Download Skull base approaches to the basilar artery

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
The BA begins at the juncture of the VAs at the pontomedullary sulcus. It ascends anteriorly to the brainstem behind the clivus and terminates when it bifurcates at the interpeduncular cistern. There it gives rise to both PCAs at
the pontomesencephalic junction.
The BA gives rise to the arteries that supply the posterior fossa. The SCA, which originates at the pontomesencephalic sulcus, encircles the midbrain to supply the cerebral
peduncles before it courses superiorly and medially to supply the tentorial surface of the cerebellum. The AICA
originates near the pontomedullary sulcus and courses
around the pons, typically below or between the fascicles
of the abducent nerve, to the cerebellopontine angle. There,
it passes between or around the seventh and eighth cranial
nerves, which course toward the acoustic meatus. The
AICA sends branches to these nerves and to the choroid
plexus before it courses posterolaterally to supply the petrosal surface of the cerebellum.
The length of the BA varies, which determines the position of the bifurcation. It can extend as rostrally as the
Abbreviations used in this paper: AICA = anterior inferior cerebellar artery; BA = basilar artery; CA = carotid artery; ICA = internal carotid artery; PCA = posterior cerebral artery; PCoA = posterior communicating artery; PICA = posterior inferior cerebellar
artery; SCA = superior cerebellar artery; STA = superior temporal
artery; VA = vertebral artery.
Neurosurg. Focus / Volume 19 / August, 2005
mammillary bodies and the floor of the third ventricle, or it
can be short, with a “low” bifurcation below the pontomesencephalic junction. The BA also can vary in the coronal
plane. It can be straight or tortuous with extreme loops on
both sides. In addition to this complex intrinsic vascular
anatomy, its relationships to bone at each level help determine the suitability of a given approach within the armamentarium to treat these complex lesions.
Conceptually, the BA can be divided into fifths. At its
upper two fifths, the main osseous structures that can obscure its visualization are the anterior and posterior clinoid
processes and the dorsum sellae (upper clivus). Usually, aneurysms at this level can be reached via an orbitozygomatic approach or a modification thereof. Occasionally, aneurysms are located higher than the petrous edge and lower
than the floor of the sella. In such cases a lateral exposure
via a subtemporal approach is indicated. The main structures that help decide the approach to midbasilar (middle
fifth of the BA) aneurysms are the free edge of the petrous
bone and the relationship between the aneurysm and internal auditory canal. The mid-BA can be reached via one of
the transpetrosal approaches or via an extended middle fossa approach. Sometimes a retrosigmoid approach is adequate. If the aneurysm is located at the bottom two fifths of
the artery (lower BA), the main bone structures of concern
are the jugular tubercle and the occipital condyle. Aneurysms at this level can usually be reached via a far-lateral
approach27 or its variants.
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L. F. Gonzalez, et al.
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 similar 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 depends 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. (Reprinted with permission from Barrow Neurological Institute.)
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Skull base approaches to the basilar artery
associated with the general risks of any vascular surgery in
the posterior circulation, in addition to the more specific
risks of graft failure and secondary ischemia.
Vascular Control
During aneurysm surgery, adequate proximal and distal
control must be established. Doing so is particularly challenging in the posterior circulation. For the upper third of
the BA, proximal control is obtained by placing a temporary clip below the point where the SCAs emerge. Usually
this maneuver softens the aneurysm enough to proceed with
the dissection. Typically, adequate control is obtained with
a temporary clip on the BA, and it is unnecessary to occlude
the PCA or SCA. Alternatives to placement of a temporary
clip to obtain adequate proximal control of the upper third
of the BA are endovascular balloon occlusion and hypothermic circulatory arrest. Balloon occlusion provides proximal control before dissection of the aneurysm begins, without the inconvenience of a temporary clip that can obstruct
visualization of the lesion in a narrow corridor.31
Circulatory arrest collapses the aneurysm, making its dissection from the brainstem feasible. A large, firm sac is
converted into a soft, pliable one that allows visualization
of the critical anatomy. This procedure makes it easier to
differentiate the neck of the aneurysm from adjacent perforating vessels. It eliminates the necessity of obtaining control, which is difficult in the midbasilar trunk. It also facilitates manipulation of the aneurysm clip around the neck of
the lesion.26 Furthermore, hypothermia and barbiturate therapy provide cerebral protection.29
The main disadvantages of circulatory arrest are related
to the coagulopathy caused by heparinization and platelet
trauma when the patient is on the bypass pump.6 Circulatory arrest itself is associated with a risk of cerebral ischemia, which hypothermia and barbiturate drugs counteract
to some degree. Meticulous closure is necessary to prevent
postoperative hematomas.
