermore, less cerebral retraction is required to reach deep brain structures.

(Courtesy of Barrow Neurological Institute, Phoenix, AZ.)

Orbitozygomatic osteotomy is usually performed after a traditional pterional craniotomy is completed.⁶⁶ A series of cuts through the lateral orbit, orbital roof, and maxillary and zygomatic roots frees the orbital bar from the adjacent skull base (Fig. 378-3). This piece of bone is preserved for later reinsertion. In addition, bone surrounding the superior orbital fissure can be removed with a high-speed drill or microrongeurs. Opening the dura with its flap based inferiorly allows the dural flap to gently depress the globe inferiorly and laterally, again enhancing exposure.⁴⁰ In preparation for orbitozygomatic osteotomy, we also recommend subfascial or interfascial dissection of the temporalis fascia to protect the frontalis branch of the facial nerve.^66,⁶⁷

FIGURE 378-3 Orbitozygomatic osteotomy. A and B, The orbitozygomatic bar is freed from the skull base with a series of cuts, including those through the zygomatic root (1), the malar eminence (2 and 3), and the orbital roof just lateral to the supraorbital notch and extending back to the superior orbital fissure (4) and finally connecting the superior and inferior orbital fissures (5 and 6). C, The resulting orbitozygomatic bar can then be freed in one piece.

(Courtesy of Barrow Neurological Institute, Phoenix, AZ.)

The risks associated with orbitozygomatic osteotomy include periorbital bruising and swelling, pulsatile enophthalmos, injury to the frontalis nerve, orbital entrapment, diplopia from injury to an extraocular muscle or nerve, and blindness. However, the overall incidence of these complications is low. Typically, the advantages provided by the orbitozygomatic approach more than offset its theoretical risks.^66,⁶⁸

Orbitozygomatic osteotomy may be ideal for almost all giant and complex aneurysms originating from the circle of Willis, including ophthalmic, paraclinoid, superior hypophysial, posterior communicating, anterior choroidal, and ICA bifurcation aneurysms. Sometimes, additional exposure by drilling the anterior clinoid process and exposing the carotid artery in Glasscock’s triangle further enhances the exposure of certain lesions. At our institution, we also use orbitozygomatic osteotomy to approach giant and complex lesions of the MCA because the trajectory of the approach to the proximal M1 segment and the aneurysm neck is optimized (Fig. 378-4).

FIGURE 378-4 Computer reconstruction of the pterional (A) and orbitozygomatic-pterional (B) approaches to middle cerebral artery (MCA) aneurysms. When compared with the pterional approach, the orbitozygomatic approach provides a lower trajectory along the skull base floor, and the supraorbital trajectory facilitates proximal control of giant and complex MCA aneurysms. The approach provides a perpendicular trajectory to the M1 segment and aneurysmal neck (compare the insets).

Interhemispheric Approach

To access lesions involving the distal anterior cerebral arteries, a bifrontal interhemispheric approach may be necessary. This approach can be combined with pterional and orbitozygomatic osteotomies to widen the degree of proximal control, as well as to secure distal control.

Posterior Circulation Approaches

The skull base approaches used for giant aneurysms of the posterior fossa include the orbitozygomatic approach, transpetrosal approaches, and the far lateral approach. Combinations of these approaches can be used for more extensive lesions. Selection of the appropriate skull base approach depends on the aneurysm’s location (Fig. 378-5). Conceptually, the basilar artery can be divided into thirds. The orbitozygomatic approach is optimal for lesions of the upper third, and the vertebrobasilar area is best accessed with the far lateral approach. Lesions involving only the midbasilar zone may require transpetrosal or extended retrosigmoid approaches.

FIGURE 378-5 Skull base approaches to posterior circulation aneurysms. Access to the basilar artery zones and brainstem is provided by the skull base approaches shown here. The shaded areas demonstrate the degree of exposure provided by each approach.

(Courtesy of Barrow Neurological Institute, Phoenix, AZ.)

