Introduction

Mid- and lower-basilar artery aneurysms represent challenging surgical entities. Midbasilar aneurysms can be defined as those affecting the basilar trunk below the level of the superior cerebellar arteries (SCAs) and extending to the anterior inferior cerebellar artery (AICA). Lower basilar aneurysms affect the inferior basilar trunk and the vertebrobasilar junction. Anatomic variability of the vertebrobasilar tree and its branches in relationship to the surrounding anatomy and the configuration, size, and orientation of the aneurysms in this region preclude the universal application of a single operative approach. Each case requires consideration of the particular features of the lesion and selection of an approach that provides the necessary surgical corridor. Endovascular treatments have become increasingly important in the management of these difficult aneurysms, but this chapter focuses on surgical management.

Microsurgical anatomy

The basilar artery originates at the junction of the vertebral arteries at the level of the pontomedullary sulcus and ascends anterior to the pons toward its apex in the interpeduncular fossa. The vertebrobasilar junction is typically located near the midline of the clivus at the level of the pontomedullary junction. Numerous perforating branches, in addition to the larger main branches, exit along its course.

The AICA originates near the pontomedullary sulcus and courses around the pons, typically below or between the fascicles of the abducens nerve, to the cerebellopontine angle (CPA). There, it passes between or around cranial nerves (CNs) VII and VIII, which head toward the acoustic meatus. The AICA sends branches to these nerves and to the choroid plexus before it courses posterolaterally to supply the petrosal surface of the cerebellum. The SCA, which originates at the pontomesencephalic sulcus, encircles the midbrain to supply the cerebral peduncles before it courses superiorly and medially to supply the tentorial surface of the cerebellum.

Numerous perforators arise along the entire course of the posterior and lateral surfaces of the basilar artery, but not along its anterior surface. The perforators supply cranial nerve nuclei, reticular centers, and input-output pathways for the cerebrum and cerebellum. Along the midbasilar portion, the long lateral pontine arteries exit and course laterally to supply the paramedian and lateral pons. The medial branches enter the pons near the midline.

Clinical presentation

Mid- and lower-basilar aneurysms are uncommon lesions, accounting for fewer than 1% of all aneurysms in most neurosurgical series.^1,2 Yamaura et al. reported a frequency of 5% (10/202) for posterior circulation aneurysms.³ In contrast, Drake and Peerless and Peerless et al. reported that mid- and lower-basilar aneurysms accounted for 15.2% (193/1266) of all posterior circulation aneurysms.^4,5 Undoubtedly, however, this higher incidence reflected the referral bias of their institution.

The clinical features of ruptured aneurysms in the mid- and lower-basilar artery location tend to be indistinguishable from subarachnoid hemorrhage (SAH) from other cerebral aneurysms. As with any SAH, sudden headache, nuchal pain, altered mental status, nausea, and vomiting are the typical manifestations. Intraparenchymal bleeding, which can be associated with anterior circulation aneurysms, is rarely a feature of these aneurysms. As with other posterior circulation aneurysms, the natural history of these aneurysms appears to be that of high rupture rates.⁵ Unruptured giant aneurysms can produce neurological symptoms specific to their anatomic location, ranging from isolated cranial neuropathies to brainstem compression syndromes.

Fusiform and dissecting aneurysms are more common in the posterior circulation than in the anterior circulation and are associated with a poor prognosis.⁶ Large dolichoectatic aneurysms involving the basilar trunk and vertebral arteries may be the result of dissection with subsequent fusiform degeneration of the artery, progressive enlargement, and luminal thrombosis or may be a consequence of intracranial atherosclerosis. These lesions are particularly challenging and often require indirect approaches for their obliteration compared to the standard direct clipping techniques most often used for saccular aneurysms.