The implementation of cardiac standstill is complex. It
requires the close cooperation of an organized team, including cardiac surgeons, and the expertise of both cardiac and
neuroanesthesia personnel. Therefore, the technique must
be used judiciously only in selected cases. In the senior author’s experience with more than 50 procedures in which
hypothermic cardiac arrest was used, the mortality rate was
almost 10%. This rate reflects the complex premorbid status of the patients more than use of the technique itself.26
For lower BA aneurysms, proximal control is easily obtained by placing a temporary clip across the VA. Distal
control is harder to obtain, even when endovascular techniques are used.29
SURGICAL APPROACHES
Orbitozygomatic Approach
The pterional approach is a traditional and versatile one
that allows access to aneurysms in the anterior circulation.
This is the basis for the orbitozygomatic approach (Fig. 2),
which combines the pterional approach with different osteotomies that remove the superior and lateral walls of the ipsilateral orbit and zygomatic arch. This extra bone resection is
grounded in the skull base surgical principle of maximizing
bone resection to minimize brain retraction (Fig. 3).15
Neurosurg. Focus / Volume 19 / August, 2005
Yaşargil popularized the use of the pterional approach for
the treatment of anterior and posterior circulation aneurysms. The main advantages of this approach are the ability to attack the aneurysm from its neck, with a good perspective on the interpeduncular cistern and the ability to
establish different surgical corridors. These corridors are
established based on the local anatomy: through the opticocarotid triangle, laterally between the ICA and third cranial
nerve, and the window between the A1, M1, and optic tract
can be used as well (Fig. 4). A wide opening of the sylvian
fissure limits the need to retract the brain.21
The relationship between the aneurysm and upper clivus
(posterior clinoid process) is key. Aneurysms located within 5 mm of the dorsum sellae are considered normal. If they
occur higher or lower than that point, they are considered
high-lying or low-lying, respectively.11
To reach normal or high-lying aneurysms, we prefer the
orbitozygomatic approach, which allows good exposure of
the interpeduncular and prepontine cisterns. There are three
osseous obstacles to the BA apex: the anterior clinoid process, posterior clinoid process, and dorsum sellae.29 Resection of these three obstacles makes the extended orbitozygomatic approach especially useful for low-lying aneurysms.
Preoperative preparation includes placing a femoral
sheath for intraoperative angiography and an electrode to
record the electroencephalographic and somatosensory
evoked potential responses. The patient’s head is placed in
a radiolucent Mayfield headholder, slightly extended, and
rotated 10 to 15˚ to the contralateral side. Specific details
about the approach are reported elsewhere.42 The carotid
cistern is opened wide, and the PCoA is followed posteriorly to the junction between P1 and P2. The Liliequist membrane is then opened. The exposure should allow the identification of both PCAs and both SCAs. Sometimes the
lamina terminalis is opened.Decreasing the degree of rotation of the head helps to bring the contralateral side into
view. Sometimes it is necessary to sacrifice the PCoA to
create more space21 to ease the manipulation of instruments
and the placement of temporary and permanent aneurysm
clips.
