Neurodiagnostic Studies

Careful review of various radiographic studies including computed tomography (CT), magnetic resonance imaging (MRI), CT angiography (CTA), MR angiography (MRA), and cerebral angiography permits accurate preoperative planning and a full understanding of relevant anatomy. Often a combination of imaging techniques is required, particularly for more complex aneurysms, to allow for: (1) correct selection of surgical or endovascular occlusion, (2) selection of a surgical approach, (3) preparation of surgical adjuncts (e.g., cervical internal carotid artery (ICA) exposure or need for revascularization), and (4) understanding possible complications and how they may be prevented or managed. Preoperative imaging, and in particular, three-dimensional (3-D) digital subtraction angiography (DSA) or reconstructed CTA can help surgical planning through provision of a 3-D or “operative” view. In addition, aneurysms may be associated with anatomic variations (e.g., hypoplastic A1), fetal posterior cerebral artery (PCA), fenestrated vertebral basilar (VB) junction, persistent carotid-to-basilar anastomoses, and other vascular abnormalities (e.g., aneurysms and arteriovenous malformations [AVMs]).

Anesthesia

There is no standard anesthetic regimen for all aneurysm surgery and the technique should be individualized (see Chapter 21). There are several basic principles^9,10 and goals (Table 365-2). First, anesthesia should be titratable and short acting to permit a prompt controlled wakeup. Some commonly used agents include infusions of remifentanil, propofol, or inhaled anesthetics such as sevoflurane or desflurane. Second, drugs that reduce cerebral blood flow (CBF) or increase intracranial pressure (ICP) should be avoided. Third, careful blood pressure control is necessary. In particular, large changes in blood pressure that cause an increase in transmural pressure and so increase aneurysm rupture risk need to be avoided during induction. Normotension usually is preferred at surgery; however, mean arterial blood pressure should be increased by 10% to 20% from the baseline if temporary arterial occlusion is applied. Consequently, invasive blood pressure monitoring is necessary in each patient. Fourth, arterial blood gases should be checked during surgery to obtain adequate oxygenation and to maintain PaCO₂ levels between 30 and 35 mm Hg. Lower levels of PaCO₂ may decrease CBF, particularly in patients with vasospasm. The role of hypothermia and other neuroprotective strategies for aneurysms remains unclear.^11–15

TABLE 365-2 Perioperative Anesthetic Goals During Cerebral Aneurysm Surgery

Brain Relaxation

The incidence of injury from brain retraction is estimated at 5% in intracranial aneurysm procedures¹⁶ and is more likely when the brain is swollen (e.g., poor-grade SAH) or when a large complex aneurysm is treated. Excessive retraction can cause ischemia or contusions. The need for retraction can be reduced through adequate exposure and proper brain relaxation that can be accomplished by several methods.

The role of hypothermia remains controversial.

Positioning

Proper positioning can facilitate aneurysm exposure, reduce the risk of retraction injury, and improve surgeon comfort. Aneurysm morphology and the type of incision and bone flap determine the specific head position; specific positions for each approach are reviewed below. Proper head position should allow the brain to naturally fall away from the surgical trajectory; this enhances exposure and reduces the need for brain retraction (Fig. 365-1). Many aneurysms are approached through a pterional craniotomy or modifications of this. For this craniotomy, patients are in a supine position with slight hip flexion and knees bent. The neck should not be rotated or flexed in such a manner that it obstructs venous return. For most pterional craniotomies, the head is positioned such that the frontal lobe falls away from the floor of the anterior fossa and the temporal lobe falls away from the sylvian fissure (i.e., the malar eminence is highest).

FIGURE 365-1 Patient positions and incisions for common surgical approaches used during aneurysm surgery. A, Pterional. B, Subtemporal. C, Frontal parasagittal. D, Lateral suboccipital. Three incision options are illustrated for the lateral suboccipital approach.

