As a group, basilar artery aneurysms are

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1 OPERATIVE NUANCES ORBITOZYGOMATIC APPROACH TO BASILAR APEX ANEURYSMS Frank P.K. Hsu, M.D., Ph.D. Division of Neurological Surgery, St. Joseph s Hospital and Medical Center, Phoenix, Arizona Richard E. Clatterbuck, M.D., Ph.D. Division of Neurological Surgery, St. Joseph s Hospital and Medical Center, Phoenix, Arizona Robert F. Spetzler, M.D. Division of Neurological Surgery, St. Joseph s Hospital and Medical Center, Phoenix, Arizona Reprint requests: Robert F. Spetzler, M.D., c/o Neuroscience Publications, 350 West Thomas Road, Phoenix, AZ neuropub@chw.edu Received, December 18, Accepted, April 7, BASILAR APEX REGION aneurysms are among the most complex cerebral aneurysms. They are not, however, among the most common aneurysms, and increased use of endovascular treatment has further decreased the number of patients with these lesions who undergo surgery. Nonetheless, not all basilar apex aneurysms are amenable to coil embolization, and neurosurgeons must be prepared to treat patients with basilar apex aneurysms surgically. We prefer an orbitozygomatic craniotomy and transsylvian approach. Meticulous exercise of the basic tenets of aneurysm surgery (proximal vascular control, sharp dissection, and preservation of perforating vessels) is crucial to optimal patient outcomes. KEY WORDS: Aneurysm, Basilar artery, Microsurgery Neurosurgery 56[ONS Suppl 1]:ONS-172 ONS-177, 2005 As a group, basilar artery aneurysms are perhaps the most complex aneurysms faced by neurosurgeons. Their relative rarity and evolving endovascular treatment options mean that the frequency of surgical treatment for patients with basilar artery aneurysms is much less than that of patients with more common aneurysms, and the rate is decreasing rapidly. However, not all of these lesions are amenable to endovascular treatment. Therefore, vascular neurosurgeons must maintain their skills in treating patients with these complex lesions, which may be made more complex by partial embolization. The central tenets of all aneurysm surgery apply to the treatment of patients with basilar apex aneurysms. These include proximal vascular control before aneurysm dissection, the use of sharp dissection techniques, adequate visualization and preservation of perforating vessels near the aneurysm, and preservation of the parent vessels. Adjuncts that include neuroanesthetic care based on cerebral protection, electrophysiological monitoring, and intraoperative angiography also are key elements in the intraoperative care of patients with basilar apex aneurysms. Treatment of patients with giant basilar artery apex aneurysms under cardiac bypass and low blood flow or in full cardiac arrest is beyond the scope of this discussion and is not addressed. Endovascular techniques have an important role to play in the treatment of basilar artery DOI: /01.NEU D1 aneurysms and are the appropriate treatment choice in selected patients. However, direct microsurgical repair is the appropriate treatment for many patients, which should not be supplanted by endovascular therapy simply because of the perceived difficulties and risks associated with surgery for basilar artery aneurysms. This discussion is based on the surgical decision-making and technique of the senior author (RFS). PATIENT SELECTION Careful patient selection is important to assuring optimal outcomes. Data from the International Study of Unruptured Intracranial Aneurysms suggests that the natural history of unruptured posterior circulation aneurysms, particularly at the basilar apex, is more aggressive than that of similarly sized anterior circulation aneurysms (2). The study results, however, also suggest that the morbidity and mortality associated with the treatment of these aneurysms is greater (2). Young patients with unruptured basilar aneurysms should be treated, given the accumulated risk of rupture during a period of many years. Although endovascular treatment options always should be considered, higher partial obliteration rates and recurrence rates make microsurgical obliteration superior in relatively young patients without extenuating medical circumstances. In older patients or in patients with poor Hunt ONS-172 VOLUME 56 OPERATIVE NEUROSURGERY 1 JANUARY

