Skull Base Approaches to the Basilar Artery

Home , Anterior clinoid process, Clivus (anatomy), Dorsum sellae, Jugular tubercle

Neurosurg Focus 19 (2):E3, 2005

Skull base approaches to the basilar artery

L. FERNANDO GONZALEZ, M.D., SEPIDEH AMIN-HANJANI, M.D., NICHOLAS C. BAMBAKIDIS, M.D., AND ROBERT F. SPETZLER, M.D. Division of Neurological Surgery, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, Arizona

Posterior circulation lesions constitute approximately 10% of all intracranial aneurysms. Their distribution includes the basilar artery (BA) bifurcation, superior cerebellar artery, posterior inferior cerebellar artery, and anterior inferior cerebellar artery. The specific features of a patient’s aneurysm and superb anatomical knowledge help the surgeon to choose the most appropriate approach and to tailor it to the patient’s situation. The main principle that must be applied is maximization of bone resection. This allows the surgeon to work within a wider corridor, which facilitates the use of surgical instruments and minimizes retraction of the brain. The management of aneurysms within the posterior circulation requires expertise in skull base and vascular surgery. Endovascular treatments have become increasingly important, but in this paper the authors focus on the surgical management of these difficult aneurysms. The paper is divided into three parts: the first section is a brief review of the anatomy of the BA; the second part is a review of the techniques associated with the management of posterior fossa aneurysms; and in the third section the authors describe the different approaches, their nuances and indications based on the location of the aneurysm, and its relationship to the surrounding bone (especially the clivus, dorsum sellae, and the free edge of the petrous apex).

KEY WORDS • basilar artery aneurysm • skull base approach • far-lateral approach • orbitozygomatic region • middle fossa • transpetrosal approach • revascularization • flow reversal

ANATOMY OF THE BA mammillary bodies and the floor of the third ventricle, or it The BA begins at the juncture of the VAs at the ponto- can be short, with a “low” bifurcation below the pontomesencephalic junction. The BA also can vary in the coronal medullary sulcus. It ascends anteriorly to the brainstem be- plane. It can be straight or tortuous with extreme loops on hind the clivus and terminates when it bifurcates at the in- both sides. In addition to this complex intrinsic vascular terpeduncular cistern. There it gives rise to both PCAs at anatomy, its relationships to bone at each level help deter- the pontomesencephalic junction. mine the suitability of a given approach within the arma- The BA gives rise to the arteries that supply the posteri- mentarium to treat these complex lesions. or fossa. The SCA, which originates at the pontomesence- Conceptually, the BA can be divided into fifths. At its phalic sulcus, encircles the midbrain to supply the cerebral upper two fifths, the main osseous structures that can ob- peduncles before it courses superiorly and medially to sup- scure its visualization are the anterior and posterior clinoid ply the tentorial surface of the cerebellum. The AICA processes and the dorsum sellae (upper clivus). Usually, an- originates near the pontomedullary sulcus and courses eurysms at this level can be reached via an orbitozygomat- around the pons, typically below or between the fascicles ic approach or a modification thereof. Occasionally, an- of the abducent nerve, to the cerebellopontine angle. There, eurysms are located higher than the petrous edge and lower it passes between or around the seventh and eighth cranial than the floor of the sella. In such cases a lateral exposure nerves, which course toward the acoustic meatus. The via a subtemporal approach is indicated. The main struc- AICA sends branches to these nerves and to the choroid tures that help decide the approach to midbasilar (middle plexus before it courses posterolaterally to supply the pet- fifth of the BA) aneurysms are the free edge of the petrous rosal surface of the cerebellum. bone and the relationship between the aneurysm and inter- The length of the BA varies, which determines the posi- nal auditory canal. The mid-BA can be reached via one of tion of the bifurcation. It can extend as rostrally as the the transpetrosal approaches or via an extended middle fossa approach. Sometimes a retrosigmoid approach is ade- Abbreviations used in this paper: AICA = anterior inferior cerebellar artery; BA = basilar artery; CA = carotid artery; ICA = inter- quate. If the aneurysm is located at the bottom two fifths of nal carotid artery; PCA = posterior cerebral artery; PCoA = poste- the artery (lower BA), the main bone structures of concern rior communicating artery; PICA = posterior inferior cerebellar are the jugular tubercle and the occipital condyle. Aneu- artery; SCA = superior cerebellar artery; STA = superior temporal rysms at this level can usually be reached via a far-lateral artery; VA = vertebral artery. approach27 or its variants.

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SPECIAL TECHNIQUES Aneurysm Trapping, Flow Reversal, and Revascularization Although the primary surgical goal in the management of aneurysms is direct clip obliteration with preservation of the parent vessel, not all lesions are amenable to this strategy. Large aneurysms especially can incorporate the parent vessel to such an extent that clip reconstruction is not feasible. The most extreme example would be fusiform vertebrobasilar dolichoectatic aneurysms or dissecting aneurysms, which are common in the posterior fossa. Nevertheless, even large, saccular aneurysms can behave in this fashion. Under these circumstances, the remaining options are trapping of the affected segment or proximal occlusion to alter the flow dynamics in the region of the aneurysm, causing it to regress and thrombose spontaneously (Fig. 1A).10 For mid- and lower basilar lesions, direct trapping may be less desirable because of the potential risk of injuring adjacent perforating vessels. Nevertheless, proximal or even distal BA occlusion can effectively change the flow dynamics, thereby abolishing the aneurysm (Fig. 1B). For large fusiform dolichoectatic aneurysms, proximal occlusion of one or both VAs can accomplish “flow reversal” in a simi- lar fashion. Removing hemodynamic stress from the vessel wall is thought to play an important part in the arrest of growth in these lesions. At this point it is necessary to establish whether preoperative revascularization is indicated. In general, a nondominant VA can be occluded with impunity, whereas the collateral circulation must be relied on if the artery is dominant. A PCoA with a caliber larger than 1 mm8 usually represents adequate collateral blood flow. When collateral flow is inadequate, different techniques for revascularization are available.10 The purpose of using a bypass is to ensure adequate blood flow to the upper basilar territory after the BA trunk has been clipped, unless the caliber of the PCoAs is adequate to provide this flow.10,28 In such cases, the choice of donor and recipient vessels de- pends on the requirements for blood flow and on the available suitable vessels. The primary arterial donor vessels are the STA and occipital artery, or interposition of a graft from the external carotid artery anastomosed to the PCA or SCA. Use of the nondominant radial artery is recommended because its caliber matches that of the PCA. A posterior circulation bypass is demanding because of the depth at which the anastomosis must be performed. It is

