Background

In the early 1940s, following surgery for a posterior fossa aneurysm that ultimately took the life of the patient, Walter Dandy stated, “I know of no successful outcome from operative attack upon an aneurysm of the posterior cranial fossa, but for those upon the vertebral and posterior inferior cerebellar arteries, which afford good exposure, cures will certainly come in time.”¹

As time passed, Dandy’s prediction came to fruition. Neurosurgeons overcame the obstacles that once challenged great surgeons who operated on posterior fossa aneurysms. Over the past several decades, outcomes in the surgical treatment of posterior circulation aneurysms have steadily improved. This success has been largely due to the development of the field of microsurgery with the advent of the surgical microscope and microinstruments. Success is also attributed to the numerous adjuncts to microsurgery, such as advances in neuroanesthesia and brain protection, modern aneurysm clips, intraoperative imaging, bypass techniques, skull base surgery, and specialty training of individuals allowing them to concentrate on cerebrovascular disease. Posterior circulation aneurysms, including those of the basilar apex (BA), have been among the most challenging to treat. Surgery in this area is especially treacherous due to the deep location, as well as the close proximity of perforators to the brain stem. Basilar tip aneurysms are also less frequently encountered than aneurysms of the anterior circulation, making it more difficult to master the nuances of their surgical treatment. The reduced exposure to basilar aneurysms has been exacerbated by the increasing utilization of endovascular therapy over the past decade.

The first description of a basilar aneurysm dates back to 1779, when Morgagni presented a case of an aneurysm involving the basilar artery and both posterior cerebral arteries (PCAs).² Little progress was made concerning the identification and treatment of posterior circulation aneurysms until the mid-20th century. In 1954, Herbert Olivecrona was the first surgeon to describe clipping of a basilar artery aneurysm through a subtemporal approach.³ It was during this time that Charles Drake emerged as a pioneer in the surgical management of basilar tip aneurysms. In 1959, he presented his experience with four patients presenting with a subarachnoid hemorrhage (SAH) from basilar bifurcation aneurysms at the American Academy of Neurological Surgeons in Pebble Beach, CA.⁴ Although two of his patients succumbed to complications due to their initial hemorrhage, his case description laid the groundwork for standards of BA aneurysm treatment. Review of the literature over 2 decades prior to the publication of Drake’s paper had identified 38 cases of aneurysms involving the vertebrobasilar (VB) circulation. Following Drake’s publication, a plethora of literature were published concerning the treatment and outcome of basilar aneurysms. Kenneth Jamieson reviewed his operative experience of 19 patients with VB aneurysms.⁵ Only four of those patients made sufficient recovery to return to work. As Drake’s experience with basilar aneurysms grew, his results also improved in parallel.

Part of his success was also attributable to advances in microinstruments and microsurgical technique, including the use of the surgical microscope. Ultimately, these factors culminated in improved surgical outcome for Drake’s patients harboring BA aneurysms. In a 1968 report, Drake described his experience after treating 17 basilar artery aneurysms with only one fatality.⁶ One year later, Drake presented a review of 43 operative cases involving the VB circulation with 70% of patients attaining “satisfactory outcome.”⁷ As part of his legacy, Drake published his 25 years of experience treating 1767 aneurysms of the VB system 2 years prior to his death in 1998.⁸ The vast majority of these patients (1286) harbored aneurysms at the basilar bifurcation or PCAs. Drake reported excellent outcomes in 70% to 80% of his patients who initially presented with a good-grade SAH and those who harbored nongiant BA aneurysms. Drake’s 1996 chapter continues to be the standard that other series involving basilar artery aneurysms are judged against.

The approach to posterior circulation aneurysms has been significantly affected since the introduction of Guglielmi detachable coils (GDCs) in 1991. In the current era of minimally invasive procedures, the treatment of VB circulation aneurysms is different from what was encountered by Drake and his colleagues.

Neuroanesthesia

Improvements in the field of neuroanesthesia have allowed neurosurgeons to more safely attack aneurysm in the BA region. Our patients undergo a thorough preoperative evaluation by the anesthesiology team that includes review of the patients’ relevant medical history, as well as their general surgical risk factors. Most patients receive anxiolytic medications (e.g., benzodiazepines) prior to transport into the operating room. The patient may also receive light opioid sedation before insertion of a radial arterial line (many patients also receive a central venous catheter). After placement of standard monitors (pulse oximeters, electrocardiogram leads, noninvasive blood pressure cuff, and temperature probe), induction of anesthesia is performed. An intravenous anesthetic agent such as propofol or thiopental is typically administered, along with opioids and intravenous lidocaine, to blunt the hemodynamic response to intubation.