2 ORBITOZYGOMATIC APPROACH TO BASILAR APEX ANEURYSMS and Hess grades after subarachnoid hemorrhage, endovascular treatment may be a better option. PREOPERATIVE CONSIDERATIONS Cerebral angiography has been the cornerstone of the preoperative evaluation of patients with basilar artery aneurysms. Although computed tomographic (CT) angiography and magnetic resonance angiography continue to improve, cerebral angiography remains a crucial element in preoperative evaluation. It is still the most sensitive modality for the diagnosis of cerebral aneurysms. The identification of small anterior circulation aneurysms not revealed by CT or magnetic resonance angiography may alter the side of the intended approach, allowing multiple aneurysms to be treated during a single procedure. CT angiography has become important in the treatment of patients with these lesions. The reconstructed images of the aneurysm with related bony surfaces that can be rotated and viewed from multiple vantages can help the surgeon to understand an individual patient s anatomy. Similarly, three-dimensional reconstructions created from rotational angiography data can facilitate surgical planning and may be superior to CT angiography for understanding crucial perforator anatomy. Neuroanesthesia and Cerebral Protection Mild hypothermia and barbiturate-induced electroencephalographic burst suppression are essential elements in the neuroanesthetic plan for patients undergoing microsurgical repair of basilar apex aneurysms. Mild hypothermia (33 35 C) provides modest cerebral protection and is commonly used in the surgical treatment of intracranial aneurysms. Thiopental titrated to burst suppression not only decreases the cerebral metabolic rate providing cerebral protection; it also decreases cerebral blood volume, decreasing intracranial volume and the subsequent need for brain retraction. Both techniques are essential to the successful use of temporary clipping as an adjunct to final aneurysm dissection and permanent clipping. During temporary clipping and related ischemia, careful avoidance of hypotension and the judicious use of induced hypertension provide additional cerebral protection. Such needs must be carefully communicated to the anesthesiologist. Although it is beyond the scope of this discussion, hypothermic circulatory arrest provides ultimate cerebral protection during basilar apex aneurysm surgery. We have previously published our technique for circulatory arrest (1). MICROSURGICAL OBLITERATION (see video at web site) Orbitozygomatic Craniotomy Preparation of the anesthetized patient for surgery includes placement of a femoral sheath for intraoperative angiography and electrophysiological monitoring leads, including leads for motor evoked potentials. The patient s head is placed in a radiolucent Mayfield head frame in slight extension and rotated 15 to 35 degrees to the contralateral side of the approach. Under typical circumstances, less rotation is applied for basilar apex aneurysms to decrease the intrusion of the temporal lobe into the transsylvian operative corridor. After the head is fixated to the table, the back of the table is elevated slightly. We approach aneurysms at the basilar apex through an orbitozygomatic craniotomy (3). A curvilinear incision is planned from just anterior to the ipsilateral tragus up to the superior temporal line. The incision then gently curves to terminate at the hairline superior to the contralateral midpupillary line. A small strip of hair is shaved with clippers along the course of the planned incision. Before the final preparation of the head and placement of the drape, the planned incision site is infiltrated with approximately 10 ml of local anesthetic containing epinephrine to allow enough time for vasoconstriction of scalp vessels before the skin incision is made. The skin is incised and hemostasis is obtained with Raney clips. The inferior limb of the incision is completed after the scalp is dissected from the temporalis fascia with a periosteal elevator. Care is taken to preserve the main trunk of the superficial temporal artery if possible. The scalp flap is mobilized anteriorly and the temporalis fascia is exposed. The fascia is sharply incised and elevated separately in a subfascial dissection to protect the frontalis branch of the facial nerve running along the superficial surface of this fascial plane. The dissection continues anteriorly to expose the orbital rim, malar eminence, and zygomatic arch. The temporalis muscle is raised separately, exposing the zygomatic root and pterion. The muscle flap is left attached to the cranium at its vascular pedicle in the infratemporal fossa. A series of surgical fishhooks attached to a Leyla bar (V. Mueller, Deerfield, IL) are used to retract the scalp flap and temporalis muscle anteriorly and inferiorly. A Midas Rex (Fort Worth, TX) drill is used to place a burr hole in the temporal bone over the root of the zygoma, and a pterional craniotomy is performed. A series of small holes is created along the superior and posterior edges of the craniotomy, and the dura is tacked to the cranium along these edges. The periorbital contents are freed from the undersurface of the orbit with a Penfield no. 1 dissector. As described elsewhere (3), a series of osteotomies is completed