(Modified from LeRoux P, Winn HR, Newell D. Management of Cerebral Aneurysms. Philadelphia: Saunders; 2003.)

Neuromonitoring

Intraoperative neuromonitoring can be a useful adjunct during aneurysm surgery. Electroencephalography (EEG) is useful when temporary occlusion is envisaged and is used to monitor for ischemia and to ensure burst suppression when some neuroprotective strategies are used. Somatosensory evoked potentials (SSEPs) also can be used to monitor for depth of anesthesia and can indicate the presence of ischemia. A role for EEG and SSEPs is better defined in anterior rather than posterior circulation aneurysms. Specialized monitoring is used for specific aneurysms and approaches (e.g., cranial nerve monitoring for posterior fossa lesions).

Craniotomy Selection

Selection of the correct approach enhances the safety and efficacy of aneurysm occlusion. The type and size of the craniotomy (Fig. 365-2) chosen for a specific aneurysm is influenced by four factors: (1) the aneurysm type (e.g., location and size); (2) aneurysm configuration and anatomy of the associated vessels and surrounding osseous and neural structures; (3) the patient’s clinical status; and (4) surgeon preference. Of these factors, the type of aneurysm and its configuration most influence what craniotomy is used. A three-dimensional understanding of the relevant neuroanatomy is needed to plan the appropriate trajectory to the aneurysm and the possible trajectories of any clip application.¹⁸ While unnecessary large craniotomies should be avoided, adequate bone removal is essential to reduce the need for brain retraction or even placement of retractors.

FIGURE 365-2 Surgical approaches to intracranial aneurysms. The shaded areas indicate each approach and amount of exposure.

(With permission from Barrow Neurological Institute, Phoenix.)

Positioning

The head is placed in a Mayfield 3-pin head frame with the solitary pin over the contralateral frontal bone behind the hairline to avoid pins in the forehead. Alternatively, the solitary pin may be placed just behind the ipsilateral ear. For a standard pterional craniotomy, the patient is positioned supine with a roll under the ipsilateral shoulder. The head is rotated about 20 degrees to the side opposite the aneurysm. The degree of rotation depends in part on aneurysm location and anatomy with less rotation for an AcomA aneurysm and more rotation for some MCA aneurysms. The head also is extended 10 to 20 degrees by rotating the vertex toward the floor. In addition, the head is translated forward. The goal of positioning is to have the malar eminence of the zygoma at the highest point in the operative field. This brings the sylvian fissure to the highest point and allows gravity to assist in frontal and temporal lobe “retraction” (i.e., the frontal lobe should fall away from the anterior fossa floor and the temporal should fall posteriorly from the sylvian fissure). When correctly positioned and when brain relaxation is adequate, brain retraction may not be necessary.

Scalp and Soft Tissue

A curvilinear frontal-temporal skin incision is made that starts just anterior to the tragus at the root of the zygoma (Fig. 365-3A). It is carried superiorly to the temporal crest and curved forward to the midline just behind the hairline. While the bone flap does not need to extend to the midline, the skin incision should just cross the midline. This allows the scalp flap to be retracted anterior and inferior to expose fully the frontal zygomatic junction and pterion. The scalp and temporalis muscle can be elevated together and retracted forward using low-profile perforating towel clips or fishhooks that then are secured with rubber bands to bars from the Greenberg system connected to the Mayfield or to the Sugita system when used. A small fascial cuff may be left along the temporalis muscle insertion for later closure (Fig. 365-3B). The pericranium should be separately mobilized if the frontal sinus is likely to be opened during the craniotomy (e.g., with a more medial exposure for some AcomA aneurysms). Extension of the incision inferiorly (below the zygoma root) or subgaleal dissection antero-inferiorly should be avoided, to prevent injury to the facial nerve, which is found about 1.5 cm below the zygomatic arch origin¹⁹ and crosses about 2 cm anterior to its origin in an oblique superior-anterior direction. The facial nerve frontal branch that innervates the frontalis muscle courses in the subgaleal fat pad; interfascial dissection of the triangular fat pad or elevation of the temporalis muscle and scalp flap together may help avoid nerve injury. Care should be taken to preserve the superficial temporal artery with at least one of its major branches to supply the temporalis muscle or, in some cases, for a bypass procedure.

