Vertebral Confluence and Midbasilar Aneurysms Including Transpetrosal Approach

Vertebral confluence and midbasilar aneurysms are uncommon but surgically challenging lesions due in large part to the ventral location of pathology in the posterior fossa and the important downstream territory this vascular tree supplies. Four general open surgical approaches are used to treat these lesions: the orbitozygomatic/subtemporal approach, the extended middle fossa (Kawase)/transpetrosal approach, the retrosigmoid approach, and the far-lateral approach. Each provides access to different segments of the posterior circulation. In this chapter, we review important surgical and clinical pearls in the treatment of these lesions.

Keywords: aneurysms, basilar artery, clipping, transpetrosal, vertebral artery

17.1 Introduction

Vertebral confluence and midbasilar aneurysms are uncommon but surgically challenging lesions due in large part to the ventral location of pathology in the posterior fossa and the important territory this vascular tree supplies. Surgical principles paramount to successful surgical obliteration of these lesions include careful analysis of preoperative vascular imaging to delineate location of critical brainstem perforating vessels, maximizing surgical visualization of the lesion with extensive bony resection and dural opening, careful and early dissection of proximal and distal vessels, preservation of all vascular perforators, and development of contingency plans for use when primary clip occlusion is not feasible.

17.2 Patient Selection

17.2.1 Diagnosis

Vertebral confluence and midbasilar aneurysms are defined by their location anywhere along the lower three-fifths of the basilar artery. This includes lesions from the vertebrobasilar junction into the origin of the anterior inferior cerebellar artery (AICA). The gold standard in diagnosis remains formal, four-vessel angiography in order to define the morphology of the aneurysm as well as to delineate the relationship of important perforating vessels to the aneurysm dome. However, perforators may not always be immediately apparent and are often obscured when treating giant aneurysms. Furthermore, injections of the internal and external carotid and vertebral arteries and three-dimensional rotational views can help determine the caliber of vessels and visualize the posterior communicating arteries in order to develop contingency plans for use when primary clipping may not be feasible and bypass may be necessary. Computed tomography (CT), CT angiography, and magnetic resonance imaging including magnetic resonance angiography are important supplementary studies that can be used during intraoperative neuronavigation, to determine the degree of bony opening and surgical approach during presurgical planning, as well as to detect the presence of calcifications and thrombus in the aneurysm. However, these studies should not obviate or replace the formal angiogram that can provide much greater anatomical detail.

17.2.2 Indications/Contraindications

Surgical indications include lesions not amenable to neuroendovascular treatment, including wide-necked aneurysms, and aneurysms with important brainstem perforators within the aneurysm dome among patients with the potential for excellent functional recovery. The risk of surgical morbidity from treating unruptured lesions in functionally asymptomatic patients needs to be weighed against the risk of the natural history of these lesions. The risk of rupture causing irreversible morbidity is directly related to the size of the aneurysm dome, irregular appearance of the aneurysm and parent vessel, and the patient’s personal or family history of aneurysmal subarachnoid hemorrhage. Contraindications for open surgical treatment include lesions in patients with extensive comorbid medical conditions that would place them at higher risk of medical complications from prolonged anesthesia or blood loss. Patients with extensive cardiopulmonary or airway issues as well as advanced age should be counseled on the extensive risk of surgery, and should proceed only if the risk of the natural history outweighs the risk of surgical treatment.

17.2.3 Timing, Alternatives, and Risks

Like many ruptured aneurysms, the risk of rerupture is greatest in the first 24 to 48 hours after the initial event, and surgical obliteration procedures should be performed early in these patients. Patients with unruptured aneurysms should be treated after a comprehensive medical and radiological workup is completed, including an assessment of the risk of medical complications with cardiac standstill procedures. The main alternatives are close radiological observation or endovascular treatment, which requires input from an endovascular neurosurgeon or neurointerventional radiologist.

17.2.4 Surgical Approaches and Anatomical Considerations for Each Approach

There are four general approaches to the treatment of vertebral confluence and midbasilar aneurysms: the orbitozygomatic/subtemporal approach, the extended middle fossa (Kawase)/transpetrosal approach, the retrosigmoid approach, and the far-lateral approach. The orbitozygomatic and subtemporal approaches allow access to the upper two-fifths of the basilar artery and are generally appropriate for treating aneurysms located at the basilar bifurcation, precommunicating segment of the posterior cerebral artery, or superior cerebellar artery. The extended middle fossa (Kawase) and transpetrosal approaches are appropriate for lesions located on the middle fifth of the basilar artery, typically AICA lesions. The retrosigmoid approach is used for aneurysms of the vertebrobasilar junction up to the AICA (lower three-fifths of the basilar artery), and the far-lateral approach is used for lesions of the vertebral artery from its intradural origin to the vertebrobasilar junction. In our experience, we have found that the orbitozygomatic, retrosigmoid, and far-lateral approaches generally provide plentiful visualization of the majority of lesions.

17.3 Preoperative Preparation

Due to the potential for high-volume blood loss, we routinely place central venous lines and use continuous blood pressure monitoring with arterial lines during surgery for aneurysms. The patient is typically typed for blood group and screened prior to surgery. If cardiac standstill is planned, a Swan-Ganz catheter is placed. Perioperative antibiotics are administered. Electroencephalography and somatosensory evoked potential (SSEP) monitoring are routinely used for both surgical positioning with neck rotation and intraoperative monitoring for adequate blood flow after clip reconstruction. Cranial nerve monitoring is not routinely used, but may include seventh and/or eighth nerve monitoring of brainstem auditory evoked potentials in order to monitor brainstem function. Motor evoked potentials are used for patients with aneurysms involving the basilar apex and for cardiac standstill cases.