to free the lateral orbital roof and rim, the top of the malar eminence, and zygomatic arch from the cranial base in a single piece (Fig. 1). Removal of this additional bone along the cranial base decreases the needed retraction and increases the angle over which structures at the depth of the microsurgical corridor can be viewed. It also decreases the working depth by allowing the surgeon s hands closer to the vessels in the circle of Willis. The advantages gained in ease of dissection and manipulation in the depth of the field and the decrease in needed retraction more than offset the additional time expended and slight risk to the orbital contents. Sylvian Fissure and Subfrontal Dissection A semilunar dural opening is created. The dura is reflected anteriorly and inferiorly over the periorbita and temporalis and tacked to the fishhook retraction system. In this fashion, NEUROSURGERY VOLUME 56 OPERATIVE NEUROSURGERY 1 JANUARY 2005 ONS-173

3 HSU ET AL. FIGURE 1. Illustration of orbitozygomatic osteotomy. A, the series of osteotomy cuts (1 6) required for en bloc removal of the orbitozygomatic complex (modified from, Zabramski JM, Kiris T, Sankhla SK, Cabiol J, Spetzler RF: Orbitozygomatic craniotomy: Technical note. J Neurosurg 89: , 1998 [3]) (With permission from Journal of Neurosurgery). B, the exposure afforded by removal of this portion of the cranial base (With permission from the Barrow Neurological Institute). the profile of the periorbital contents is flattened, increasing the exposure. The sylvian fissure then is opened microsurgically. The superficial sylvian cistern is entered first, and the dissection proceeds proximally into the opticocarotid cistern. Cerebrospinal fluid is removed from this cistern to promote brain relaxation and to minimize the need for brain retraction. Next, attention is turned to the subfrontal dissection. The orbital surface is freed from the optic nerve and the lamina terminalis is exposed. Fenestration of the lamina terminalis is completed to allow more cerebrospinal fluid to egress from the third ventricle to obtain further brain relaxation. The deep sylvian cistern is opened and the carotico-oculomotor triangle is dissected. The temporal tip is released from the anterior aspect of the middle cranial fossa by coagulation and transection of necessary bridging veins. Arachnoid adhesions to the third cranial nerve lying along the edge of the tentorium are transected as the temporal lobe is retracted gently. The course of the posterior communicating artery is then visible as it pierces the membrane of Liliequist. Opening Liliequist s Membrane and Skeletonizing the Anterior Choroidal Artery Liliequist s membrane is opened sharply with a no. 11 blade or microscissors (Fig. 2). Through this space, the course of the posterior communicating artery can be observed to its junction with the ipsilateral posterior cerebral artery. To maximize the operative corridor, the dissection of the sylvian fissure is completed by skeletonizing the anterior choroidal artery from the internal carotid artery to the choroidal fissure. On occasion, the posterior communicating artery must be coagulated and divided to increase the visibility and surgical access through the carotico-oculomotor triangle. First, however, the surgeon should verify with the preoperative angiograms that the posterior cerebral artery fills adequately from the basilar apex. Basilar Apex Dissection and Preservation of Perforators After the Sylvian fissure has been split widely and Liliequist s membrane has been opened, the ipsilateral posterior cerebral artery should be visualized easily. It is observed proximally beyond its junction with the ipsilateral posterior communicating artery to the region of the basilar apex. Once the basilar apex has been identified, the four vessels arising at the apex should be identified (both posterior cerebral and both superior cerebellar arteries). If identification of these vessels is difficult, the rotation of the patient s head should be decreased to bring the basilar trunk and contralateral vessels into view. The basilar artery aneurysm also should be visible. Identification of the IIIrd cranial nerve between the posterior cerebral and superior cerebellar arteries is often helpful in confirming the vascular anatomy. Visualization of the basilar apex and aneurysm can be challenging when the lesion is particularly high riding or low riding relative to the posterior clinoids. For low-riding lesions, the posterior clinoid and upper portion of the clivus can be drilled off with a diamond burr, or they can be aspirated with an ultrasonic aspirator equipped with a bone curettage tip. High-riding lesions can be buried deep in the interpeduncular fossa. In this situation, the increased angle over which this region can be viewed after an orbitozygomatic craniotomy becomes crucial. The region of the basilar apex is thoroughly dissected, clearing arachnoid from around the neck of the aneurysm. Both the proximal and distal neck must be scrutinized for brainstem and thalamoperforating vessels (Fig. 3). If the perforating vessels are to be preserved, their location and relationship to the neck of the aneurysm must be fully understood before clip placement. Temporary and Permanent Clipping Before dissecting the basilar apex and aneurysm, the surgeon must prepare an area on the basilar trunk for placement ONS-174 VOLUME 56 OPERATIVE NEUROSURGERY 1 JANUARY