Skull

After placement of a bur hole and in the presence of a lumbar drain, careful CSF drainage is initiated. About 20 mL of CSF should be initially drained with subsequently larger (50 to 200 mL is not uncommon) volumes achieved. If extensive sylvian fissure dissection is likely, CSF drainage should be minimized. A single bur hole in the temporal bone at the root of the zygoma is generally all that is required. This single bur hole craniotomy is less likely to be associated with cosmetic defects than multiple bur hole craniotomies. Some surgeons place a bur hole in the pterion. In older patients, the dura is frequently adherent to the skull and frequently torn in the anterior medial frontal region; consequently, multiple bur holes are used in the elderly. The craniotomy (Fig. 365-3C) is C-shaped in an arc “around” the pterion and anterior clinoid process (ACP). The dura is dissected free at the bur hole and then with a high-speed drill, the skull is cut forward in the subtemporal region until the sphenoid wing. Then in a separate cut, the skull is opened in a curvilinear fashion in the frontal-temporal region (i.e., the cut extends superior to the temporal crest and then anterior to the midpupillary region of the supraorbital region (foramen of supraorbital nerve). This cut may extend more medial for an AcomA aneurysm. The opening then proceeds posteriorly to the pterion and sphenoid wing flat along the anterior fossa floor. The supraorbital ridge is preserved for later reconstruction. The drill may not cut all the way through the pterion and in these cases the bone in this region can be thinned and then fractured through the pterion.

Skull Base

Once the bone flap is removed, the dura is dissected free from the anterior fossa skull base. With a high-speed bur, the pterional bone is drilled away to remove the lesser wing of the sphenoid, the ACP base, and flatten the bony prominences of the anterior fossa floor, the inner table of the inferior frontal bone over the supraorbital ridge and the squasmal temporal bone (Fig. 365-3D and E). Orbitomeningeal artery bleeding indicates proximity to the superior orbital fissure (SOF); this artery can be coagulated and divided. For some MCA aneurysms, subtemporal bone may need removal to expose the middle fossa floor. Extensive bone removal creates a flat corridor with ample space under the frontal and temporal lobes. The extradural drilling can be carried down to the ACP, which can be removed before the dural opening to enhance exposure of supraclinoid ICA aneurysms.²⁰ Alternatively the ACP can be removed after the dura is opened (Fig. 365-4). The craniotomy edges then are lined with hemostatic agents and tack-up sutures are placed.

FIGURE 365-4 The pterional approach: intradural anterior clinoid process removal. A high-speed diamond bur can be used to remove the anterior clinoid process (ACP) once the dura is reflected to expose the clinoid segment of the internal carotid artery (ICA), the optic nerve (ON), and the bony optic strut. The optic nerve can be mobilized medially to expose the ophthalmic artery (OA) once the dural layer distal to the proximal dural ring and optic strut is removed.

(Modified from LeRoux P, Winn HR, Newell D. Management of Cerebral Aneurysms. Philadelphia: Saunders; 2003.)

Dural Opening and Brain Preparation

The dura is opened in a C-shaped fashion centered on the pterion and ACP (Fig. 365-3F) and retracted laterally to expose the inferior frontal lobe and anterior sylvian fissure. It is held out of the surgical field with sutures. If there has been adequate bone removal, the optic nerve and ICA at the skull base can now be approached with little brain retraction. First the brain should be protected. This is particularly important after a SAH where its surface may be friable and easily contused. We place large squares of Gelfoam under the dura in the more posterior aspect of the bony opening to prevent direct brain injury and to prevent the run down of blood into the subdural space as brain relaxation and CSF drainage proceeds. The brain surface is covered with moist surgical cottonoids or Telfa strips.

