Anatomy

The basilar bifurcation is in the interpeduncular cistern. This location is bounded by the clivus and posterior clinoid processes anteriorly, the cerebral peduncles posteriorly, the mammillary bodies and posterior perforated substance superiorly, and the medial temporal lobes laterally. On average, the bifurcation is 15 mm posterior to the carotid arteries. The precommunicating (P1) segments of the posterior cerebral artery (PCA) are the branches of the bifurcation that transition to the P2 segments beyond the junction with the posterior communicating arteries. A large posterior communicating artery when associated with a small, or absent, P1 segment is referred to as fetal configuration. Anterior thalamoperforating arteries can arise in abundance from the posterior communicating artery and may factor in to the side of the surgical approach. The oculomotor nerve will religiously follow a course between the PCA and superior cerebellar arteries (SCAs); thus, it is an indispensable landmark when surgical orientation is in question. The SCAs are single or duplicated structures that exit below the oculomotor nerve and will continue below the tentorium; the trochlear nerve will typically enter the tentorial edge anterior to the SCA descent into the posterior fossa. Pontine perforating vessels will become more abundant about 5 mm proximal to the SCA origins; this creates the frequently quoted “perforator-free zone” below the SCAs where proximal basilar clipping can occur for control or sacrifice.

11.2 Preoperative Considerations

In 1989, Batjer and Samson discussed the causes of morbidity and mortality from surgery of aneurysms of the distal basilar artery and appropriately noted that “errors in surgical timing and technical or conceptual errors [can] prove[d] to be of great consequence in patient outcomes.” ¹ Since that era, significant enhancements in noninvasive imaging have allowed for an enhanced understanding of the anatomy of the basilar apex that contributes to the creation of the surgical concept. When evaluating a surgical approach to the upper basilar, the surgeon should fundamentally understand the following:

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  The relationship of the upper basilar to the posterior clinoid process.

  
  
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  The projection of the aneurysm dome.

  
  
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  The PCA vessel that is most involved with the aneurysm neck.

  
  
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  The relationship of the aneurysm dome to the vessels of the basilar quadrification,

  
  
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  The presence, size, and relationship of the posterior communicating artery to the P1 segments.

These factors, in addition to the hand dominance of the operating surgeon, will frequently determine the side of surgical approach and dictate clip placement. A schematic representation of the basilar apex in a surgical orientation should be clearly understood ( ▶ Fig. 11.1).

Preoperative investigations for these cases should include computed tomography angiography. This will allow the surgeon to evaluate many of the points mentioned above noninvasively while also revealing the presence of calcification in wall of the aneurysm or related arteries. The relationship of the posterior clinoid process is best evaluated using sagittal reconstructions ( ▶ Fig. 11.2). In general, the lower the basilar artery bifurcation is in relation to the posterior clinoid processes, the more difficulty the surgeon will have in obtaining proximal control and the more likely that endovascular adjuncts or primary endovascular therapy may be employed. It is also for this reason that extended approaches to the upper basilar aneurysms were initially described.

While the apex of the basilar artery is commonly referred to as a quadrification, most surgeons will observe many variants to this name. Lasjaunias and colleagues provide an excellent evaluation of basilar artery anatomy and its variants. ² This provides an excellent anatomical foundation for the preoperative considerations that are essential to planning a successful transsylvian approach to the aneurysms of the upper basilar artery.

General anesthesia is needed to ensure a brain relatively protected from ischemia by neuroprotective anesthetic drugs, to ensure hemodynamic stability, and to create brain relaxation for the deeper posterior circulation approach. Routine monitoring should include an arterial line, noninvasive blood pressure cuff, five-lead electrocardiogram, pulse oximetry, esophageal stethoscope, temperature probe, Foley catheter, capnograph, peripheral nerve stimulator, and central venous catheter. Pulmonary artery catheters may be used in patients with congestive heart failure or impaired heart function. Where larger lesions may require cardiac pause or rapid ventricular pacing, it may be wise to place defibrillator pads in case of dysrhythmia during cooling. ^{3,​ 4} Electroencephalography electrodes should be placed for monitoring in case burst suppression is used. Anticonvulsants are not usually administered, perioperative antibiotics are routine, and brain relaxation is facilitated by transcranial placement of a ventricular drain rather than by spinal drainage except in the case of subtemporal approaches. ⁵ Anesthetic adjuncts for brain relaxation should include mannitol (0.5–1 g/kg) 30 minutes prior to dural opening and hyperventilation to a PaCO₂ of 25 to 30 mm Hg. Intraoperative hypothermia to 33°C may be employed at the discretion of the surgeon.

11.3 The Transsylvian Approach

Wide splitting of the sylvian fissure is a key step during the approach to the upper basilar. The success of this approach in cerebrovascular neurosurgery is dependent on careful tissue mobilization with the goal of circumferential visualization of the aneurysm and optimal illumination of what is ultimately an area that may be up to 12 to 15 mm deep to the carotid cistern. During dissection, the surgeon must exercise extreme care to minimize or create only controlled injury of the arterial, parenchymal, and venous anatomy that is encountered as the surgical corridor narrows. While a thorough understanding of the anatomy of the sylvian fissure makes this possible, we recommend the following goal-oriented approach to understanding sylvian fissure dissection for visualization of the interpeduncular cistern. The surgical goals to achieve in order, where possible, are discussed in the following.

11.3.1 Positioning for Pterional Craniotomy: Stay Away from the Shoulder, so Tilt the Head

Following fixation of the cranial clamp, the head should be rotated to the contralateral side, approximately 45 degrees. This should be followed by neck extension to place the malar eminence at the highest point. Without further modification, this position may cause the temporal lobe to obscure the surgeon’s vision and drive him/her toward the ipsilateral shoulder. By slightly tilting the nonsurgical ear toward the ipsilateral shoulder, the surgeon can continue to operate in the sagittal plane and allow a more direct visualization of the upper basilar region.

11.3.2 Craniotomy and Brain Relaxation: Muscle Mobilization and Intraoperative Ventriculostomy

In many cases, the “standard” pterional craniotomy is modified to address the preoperatively evaluated basilar anatomy. For a basilar lesion, we will typically extend the inferior aspect of the incision below the zygoma and perform a subfascial dissection to allow for dissection of the temporalis muscle from the lateral orbit; this will permit posteroinferior retraction of the muscle ( ▶ Fig. 11.3). In our experience, such muscle mobilization, along with aggressive subtemporal bony removal, obviates the need to routinely consider an orbitozygomatic osteotomy for all but very high and giant lesions. To optimize illumination and minimize brain retraction, emphasis should then be made on generous drilling of the sphenoid wing and placement of an intraoperative ventriculostomy. This technique involves inserting the ventricular catheter perpendicular to the surface of the brain at a point 2.5 cm superior to the lateral orbital roof at the sphenoid ridge and 2.5 cm above the sylvian fissure. ⁵ This technique should not supplant the anesthetic adjuncts that also maximize brain relaxation.

Related posts:

How to Repair the Intracranial Aneurysm: Clipping or Coiling Decision Making Distal Anterior Cerebral Artery Aneurysms: Anterior Interhemispheric Approach Middle Cerebral Artery Aneurysms Tentorial and Posterior Fossa Dural Arteriovenous Fistulas Superficial Temporal Artery to Middle Cerebral Artery Bypass Transsylvian Transclinoidal and Transcavernous Approach for Basilar Bifurcation Aneurysms

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