Orbitozygomatic Approach

The particulars of the orbitozygomatic osteotomy were described previously, but modifications can improve access to the upper basilar artery. Drilling the anterior and posterior clinoid processes and the clivus itself allows visualization down toward the midbasilar zone (Fig. 378-6).⁶⁹ The orbitozygomatic approach is also useful with high-riding giant aneurysms of the basilar artery because it provides exposure of the upper interpeduncular space without requiring excessive frontal lobe traction. Important with any approach to aneurysms in this area is adequate visualization of all inflow and outflow vessels, including the bilateral posterior cerebral arteries and superior cerebellar arteries, as well as the proximal basilar artery.

FIGURE 378-6 Modified orbitozygomatic approach to the upper basilar region. The orbitozygomatic approach can be modified to improve access to the upper basilar area. These modifications consist of removing the posterior clinoid processes and upper clivus with a high-speed drill (crosshatching). Arrow indicates the trajectory of the surgical approach.

(Courtesy of Barrow Neurological Institute, Phoenix, AZ.)

Transpetrosal Approaches

The anterior clivus and brainstem can be accessed by progressive removal of the petrous ridge. Depending on the degree of removal, the approaches can be defined as retrolabyrinthine, translabyrinthine, and transcochlear (Fig. 378-7).⁷⁰^–⁷⁵ The amount of petrosal drilling dictates the degree of exposure of the anterior brainstem and clivus. Drilling the semicircular canals sacrifices hearing, whereas drilling the facial canal and transposition of the facial nerve may produce temporary facial nerve palsy. The expected morbidity involving cranial nerves VII and VIII with the more aggressive petrosal approaches must be weighed against the advantages of increasing exposure in the posterior fossa. At our institution, transpetrosal approaches are reserved for complex lesions of the midbasilar zone. If possible, we prefer to use the orbitozygomatic or far lateral approach or a combination to access lesions that extend into the midbasilar zone.

FIGURE 378-7 Transpetrosal skull base approaches. The degree of bone removal defines the transpetrosal approach. The retrolabyrinthine approach preserves middle ear structures. The translabyrinthine approach removes the semicircular canals, thereby sacrificing hearing. The transcochlear approach offers the greatest exposure to the anterior brainstem by removing the cochlea and transposing the facial nerve.

(Courtesy of Barrow Neurological Institute, Phoenix, AZ.)

Far-Lateral Approach

The far lateral approach provides excellent exposure from the midbasilar zone down to the intradural vertebral artery.^18,^55,^76,⁷⁷ A lateral suboccipital craniotomy is modified in three ways to create the far lateral approach: (1) the arch of C1 is removed to the sulcus arteriosus, (2) the bony rim of the foramen magnum is resected to the occipital condyle, and (3) the posterior third of the occipital condyle is removed (Fig. 378-8). Removing less than half of the occipital condyle should not affect spinal stability,⁷⁸ nor should it pose a risk to cranial nerve XII as it courses through the hypoglossal canal. The extracranial vertebral artery may be freed from its course in the sulcus arteriosus and transverse foramen and transposed laterally to improve operative exposure. The far lateral approach is best used to access lesions of the vertebrobasilar junction, including giant aneurysms of the vertebrobasilar, vertebral, and proximal posterior inferior cerebellar arteries.

FIGURE 378-8 A far lateral suboccipital craniotomy is combined with C1 hemilaminectomy and posterior condylectomy. This exposure provides excellent access to the vertebrobasilar junction as far rostrally as the midbasilar zone.

(Courtesy of Barrow Neurological Institute, Phoenix, AZ.)

Combined Approaches

Several approaches can be combined to treat complex lesions involving multiple basilar zones or when anatomic complexity necessitates wider access and control (Fig. 378-9).⁷⁴ A combined supratentorial and infratentorial approach merges a subtemporal craniotomy with a transpetrosal approach. It can be extended farther inferiorly if a far lateral approach is added to it.⁷⁹ Sometimes, the tentorium and lateral sinus can be transected to facilitate exposure.⁸⁰ As always with skull base approaches, the risks associated with extensive bony drilling are offset by minimized brain retraction.

