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.