Fig. 1. Drawings showing lesions that are unsuitable for clip occlusion. A: This giant BA tip aneurysm is unsuitable for direct clip placement because both PCAs are involved with the neck of the lesion. The aneurysm clip is placed below the level of the SCAs in a patient with inadequate collateral flow, with a bypass performed from the STA to the PCA (arrow marked A) or to the SCA (arrow marked B). B: The other aneurysm is located at the level of the mid-BA. The involvement of this giant midbasilar aneurysm with perforating vessels makes it unsuitable for clip occlusion. The flow to the aneurysm is reversed with an aneurysm clip (placed either proximally or distally), and it is revascularized with an STA–PCA bypass. The distal circulation must be bypassed with an anastomosis from the STA or occipital artery into the PCA or SCA. (Reprint- ed with permission from Barrow Neurological Institute.)

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Unauthenticated | Downloaded 10/01/21 09:32 PM UTC Skull base approaches to the basilar artery associated with the general risks of any vascular surgery in Ya¸sargil popularized the use of the pterional approach for the posterior circulation, in addition to the more specific the treatment of anterior and posterior circulation aneu- risks of graft failure and secondary ischemia. rysms. The main advantages of this approach are the ability to attack the aneurysm from its neck, with a good per- Vascular Control spective on the interpeduncular cistern and the ability to During aneurysm surgery, adequate proximal and distal establish different surgical corridors. These corridors are control must be established. Doing so is particularly chal- established based on the local anatomy: through the optico- lenging in the posterior circulation. For the upper third of carotid triangle, laterally between the ICA and third cranial the BA, proximal control is obtained by placing a tempo- nerve, and the window between the A1, M1, and optic tract can be used as well (Fig. 4). A wide opening of the sylvian rary clip below the point where the SCAs emerge. Usually 21 this maneuver softens the aneurysm enough to proceed with fissure limits the need to retract the brain. the dissection. Typically, adequate control is obtained with The relationship between the aneurysm and upper clivus a temporary clip on the BA, and it is unnecessary to occlude (posterior clinoid process) is key. Aneurysms located with- the PCA or SCA. Alternatives to placement of a temporary in 5 mm of the dorsum sellae are considered normal. If they occur higher or lower than that point, they are considered clip to obtain adequate proximal control of the upper third 11 of the BA are endovascular balloon occlusion and hypo- high-lying or low-lying, respectively. thermic circulatory arrest. Balloon occlusion provides prox- To reach normal or high-lying aneurysms, we prefer the imal control before dissection of the aneurysm begins, with- orbitozygomatic approach, which allows good exposure of out the inconvenience of a temporary clip that can obstruct the interpeduncular and prepontine cisterns. There are three 31 osseous obstacles to the BA apex: the anterior clinoid proc- visualization of the lesion in a narrow corridor. 29 Circulatory arrest collapses the aneurysm, making its dis- ess, posterior clinoid process, and dorsum sellae. Resec- section from the brainstem feasible. A large, firm sac is tion of these three obstacles makes the extended orbitozygo- converted into a soft, pliable one that allows visualization matic approach especially useful for low-lying aneurysms. of the critical anatomy. This procedure makes it easier to Preoperative preparation includes placing a femoral differentiate the neck of the aneurysm from adjacent perfo- sheath for intraoperative angiography and an electrode to rating vessels. It eliminates the necessity of obtaining con- record the electroencephalographic and somatosensory trol, which is difficult in the midbasilar trunk. It also facili- evoked potential responses. The patient’s head is placed in tates manipulation of the aneurysm clip around the neck of a radiolucent Mayfield headholder, slightly extended, and 26 rotated 10 to 15˚ to the contralateral side. Specific details the lesion. Furthermore, hypothermia and barbiturate ther- 42 apy provide cerebral protection.29 about the approach are reported elsewhere. The carotid The main disadvantages of circulatory arrest are related cistern is opened wide, and the PCoA is followed posterior- to the coagulopathy caused by heparinization and platelet ly to the junction between P1 and P2. The Liliequist mem- trauma when the patient is on the bypass pump.6 Circula- brane is then opened. The exposure should allow the iden- tory arrest itself is associated with a risk of cerebral ische- tification of both PCAs and both SCAs. Sometimes the mia, which hypothermia and barbiturate drugs counteract lamina terminalis is opened.Decreasing the degree of rota- to some degree. Meticulous closure is necessary to prevent tion of the head helps to bring the contralateral side into view. Sometimes it is necessary to sacrifice the PCoA to postoperative hematomas. 21 The implementation of cardiac standstill is complex. It create more space to ease the manipulation of instruments requires the close cooperation of an organized team, includ- and the placement of temporary and permanent aneurysm ing cardiac surgeons, and the expertise of both cardiac and clips. neuroanesthesia personnel. Therefore, the technique must Pretemporal Transcavernous Approach be used judiciously only in selected cases. In the senior author’s experience with more than 50 procedures in which A variation of the orbitozygomatic approach includes an hypothermic cardiac arrest was used, the mortality rate was extension into the upper wall of the cavernous sinus to in- almost 10%. This rate reflects the complex premorbid sta- crease access to the anterior incisura. This modification was tus of the patients more than use of the technique itself.26 initially described by Dolenc, et al.5 It has been used for For lower BA aneurysms, proximal control is easily ob- low-lying aneurysms and facilitates access to the interpe- tained by placing a temporary clip across the VA. Distal duncular cistern from the carotid–oculomotor triangle. Se- control is harder to obtain, even when endovascular tech- oane, et al.,37 illustrated this procedure, which consists of an niques are used.29 orbitozygomatic approach with extradural drilling of the anterior clinoid process (Fig. 5A).35 The clinoid segment of SURGICAL APPROACHES the ICA is exposed and released from its distal and proximal dural rings. Once the ICA is free, the posterior clinoid Orbitozygomatic Approach process (Fig. 5B)41 is drilled and the upper clivus (dorsum The pterional approach is a traditional and versatile one sellae) is removed. The main disadvantage of this approach is the almost inevitable postoperative palsy of the third cra- that allows access to aneurysms in the anterior circulation. 11 This is the basis for the orbitozygomatic approach (Fig. 2), nial nerve. which combines the pterional approach with different osteotomies that remove the superior and lateral walls of the ipsi- Subtemporal Approach lateral orbit and zygomatic arch. This extra bone resection is This approach was popularized by Drake,7 who has pub- grounded in the skull base surgical principle of maximizing lished many papers relating his vast experience with treat- bone resection to minimize brain retraction (Fig. 3).15 ing posterior fossa aneurysms. Compared with the pterion-