FIGURE 378-9 Combined approaches. In this example a combined-combined approach is shown, with orbitozygomatic craniotomy coupled with transpetrosal drilling and far lateral exposure. The entire brainstem is exposed from the foramen magnum to the circle of Willis.

(Courtesy of Barrow Neurological Institute, Phoenix, AZ.)

Subtemporal Approach

Historically, the subtemporal approach was used routinely for aneurysms involving the upper basilar artery.⁶³^–⁶⁵ ^,69 ^,70 The transsylvian pterional approach has now supplanted it to a large degree. The subtemporal approach can still be useful with posteriorly projecting upper basilar aneurysms¹² and those located around the lateral edge of the tentorium.

Operative Techniques

The basic tenets of aneurysm surgery apply to giant intracranial aneurysms: complete vascular control, wide aneurysm exposure, careful clip application, and selective use of alternative techniques when clipping is not possible.

Vascular Control

Vascular control is imperative with giant aneurysms because it allows the neurosurgeon to respond to intraoperative rupture of the aneurysm. In addition, vascular control enables temporary clipping of the parent or afferent arteries to soften the aneurysm, dissect it from surrounding neural structures, and manipulate the dome aggressively. Temporary clipping facilitates permanent clipping because clips can be applied more easily and with better visualization to a softened aneurysm sac.

In the anterior circulation, obtaining vascular control may be routine. However, when aneurysms are located at the skull base, particularly at the point where the carotid artery exits the cavernous sinus, proximal control may be more difficult to achieve. In these instances, the aneurysm itself may obstruct the view of a proximal vessel. Alternative techniques of proximal control may be essential for giant paraclinoid, ophthalmic, and superior hypophysial artery aneurysms. In these cases, the ICA can be controlled in the neck through an additional surgical incision or along its petrous course by drilling Glasscock’s triangle.⁴⁰ Intracranial exposure can be facilitated by drilling the anterior clinoid and incising the distal dural ring, thereby accessing the clinoidal ICA, which may provide enough room for a temporary clip. Endovascular balloon techniques have been used to gain proximal control of the ICA.⁶⁰

Proximity to the skull base and complex vascular anatomy make control of the posterior circulation more difficult to achieve. Skull base approaches can provide some access to these areas, but the vascular control that can be achieved is often incomplete. Extreme caution must be exercised when placing temporary clips along the middle portion of the basilar artery because brainstem perforators are susceptible to injury. With far lateral exposure, proximal control can be achieved with temporary clips on the vertebral arteries. However, distal control beyond the aneurysm along the basilar artery can be problematic. With orbitozygomatic exposure, proximal control can be gained by placing a temporary clip on the basilar trunk, but control of the contralateral superior cerebellar artery and posterior cerebral artery can be difficult. Frequently, the mass of a giant aneurysm makes these deep points of control difficult to visualize and access.

Endovascular temporary balloon occlusion techniques can also be useful in the posterior circulation, particularly in the proximal basilar and vertebral artery regions.⁵⁹ Distal balloon occlusion of the middle basilar artery may obstruct brainstem perforator vessels, with devastating consequences. Retrograde placement of a temporary occlusion balloon into the upper basilar artery from the anterior circulation is usually difficult.⁴⁰ As with all temporary occlusion techniques, the risk for cerebral ischemia can be lessened by using cerebral protective agents, minimizing clip occlusion time, and providing alternative sources of circulation through appropriate bypass procedures, if necessary. Mild hypertension can also be induced to maximize circulation through collateral pathways.