4 ORBITOZYGOMATIC APPROACH TO BASILAR APEX ANEURYSMS FIGURE 2. Intraoperative photograph of Liliequist s membrane as observed beyond the optic nerve (CN II) and the supraclinoid portion of the internal carotid artery (ICA), which is being opened sharply with microscissors. The posterior communicating artery can be observed piercing this arachnoid membrane, and its course on the far side (projected onto Liliequist s membrane; dotted line) can be observed. FIGURE 3. Intraoperative photograph showing the basilar apex via the orbitozygomatic approach. The aneurysm (An) and the proximal and distal posterior cerebral artery (P1 and P2) are outlined (dotted line). The interval between the microsuction (left) and dissector (right) is filled with small perforators. of a temporary clip, should it become necessary. An area below the superior cerebellar arteries relatively free of circumflex perforating vessels should be identified. The angle and final location of temporary clip placement need to be clear in the surgeon s mind so that when needed, clips can be placed almost automatically. Optimal temporary clipping should not significantly obstruct the working area for permanent clip placement (Fig. 4). Visualization of posteriorly pointed aneurysms and their related perforating vessels can sometimes be improved by applying gentle pressure to the basilar artery just below the apex. This maneuver may allow clip placement on an otherwise unclippable aneurysm. With a high-riding basilar apex FIGURE 4. Intraoperative photograph showing temporary clip placement. The temporary clip (right) is positioned on the basilar trunk (BA) below the superior cerebellar arteries so that it does not obstruct the view of the basilar apex. The permanent aneurysm clip (left) can be observed obliterating the aneurysm. The ipsilateral posterior cerebral artery (P1), the contralateral IIIrd cranial nerve (CN III), and both superior cerebellar arteries are visible. aneurysm, the perforating vessels often stream down away from the aneurysm neck, decreasing the risk for their inclusion in a clip. The reverse is true of low-riding lesions, in which perforating vessels may be plastered along the dome of the aneurysm as the posterior cerebral arteries ascend along the side of the aneurysm. Perforating vessels must be dissected free of the neck of the aneurysm sharply. Although not absolutely necessary, it is preferable to dissect perforating vessels free along their entire length. This allows the perforators to be held free of the aneurysm neck more readily during clipping. If possible, final clip placement should be completed with the perforating vessels under simultaneous direct visualization to ensure their safety during clipping. Clip selection must be individualized for each patient. In general, however, long, straight clips are the most useful to clip basilar aneurysms from an orbitozygomatic approach. Occasionally, the use of bayoneted clips improves visualization during clipping. For large aneurysms, a combination of fenestrated and long straight clips is preferred. The fenestrated clip is applied first to occlude the distal end of the neck with greater closing force, and the straight clip is used to close the proximal end of the aneurysm neck. Intraoperative Angiography We routinely use intraoperative angiography to verify obliteration of the aneurysm and patency of the posterior cerebral and superior cerebellar arteries. Typically, perforating vessels cannot be resolved with the portable C-arms used in most operating rooms. Significant residual aneurysm or compromise of a major vessel exiting the basilar apex mandates repositioning of the clip or placement of another clip. Contrast NEUROSURGERY VOLUME 56 OPERATIVE NEUROSURGERY 1 JANUARY 2005 ONS-175

