Surgical Management of Terminal Basilar and Posterior Cerebral Artery Aneurysms

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.”

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.

Neuroanesthesia

Improvements in the field of neuroanesthesia have allowed neurosurgeons to more safely attack aneurysms in the BA region. Most patients receive anxiolytic medications (e.g., benzodiazepines) prior to transport to the operating room. They may also receive light opioid sedation before insertion of a radial arterial line (many patients also receive a central venous catheter). After the 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 oxygen-air mixture. Brain relaxation is crucial for maximum exposure of the surgical site and minimization of retraction during the operation. In addition to cerebrospinal fluid (CSF) drainage following dural opening, intracranial relaxation is achieved by the administration of 50 to 100 g of mannitol (0.5 to 2 g/kg body weight) prior to entering the cranium. If further relaxation is necessary, furosemide (0.5 to 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 PaCO ₂ between 32 and 44 mm Hg, with a PaO ₂ target greater than 100 mm Hg. Continuous assessment of fluid status is facilitated by the close monitoring of hourly urine output and the 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 cooled 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 aneurysmal sac, facilitating clip placement at the neck. As the aneurysm sac softens, perforator vessels are also identified more easily. To lower the risk of recurrent hemorrhage prior to surgery, we routinely place our subarachnoid hemorrhage (SAH) patients on the lysine analogue ε-aminocaproic acid (Amicar) with a 5-g intravenous bolus followed by 1 g/h maintenance. We stop this infusion following clip placement.

We do not routinely perform electroencephalography or brain stem auditory evoked potentials (BAEPs) BA aneurysms are being treated. An exception is made in treating “giant” 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. ^, 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 electroencephalographic 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 Basilar Apex 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 posterior carotid arteries (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). 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 extends 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, which 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. Owing to the extent of the vascular territories supplied by these vessels, compromise of any of these perforators can lead to devastating results.

Approaches for Basilar Apex Aneurysms

The approach to BA aneurysms largely relies on the relationship of the basilar bifurcation with the sella ( Fig. 50.1 ). Multiple surgical approaches may be used to treat aneurysms in the BA region. These craniotomy routes include subtemporal, orbitozygomatic, and pterional craniotomies, which 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). Aneurysms associated with a high bifurcation (more than 1 cm above the posterior clinoids) can be treated via an orbitozygomatic craniotomy, which 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 lesions in this location are being targeted.