When all other forms of vascular control prove insufficient, hypothermic circulatory arrest can be considered. Circulatory arrest permits complete vascular control, and the risk for intraoperative rupture of the aneurysm is eliminated. Hypothermic circulatory arrest can facilitate the surgical technique by allowing more aggressive manipulation of the aneurysm dome without fear of rupture. Absence of blood flow greatly facilitates debulking of the aneurysm and clip reconstruction. Deep hypothermia reduces the brain’s metabolic requirements for oxygen,⁵⁴ but during circulatory arrest, adequate cerebral protection with barbiturates is imperative. The risks related to hypothermic circulatory arrest are well described and include coagulopathies associated with hypothermia⁸¹ and those associated with hemodilution after blood products and intravenous fluids are replaced.⁸² Systemic heparinization is needed to place the percutaneous bypass catheter required for bypass procedures, but heparin can subject patients to hemorrhagic complications.⁸³ Finally, even when cerebral protective agents and hypothermia are used, the total duration of circulatory arrest must be minimized to avoid ischemic complications. Hypothermic circulatory arrest is associated with a morbidity rate of 13% and a mortality rate of 8%.⁸⁴ These numbers, in part, reflect the complexity of aneurysms that require hypothermic circulatory arrest for adequate vascular control.

Surgical Techniques for Clipping

Successful clipping of a giant aneurysm depends on the anatomic complexity and morphologic features of the lesion. Successful clipping of saccular aneurysms typically requires a favorable and well-defined aneurysm neck. The clip is positioned to bring the vessel walls together at the neck of the aneurysm while the patency of the parent vessel is preserved. Fusiform or dolichoectatic giant aneurysms have no definable neck, and efferent vessels may arise at various points along the aneurysm.³⁵ Simple clipping of these lesions is often impossible, and clip reconstruction techniques are frequently necessary to exclude these aneurysms from the circulation. Creation of a fenestration tube is one technical option useful for redirecting blood flow after occlusion of the aneurysm (Fig. 378-10). In addition, several smaller clips placed in tandem can be a better solution than placing one long clip (Fig. 378-11). With thrombotic aneurysms, clip placement needs to be combined with thrombectomy to relieve the mass effect (Fig. 378-12). Other aneurysms must be excised altogether because of their mass effect, thereby adding another layer of complexity to the reconstruction (Fig. 378-13). In these instances, clips must be used to reconstruct the vessel lumina to connect all inflow and outflow vessels. These techniques require an exquisite comprehension of vascular anatomy and spatial creativity.

FIGURE 378-10 Tandem clipping with a fenestration tube. Some clip reconstructions require stacked fenestrated clipping to re-create a patent lumen while excluding the aneurysm. A and B, This giant anterior communicating artery aneurysm was clipped with a stack of fenestrated clips (C), with the ipsilateral A2 artery coming out of the fenestration tube (D). Postoperative angiography (anteroposterior [E] and lateral [F] views) demonstrate preservation of this A2 artery origin and exclusion of the aneurysm.

FIGURE 378-11 Direct clipping of a giant aneurysm. Lateral (A), anteroposterior (B), and oblique (C) views of a right carotid angiogram demonstrate this supraclinoid internal carotid artery giant aneurysm. D, Axial magnetic resonance imaging demonstrates the aneurysm in cross section. Tandem angled fenestrated clips can be used to reconstruct the carotid artery along this wide aneurysm neck, as seen postoperatively on anteroposterior (E) and lateral (F) carotid angiograms. Intraoperative photographs show the aneurysm’s relationship to the optic nerve and anterior clinoid process (G), the proximal end of the neck after clinoidectomy (H), and the medial part of the neck of the aneurysm (I). J and K, Clipping required multiple clips in a heel-to-toe conformation and preservation of medial perforators to the optic nerve (L).

FIGURE 378-12 Thrombectomy and clip reconstruction. Compacted coils with intraluminal thrombus add complexity to the management of this giant ophthalmic artery aneurysm, as seen on a lateral angiogram before treatment (A) and on an anteroposterior angiogram after it recurred (B). C, Sagittal magnetic resonance imaging demonstrates the size of the aneurysm, which is considerably larger than its luminal filling indicates on angiography. D, Thrombectomy with the Cavitron ultrasonic aspirator debulked the aneurysm down to the coils to allow the softened neck to be clipped (E). F, Lateral angiogram postoperatively.