Management principles

Patients presenting with SAH from mid- or lower-basilar artery aneurysms are managed according to the same general principles as patients with aneurysmal SAH in general: cardiorespiratory and basic neurological supportive care, early ventriculostomy in patients with a poor Hunt and Hess grade, early surgery in suitable patients, and aggressive management of increased intracranial pressure and vasospasm. Patients with neurological symptoms related to the mass effect of an aneurysm are managed semi-urgently. These patients typically exhibit signs of brainstem compression or cranial nerve deficits, which must be considered when planning both the surgical approach and timing of surgery.

Standard intraoperative management is utilized. Intraoperative hypotension is avoided. In fact, intraoperative blood pressure is allowed to run mildly hypertensive, especially during temporary vessel clipping. For brain protection, all patients receive intravenous doses of barbiturates (pentobarbital) to achieve electroencephalographic burst suppression. As required, brain relaxation is achieved with hyperventilation, mannitol, barbiturates, or spinal drainage of cerebrospinal fluid (CSF). Comprehensive electrophysiological monitoring is used routinely:⁷ somatosensory-evoked potentials, brainstem auditory responses (in both ears or the contralateral ear alone if ipsilateral hearing is to be sacrificed), and monitoring of the facial nerve and other appropriate CNs.

Surgical overview

The general principles for surgical treatment of aneurysms located in this part of the basilar artery are the same as those for any aneurysm.^6,8–17 The main surgical difference relates to the anatomical considerations specific to the constrained aneurysm locations and the surrounding vital neuroanatomy. Regardless, the primary endpoint for the surgeon should be complete obliteration of the aneurysm from the circulation, with preservation of the parent vessels, particularly the perforators supplying the brainstem.

To achieve optimal aneurysm obliteration, maximal exposure of the basilar artery and the aneurysm itself is required. This exposure must be achieved without exposing the brain or critical structures to undue retraction while simultaneously obtaining adequate control of the parent vessel and aneurysm. Given the challenges of exposure in this region, specialized operative approaches are often necessary. In some cases, such approaches must be combined with the technique of hypothermic cardiac standstill.^18–22 Alternatively, additional methods may be used for revascularization to allow proximal artery occlusion or trapping of the aneurysm.

Surgical decision making

Depending on the specific anatomy and configuration of the aneurysm, the following approaches can be appropriate. Moving from superior to inferior, the following approaches provide overlapping access to the mid- and lower-basilar artery: subtemporal, extended orbitozygomatic approach, transpetrosal approach, lateral suboccipital-retrosigmoid, and far-lateral. Combinations of these approaches (combined supra- and infratentorial approach and combined-combined) can extend exposure as sometimes needed for large aneurysms.

The various approaches that provide overlapping amounts of exposure can be seen in Figure 26–1. Typically, the subtemporal and orbitozygomatic approaches are used for basilar tip lesions, but the extended orbitozygomatic approach can provide access to the upper two-fifths of the basilar artery. Some aneurysms of the basilar trunk below the SCAs can be approached in this manner. The transpetrosal provides exposure further inferiorly, allowing the middle three-fifths of the basilar trunk to be accessed. With the combined supra- and infratentorial approach, access can be extended down to the vertebrobasilar junction. The far-lateral approach gives access primarily to the lower two-fifths of the basilar artery and is suitable for vertebrobasilar junction aneurysms. Both the far-lateral and extended orbitozygomatic approaches provide a view along the aneurysm neck, allowing a clip to be applied along this line of sight. In contrast, however, the transpetrosal approach exposes the aneurysm sac between the surgeon and the neck. A right-angled clip is required, increasing the technical difficulty of clip application (Figure 26–2).

Figure 26–1 Approaches to the basilar artery, divided in thirds. The upper third can be accessed through an orbitozygomatic approach. The middle third can be approached via the transpetrosal route. The lower third is accessible through a far-lateral craniotomy.

(With permission from the Barrow Neurological Institute.)

Figure 26–2 Variations in the operative approach to the basilar artery translate to variations in surgical clip strategy. Far-lateral and extended orbitozygomatic approaches provide a view along the neck of the aneurysm, allowing a clip to be applied along this line of sight. The transpetrosal approach exposes the aneurysmal sac between the surgeon and the neck, necessitating a right-angle clip.