Prior to performing the craniotomy, mannitol (0.5 g/kg) is given in most cases. The goals for blood pressure and PaCO₂ are normotension and normocapnia during the majority of the procedure. After temporary occlusion, the blood pressure may be elevated pharmacologically to promote brain perfusion. If preoperative imaging or intraoperative inspection demonstrates a friable aneurysm neck or dome, the blood pressure can be lowered during dissection to reduce the chances of an intraoperative rupture. Anticonvulsants are seldom used. Common to all the approaches described here are the use of lumbar drainage for up to 72 hours postoperatively and preparation and draping of the abdomen and groin in case fat grafts and intraoperative angiography are used. Collaboration with a neuro-otologist is generally recommended, not just for transpetrosal approaches, but also for postoperative management of ear, nose, and throat issues.

17.4 Operative Procedures

17.4.1 Extended Middle Fossa (Kawase) Approach

The patient is positioned supine with the head turned 90 degrees contralateral to the surgical approach. Elderly patients with a risk of cervical stenosis should have pre- and postpositioning SSEP monitoring for any change in latency or amplitude. Large shoulder and hip rolls are used as needed to facilitate the head turn and to reduce the degree of neck rotation to promote venous return. The neck is laterally extended and the vertex is tilted 10 to 15 degrees toward the floor to allow the temporal lobe to fall with gravity away from the surgical corridor. To prevent the shoulder from obstructing the surgeon’s arms, the shoulder can be taped caudally, but careful attention is paid to avoid excessive traction as this increases the risk of cervical and brachial plexopathy ( ▶ Fig. 17.1). The head is placed in a three-pin radiolucent head holder in the event an intraoperative angiogram is required.

Fig. 17.1 The patient is placed in the supine position for the extended middle fossa approach, with the appropriate shoulder elevated and the head in the horizontal position, slightly extended and tilted 10 to 15 degrees toward the floor.

(Reproduced with permission from Barrow Neurological Institute, Phoenix, AZ.)

Either an anteriorly concave question-mark incision or an inferiorly concave horseshoe-shaped incision can be used ( ▶ Fig. 17.2). The inferior end of the incision immediately anterior to the tragus should reach the inferior margin of the origin of the zygomatic process of the temporal bone. The incision should remain less than 1 cm ahead of the tragus to avoid severing the frontalis branch of the facial nerve and ascending trunk of the superficial temporal artery. When a question-mark incision is used, the scalp flap is reflected anteriorly in two layers, in a standard fascia-splitting technique. A zygomatic osteotomy is optional. If one is performed, the authors detach the zygomatic process with two reciprocating saw cuts, keeping it attached to the inferiorly reflected temporalis muscle. For horseshoe-shaped incisions, the scalp flap is reflected inferiorly in one layer.

Fig. 17.2 The scalp incision options and craniotomy for the middle fossa approach. It is essential to extend the craniotomy inferiorly until it is flush with the floor of the middle fossa.

(Reproduced with permission from Spetzler RF, Koos WT, Richling B, Lang J. Approaches. In: Spetzler RF, Koos WT, eds. Color Atlas of Microneurosurgery: Microanatomy, Approaches, and Techniques. 2nd ed. Vol 2: Cerebrovascular Lesions. New York, NY: Thieme; 1997:93.)

The temporal bone exposure should be two-thirds anterior and one-third posterior to the external auditory meatus ( ▶ Fig. 17.3). A temporal craniotomy is performed and the bony opening should be drilled flush with the floor of the middle cranial fossa. An extradural dissection is then carried out from the floor of the middle cranial fossa, starting posteriorly and moving anteriorly. The following structures should be exposed: arcuate eminence, greater superficial petrosal nerve (GSPN) at the facial–geniculate hiatus, middle meningeal artery at the foramen spinosum, and posterolateral margin of the mandibular nerve at the foramen ovale ( ▶ Fig. 17.4). Continued elevation of the dura medially along the petrous ridge exposes the structures that form the triangle of Glasscock laterally and the triangle of Kawase medially ( ▶ Fig. 17.5). The bone of the Kawase triangle can then be drilled away carefully using a diamond drill. The petrous segment of the internal carotid artery, which is usually near the inferior aspect of the GSPN, is identified. This drilling exposes the posterior fossa dura. The temporal dura is opened linearly, anteriorly to posteriorly, and careful attention is given to avoid injury to the vein of Labbé. This portion of the dura is then reflected superiorly with tack-up sutures. A second dural incision is extended perpendicularly from the first incision toward the exposed dura of the posterior fossa. A third, posteriorly concave incision is made in the dura of the posterior fossa. The final incision is made through the tentorial incisura, with sacrifice of the superior petrosal sinus as needed. The trochlear nerve is identified and preserved before making the tentorial incision ( ▶ Fig. 17.6). After the arachnoid has been dissected, the superior cerebellar, posterior cerebral, and midbasilar arteries and cranial nerves III through VIII can be seen.

Fig. 17.3 The middle fossa bone exposure is shown. The posterolateral to anteromedial areas of the foramen spinosum, foramen ovale, and foramen rotundum are seen. The foramen lacerum is medial and just lateral to the posterior clinoid processes.

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