FIGURE 378-13 Aneurysm transection and clip reconstruction. A, Computed tomographic angiography reveals this thrombotic and calcified aneurysm involving the supraclinoid internal carotid artery (ICA). B, Lateral angiography shows its dolichoectatic morphology and absence of a neck. C, Anterior clinoidectomy was needed to expose the proximal ICA, and the aneurysm was transected. D and E, Fenestrated clips were stacked across the opening in the aneurysm to reconstruct the carotid artery. F, Anteroposterior angiography demonstrates the carotid reconstruction.

The technical aspects of direct clipping of giant aneurysms must not be overlooked. Aneurysm clips have mechanical limitations. Notably, the lowest closing force along the clip is located at its tip.³ Clips may also be stacked on top of one other to guard against clip migration or slippage.^69,⁷⁰ Aneurysms with atherosclerotic changes within their walls may prevent clips from closing completely. The neurosurgeon must then be careful to not loosen plaque into the parent vessel, where it can embolize to distal vascular territories.¹⁶ In such cases, some authors use crushing instruments to make the aneurysm necks more malleable.⁸⁵

Once a clip has been placed, the neurosurgeon should inspect the parent vessel and any associated neural and vascular structures. Hemodynamically significant stenoses may result if vessels kink after clips are placed.²¹ The neurosurgeon must also ascertain that no small perforating vessels are caught within the clip blades, particularly brainstem and thalamic perforators associated with basilar tip aneurysms or lenticulostriate perforators associated with ICA bifurcation and proximal MCA aneurysms. Vessel patency and blood flow can be assessed intraoperatively with micro-Doppler probes, angiography, or indocyanine green dye.

Alternative Occlusion Techniques

Because of their size, location, or morphologic features, certain giant aneurysms cannot be clipped directly. The neurosurgeon must then consider other options, which include proximal parent artery occlusion, distal parent artery occlusion, and trapping, with or without bypass.^40,⁸⁶ Aneurysm trapping excludes the aneurysm from the cerebral circulation, whereas parent artery occlusion redirects or reduces blood flow through the aneurysm to promote thrombosis and minimize its risk for rupture. These latter techniques are inferior to direct clipping and are usually considered only for aneurysms that fail direct clipping.

Parent artery occlusion can be either proximal or distal to the aneurysm. Hunterian ligation of parent vessels harboring giant aneurysms is a viable surgical alternative (Fig. 378-14).⁸⁷ These techniques are particularly useful for aneurysms with no definable necks, such as fusiform or dolichoectatic aneurysms. Surgical alteration of blood flow dynamics through these lesions may result in thrombosis and resolution,⁸⁸^–⁹⁰ but spontaneous revascularization of a previously thrombosed aneurysm is possible.⁹¹^–⁹³ Collateral blood flow upstream of the parent vessel occlusion must be available through the circle of Willis, leptomeningeal connections, or surgical augmentation. In the latter case, numerous bypass procedures may be able to provide blood flow to the distal circulation (Fig. 378-15). Preoperatively, collateral circulation can be assessed by cerebral angiography and temporary balloon occlusion tests to determine neurological status and cerebral blood flow. None of these studies, however, are completely reliable in ascertaining which patients will tolerate permanent vessel occlusion without a bypass.^24,^85,^87,⁹⁴