(With permission from the Barrow Neurological Institute.)

There is still a role for the standard subtemporal craniotomy and lateral suboccipital/retrosigmoid craniotomy in approaching the upper-mid and mid-lower basilar artery, respectively, especially given that the more extensive approaches such as radical petrosectomy are associated with inherent morbidity. These standard craniotomies can be used for small, favorably located, unruptured aneurysms. Adequate brain relaxation is required. A combination of diuretics and CSF drainage can be used to allow clipping without the application of undue retraction.

At our institution, a standard retrosigmoid craniotomy has been found to be adequate for small-to-medium aneurysms associated with the AICA along the lower two-thirds of the clivus. For larger aneurysms, however, the routine lateral suboccipital or retrosigmoid approach may not adequately expose the mid- or lower-basilar trunk without an unacceptable amount of cerebellar or brainstem retraction. For aneurysms involving the basilar trunk above the vertebral bifurcation, the transpetrosal approaches increase rostral exposure for distal control and decrease the operative distance to the lesion. Even the extent of this exposure, especially for larger aneurysms, can be limited, leading to the utility of the combined approaches. The combined supra- and infratentorial approach with its appropriate variations permits exquisite surgical exposure for dealing with most aneurysms involving the mid- and lower-basilar artery. The far-lateral approach, alone or in combination with other approaches, provides excellent access to the lower basilar and vertebrobasilar junction.

Operative approaches

Aneurysms of the mid- and lower-basilar artery pose a difficult challenge because they are located in a small, restricted area encased with bone and situated within a limited subarachnoid space filled with a dense collection of vital neurovascular structures. All these elements must be considered when an approach is planned. The basic principles of aneurysm surgery still apply: (1) proximal and distal vascular control, (2) preservation of parent vessels and all perforators, and (3) complete obliteration of the aneurysm.

Next, the anatomical concerns specific to this region must be accommodated. In the past, these aneurysms were treated through a subtemporal-transtentorial approach or through the suboccipital approach. A transoral or transmaxillary-transclival approach has also been used,²³ but exposure is limited and the risks of postoperative CSF leakage and meningitis are significant. Although these approaches can provide access to aneurysms of the mid- and lower-basilar artery, they seldom provide maximal exposure with minimal retraction. Consequently, the subtemporal and lateral suboccipital/retrosigmoid approaches are often superseded by more extensive skull base approaches that maximize lateral bone removal to provide a shorter and flatter route of access to the anterior brainstem and basilar artery (Figure 26–3). The basic tenets of these approaches and their application to mid- and lower-basilar artery aneurysms are reviewed.

Figure 26–3 Microsurgical corridors to the skull base and basilar artery.

(With permission from the Barrow Neurological Institute.)

Transpetrosal

The transpetrosal approach encompasses a range of approaches with various extents of bony dissection that provide differential degrees of exposure to the anterior brainstem and clival region (Figure 26–4).^28–30 Overall, this approach provides a lateral trajectory to the basilar trunk through a presigmoid corridor by drilling through the petrous aspect of the temporal bone. This approach can afford good proximal and distal control and is usually undertaken ipsilateral to the side of the aneurysm. The lateral angle of approach can place the aneurysm dome between the surgeon and the site of clip application at the neck of the aneurysm. However, a contralateral approach often mandates more complex clip strategies, including the use of fenestrated clips and a subsequent increase in the potential for perforator injury. The trajectory places the CNs in the foreground of the operative field, although this is true of all exposures to this region other than the direct anterior transoral or transclival.

Figure 26–4 Transpetrosal approaches to the basilar trunk.

(With permission from the Barrow Neurological Institute.)