Pretemporal Transcavernous Approach
A variation of the orbitozygomatic approach includes an
extension into the upper wall of the cavernous sinus to increase access to the anterior incisura. This modification was
initially described by Dolenc, et al.5 It has been used for
low-lying aneurysms and facilitates access to the interpeduncular cistern from the carotid–oculomotor triangle. Seoane, et al.,37 illustrated this procedure, which consists of an
orbitozygomatic approach with extradural drilling of the
anterior clinoid process (Fig. 5A).35 The clinoid segment of
the ICA is exposed and released from its distal and proximal dural rings. Once the ICA is free, the posterior clinoid
process (Fig. 5B)41 is drilled and the upper clivus (dorsum
sellae) is removed. The main disadvantage of this approach
is the almost inevitable postoperative palsy of the third cranial nerve.11
Subtemporal Approach
This approach was popularized by Drake,7 who has published many papers relating his vast experience with treating posterior fossa aneurysms. Compared with the pterion3
L. F. Gonzalez, et al.
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
better visualization of the perforating arteries of the thalamus (Fig. 6). It is also feasible to establish proximal and
distal control via the subtemporal approach. Its main disad4
vantages are the intrinsic need to retract the temporal lobe
and the potential for injury to the third and fourth cranial
nerves during manipulation of the tentorium.
With the patient positioned laterally, a craniotomy is perNeurosurg. Focus / Volume 19 / August, 2005
Skull base approaches to the basilar artery
Fig. 3. Illustration showing the difference in working angle
obtained when the pterional approach (light blue) is converted to an
orbitozygomatic approach (dark blue). The net difference is 10˚,
which provides a more shallow and wider exposure when the lateral wall and orbital roof are removed. (Reprinted with permission
from Barrow Neurological Institute.)
formed in front of the ear. It is usually performed from the
right side unless there are special circumstances such as the
projection of the aneurysm, compromise of the third cranial
nerve, or right hemiparesis.9 Once the craniotomy is completed, the floor of the middle fossa must be flush with the
zygomatic arch. All of the squamous bone is resected, and
the temporal lobe is retracted (the temporal vein should be
preserved if possible). The surgical corridor is established
perpendicular to the axis of the head (Fig. 7). A combined
subtemporal–orbitozygomatic approach can be obtained by
performing a wide craniotomy and drilling the floor of the
middle fossa, then the microscope can be swiveled back and
forth to obtain the visual advantages of each approach. Once
the tentorium is reached, it can be retracted with one suture
stitch and pulled or cut to obtain proximal control. The temporary clip is placed below the SCA because this area is relatively free of circumflex perforating vessels. Exposing the
tentorium provides access to the fourth cranial nerve. The
arachnoid is opened and then the Liliequist membrane is
opened, exposing the SCAs and PCAs. This approach
should be reserved for cases in which the CA is short, the
PCoA is dominant, and the BA bifurcation is below the dorsum sellae and above the internal auditory canal.37
Extended Middle Fossa or Kawase Approach
The middle fossa approach was developed by House and
Crabtree20 to treat lesions of the seventh and eighth cranial
nerves in the internal auditory canal.
Conceptually, the floor of the middle fossa has been divided into two triangular areas, the posterolateral and posteromedial triangles. The posterolateral triangle (Fig. 8) is
the bone surface defined by the foramen spinosum, cochlea,
and mandibular nerve near its intersection with the greater
superficial petrosal nerve. This area was originally defined
by Glasscock.13 It is a key landmark for exposure of the horizontal intrapetrous segment of the ICA. The posteromediNeurosurg. Focus / Volume 19 / August, 2005
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 midBA. 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
access to it.2
Transpetrosal Approach
The retrolabyrinthine, translabyrinthine, and transcochlear 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
the gradually increasing amount of resection of the mastoid
process. Progressive drilling of the petrous bone exposes
the anterior aspect of the brainstem, minimizing retraction
and leaving the surgeon closer to the target.18
Transpetrosal approaches are described in more detail
elsewhere in this issue. The patient is placed supine, with
the head rotated to the contralateral side, parallel to the
floor, and declined slightly. The head is held in three-pin
Mayfield fixation. A soft shoulder roll is placed under the
ipsilateral shoulder to facilitate rotation. The head should
be flexed slightly. The shoulder is taped caudally to increase the surgeon’s working room. If the patient’s habitus
precludes adequate depression of the shoulder, the park
bench position is used. The skin incision begins 1 cm anterior to the ear and continues in a gentle curve around the
ear to just below the mastoid tip in a C-shaped fashion.