Initial Vascular Dissection

The frontal lobe is mobilized gently close the sylvian fissure. This exposes the olfactory tract and may be done with a No. 5 sucker against a cottonoid, if there is adequate brain relaxation and bone exposure. If the brain remains swollen (e.g., after severe SAH), additional relaxation with further CSF drainage or barbiturates may be needed. When extensive sylvian fissure dissection is planned (e.g., with an MCA aneurysm), we prefer not to drain too much CSF because the fissure is easier to split when it is filled with CSF. Using sharp, careful arachnoid dissection, the fissure may be split from lateral to medial or medial to lateral depending on aneurysm location and morphology and length of the ICA and MCA (M1) segments, but usually on the medial (frontal) side of the sylvian veins. The extent of sylvian fissure dissection also depends on aneurysm location and brain condition. Transsylvian veins should be preserved, in particular those that drain into the sphenoparietal sinus, to limit venous congestion. The goal of initial dissection is to define the proximal ICA and optic nerve and then open arachnoid over these structures (i.e., the anterior and medial portions of the chiasmatic and carotid cisterns) using a sharp arachnoid knife (Fig. 365-3G). This allows further CSF drainage and brain relaxation such that the frontal lobe falls away from the skull base. The surgical approach thereafter depends on the aneurysm to be treated. A detailed description of how aneurysms in different locations should be dissected or occluded is beyond the scope of this chapter. A brief review of select techniques is provided.

Carotid Ophthalmic Artery and Paraclinoid ICA Aneurysms

Aneurysms that involve the ICA as it winds around the ACP (paraclinoid aneurysms) all involve or are near the carotid ring, ACP, optic strut, and optic nerves. Proximal control is important and therefore the ipsilateral neck should be included into the sterile field to provide access to the cervical carotid bifurcation for proximal ICA control, retrograde suction decompression (Fig. 365-5), or a saphenous vein bypass graft when required.²¹ There should be a low threshold for neck opening throughout the case. We usually obtain cervical ICA exposure in all patients with a clinoidal ICA aneurysm, a complex or giant aneurysm, or aneurysm of the ophthalmic segment.

FIGURE 365-5 Suction-decompression technique. Illustrated is a low paraclinoid aneurysm that is trapped by occlusion of the proximal carotid artery in the neck and the intracranial internal carotid artery distal to the aneurysm. A needle is inserted into the extracranial internal carotid artery and blood aspirated to empty the aneurysm.

(Modified from LeRoux P, Winn HR, Newell D. Management of Cerebral Aneurysms. Philadelphia: Saunders; 2003.)

Next the ACP and optic strut (OS) is removed. This is essential to obliterate all aneurysms of the clinoid or opthalmic ICA segments. Dorsal carotid wall aneurysms, however, often can be occluded without extensive skull base resection. ACP removal can be performed through an extradural or intradural approach determined primarily by the relationship between the aneurysm and ACP. Extradural ACP removal is feasible for most opthalmic segment aneurysms, but intradural ACP removal is preferred for large, complex, or ruptured aneurysms. In addition, an intradural approach is recommended for clinoidal segment aneurysms, especially those of the anterolateral segment because these lesions may adhere to or erode into the ACP.

Extradural ACP Removal

Before the dura is opened, the posterior half of the roof and lateral wall of the orbit and the sphenoid ridge over the SOF are removed to define the orbital portion of the optic nerve. The ACP then is internally cavitated using a small diamond bur, and the remaining thin remnants are removed with small rongeurs and curets. Bleeding is controlled with a combination of bone wax, Surgicel, and Gelfoam.^20,22 Slight elevation of the head will frequently result in arrest of venous bleeding.