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Fig. 2. Drawings showing the pterional craniotomy, the basic approach to the BA. A: The osteotomies (blue) are performed to detach the roof and lateral wall of the orbit. The superior and inferior orbital fissures are in communication and the malar eminence is cut. The remaining sphenoid wing (pink), anterior clinoid process (pink), and posterior clinoid process (brown) are visible. B: Appearance of the skull after the osteotomies have been performed. C: Close-up view of the structures in panel B showing the relationship of the BA trunk with the anterior (pink) and posterior clinoid processes (brown) and with the dorsum sellae (brown). Note the obstruction caused by the anterior and posterior clinoid processes (trajectory indicated by arrow). D: Bone obstacles must be eliminated for access to the BA. The anterior clinoid process is not a real obstacle, but it shadows the region of interest. Note the wide exposure of the upper and mid-BA (Reprinted with permission from Barrow Neurological Institute.) al and orbitozygomatic approaches, its main advantage is vantages are the intrinsic need to retract the temporal lobe better visualization of the perforating arteries of the thala- and the potential for injury to the third and fourth cranial mus (Fig. 6). It is also feasible to establish proximal and nerves during manipulation of the tentorium. distal control via the subtemporal approach. Its main disad- With the patient positioned laterally, a craniotomy is per-

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al triangle was named by Fukushima after its description by Kanzaki and colleagues.22–25 The cochlea, trigeminal groove, and horizontal intrapetrous ICA delimit this area of bone. It defines a shallow volume of petrous apex that can be resected with impunity because no structures are present to establish a surgical corridor into the posterior fossa from the middle fossa. The advantage of this approach is that most of the exposure is achieved through resection of extradural bone; hence, brain retraction is minimized. The middle fossa approach is a useful route for resection of intracanalicular vestibular schwannomas. This corridor was initially used to resect large tumors of this type.22 Later, the same approach was used to treat aneurysms of the mid- BA. Detailed morphometric analysis of the local bone anatomy (dorsum sellae, internal auditory canal, jugular tubercle) has been proposed to identify an aneurysm’s location and to determine the feasibility of using this approach for Fig. 3. Illustration showing the difference in working angle access to it.2 obtained when the pterional approach (light blue) is converted to an orbitozygomatic approach (dark blue). The net difference is 10˚, Transpetrosal Approach which provides a more shallow and wider exposure when the lateral wall and orbital roof are removed. (Reprinted with permission The retrolabyrinthine, translabyrinthine, and transcochle- from Barrow Neurological Institute.) ar approaches, which constitute the transpetrosal approaches, are typically used to treat lesions located in front of the brainstem, such as tumors or midbasilar aneurysms, following the principle of removing bone rather than retracting neural tissue. The difference between these approaches is formed in front of the ear. It is usually performed from the the gradually increasing amount of resection of the mastoid right side unless there are special circumstances such as the process. Progressive drilling of the petrous bone exposes projection of the aneurysm, compromise of the third cranial 9 the anterior aspect of the brainstem, minimizing retraction nerve, or right hemiparesis. Once the craniotomy is com- and leaving the surgeon closer to the target.18 pleted, the floor of the middle fossa must be flush with the Transpetrosal approaches are described in more detail zygomatic arch. All of the squamous bone is resected, and elsewhere in this issue. The patient is placed supine, with the temporal lobe is retracted (the temporal vein should be the head rotated to the contralateral side, parallel to the preserved if possible). The surgical corridor is established floor, and declined slightly. The head is held in three-pin perpendicular to the axis of the head (Fig. 7). A combined Mayfield fixation. A soft shoulder roll is placed under the subtemporal–orbitozygomatic approach can be obtained by ipsilateral shoulder to facilitate rotation. The head should performing a wide craniotomy and drilling the floor of the be flexed slightly. The shoulder is taped caudally to in- middle fossa, then the microscope can be swiveled back and crease the surgeon’s working room. If the patient’s habitus forth to obtain the visual advantages of each approach. Once precludes adequate depression of the shoulder, the park the tentorium is reached, it can be retracted with one suture bench position is used. The skin incision begins 1 cm ante- stitch and pulled or cut to obtain proximal control. The tem- rior to the ear and continues in a gentle curve around the porary clip is placed below the SCA because this area is rel- ear to just below the mastoid tip in a C-shaped fashion. atively free of circumflex perforating vessels. Exposing the The retrolabyrinthine approach only compromises the tentorium provides access to the fourth cranial nerve. The bone anterior to the sigmoid sinus and posterior to the semi- arachnoid is opened and then the Liliequist membrane is circular canals (Fig. 9). To preserve hearing, the latter are opened, exposing the SCAs and PCAs. This approach skeletonized but not opened. Ligation and division of the should be reserved for cases in which the CA is short, the sigmoid sinus provide additional exposure by opening a PCoA is dominant, and the BA bifurcation is below the dor- 12 37 wider angle between the cerebellum and petrous bone. sum sellae and above the internal auditory canal. The translabyrinthine approach involves drilling the entire Extended Middle Fossa or Kawase Approach otic capsule, providing a progressively more anterior view of the brainstem; however, hearing is sacrificed. An inter- The middle fossa approach was developed by House and mediate step is the partial labyrinthectomy, which opens on- Crabtree20 to treat lesions of the seventh and eighth cranial ly the common crus. The theoretical advantage in this ap- nerves in the internal auditory canal. proach is preservation of hearing.19,36 Conceptually, the floor of the middle fossa has been di- The transcochlear approach involves further resection of vided into two triangular areas, the posterolateral and pos- the petrous bone. The external auditory canal is closed. The teromedial triangles. The posterolateral triangle (Fig. 8) is otic capsule is drilled completely. The facial nerve is trans- the bone surface defined by the foramen spinosum, cochlea, located, and the cochlea is drilled. The chorda tympani and and mandibular nerve near its intersection with the greater the greater petrosal superficial nerve are cut. The transotic superficial petrosal nerve. This area was originally defined approach is a variant of the transcochlear, in which the fa- by Glasscock.13 It is a key landmark for exposure of the hor- cial nerve is not mobilized, and therefore is highly pre- izontal intrapetrous segment of the ICA. The posteromedi- served. The result is a wide avenue with the straightest pos-