The retrolabyrinthine approach only compromises the
bone anterior to the sigmoid sinus and posterior to the semicircular canals (Fig. 9). To preserve hearing, the latter are
skeletonized but not opened. Ligation and division of the
sigmoid sinus provide additional exposure by opening a
wider angle between the cerebellum and petrous bone.12
The translabyrinthine approach involves drilling the entire
otic capsule, providing a progressively more anterior view
of the brainstem; however, hearing is sacrificed. An intermediate step is the partial labyrinthectomy, which opens only the common crus. The theoretical advantage in this approach is preservation of hearing.19,36
The transcochlear approach involves further resection of
the petrous bone. The external auditory canal is closed. The
otic capsule is drilled completely. The facial nerve is translocated, and the cochlea is drilled. The chorda tympani and
the greater petrosal superficial nerve are cut. The transotic
approach is a variant of the transcochlear, in which the facial nerve is not mobilized, and therefore is highly preserved. The result is a wide avenue with the straightest pos5
L. F. Gonzalez, et al.
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, 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. Panel C reprinted with permission from Barrow Neurological Institute.)
sible trajectory (widest angle) to the pons and mid-BA. As
bone is resected more aggressively, the risk of complications (primarily cerebrospinal fluid leaks, transitory or
definitive facial nerve paralysis, and deafness) increases.
The senior author’s preference is to obtain as much exposure as possible from the orbitozygomatic or retrosigmoid approach. A far-lateral exposure is used to access the
lower portions of the BA as much as possible. The midbasilar region is accessed from a retrosigmoid craniotomy
when possible. Because of their associated morbidity, use
6
of the transpetrosal approaches is limited to selected cases
of large and giant midbasilar trunk aneurysms. Furthermore, the angle of attack exposes the dome of the aneurysm
rather than its neck. The neck can be exposed using a contralateral approach, but more complex clip placement techniques, including the use of fenestrated clips, become necessary. Consequently, the potential for injury to perforating
vessels increases. In contrast, the orbitozygomatic or farlateral approach exposes the aneurysm’s neck in an almost
parallel trajectory with the parent vessel.
Neurosurg. Focus / Volume 19 / August, 2005
Skull base approaches to the basilar artery
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 orbitozygomatic approach has been performed and the dura mater has been
opened; the carotid cistern and sylvian fissure are wide open. Removal of the anterior clinoid process exposes the clinoid segment
of the ICA. The roof of the cavernous sinus is exposed. A: The
posterior clinoid process and upper aspect of the clivus obstruct the
view to the BA. B: Complete visualization of the tip of the BA,
including both PCAs and both SCAs after the anterior and posterior clinoid processes have been removed. This perspective offers
the room to obtain proximal and distal control of the lesion before
dissection of the aneurysm is completed. CN III = oculomotor
nerve; PComA = PCoA. (From Ferreira MAT, Tedeschi H, Wen
HT, et al: Posterior circulation aneurysms: Guideline to management. Oper Tech Neurosurg 3:169–178, 2000. Reprinted with
permission from Elsevier.)
Retrosigmoid Approach
The retrosigmoid craniotomy provides a narrow opening
with an angle that parallels the petrous bone. Exposure of
the lateral or anterior medulla is limited. The craniotomy
must be extended as laterally as possible. The sigmoid sinus is outlined so that it can be retracted to gain a couple
Neurosurg. Focus / Volume 19 / August, 2005
Fig. 7. Drawing showing the main difference between the approach angle when an upper BA aneurysm is accessed from an orbitozygomatic approach rather than from a subtemporal one. The
wide craniotomy and flush drilling of the floor of the middle fossa
combined with an orbitozygomatic osteotomy provide the advantages associated with each approach. (Reprinted with permission
from Barrow Neurological Institute.)
more millimeters of exposure (Fig. 10). This approach is
most often used during the resection of posterior fossa tumors, in the rare cases of aneurysms involving the AICA
where it emerges from the BA,14 or for midbasilar trunk aneurysms. 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
a far-lateral approach to obtain a wide corridor to the anterior brainstem.