Intradural ACP Removal

The roof and lateral wall of the orbit and the sphenoid ridge are removed extradurally. Once the dura is opened, the sylvian fissure is split widely from lateral to medial to expose the ICA and the ACP. A longitudinal dural incision then is made from the ACP to the resected edge of the medial sphenoid ridge. The falciform ligament is cut to decompress the optic nerve. The ACP and optic canal roof and lateral wall then are thinned with a small diamond bur, and the remaining bone removed with small rongeurs. Irrigation is necessary to prevent thermal injury and to clear bone dust while drilling. Finally, the optic strut is drilled to expose the anterior border of the ICA clinoidal segment. This exposes the ophthalmic artery (OA) and permits the optic nerve to be mobilized superiorly off the OA/ICA (see Fig. 365-4). Bleeding from the cavernous sinus can be controlled by Surgicel or Gelfoam.

Internal Carotid and Posterior Communicating Artery Aneurysms

Twenty-five percent of intracranial aneurysms arise from the ICA at the PcomA origin, making this site the second most common location after AcomA aneurysms. The same basic approach is used for aneurysms that involve the PcomA, anterior choroidal artery (AChA), and ICA bifurcation.

Posterior Communicating Artery

The PcomA arises from the posterolateral ICA wall within the carotid cistern. Here the PComA is encased by its own arachnoid trabeculae and then it pierces Liliequist’s membrane to enter the interpeduncular cistern. The oculomotor nerve that enters the dura lateral to the posterior clinoid process and medial to the dural band passing from the tentorium toward the ACP is an important landmark. The PComA usually runs medial to the oculomotor nerve. The PComA origin usually is seen as a slight bulge just proximal to the aneurysm on the posterolateral ICA wall. The AChA often is seen before the PComA because the supraclinoid ICA courses upward in a posterolateral direction. The ACP may need to be removed when there is a proximal PComA origin, a very short and lateral supraclinoid ICA course, or a long and prominent ACP. Most PcomA aneurysms arise from the posterior and lateral ICA surface and are oriented posteriorly and inferiorly into the temporal lobe. Some may project onto the tentorium and III nerve or under the tentorium. There are four important surgical principles:

Anterior Choroidal Artery

The AChA arises from the ICA 2 to 5 mm distal to the PComA from the posterolateral ICA wall and courses posterior and lateral beside the optic tract. Its origin is usually more lateral on the posterior ICA wall than the PComA origin and so may be easier to identify. However, it is important to define whether the AChA arises as a single trunk or multiple vessels. The initial approach to AChA aneurysms is identical to that for PComA aneurysms. AChA aneurysms usually point laterally and may be adherent to the medial temporal lobe (i.e., avoid temporal lobe retraction).

Internal Carotid Artery Bifurcation

After the AChA, the ICA continues to then bifurcate into the anterior cerebral artery (ACA) and MCA just below the anterior perforated substance. A wide sylvian fissure opening and medial dissection of the orbitobasal aspect of the frontal lobe are important during the initial approach to ICA bifurcation aneurysms. Excessive frontal lobe retraction should be avoided because the aneurysm dome may adhere to or be buried into the orbitobasal aspect of the frontal lobe. The ICA between the PComA and AChA is prepared for a temporary clip because a clip placed on the ICA distal to the AChA may perforate the posterior aneurysm wall or occlude perforating arteries that course underneath the bifurcation. The proximal MCA trunk and proximal ACA then are exposed. This is crucial because perforating branches from the A1 and the lenticulostriate arteries from M1 segments are frequently compressed by or adherent to the dome of an ICA bifurcation aneurysm.

Posterior ICA Wall Aneurysms

There is usually no identifiable neck in these aneurysms and they may involve ICA branches. In addition, when the surgeon first approaches these aneurysms, it appears that the entire artery is involved, similar to a fusiform aneurysm. Ideally these lesions are occluded with angled fenestrated clips placed over the ICA that is left in the fenestration (i.e., “reconstructing” the ICA lumen). Sometimes multiple clips are necessary (Fig. 365-6). Exposure of the cervical ICA and use of a temporary ICA occlusion in the neck is useful as is intraoperative angiography or fluorescein angiography.