FIGURE 378-14 Parent artery ligation. Hunterian ligation of parent vessels harboring giant aneurysms is a viable surgical alternative in some cases, as with this basilar apex aneurysm, which is filled with thrombi (A, computed tomographic angiography, sagittal view) and is dolichoectatic, with both the posterior cerebral and superior cerebellar arteries coming from the aneurysm (catheter-based angiogram, anteroposterior [B], lateral [C], and three-dimensional reconstructed [D] views). Parent artery occlusion can be either proximal or distal to the aneurysm, and collateral blood flow upstream of the parent vessel occlusion must be available through the circle of Willis, leptomeningeal connections, or surgical augmentation. This giant basilar apex aneurysm was occluded proximally with a single clip across the basilar trunk, as seen postoperatively on a left vertebral artery angiogram (E). F, Postoperative angiography demonstrated sufficient collateral flow through the circle of Willis. Intraoperative photographs demonstrate the dolichoectatic basilar apex (G), the thrombotic fundus (H and I), and the proximal clip (J).

FIGURE 378-15 Bypass and aneurysm occlusion. A, Giant fusiform aneurysms, such as this basilar trunk aneurysm, are seldom amenable to direct surgical clipping or trapping. B, The distal blood supply is augmented surgically with a superficial temporal artery–to–superior cerebellar artery bypass. C, The aneurysm can then be occluded distally to reduce flow through the aneurysm while preserving flow to perforators. High-flow bypass can also be used, as with this petrous carotid artery aneurysm seen on a sagittal computed tomographic angiogram (D) and on anteroposterior (E) and lateral (F) angiograms. G, Hemispheric perfusion was severely diminished during balloon test occlusion, so a bypass from the external carotid artery to the middle cerebral artery was performed (anteroposterior [H] and lateral [I] angiograms). J, Inflow from the internal carotid artery was then occluded with coils. Intraoperative photographs demonstrate the distal anastomosis (K to N), the proximal anastomosis (O), and the course of the radial artery graft (P).

Trapping an aneurysm removes it from the cerebral circulation and, by definition, involves occlusion of the parent vessel proximal and distal to the giant aneurysm. Again, collateral circulation distal to the trapped aneurysm must be assessed. Furthermore, trapping may be impossible if important vascular territories are fed by branches incorporated in the giant aneurysm itself. For example, brainstem perforators intrinsically involved in a giant fusiform midbasilar artery aneurysm will have no blood supply if such a lesion is trapped, and brainstem infarction will ensue. In such cases, proximal or distal parent vessel occlusion may be considered.

Revascularization procedures and bypass techniques must be used when the collateral circulation is insufficient to replace blood flow after permanent vessel occlusion.^13,^95,⁹⁶ Various bypass techniques have been described for both the anterior and posterior circulations. Sometimes, altering hemodynamics with a bypass alone may result in aneurysm thrombosis and resolution.^88,⁹⁰ In many cases, bypass procedures involve supplying the intracranial circulation through a mobilized pedicle of the external carotid circulation. The superficial temporal arteries and occipital arteries have been used in this manner to supply blood to branches of the MCA, posterior cerebral artery, and superior cerebellar artery (Fig. 378-16).^14,¹⁵ With these techniques, the neurosurgeon must be aware of mismatches in blood flow because some bypass techniques are limited by the small caliber of donor vessels. For example, if the ICA is to be sacrificed, superficial temporal artery–to-MCA bypass may not provide enough compensatory blood flow. High-flow bypasses can be created by using interposition saphenous vein grafts,^88,⁹⁷ or “double-barrel” bypasses can connect the anterior and posterior branches of the superficial temporal artery to two recipient vessels.⁴⁰ Likewise, interposition vein grafts can be used to bypass diseased segments of large-caliber vessels (Fig. 378-17), such as petrous-to-supraclinoid bypass procedures of the ICA. The radial artery can be used as an arterial interposition graft.⁹⁸ Finally, certain vascular territories within the brain are amenable to revascularization via in situ anastomoses, particularly the paired anterior cerebral and posterior inferior cerebellar arteries (Fig. 378-18). In this situation, blood flow distal to the trapped aneurysm can be augmented with a side-to-side anastomosis from the contralateral circulation. The anterior temporal branch of the MCA can sometimes be mobilized to bypass diseased segments just distal to it.