5 HSU ET AL. runs should be observed sufficiently far to ensure that during later angiographic phases, contrast does not pool in the aneurysm from subtle residual blood flow into the aneurysm. An exception to our use of intraoperative angiography for basilar aneurysms is a patient undergoing repair during circulatory arrest. CONCLUSION Basilar apex region aneurysms are among the most complex cerebral aneurysms. Not all basilar apex aneurysms are amenable to coil embolization, and for the immediate future, neurosurgeons must be prepared to treat basilar apex aneurysms surgically. We use an orbitozygomatic craniotomy and transsylvian approach to access such lesions. Meticulous exercise of the basic tenets of aneurysm surgery (proximal vascular control, sharp dissection, and preservation of perforating vessels) is crucial to optimize patient outcomes. REFERENCES 1. Spetzler RF, Hadley MN, Rigamonti D, Carter LP, Raudzens PA, Shedd SA, Wilkinson E: Aneurysms of the basilar artery treated with circulatory arrest, hypothermia, and barbiturate cerebral protection. J Neurosurg 68: , Wiebers DO, Whisnant JP, Huston J III, Meissner I, Brown RD Jr, Piepgras DG, Forbes GS, Thielen K, Nichols D, O Fallon WM, Peacock J, Jaeger L, Kassell NF, Kongable-Beckman GL, Torner JC: International Study of Unruptured Intracranial Aneurysms Investigators: Unruptured intracranial aneurysms Natural history, clinical outcome, and risks of surgical and endovascular treatment. Lancet 362: , Zabramski JM, Kiris T, Sankhla SK, Cabiol J, Spetzler RF: Orbitozygomatic craniotomy: Technical note. J Neurosurg 89: , COMMENTS Orbitozygomatic craniotomy has become an important part of the surgical approach to basilar apex aneurysms. There is some additional immediate postoperative discomfort and minimal trauma to the orbital contents accompanying an orbitozygomatic craniotomy compared with a simple pterional craniotomy. The additional invasion seems to be necessary in reducing potential morbidity from basilar apex aneurysm surgery. Brain retraction is reduced, the working distances are decreased, and the amount of light is improved. Robert A. Solomon New York, New York This is an excellent operative nuance. The authors correctly allude to the point that microvascular neurosurgeons must continue to develop these skills, maintain existing skills, and develop new and creative approaches to these complex problems. Increasingly, there is a tendency toward abdication of surgical responsibility if a given problem is difficult. This argument flies in the face of existing data. Even in the best endovascular neurosurgical centers, the complete obliteration rate of basilar apex aneurysms remains poor. In addition, the high early (and unknown late) recurrence risks bring many of these patients back to microsurgical attention. At that point, the problems are more difficult and complex than they were before treatment was initiated. The years of experience that went into the concepts put forward in this article should be carefully scrutinized by those interested in microvascular neurosurgery. These principles were learned through trial and error, and neurosurgeons should not have to duplicate or relearn these techniques to the detriment of our patients. In addition to the key points illustrated by the authors, I would mention a few others: 1. Division of the posterior communicating artery: This technique can be quite helpful in untethering the anterior from the posterior circulation and thus expanding the working space. Conversely, it should be done only judiciously. One never knows when technical problems may arise, and it might be necessary because of a tear or other issues to sacrifice the origin of the ipsilateral P1 segment. Division of the posterior communicating artery eliminates that potential exit strategy. 2. Reexamination of the middle cerebral artery: During the dissection and ultimate clip reconstruction, it is important for the surgeon to look back carefully at the M1 segment all the way through to its bifurcation. Adjustments of retractors and other manipulations can inadvertently kink the middle cerebral artery. I have had one case of delayed M1 dissection and occlusion. 3. Dissection along the inferior border of the P1 segment: In a packed interpeduncular cistern soon after subarachnoid hemorrhage, the anatomy can be somewhat difficult to sort out, particularly in contemporary times when it is not observed very often. Once the P1 segment is identified, if the surgeon dissects along the inferior border of the P1, it can be safely followed back to the basilar trunk, where a temporary clip site can be developed. 4. Posterior neck dissection: Critical posterior dissection is the essence of safe basilar aneurysm surgery. If the surgeon begins working on that posterior plane beneath the P1 origin rather than through the true surgical neck, the perforators are less adherent and less stretched by the aneurysm tissue. Virtually all of these aneurysms have a more posterior projection than is appreciated on the initial angiogram. This plane beneath the P1 can be followed all the way across to the contralateral side. 5. Perforators: In my opinion, the posterior thalamic perforating branches should be freed not only in the neck but also up to their adhesion to the fundus. It is possible that during clip occlusion of the neck, the resulting traction and aneurysm decompression can kink these branches distal to the clip site. 6. Consideration given to not aspirating the sac: A significant change during the past several years in my practice has been that for basilar apex lesions, particularly if they are ruptured, I tend not to aspirate the sac after final clipping. Each additional clip added in the setting increases the risk of inadvertent perforator damage, as the ONS-176 VOLUME 56 OPERATIVE NEUROSURGERY 1 JANUARY