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Fig. 4. Drawings showing the anatomy accessed via the orbitozygomatic approach. A: View from an orbitozygomatic approach once the carotid cistern, lamina terminalis, and sylvian cistern are wide open. An advantage of this approach is that the BA can be accessed through three different windows, depending on the patient’s local anatomy. The window through the opticocarotid triangle (arrow 1) is ideal when the CA is long and separated from the optic nerve. Arrow 2 designates the opening using the optic tract-A1-M1 interval. The window between the oculomotor nerve and the CA (arrow 3) (carotid–oculomotor triangle) is ideal. B: When the CA is long and separated from the optic nerve, a natural window at the opticocarotid triangle is opened. Note the narrow space between the CA and oculomotor nerve. The space between A1, M1, and the optic tract is reduced. C: Unlike in panel B, the supraclinoid CA is short and located near the optic nerve. The best window is between the CA and oculomotor nerve at the carotid–oculomotor triangle. Note the limited space available in the other two windows. D: Wide opening of the space between A1, M1, and optic tract. A. = artery; n. = nerve. (Panels A, B, and D from Wascher TM, Spetz- ler RF: Saccular aneurysms of the basilar bifurcation, in Carter LP, Spetzler RF, Hamilton MG (eds): Neurovascular Surgery. New York: McGraw-Hill, 1995, pp 729–752. Reprinted with permission from McGraw-Hill. Panel C reprinted with permission from Barrow Neuro- logical Institute.) sible trajectory (widest angle) to the pons and mid-BA. As of the transpetrosal approaches is limited to selected cases bone is resected more aggressively, the risk of complica- of large and giant midbasilar trunk aneurysms. Further- tions (primarily cerebrospinal fluid leaks, transitory or more, the angle of attack exposes the dome of the aneurysm definitive facial nerve paralysis, and deafness) increases. rather than its neck. The neck can be exposed using a con- The senior author’s preference is to obtain as much ex- tralateral approach, but more complex clip placement tech- posure as possible from the orbitozygomatic or retrosig- niques, including the use of fenestrated clips, become nec- moid approach. A far-lateral exposure is used to access the essary. Consequently, the potential for injury to perforating lower portions of the BA as much as possible. The mid- vessels increases. In contrast, the orbitozygomatic or far- basilar region is accessed from a retrosigmoid craniotomy lateral approach exposes the aneurysm’s neck in an almost when possible. Because of their associated morbidity, use parallel trajectory with the parent vessel.

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Fig. 6. Drawing depicting approach in which the subtemporal craniotomy is performed, and the dura mater and sylvian fissure are opened. The temporal lobe is retracted, showing the view of the perforating vessels. This view is the main advantage of this approach compared with the orbitozygomatic one. (From Wascher TM, Spetzler RF: Saccular aneurysms of the basilar bifurcation, in Carter LP, Spetzler RF, Hamilton MG (eds): Neurovascular Surgery. New York: McGraw-Hill, 1995, pp 729–752. Reprinted with permission from McGraw-Hill.)