FIGURE 378-16 Extracranial-to-intracranial bypass. The mobilized pedicle of one superficial temporal artery is anastomosed to a distal branch of the middle cerebral artery (MCA) distribution. In this manner, blood flow to the ipsilateral MCA distribution and ipsilateral hemisphere is augmented.

(Courtesy of Barrow Neurological Institute, Phoenix, AZ.)

FIGURE 378-17 Vein bypass. When large intracranial vessels such as the internal carotid artery (ICA) must be sacrificed, the caliber of the bypassing vessel must approximate the caliber of the original artery to avoid ischemia from perfusion mismatch. The saphenous vein is harvested and anastomosed proximally to the petrous ICA and distally to the supraclinoid ICA in this short-segment bypass for a giant cavernous carotid aneurysm.

(Courtesy of Barrow Neurological Institute, Phoenix, AZ.)

FIGURE 378-18 In situ bypass. At certain intracranial locations, paired arterial branches coexist and are amenable to side-to-side bypass techniques. Here, the A2 segment of the anterior cerebral artery is anastomosed to the contralateral vessel to provide distal flow once the parent vessel and associated giant aneurysm are sacrificed.

(Courtesy of Barrow Neurological Institute, Phoenix, AZ.)

Aneurysmectomy may be considered for giant intracranial aneurysms involving vessels that can be mobilized and reanastomosed after excision of the aneurysm (Fig. 378-19), such as giant aneurysms involving the MCA bifurcation.^99,¹⁰⁰ Excision is precluded if the aneurysm directly involves significant perforators or outflow vessels.

FIGURE 378-19 Aneurysmectomy. A, Giant aneurysms of the middle cerebral artery can sometimes be excised directly with reanastomosis of the vessel. B, Complete vascular control of all outflow and tributary vessels must be achieved. C, After the aneurysm dome is excised, enough tissue must be preserved to reconstruct the vessel with sutures or clips. D, The resulting anastomosis preserves the blood supply to all the original territory subserved by the aneurysm.

(From Bojanowski WM, Spetzler RF, Carter LP. Reconstruction of the MCA bifurcation after excision of a giant aneurysm. J Neurosurg. 1988;68:974-977.)

When an aneurysm can be clipped directly or trapped, it may be possible to reduce the mass effect of the aneurysm by puncturing it ²¹ or by opening the aneurysm and removing associated thrombus. When an aneurysm cannot be excluded from the circulation, it may not be possible to debulk it. However, some aneurysms that can be controlled completely with temporary clips can be opened, thrombectomized, and closed to reduce any mass effect (Fig. 378-20).^41,¹⁰¹ In other circumstances, clip reconstruction can be combined with thrombectomy of the aneurysm (Fig. 378-21).

FIGURE 378-20 Aneurysmorrhaphy. A, A giant fusiform aneurysm of the vertebral artery is exerting a mass effect and must be decompressed. B, The lesion is incised directly to remove thrombus. C, The thrombus in the aneurysm is removed with an ultrasonic aspirator. D, The aneurysmorrhaphy site is reconstructed with clips, thereby lessening the local mass effect of the aneurysm.

(Courtesy of Barrow Neurological Institute, Phoenix, AZ.)

FIGURE 378-21 Thrombectomy for medullary decompression. Thrombotic aneurysms require special consideration because of mass effect, as seen on axial (A) and sagittal (B) magnetic resonance images of this distal vertebral artery giant aneurysm. C, Intraoperative photographs show the aneurysm’s relationship to the lower cranial nerves. D, Thrombectomy with the Cavitron ultrasonic aspirator decompresses the aneurysm and allows it to be mobilized away from the medulla. E, Stacked clips close the aneurysm. F, Postoperative computed tomography shows the thrombectomy and reduction in mass effect.