FIGURE 365-6 Vessel reconstruction: Posterior internal carotid artery (ICA) wall aneurysms are occluded by “reconstruction” of the vessel (i.e., the ICA). This may be achieved with an angled fenestrated clip (A). Sometimes multiple clips are necessary (B). These tandem clips may be applied in opposite directions and re-enforced with a third.

(A, Modified from Ojeman RG, Ogilvy CS, Crowell RM, et al, eds. Surgical Management of Neurovascular Disease. Baltimore: Williams & Wilkins; 1995; B, modified from LeRoux P, Winn HR, Newell D. Management of Cerebral Aneurysms. Philadelphia: Saunders; 2003.)

Anterior Cerebral and Anterior Communicating Artery Aneurysms

The anterior communicating artery (AcomA) is the most common location of cerebral aneurysms. However, aneurysms in this location can be missed on angiography: multiple angiographic views therefore are needed to define AcomA aneurysms. These aneurysms also have great variations in morphology, size, projection, and relationship to surrounding neurovascular structures. In addition, AcomA aneurysms often are associated with vascular anomalies and the AcomA complex exhibits considerable anatomic variation. In particular, “dominance” of one A1 segment (i.e., a large-caliber A1 vessel supplies the aneurysm, whereas the opposite A1 is hypoplastic) is seen. The aneurysm usually points in the direction of flow (i.e., away from the dominant A1 and toward the opposite hemisphere) and so may be approached best from the dominant A1. When there is ICH, particularly one that causes mass effect in the basal-frontal region, the approach is from the side of the ICH to remove the clot to help brain relaxation and reduce potential bifrontal injury.

The usual pterional craniotomy is extended more medial and the sphenoid wing and anterior fossa floor are flattened. An orbital zygomatic (OZ) approach may be helpful for superior-oriented AcomA aneurysms. Aneurysm dissection and occlusion occurs under the frontal lobe; wide sylvian opening improves frontal lobe mobility. The initial dissection then occurs on the anterior surface of the A1 segment taking care to avoid the recurrent artery of Heubner. The key to successful AcomA aneurysm occlusion is complete appreciation of the AcomA complex anatomy; aneurysm morphology influences how this may be achieved. The AcomA is formed by the confluence of the two A1s to form the anterior aspect of the Circle of Willis and generally is on the optic chiasm at the lamina terminalis level. The AcomA marks the origin of the A2 segments bilaterally. Small perforating vessels arise from the A1 and A2 segments and the AcomA; these supply the hypothalamus, anterior perforating substance, dorsal optic chiasm, suprachiasmatic area, anterior third ventricle, frontal lobe, and gyrus rectus. The recurrent artery of Heubner arises at, or near, the A1 and A2 junction and has a retrograde course posterior or superior to the ipsilateral A1 and sometimes may be identified before the A1 during microdissection. In addition, frontal-polar branches need to be defined, and ideally both optic nerves seen. A gyrus rectus resection adjacent to the AcomA complex can be helpful, especially with superior-oriented aneurysms, and can help visualize the A2s. Further dissection to define all the vessels then depends on aneurysm orientation.²³ AcomA aneurysms can project at any angle in three-dimensional space; however, the projection is best understood in an anterior-posterior or superior-inferior view of perpendicular two-dimensional planes. There is a difference between the perspective in true anatomic space and the pterional surgical exposure; (e.g., an aneurysm that projects anteriorly may appear superiorly directed in the surgical field. Angiographic perspective further differs, and depends on the image projection and obliquity. To understand dissection, AcomA aneurysms may be classified into one of four projections based on their orientation in true anatomic space (Fig. 365-7):