Fig. 5. Drawing depicting the anatomy after the orbitozygomat- Fig. 7. Drawing showing the main difference between the ap- ic approach has been performed and the dura mater has been proach angle when an upper BA aneurysm is accessed from an or- opened; the carotid cistern and sylvian fissure are wide open. Re- bitozygomatic approach rather than from a subtemporal one. The moval of the anterior clinoid process exposes the clinoid segment wide craniotomy and flush drilling of the floor of the middle fossa of the ICA. The roof of the cavernous sinus is exposed. A: The combined with an orbitozygomatic osteotomy provide the advan- posterior clinoid process and upper aspect of the clivus obstruct the tages associated with each approach. (Reprinted with permission view to the BA. B: Complete visualization of the tip of the BA, from Barrow Neurological Institute.) including both PCAs and both SCAs after the anterior and posteri- or clinoid processes have been removed. This perspective offers the room to obtain proximal and distal control of the lesion before more millimeters of exposure (Fig. 10). This approach is dissection of the aneurysm is completed. CN III = oculomotor most often used during the resection of posterior fossa tu- nerve; PComA = PCoA. (From Ferreira MAT, Tedeschi H, Wen mors, in the rare cases of aneurysms involving the AICA HT, et al: Posterior circulation aneurysms: Guideline to manage- 14 ment. Oper Tech Neurosurg 3:169–178, 2000. Reprinted with where it emerges from the BA, or for midbasilar trunk an- permission from Elsevier.) eurysms. When a more anterior or inferior exposure is required to reach the lower two fifths of the BA, the far-lateral approach with its variants is the procedure of choice. Sometimes the retrosigmoid craniotomy is combined with Retrosigmoid Approach a far-lateral approach to obtain a wide corridor to the ante- The retrosigmoid craniotomy provides a narrow opening rior brainstem. with an angle that parallels the petrous bone. Exposure of Far-Lateral Approach the lateral or anterior medulla is limited. The craniotomy must be extended as laterally as possible. The sigmoid sin- The far-lateral approach and its variants consist of a low us is outlined so that it can be retracted to gain a couple suboccipital craniotomy and removal of part of the ipsilat-

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Fig. 8. Photographs of a cadaveric dissection showing the right middle fossa. A: The exposure shows the Kawase area (K) after the petrous apex has been drilled. This area represents the “door” to the posterior fossa from the middle fossa. B: Once the door to the posterior fossa has been opened, the midbasilar trunk is exposed. Note the close relationship between the AICA and abducent nerve (CN VI). CN

VII = facial nerve; FS = foramen spinosum; GG = geniculate ganglion; GSPN = greater superficial petrosal nerve; V1 = ophthalmic branch of the trigeminal nerve; V2 = maxillary branch of the trigeminal nerve; V3 = mandibular branch of the trigeminal nerve. eral posterior arch of C-1. This combination provides a si- myocutaneous flap. This type of incision, however, is as- multaneously oblique and inferior trajectory that facilitates sociated with a higher risk of injury to the VA at the inferi- exposure of the anterior rim of the foramen magnum, odon- or aspect of the incision. toid process, anterior and lateral aspects of the medulla, and The lateral mass of C-1 and the VA are exposed, from the the VA without the need to retract the cerebellum.16 This approach offers unique access to the anterior brainstem and provides excellent proximal control of the ipsilateral VA. Multiple variations of this approach have been described and summarized by Salas, et al.33 Exposure of the occipital condyle ranges from nonresection during the retrocondylar approach (without drilling the occipital condyle), to progressive removal of the condyle (transcondylar approach), to the extreme-lateral transjugular approach in which the exposure includes extradural resection of the jugular tubercle above the occipital condyle (supracondylar approach). In cases of a high-riding vertebrobasilar junction, the jugular tubercle must be exposed, usually in an extradural fashion, because this bone prominence blocks access to the midclival region. Rarely, this exposure is required to access a normal vertebrobasilar junction. The patient is placed laterally in a modified park bench position with the side to be treated facing up, and the head is placed in a three-pin Mayfield frame. The patient’s de- pendent arm is allowed to drop off the edge of the table and is carefully cradled in foam. A foam axillary roll is placed at the edge of the table. With the patient in this position, the ipsilateral mastoid process is the highest point in the operative field. The upper shoulder is taped, pulling it down- ward toward the feet and increasing the working space for the surgeon. An inverted hockey stick incision is started at the mastoid prominence and proceeds under the superior nuchal line to Fig. 9. Drawing showing presigmoid retrolabyrinthine craniec- the midline (Fig. 11). From there, it follows the midline tomy with complete skeletonization of the sigmoidal sinus, which can be transected if adequate drainage is present contralaterally on down to the C-3 or C-4 spinous process. A 1-cm border of the preoperative angiogram or after pressure within the sinus has nuchal fascia and muscle is left at the edge of the upper inci- been determined before and after its temporary occlusion. CN V = sion to permit anatomical closure of the wound. The myo- trigeminal nerve; CN VI = abducent nerve; CN VIII = vestibulo- cutaneous flap is retracted inferiorly and laterally with fish- cochlear nerve; CN IX = glossopharyngeal nerve; CN X = vagus hooks attached to a Leyla bar. Alternatively, a linear lateral nerve. (Reprinted with permission from Barrow Neurological Insti- incision can achieve the same exposure without a bulky tute.)