Far-Lateral Approach
The far-lateral approach and its variants consist of a low
suboccipital craniotomy and removal of part of the ipsilat7
L. F. Gonzalez, et al.
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 simultaneously oblique and inferior trajectory that facilitates
exposure of the anterior rim of the foramen magnum, odontoid process, anterior and lateral aspects of the medulla, and
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 dependent 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 downward 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
the midline (Fig. 11). From there, it follows the midline
down to the C-3 or C-4 spinous process. A 1-cm border of
nuchal fascia and muscle is left at the edge of the upper incision to permit anatomical closure of the wound. The myocutaneous flap is retracted inferiorly and laterally with fishhooks attached to a Leyla bar. Alternatively, a linear lateral
incision can achieve the same exposure without a bulky
8
myocutaneous flap. This type of incision, however, is associated with a higher risk of injury to the VA at the inferior aspect of the incision.
The lateral mass of C-1 and the VA are exposed, from the
Fig. 9. Drawing showing presigmoid retrolabyrinthine craniectomy with complete skeletonization of the sigmoidal sinus, which
can be transected if adequate drainage is present contralaterally on
the preoperative angiogram or after pressure within the sinus has
been determined before and after its temporary occlusion. CN V =
trigeminal nerve; CN VI = abducent nerve; CN VIII = vestibulocochlear nerve; CN IX = glossopharyngeal nerve; CN X = vagus
nerve. (Reprinted with permission from Barrow Neurological Institute.)
Neurosurg. Focus / Volume 19 / August, 2005
Skull base approaches to the basilar artery
sulcus arteriosus of C-1 to its point of dural entry at the posterior fossa (Fig. 12A). Bleeding from the venous plexus
surrounding the VA is controlled with small pieces of hemostatic material. A C-1 hemilaminectomy is performed medial to the sulcus arteriosus (Fig. 12B).
A limited suboccipital retrosigmoid craniotomy is performed (Fig. 13A). The opening extends from the margin
of the foramen magnum just beyond midline as far laterally as possible and inferiorly to the foramen magnum just
medial to the level at which the VA enters the dura mater.
The height of the craniotomy can extend to the level of the
transverse sinus as needed (combined retrosigmoid–far lateral approach), depending on the location and size of the
lesion.
The lateral rim of the foramen magnum is removed with
a bone rongeur or a pneumatic drill. The occipital condyle
and superior lateral mass and facet of C-1 are removed with
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. 10. Drawing showing a retrosigmoid craniotomy with
complete exposure of the sigmoid sinus. Opening the dura mater as
close as possible to the sinus allows it to be pulled and the sinus
to be retracted to maximize use of this approach. The BA is located deep at the bottom of the approach. (Reprinted with permission
from Barrow Neurological Institute.)
Neurosurg. Focus / Volume 19 / August, 2005
stability of the craniovertebral junction is based on the integrity of articular capsules and ligaments. The ligaments insert at the occipital condyle, especially the alar ligament.
Therefore, progressive removal of the condyle39 (. 50%)
usually involves sectioning of the ligament and will cause
instability. Occipitocervical fusion then becomes necessary.
Bleeding from condylar vessels can be readily controlled with bone wax and hemostatic agents. The jugular
tubercle can be drilled extradurally because its presence
obscures the vertebrobasilar junction (Fig. 14A).
The dura mater is opened in a curvilinear fashion with
its base hinged laterally and pulled tight against the lateral
aspect of the craniotomy with sutures (Fig. 14B). The extensive extradural bone removal and dural retraction allow
visualization of the anteroinferior brainstem without retraction, as far rostrally as the pontomedullary junction
(Fig. 14C).
Fig. 11. Illustration showing two different types of skin incisions. 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
at the mastoid process and a longer limb that extends toward the
midline up to C-3 or C-4. (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.)
9
L. F. Gonzalez, et al.
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 localize the artery. A pneumatic drill is used to perform the laminectomy. B: Once the posterior arch of C-1 is removed, the VA is
exposed. If further exposure is required, the foramen transversarium 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 supraand 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.)