Endovascular Treatment

Endovascular techniques, which include the use of detachable balloons, embolic polymers, and detachable thrombogenic coils,^2,¹⁰²^–¹⁰⁴ have been used to occlude giant intracranial aneurysms directly and to exclude them from the cerebral circulation. A combination of stents and detachable coils can also induce thrombosis of the aneurysm.^2,¹⁰⁵ Significant improvements have made it possible to exclude some aneurysms completely from the cerebral circulation with endovascular techniques alone. Frequently, the lesions most amenable to endovascular treatment are also those that are best treated by surgical techniques, namely, those with well-defined, small, narrow necks.^2,¹⁰⁶

Unfortunately, occlusion is incomplete in a considerable percentage of endovascular treatments.^2,^106,¹⁰⁷ Even if a significant portion of an aneurysm’s lumen is embolized or thrombosed, the patient is still subject to risk for SAH from that lesion.^2,¹⁰³ In fact, endothelialization of the aneurysm’s orifice does not always follow coil occlusion.¹⁰⁸ Furthermore, follow-up studies have demonstrated refilling and recurrence of aneurysms thought to have been completely occluded.²

There are a number of hypotheses about how the lumen of an aneurysm becomes recanalized. The water hammer hypothesis proposes that the pulsatile hemodynamic blood flow compacts the thrombus and coil mass within the lumen, impacts them up toward the dome, and allows filling around the neck. Other mechanisms might include recanalization of a blood flow channel around the outer surface of the coil and thrombus mass next to the aneurysmal wall.² However, recurrence rates in large surgical series are low,¹⁰⁹ and endovascular techniques must be measured against this standard when their overall efficacy is assessed. One must weigh the risks associated with the procedure itself against the natural history of partially treated or recurrent aneurysms in determining the morbidity and mortality associated with either surgical or endovascular procedures.

Endovascular techniques have their own complications. Embolization and infarction from cerebral catheterization are risks.^58,⁶⁰ Coil migration into a parent vessel lumen with subsequent thrombosis or embolization is a rare but real risk.² Moreover, a partially treated, coiled aneurysm poses a greater surgical challenge to the neurosurgeon than if no endovascular intervention had been undertaken.

Endovascular techniques are often inadequate to address the mass effect of a giant aneurysm, the probable manifestation in most patients. Sacrifice of the parent vessel with detachable endovascular balloons can promote thrombosis and resolution of the aneurysm. Endovascular studies have demonstrated resolution of mass effect symptoms with aneurysmal coiling. Coil occlusion of the aneurysm may reduce the hemodynamic pulsations of the aneurysm on surrounding neural structures.^102,^103,¹¹⁰ Nonetheless, deterioration after coiling procedures and signs of worsening mass effect have been described.^110,¹¹¹

Endovascular techniques have revolutionized the treatment of giant aneurysms of the cavernous sinus. Direct surgical approaches through the cavernous sinus can be associated with significant morbidity,⁸⁵ and endovascular coiling of the lesion or sacrifice of the parent vessel by balloon occlusion, if tolerated, has reduced the need for such aggressive surgery.^112,¹¹³ Furthermore, the need to completely occlude aneurysms outside the subarachnoid space is less pronounced because the morbidity and mortality associated with their rupture are much lower.

Endovascular techniques are rapidly becoming an indispensable adjunct to microsurgical treatment of giant cerebral aneurysms. Proximal or distal parent vessel occlusion can be performed with detachable endovascular balloons if an amenable vascular territory is involved (Fig. 378-22).^2,^61,¹⁰⁴ Likewise, proximal vascular control is sometimes readily achieved with endovascular balloon occlusion techniques.^56,^58,⁶⁰ The results are impressive when endovascular techniques are combined with direct surgical treatment of giant cerebral aneurysms.^57,^59,¹¹⁴ For example, endovascular balloon occlusion has been combined with suction decompression of the aneurysm to facilitate surgical manipulation and clipping of paraclinoid aneurysms.^56,^58,⁶⁰ Undoubtedly, as endovascular technology advances, less invasive surgical and endovascular options will be applied to the treatment of these problematic lesions.