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Unauthenticated | Downloaded 10/01/21 09:32 PM UTC Skull base approaches to the basilar artery sulcus arteriosus of C-1 to its point of dural entry at the pos- stability of the craniovertebral junction is based on the inte- terior fossa (Fig. 12A). Bleeding from the venous plexus grity of articular capsules and ligaments. The ligaments in- surrounding the VA is controlled with small pieces of hemo- sert at the occipital condyle, especially the alar ligament. static material. A C-1 hemilaminectomy is performed medi- Therefore, progressive removal of the condyle39 (Ͼ 50%) al to the sulcus arteriosus (Fig. 12B). usually involves sectioning of the ligament and will cause A limited suboccipital retrosigmoid craniotomy is per- instability. Occipitocervical fusion then becomes necessary. formed (Fig. 13A). The opening extends from the margin Bleeding from condylar vessels can be readily con- of the foramen magnum just beyond midline as far lateral- trolled with bone wax and hemostatic agents. The jugular ly as possible and inferiorly to the foramen magnum just tubercle can be drilled extradurally because its presence medial to the level at which the VA enters the dura mater. obscures the vertebrobasilar junction (Fig. 14A). The height of the craniotomy can extend to the level of the The dura mater is opened in a curvilinear fashion with transverse sinus as needed (combined retrosigmoid–far lat- its base hinged laterally and pulled tight against the lateral eral approach), depending on the location and size of the aspect of the craniotomy with sutures (Fig. 14B). The ex- lesion. tensive extradural bone removal and dural retraction allow The lateral rim of the foramen magnum is removed with visualization of the anteroinferior brainstem without re- a bone rongeur or a pneumatic drill. The occipital condyle traction, as far rostrally as the pontomedullary junction and superior lateral mass and facet of C-1 are removed with (Fig. 14C). a high-speed drill (Fig. 13B). During drilling the surgeon must pay special attention to identify cortical bone first, followed by cancellous bone, and again by cortical bone as the hypoglossal canal is approached. Additional removal threatens the hypoglossal canal situated in the anterior medial third of the occipital condyle and may destabilize the condylar joint. Progressive drilling of the occipital condyle widens the surgical exposure, increases anterior access to the brainstem, and increases the angle of exposure by 30 and 40% if the occipital condyle is resected 25 or 50%, respectively, compared with a retrocondylar exposure.40 The

Fig. 11. Illustration showing two different types of skin inci- sions. A: A straight-line incision minimizes operative time but it is harder to maintain orientation during the procedure. The VA can be accessed quickly but is at risk during dissection of the muscle. B: A hockey stick incision typically includes a short limb that starts Fig. 10. Drawing showing a retrosigmoid craniotomy with at the mastoid process and a longer limb that extends toward the complete exposure of the sigmoid sinus. Opening the dura mater as midline up to C-3 or C-4. (From Baldwin HZ, Miller CG, van close as possible to the sinus allows it to be pulled and the sinus Loveren HR, et al: The far lateral/combined supra- and infratento- to be retracted to maximize use of this approach. The BA is locat- rial approach. A human cadaveric prosection model for routes of ed deep at the bottom of the approach. (Reprinted with permission access to the petroclival region and ventral brain stem. J Neuro- from Barrow Neurological Institute.) surg 81:60–68, 1994.)

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Fig. 12. A: Illustration showing the VA at its horizontal portion and its dural entrance. Between C-1 and C-2, the VA is under the root of C-2. During surgery this landmark can be used to local- ize the artery. A pneumatic drill is used to perform the laminecto- my. B: Once the posterior arch of C-1 is removed, the VA is exposed. If further exposure is required, the foramen transversari- um of C-1 can be drilled with a diamond drill. The VA is then mobilized medially to provide access to the occipito–C1 joint. PCEV = posterior condylar emissary vein. (From Baldwin HZ, Miller CG, van Loveren HR, et al: The far lateral/combined supra- and infratentorial approach. A human cadaveric prosection model for routes of access to the petroclival region and ventral brain stem. J Neurosurg 81:60–68, 1994.) Fig. 13. A: Illustration of the exposure obtained through the far-lateral retrocondylar approach. The VA is exposed from the occipital condyle to its entrance into the dura mater of the posteri- or fossa. To begin the craniotomy, the drill is used with a foot plate. B: Illustration showing the craniotomy, including removal of the ipsilateral posterior arch of C-1 and exposure of the dura mater. Combined Supratentorial and Infratentorial Approach The VA is protected while the occipital condyle is drilled. When The exposure of the BA provided by the transpetrosal the occipital condyle is drilled, cortical bone is encountered first, approach can be dramatically enhanced when combined followed by cancellous bone, and then cortical bone again as the hypoglossal canal is approached. (From Baldwin HZ, Miller CG, with a supratentorial approach. This combined supra- and van Loveren HR, et al: The far lateral/combined supra- and infra- infratentorial surgical approach can provide exposure that tentorial approach. A human cadaveric prosection model for routes extends from the petrous ridge and cavernous sinus to the of access to the petroclival region and ventral brain stem. J Neuro- foramen magnum. The key elements needed for this ap- surg 81:60–68, 1994.) proach are variable amounts of mastoidectomy in conjunc- tion with a supra- and infratentorial craniotomy. Finally, the tentorium is divided to connect the supra- and infratentori- tion. Variations of this combined approach have been de- al compartments. These maneuvers allow extensive expo- scribed for many years.17,30,32 sure of the clivus, medial petrous region, and associated A number of authors have presented their experience neural and vascular structures with minimal brain retrac- with variations of the combined infratentorial and posterior

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Fig. 14. A: Illustration showing the extensive drilling of the occipital condyle that leads to extradural exposure of the jugular tubercle. Sometimes the jugular tubercle shadows the visualization of the vertebrobasilar junction or is in the way when attempting to obtain distal vascular control during dissection of the aneurysm. B: Illustration showing the surgical exposure once the dura mater has been opened. The in- tradural VA is visible, and the dura is retracted with stitches in the soft tissues. C: Magnification of panel B showing that the PICA can be followed from its emergence from the VA to its lateral and tonsillar segments. Note the exposure obtained from the anterior and lateral aspects of the cervicomedullary junction and the proximity of the spinal accessory nerve (CN XI) to the dura mater. The hypoglossal nerve (CN XII) is shown in multiple fascicles. CN IX = glossopharyngeal nerve; CN X = vagus nerve. (From Baldwin HZ, Miller CG, van Loveren HR, et al: The far lateral/combined supra- and infratentorial approach. A human cadaveric prosection model for routes of access to the petroclival region and ventral brain stem. J Neurosurg 81:60–68, 1994.)