6 ORBITOZYGOMATIC APPROACH TO BASILAR APEX ANEURYSMS amount of visibility increasingly deteriorates. Postoperative angiography is typically performed before the patient is discharged, and if a minor flaw in the clip reconstruction is present, endovascular options are usually successful at correcting the defect. H. Hunt Batjer Chicago, Illinois The authors have presented an excellent article on the orbitozygomatic approach to basilar apex aneurysms. As they stress, basilar bifurcation aneurysms are the most complex aneurysms faced by neurosurgeons. Although they are relatively rare (only approximately 5% of all intracranial aneurysms), the basilar apex is the most frequent site for posterior fossa aneurysms (approximately 50% of posterior circulation aneurysms are located at the basilar apex). As also emphasized by the authors, it is concerning to us as vascular neurosurgeons that the majority of basilar tip aneurysms are treated nowadays by an endovascular approach, such that the number of patients surgically treated is shrinking at a rapid pace. It is likely to be a great problem for future generations of vascular neurosurgeons to learn how to approach these lesions surgically in cases not amenable to endovascular treatment. The orbitozygomatic approach is in reality a modified pterional craniotomy that has the great advantage of allowing the use of multiple surgical routes in the same craniotomy. The surgeon can choose the best approach during surgery and change the angle of view depending on the specific case. A view of the interpeduncular cistern can be achieved from a transsylvian, subtemporal, or anterior temporal surgical perspective. The additional bone removal carried out using such an approach improves the surgeon s angle of view to the lesion, however, without creating a wider working space in the depths of the interpeduncular and prepontine cisterns. This craniotomy technique used by the majority of vascular neurosurgeons for basilar bifurcation aneurysms is also useful for giant anterior communicating artery aneurysms. It can be associated with sacrifice of the veins of the temporal tip to permit retraction of the temporal lobe, as first described by Sano (2) in 1980 as the temporopolar approach and, lately, as the half-and-half approach by others. In cases of more complex basilar apex aneurysms or low-lying lesions, the orbitozygomatic craniotomy can be associated with the transcavernous transsellar approach, as described by Dolenc et al. (1) in Another possibility for improving the view of the interpeduncular fossa without sacrifice of the veins of the temporal tip is the resection of the uncus, a procedure similar to the gyrus rectus resection for anterior communicating artery aneurysms. The authors recommend the division of the posterior communicating artery in cases in which there is a need to increase the visibility and surgical access. This maneuver detaches the carotid artery from the posterior cerebral artery, thus improving access through a transsylvian dissection. Other components of the circle of Willis such as the P1 segment of the posterior cerebral artery or the A1 segment of the anterior cerebral artery can also be sacrificed to allow a better view of the basilar bifurcation. Atos Alves de Sousa Belo Horizonte, Brazil 1. Dolenc VV, Skrap M, Sustersic J, Skrbec M, Morina A: A transcavernoustranssellar approach to the basilar tip aneurysms. Br J Neurosurg 1: , Sano K: Temporopolar approach to aneurysms of the basilar artery at and around the distal bifurcation: Technical note. Neurol Res 2: , Future Meetings Congress of Neurological Surgeons The following are the planned sites and dates for future annual meetings of the Congress of Neurological Surgeons: Boston, MA Chicago, IL San Diego, CA Orlando, FL October 8 13 October 7 12 September September Future Meetings American Association of Neurological Surgeons The following are the planned sites and dates for future annual meetings of the American Association of Neurological Surgeons: New Orleans, LA San Francisco, CA Washington, DC Chicago, IL April April April March 29 April 3 NEUROSURGERY VOLUME 56 OPERATIVE NEUROSURGERY 1 JANUARY 2005 ONS-177