Combined Supratentorial and Infratentorial Approach
The exposure of the BA provided by the transpetrosal
approach can be dramatically enhanced when combined
with a supratentorial approach. This combined supra- and
infratentorial surgical approach can provide exposure that
extends from the petrous ridge and cavernous sinus to the
foramen magnum. The key elements needed for this approach are variable amounts of mastoidectomy in conjunction with a supra- and infratentorial craniotomy. Finally, the
tentorium is divided to connect the supra- and infratentorial compartments. These maneuvers allow extensive exposure of the clivus, medial petrous region, and associated
neural and vascular structures with minimal brain retrac10
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 posterior 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.
The VA is protected while the occipital condyle is drilled. When
the occipital condyle is drilled, cortical bone is encountered first,
followed by cancellous bone, and then cortical bone again as the
hypoglossal canal is approached. (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.)
tion. Variations of this combined approach have been described for many years.17,30,32
A number of authors have presented their experience
with variations of the combined infratentorial and posterior
Neurosurg. Focus / Volume 19 / August, 2005
Skull base approaches to the basilar artery
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 intradural 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
the clivus and medial petrous region.1,34,38 Some have advocated preserving the major dural sinus. Others, with appropriate consideration for the patency of the opposite transverse sinus, often sacrifice the sigmoid or transverse sinus.
Sacrifice of the sigmoid sinus can be considered after obtaining angiographic verification of the patency of the contralateral transverse and sigmoid sinuses. These structures
must communicate with the sagittal sinus and the ipsilateral transverse sinus through a patent torcular herophili. To
further ensure that the sigmoid sinus can be sacrificed safely, intravascular pressure within the proximal sinus can be
measured before and after its temporary occlusion. The
measurement is performed after ligation of the superior petrosal sinus by inserting a 25-gauge needle into the sigmoid
sinus just proximal to the temporary clip location. If pressure rises more than 10 mm Hg during temporary sinus occlusion, the structure should not be divided. This approach
is rarely used during the treatment of BA aneurysms.1,34,38
Far-Lateral Combined Supra- and Infratentorial Approach
(Combined–Combined)
Occasionally, an extensive lateral exposure of the clivus
is required to treat giant aneurysms of the midbasilar or
lower BA. Such an exposure can be achieved by combining
the far-lateral and combined supra- and infratentorial approaches.3,4,27 This so-called combined–combined approach
uses the same principles previously outlined for each individual procedure.
The anterior and lateral aspect of the brainstem can be
exposed with this complicated technique. This approach is
rarely used to treat BA aneurysms; the technique has been
detailed elsewhere.3,4,26 The combined approaches are explained in detail elsewhere in this issue.
CONCLUSIONS
Posterior fossa aneurysms are challenging lesions that
Neurosurg. Focus / Volume 19 / August, 2005
require expertise in vascular and skull base surgery for their
treatment. The choice of approach is based on the specific
features of each aneurysm. Multiple trajectories and angles
of approach are available and should be tailored on a caseby-case basis. The preferred method is to work with an angle that runs parallel to the parent vessel (that is, orbitozygomatic and far-lateral approaches). The transpetrosal and
retrosigmoid approaches are limited to selected cases.
The traditional principles of skull base surgery in terms
of maximizing bone resection to minimize brain retraction
and to facilitate instrument manipulation apply. In aneurysm surgery, the ultimate goal is to clip the aneurysm and
to exclude it from the circulation. If these goals cannot be
achieved, the aneurysm can be trapped or the flow dynamics changed. It is crucial to identify preoperatively whether
revascularization is indicated.
Hypothermic cardiac arrest facilitates dissection of an
aneurysm from perforating arteries. The aneurysm is deflated and transmural pressure is minimal. The technique
eliminates the risk of rupture and the hypothermia induced
with barbiturate therapy protects the brain. Nevertheless,
this complex technique is associated with high mortality
and morbidity rates and should be reserved for selected patients treated only at specialized centers.
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Manuscript received June 20, 2005.
Accepted in final form July 15, 2005.
Address reprint requests to: Robert F. Spetzler, M.D., Neuroscience Publications, Barrow Neurological Institute, 350 West Thomas
Road, Phoenix, Arizona 85013. email: [email protected].
Neurosurg. Focus / Volume 19 / August, 2005