fossa approach for the removal of large lesions involving require expertise in vascular and skull base surgery for their the clivus and medial petrous region.1,34,38 Some have advo- treatment. The choice of approach is based on the specific cated preserving the major dural sinus. Others, with appro- features of each aneurysm. Multiple trajectories and angles priate consideration for the patency of the opposite trans- of approach are available and should be tailored on a case- verse sinus, often sacrifice the sigmoid or transverse sinus. by-case basis. The preferred method is to work with an an- Sacrifice of the sigmoid sinus can be considered after ob- gle that runs parallel to the parent vessel (that is, orbitozy- taining angiographic verification of the patency of the con- gomatic and far-lateral approaches). The transpetrosal and tralateral transverse and sigmoid sinuses. These structures retrosigmoid approaches are limited to selected cases. must communicate with the sagittal sinus and the ipsilater- The traditional principles of skull base surgery in terms al transverse sinus through a patent torcular herophili. To of maximizing bone resection to minimize brain retraction further ensure that the sigmoid sinus can be sacrificed safe- and to facilitate instrument manipulation apply. In aneu- ly, intravascular pressure within the proximal sinus can be rysm surgery, the ultimate goal is to clip the aneurysm and measured before and after its temporary occlusion. The to exclude it from the circulation. If these goals cannot be measurement is performed after ligation of the superior pet- achieved, the aneurysm can be trapped or the flow dynam- rosal sinus by inserting a 25-gauge needle into the sigmoid ics changed. It is crucial to identify preoperatively whether sinus just proximal to the temporary clip location. If pres- revascularization is indicated. sure rises more than 10 mm Hg during temporary sinus oc- Hypothermic cardiac arrest facilitates dissection of an clusion, the structure should not be divided. This approach aneurysm from perforating arteries. The aneurysm is de- is rarely used during the treatment of BA aneurysms.1,34,38 flated and transmural pressure is minimal. The technique eliminates the risk of rupture and the hypothermia induced Far-Lateral Combined Supra- and Infratentorial Approach with barbiturate therapy protects the brain. Nevertheless, (Combined–Combined) this complex technique is associated with high mortality Occasionally, an extensive lateral exposure of the clivus and morbidity rates and should be reserved for selected pa- is required to treat giant aneurysms of the midbasilar or tients treated only at specialized centers. lower BA. Such an exposure can be achieved by combining the far-lateral and combined supra- and infratentorial ap- References 3,4,27 proaches. This so-called combined–combined approach 1. Al-Mefty O, Fox JL, Rifai A, et al: A combined infratemporal uses the same principles previously outlined for each indivi- and posterior fossa approach for the removal of giant glomus dual procedure. tumors and chondrosarcomas. Surg Neurol 28:423–431, 1987 The anterior and lateral aspect of the brainstem can be 2. Aziz KM, van Loveren HR, Tew JM Jr, et al: The Kawase ap- exposed with this complicated technique. This approach is proach to retrosellar and upper clival basilar aneurysms. Neu- rarely used to treat BA aneurysms; the technique has been rosurgery 44:1225–1234, 1999 detailed elsewhere.3,4,26 The combined approaches are ex- 3. Baldwin HZ, Miller CG, van Loveren HR, et al: The far later- plained in detail elsewhere in this issue. al/combined supra- and infratentorial approach. A human cadaveric prosection model for routes of access to the petroclival region and ventral brain stem. J Neurosurg 81:60–68, 1994 CONCLUSIONS 4. Baldwin HZ, Spetzler RF, Wascher TM, et al: The far lateral- combined supra- and infratentorial approach: clinical experi- Posterior fossa aneurysms are challenging lesions that ence. Acta Neurochir (Wien) 134:155–158, 1995