Neuromuscular blocking agents are also used to facilitate placement of the endotracheal tube. A deep plane of anesthesia is typically maintained with a volatile anesthetic such as isoflurane or sevoflurane in an O₂–air mixture. Brain relaxation is crucial for maximum exposure of the surgical site and minimization of retraction during the operation. In addition to CSF drainage following dural opening, intracranial relaxation is achieved by administration of 50 to 100 g of mannitol (0.5-2 g per kilogram of body weight) prior to entering the cranium. If further relaxation is necessary, furosemide (0.5-1 mg/kg) is given intravenously. If necessary, sympathetic agonists are used to maintain mean arterial pressure (MAP) within 20% of the patient’s baseline pressure. Respiratory parameters are adjusted to keep the PaCO₂ between 32 and 44 mm Hg, with a PaO₂ target greater than 100 mm Hg. Continuous assessment of fluid status is facilitated by closely monitoring hourly urine output and central venous pressure. When the use of temporary clip occlusion of a parent vessel is necessary, a set of standard steps is initiated prior to clip placement. The MAP is kept above 90 mm Hg to augment collateral blood flow. Burst suppression for cerebral protection is achieved by supplementing the anesthetic with intravenous pentobarbital or propofol. Permissive hypothermia is closely monitored, and the patient is warmed to maintain a body temperature of at least 32°C. Temporary vessel occlusion is limited to 10-minute intervals to prevent ischemic injury. This technique softens the aneurysm sac, facilitating ease of clip placement at the neck. As the aneurysm sack softens, identification of perforator vessels is also achieved more easily. To lower the risk of recurrent hemorrhage prior to surgery, we routinely place our SAH patients on the lysine analogue, ε-aminocaproic acid (Amicar, Xanodyne Pharmaceuticals) with a 5-g intravenous bolus followed by 1 g/hr maintenance. We stop this infusion following clip placement.

We do not routinely perform electroencephalography or brain stem auditory evoked potentials (BAEPs) when treating BA aneurysms. An exception is made when we have treated “giants” or complex BA aneurysms using complete circulatory arrest. This technique has been shown to be a useful adjunct in the treatment of complex intracranial aneurysms, particularly when prolonged cerebral hypoperfusion is needed.^9,10 In these situations, we use electroencephalography, spontaneous somatosensory evoked potentials (SSEPs), and BAEPs, in conjunction with deep hypothermic (18°C) circulatory arrest. The suppression of electroencephalogram activity by barbiturates is used to titrate an effective dose for cerebral protection. The SSEPs are an indicator of intact sensory pathway conduction and persist despite burst suppression. A BAEP is a useful tool, especially if manipulation of brain stem structures is anticipated during the procedure.

Surgical Strategies for BA Aneurysms

Surgical treatment of BA aneurysms is extremely challenging due to the complex anatomy in and around the interpeduncular cistern. Surgeons are forced to navigate through deep and narrow channels that make visualization of the anatomy in this region particularly difficult. The basilar tip and PCAs are located in the confined spaces of the interpeduncular cistern. They are enclosed by the posterior clinoids and clivus anteriorly, mesiotemporal lobes laterally, cerebral peduncles posteriorly, and mammillary bodies superiorly. The BA is approximately 15 mm posterior to the internal carotid artery (ICA).¹¹ The termination of the basilar artery gives rise to bilateral PCAs. The superior cerebellar arteries (SCAs) arise immediately proximal to the basilar bifurcation. The oculomotor nerve exits between the PCAs and the SCAs and is therefore vulnerable to injury during surgery. The segment of the PCA from its origin from the BA to the ostium of the posterior communicating artery (Pcomm) is referred to as the P1 segment of the PCA. The PCA distal to the Pcomm is also known as the P2 segment. The P2 segment extents from the Pcomm to the posterior edge of the midbrain. Flow to the PCA territory may be predominantly from the Pcomm in cases of a fetal Pcomm that occurs in 15% to 40% of the population. Anterior thalamoperforators typically arise from the Pcomm. These vessels supply a portion of the cerebral peduncles, posterior thalamus, subthalamic nucleus, optic chiasm, tuber cinereum, and mammillary bodies. The posterior thalamoperforators usually arise from the BA or the proximal P1 segments. These vessels supply the thalamus, hypothalamus, posterior limb of the internal capsule, and subthalamic nucleus. There may be considerable variation in the configuration and areas supplied by the anterior and posterior thalamoperforators. Due to the extent of the vascular territories supplied by these vessels, compromise of any of these perforators can lead to devastating results.