The transpetrosal approach can be divided into three variations (see Figure 26–4): the retrolabyrinthine approach (which involves petrous bone resection between the semicircular canals anteriorly and the posterior fossa dura on the posterior aspect of the temporal bone). Hearing is preserved. The translabyrinthine approach removes more of the petrous bone (including the semicircular canals, thereby increasing exposure anteriorly to the internal auditory canal), and sacrifices hearing. The transcochlear approach involves maximal petrous bony resection by transposing the facial nerve posteriorly and allowing access for removal of the cochlea. Hearing is sacrificed and injury to the facial nerve is a risk. In each variation the amount of petrous bone resection increases with a resultant increase in exposure of the anterior brainstem and basilar trunk.

Revascularization

Although the primary surgical goal in the management of aneurysms is direct clip obliteration with preservation of the parent vessel, not all aneurysms are amenable to this approach.^31–33 Large aneurysms especially can incorporate the parent vessel to such an extent that clip reconstruction is infeasible. The extreme example of this situation is fusiform vertebrobasilar dolichoectatic aneurysms or dissecting aneurysms, but even large saccular aneurysms can behave in this fashion. Under these circumstances, the remaining options are trapping of the aneurysmal segment or proximal occlusion to alter the flow dynamics in the region of the aneurysm to induce regression and spontaneous thrombosis.^34,35

For mid- and lower-basilar lesions, direct trapping may be less desirable because of the potential risk to adjacent perforators.³⁶ However, proximal or even distal basilar occlusion can serve to abolish the aneurysm effectively.³⁷ For large fusiform dolichoectatic aneurysms, proximal occlusion of one or both vertebral arteries can reverse blood flow in a similar fashion by removing the hemodynamic wall stress thought to play a role in the growth of these degenerative lesions. If the caliber of the posterior communicating arteries is insufficient to provide adequate blood flow, this strategy must be combined with a bypass to the upper basilar territory. In such cases, the donor and recipient vessels depend on blood flow requirements and the availability of suitable vessels. For basilar trunk lesions, the distal basilar territory typically needs to be revascularized.^38,39 Doing so can be accomplished with a STA donor or interposition graft from the external carotid artery anastomosed to the posterior cerebral artery or SCA. Posterior circulation bypasses tend to be technically demanding due to the depth at which the anastomosis must be performed. These procedures are associated with the general risks of any vascular surgery within the posterior circulation in addition to the more specific risks of graft failure and ischemia. In certain cases, however, revascularization and proximal occlusion strategy are the best feasible options.

Surgical approaches to the revascularization of the posterior circulation include revascularization of mid-basilar artery lesions with an STA-PCA or STA-SCA bypass. A vertebral artery aneurysm can be trapped and revascularized with a PICA-PICA in situ bypass or with an OA-PICA bypass. In either scenario, endovascular coils placed within the vertebral artery can be used to occlude the vertebral artery once the bypass has been achieved. Using the vast armamentarium of available surgical and endovascular techniques, most giant aneurysms can be treated successfully.

Flow redirection

Flow-redirection techniques can be used for aneurysms involving the basilar artery.⁴⁰ When an aneurysm cannot be removed from the circulation, the goal is to alter the hemodynamics at the lesion by changing the direction of blood flow. One approach involves a two-stage treatment paradigm. The first procedure consists of revascularization through an STA-SCA bypass, followed by a two-stage endovascular treatment. Another approach is unilateral occlusion of the vertebral artery proximal to the PICA, followed by contralateral vertebral artery occlusion distal to the PICA. The goal in delaying occlusion of the second vertebral artery is to provide the graft with time to mature. Ultimately, one PICA depends on back-flow from the basilar artery, and this demand is thought to help prevent basilar thrombosis, thus maintaining flow from the bypass to the basilar artery and the brainstem-perforating vessels.

Conclusion

Recent developments in endovascular techniques now provide additional options for the management of these difficult lesions. Nonetheless, for many lesions, surgical management remains an important and necessary aspect of treatment. An understanding of the unique anatomical considerations and choices regarding surgical access is therefore still paramount.