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5. Dolenc VV, Skrap M, Sustersic J, et al: A transcavernous-trans- for aneurysms of the lower basilar artery. J Neurosurg 63: sellar approach to the basilar tip aneurysms. Br J Neurosurg 1: 857–861, 1985 251–259, 1987 26. Lawton MT, Daspit CP, Spetzler RF: Technical aspects and 6. Doutremepuich C: Haemostasis defects following cardio-pul- recent trends in the management of large and giant midbasilar monary by-pass based on a study of 1350 patients. Thromb artery aneurysms. Neurosurgery 41:513–520, 1997 Haemost 39:539–541, 1978 27. Lawton MT, Daspit CP, Spetzler RF: Transpetrosal and combi- 7. Drake CG: Further experience with surgical treatment of aneu- nation approaches to skull base lesions. Clin Neurosurg 43: rysm of the basilar artery. J Neurosurg 29:372–392, 1968 91–112, 1996 8. Drake CG, Peerless SJ: Giant fusiform intracranial aneurysms: 28. Lawton MT, Hamilton MG, Morcos JJ, et al: Revascularization review of 120 patients treated surgically from 1965 to 1992. J and aneurysm surgery: current techniques, indications, and out- Neurosurg 87:141–162, 1997 come. Neurosurgery 38:83–92, 1996 9. Drake CG, Peerless SJ, Hernesniemi JA: Surgery of Vertebro- 29. Lawton MT, Spetzler RF: Surgical strategies for giant intracra- basilar Aneurysms: London, Ontario, Experience on 1767 nial aneurysms. Neurosurg Clin N Am 9:725–742, 1998 Patients. Wien: Springer-Verlag, 1996 30. Malis LI: Surgical resection of tumors of the skull base, in Wil- 10. Evans JJ, Sekhar LN, Rak R, et al: Bypass grafting and revas- kins RH, Rengachary SS (eds): Neurosurgery. New York: Mc- cularization in the management of posterior circulation aneu- Graw-Hill, 1985, pp 1011–1021 rysms. Neurosurgery 55:1036–1049, 2004 31. Mizoi K, Yoshimoto T, Takahashi A, et al: Direct clipping of 11. Ferreira MAT, Tedeschi H, Wen HT, et al: Posterior circulation basilar trunk aneurysms using temporary balloon occlusion. J aneurysms: guideline to management. Oper Tech Neurosurg Neurosurg 80:230–236, 1994 3:169–178, 2000 32. Morrison AW, King TT: Experiences with a translabyrinthine- 12. Giannotta SL, Maceri DR: Retrolabyrinthine transsigmoid transtentorial approach to the cerebellopontine angle. Technical approach to basilar trunk and vertebrobasilar artery junction an- note. J Neurosurg 38:382–390, 1973 eurysms. Technical note. J Neurosurg 69:461–466, 1988 33. Salas E, Sekhar LN, Ziyal IM, et al: Variations of the extreme- 13. Glasscock ME III: Middle fossa approach to the temporal bone. lateral craniocervical approach: anatomical study and clinical An otologic frontier. Arch Otolaryngol 90:15–27, 1969 analysis of 69 patients. J Neurosurg 90:206–219, 1999 14. Gonzalez LF, Alexander MJ, McDougall CG, et al: Anteroin- 34. Samii M, Ammirati M, Mahran A, et al: Surgery of petroclival ferior cerebellar artery aneurysms: surgical approaches and out- meningiomas: report of 24 cases. Neurosurgery 24:12–17, 1989 comes—a review of 34 cases. Neurosurgery 55:1025–1035, 35. Sato S, Sato M, Oizumi T, et al: Removal of anterior clinoid 2004 process for basilar tip aneurysm: clinical and cadaveric analy- 15. Gonzalez LF, Crawford NR, Horgan MA, et al: Working area sis. Neurol Res 23:298–303, 2001 and angle of attack in three cranial base approaches: pterional, 36. Sekhar LN, Schessel DA, Bucur SD, et al: Partial labyrinthec- orbitozygomatic, and maxillary extension of the orbitozygo- tomy petrous apicectomy approach to neoplastic and vascular matic approach. Neurosurgery 50:550–555, 2002 lesions of the petroclival area. Neurosurgery 44:537–550, 16. Heros RC, Debrun GM, Ojemann RG, et al: Direct spinal arte- 1999 riovenous fistula: a new type of spinal AVM. Case report. J Neurosurg 64:134–139, 1986 37. Seoane E, Tedeschi H, de Oliveira E, et al: The pretemporal tran- 17. Hitselberger WE, House WF: A combined approach to the cere- scavernous approach to the interpeduncular and prepontine cis- bellopontine angle. A suboccipital-petrosal approach. Arch terns: microsurgical anatomy and technique application. Neuro- Otolaryngol 84:267–285, 1966 surgery 46:891–899, 2000 18. Horgan MA, Anderson GJ, Kellogg JX, et al: Classification and 38. Spetzler RF, Daspit CP, Pappas CT: The combined supra-and in- quantification of the petrosal approach to the petroclival region. fratentorial approach for lesions of the petrous and clival regions: J Neurosurg 93:108–112, 2000 experience with 46 cases. J Neurosurg 76:588–599, 1992 19. Horgan MA, Delashaw JB, Schwartz MS, et al: Transcrusal ap- 39. Vishteh AG, Crawford NR, Melton MS, et al: Stability of the proach to the petroclival region with hearing preservation. Tech- craniovertebral junction after unilateral occipital condyle resec- nical note and illustrative cases. J Neurosurg 94:660–666, 2001 tion: a biomechanical study. J Neurosurg 90:91–98, 1999 20. House WF, Crabtree JA: Surgical exposure of petrous portion 40. Wanebo JE, Chicoine MR: Quantitative analysis of the trans- of 7th nerve. Arch Otolaryngol 81:506–507, 1965 condylar approach to the foramen magnum. Neurosurgery 49: 21. Hsu FPK, Clatterbuck RE, Spetzler RF: Orbitozygomatic ap- 934–941, 2001 proach to basilar apex aneurysms. Neurosurgery 56:172–177, 41. Ya¸sargil MG: Microneurosurgery. Clinical Considerations, 2005 Surgery of the Intracranial Aneurysms, and Results. Stutt- 22. Kanzaki J, Kawase T, Sano K, et al: A modified extended mid- gart: Georg Thieme Verlag, 1984, dle cranial fossa approach for acoustic tumors. Arch Otorhino- 42. Zabramski JM, Kiris T, Sankhla SK, et al: Orbitozygomatic laryngol 217:119–121, 1977 craniotomy. Technical note. J Neurosurg 89:336–341, 1998 23. Kawase T, Bertalanffy H, Otani M, et al: Surgical approaches for vertebro-basilar trunk aneurysms located in the midline. Ac- ta Neurochir (Wien) 138:402–410, 1996 24. Kawase T, Shiobara R, Toya S: Anterior transpetrosal-transten- Manuscript received June 20, 2005. torial approach for sphenopetroclival meningiomas: surgical Accepted in final form July 15, 2005. method and results in 10 patients. Neurosurgery 28:869–875, Address reprint requests to: Robert F. Spetzler, M.D., Neurosci- 1991 ence Publications, Barrow Neurological Institute, 350 West Thomas 25. Kawase T, Toya S, Shiobara R, et al: Transpetrosal approach Road, Phoenix, Arizona 85013. email: [email protected].

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