FIGURE 365-7 Anterior communicating artery (AcomA) aneurysm classification: AcomA aneurysms may be classified into one of four projections based on their orientation in true anatomic space. The arrows indicate spatial orientation and projection of the aneurysm fundus. A, Superior. The aneurysm projects superiorly and obscures most of the left A1 segment and the proximal left A2 segment. The right A1 segment is dominant. B, Anterior. The aneurysm projects into the optic chiasm and indents the left optic nerve. These aneurysms may adhere to the basal dura. The right A1 segment is absent and Heubner’s artery on the left is duplicated. C, Posterior. The aneurysm projects between the two A2 segments and away from the optic apparatus. Perforators may be associated with the aneurysm base. The contralateral A1 segment is well seen. D, Inferior. This aneurysm projects inferiorly and distorts the AcomA complex. However there is an unobstructed view of the entire AcomA complex.

(Modified from Chalif DC, Weinberg JS. Surgical treatment of aneurysms of the anterior cerebral artery. Neurosurg Clin N Am. 1998;9:799.)

Middle Cerebral Artery Aneurysms

Surgical anatomy is detailed in Yasargil.¹ The ICA bifurcates superolateral to the optic chiasm, inferior to the anterior perforated substance, and posterior to the olfactory tract division. The MCA is the larger terminal ICA branch and courses laterally from the medial sylvian fissure. It makes a sharp turn at the limen insulae to lie on the insular cortex in the superficial part of the sylvian fissure. The M1 or sphenoidal segment lies in the medial sylvian fissure until the limen insulae but in common use “M1” refers to the MCA from its origin to where it first divides. The M1/M2 junction is located at, or just before, the junction of the sphenoidal and operculoinsular sections of the sylvian fissure, where the MCA branches turn 90 degrees to run over the insula. The M2, or “insular,” segment includes the MCA branches from the M1 division to where each branch turns again at the circular sulcus. Branches continue as the M3 (opercular) segments that course over the opercula to the convexity. The cortical branches on the hemisphere surface are the M4 segments.

The MCA is the third most common location of ruptured aneurysms (about 20%) and usually arise at the M1/M2 junction. The direction of parent vessel blood flow generally determines the direction that an aneurysm will project (i.e., most bifurcation aneurysms project laterally or forward toward the temporal lobe). Consequently, MCA aneurysm rupture can cause an ICH in the sylvian fissure or temporal lobe. There are three basic approaches:

Closure

To assess successful aneurysm clipping (complete aneurysm obliteration and parent vessel patency) may be difficult through direct external inspection alone. In particular this is difficult for paraclinoid and complex aneurysms. Once certain that the aneurysm is occluded, without adjacent vessel compromise or compression of neural structures, the aneurysm can be punctured with a 23- or 25-gauge needle to confirm absent blood flow in the dome. Microvascular Doppler also can be used to confirm aneurysm occlusion and flow in the vessels. Intraoperative angiography (DSA or fluorescein) is invaluable for large or complex aneurysms particularly those of the paraclinoid region. Residual blood clot in the cisterns or in the brain parenchyma is removed when possible using gentle irrigation. Papaverine-soaked Gelfoam can be placed on narrowed exposed vessels. The operative field and brain are carefully inspected under the microscope at different angles to ensure complete hemostasis and the field irrigated. Dissected brain surfaces or an ICH cavity, if present, are covered with Surgicel. When the ACP is removed any communication with sphenoid sinus can be occluded using a small muscle plug, Gelfoam, or wax. Tack-up sutures are placed before dural closure to ensure that the needle does not injure the brain and the dura closed using 4/0 Nurolon suture. Dural defects can be corrected with pericranium or artificial dural substitutes. If the frontal sinus was entered during the craniotomy it should be covered with a vascularized pericranium flap. The bone is replaced using low-profile plates and screws and the pterional region reconstructed with mesh cranioplasty to limit indentation associated with temporalis atrophy (see Fig. 365-3H). The temporalis muscle is reattached with 3-0 or 2-0 Vicryl sutures and then the temporalis fascia is resutured (see Fig. 365-3I). The subgaleal space is again irrigated with saline to clean blood clots, bone, and tissue debris and a subgaleal drain placed. The galea is closed with inverted 2-0 Vicryl sutures and the skin is closed with a running nylon suture or with staples.