Approaches for BA Aneurysms

The approach to BA aneurysms largely relies on the relationship of the basilar bifurcation to the sella (Fig. 76-1). Multiple surgical approaches may be used to treat aneurysms in the BA region. These craniotomy routes include subtemporal, orbitozygomatic, and pterional craniotomies. These approaches describe an increasingly anterior trajectory to approaching basilar bifurcation aneurysms. Aneurysms arising at or below the middle depth of the sella are best approached via a subtemporal craniotomy, often combined with additional skull base techniques such as removal of the petrous apex (Kawase approach).¹² Aneurysms associated with a high bifurcation (more than 1 cm above the posterior clinoids) can be treated via an orbitozygomatic craniotomy that allows for better superior visualization due to more aggressive bone removal. Aneurysms that arise above the sella and up to 1 cm above the clinoids are accessible via a trans-sylvian pterional craniotomy. We have employed all three techniques in treating BA aneurysms at our institution. However, we favor the use of a pterional trans-sylvian craniotomy, in combination with some elements of the subtemporal craniotomy (a half-and-half approach), when targeting lesions in this location.

FIGURE 76-1 The approach to basilar tip aneurysms relies largely on the relationship of the aneurysm to a region between the midsella and 1 cm above the posterior clinoids. A majority of BA aneurysms are located in this region and can be approached by a half-and-half approach.

Subtemporal Approach

The subtemporal approach was popularized by Peerless and Drake, who were pioneers in successfully treating aneurysms in the basilar bifurcation region.^13,14 This approach targets the aneurysm from a lateral trajectory as the temporal lobe is elevated. We utilize this approach for aneurysms originating from below the middle depth of the sella turcica and for posteriorly projecting aneurysms. Advantages to the subtemporal approach are many. Lateral trajectory facilitates visualization and dissection of posterior perforators, especially for posteriorly projecting aneurysms. Preservation of these perforators is perhaps the most crucial objective of these procedures. Proximal control is also easy to obtain with this approach. As the surgeon is working along the axis of the aneurysm neck, aneurysm clips can be placed with optimal visualization of the aneurysm neck as well as the thalamoperforators. Lastly, division of the tentorium and even removal of the petrous apex through this approach allows for exposure of the upper third of the clivus for access to low-lying bifurcations. The subtemporal craniotomy does have some disadvantages: there is poor visualization of the contralateral P1; cranial nerve III (CN III) is centered in the field, which often leads to postoperative oculomotor nerve palsies; and excessive retraction may lead to temporal lobe injury.

In most instances, a right-sided approach is used to avoid injury to a dominant temporal lobe. A left-sided approach is used when there is a preexisting left CN III palsy or right hemiparesis or if aneurysmal anatomy favors a left-sided approach.

Positioning and Scalp Incision

The patient is positioned supine with a shoulder roll placed on the right side. The head is fixed in a radiolucent Mayfield headrest. One pin is positioned over the forehead, and two pins are positioned over the occiput (Fig. 76-2). The head is rotated until the midline plane parallels the floor. The head is angled approximately 15 to 20 degrees until the floor of the middle cranial fossa is parallel to the line of sight. This maneuver also allows the temporal lobe to fall away from the middle cranial fossa, minimizing retraction during the procedure.

FIGURE 76-2 Patient positioning for a subtemporal approach.

A question mark–shaped incision is made over the right temporal region (Fig. 76-3). The incision originates 1 cm anterior to the tragus and extends superiorly approximately 4 cm. The incision is then curved anteriorly to a point 1 cm behind the hairline. The line then follows the path of the superior temporal line, terminating just superior and posterior to the mastoid. The scalp flap is retracted anteriorly and secured with fishhooks connected to a Leyla bar. The temporalis muscle is divided and retracted anteriorly. An approximately 4 × 4 cm bone flap is made, based over the zygomatic root, using a high-speed pneumatic drill. Our craniotomies are slightly larger for patients with a SAH. We drill the temporal bone inferiorly and anteriorly until it is flush with the floor of the middle fossa to visualize the middle fossa floor, as well as the temporal tip. The dura is incised in a square based at the floor of the middle fossa (Fig. 76-4). The dura is retraced and secured to the overlying tissue using a 3-0 Vicryl suture. A ventricular catheter or lumbar drain, if present, is opened, allowing for cerebrospinal fluid (CSF) drainage and brain relaxation. If sufficient brain relaxation is not achieved with CSF diversion, diuresis or hyperventilation subpial resection of the inferior temporal gyrus may be performed. The Budde Halo retraction system (Integra) is used to elevate the temporal gyrus, exposing the tentorial incisura. Incising the tentorium may be necessary for low-lying aneurysms. The surgical microscope is now brought into the field. It may be helpful at this point to place a 3-0 Vicryl suture into the free edge of the tentorium, anchoring it to the floor of the middle cranial fossa. This maneuver provides 3 to 4 mm of additional exposure in the surgical field.

FIGURE 76-3 Skin incision and craniotomy for a subtemporal approach.

FIGURE 76-4 Subtemporal approach: dural opening and tack up, allowing for retraction of the temporal lobe.

Subarachnoid Dissection and Clip Application

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