Subtemporal

The subtemporal approach proceeds from a lateral trajectory under the temporal lobe and along the middle fossa floor.²⁶ This approach provides direct lateral access along what often is the widest axis of the aneurysm neck and often the optimal direction for clip placement. The area behind the aneurysm, including the perforators, whose preservation is essential, usually is seen best from this approach (Fig. 365-10). The approach also may be suitable for some distal basilar artery (BA), SCA, and posterior cerebral artery (PCA) aneurysms. In general, a right-sided approach is used (but see previous discussion). A relaxed brain achieved through a combination of techniques is crucial when using a subtemporal approach, including direct CSF removal from the opened cisterns, once the temporal lobe is elevated. There are several disadvantages to the subtemporal approach: (1) the operating field is small; (2) excess temporal lobe retraction may be necessary; (3) the ipsilateral P1 lies between the surgeon and the aneurysm, which may limit dissection or clip application; (4) the aneurysm, particularly when large, needs to be retracted to see the opposite P1; and (5) a high-lying bifurcation may be difficult to approach.

FIGURE 365-10 Subtemporal approach. A, Illustration of lateral head position and bone flap. Temporal bone is removed inferiorly to make the cranial exposure flush with the middle fossa floor (cross-hatching). B, Microsurgical anatomy of the tentorial incisura once the temporal lobe is elevated. This trajectory provides a good view of the posterior perforating arteries and the back of the aneurysm’s neck.

(With permission from Barrow Neurological Institute, Phoenix.)

The patient is placed in the lateral decubitus or in the supine position with a shoulder roll. The head is rotated until the midline plane (superior sagittal sinus) is parallel to the floor, and the vertex is angled 15 to 20 degrees downward to achieve a line of sight parallel to the floor of the middle fossa. One of two incisions may be used: a 7- to 10-cm linear incision that extends up from a point 1 cm anterior to the tragus at the zygomatic arch or a question mark that starts just anterior to the tragus and curves above the ear to the superior temporal line (Fig. 365-10). A 4- × 4-cm craniotomy is made and temporal squamosal bone removed inferiorly with a bur until flush with the middle fossa floor. Under the operating microscope, a self-retaining retractor is positioned to elevate the temporal lobe and expose the tentorial incisura. Dissection then is directed between the medial temporal lobe and the tentorial edge. The arachnoid is opened to enter the interpeduncular and ambient cisterns, and cranial nerve (CN) III is identified. The edge of the tentorium can be pulled laterally with a tacking suture between CNs III and IV to enlarge the opening into the interpeduncular cistern. The tentorium also may be divided behind the entry of CN IV for additional inferior exposure. The SCA and PCA then are followed medially to the basilar apex with minimal third nerve manipulation.

Orbitozygomatic-Pterional Approach

The orbitozygomatic approach adds two steps to the pterional approach: (1) soft tissue dissection to expose the orbitozygomatic unit (the orbital rim, orbital roof, lateral orbital wall, and zygomatic arch), and (2) osteotomies to free it, (i.e., the superior and lateral portions of the orbit [Figs. 365-8 and 365-11] are removed). This approach is useful for a high bifurcation and provides a more anterior trajectory, a higher view above the posterior clinoid process, and greater space in the operative corridor than a standard pterional craniotomy.

FIGURE 365-11 Orbitozygomatic craniotomy. A and B, The orbitozygomatic approach combines a pterional craniotomy and removal of the orbitozygomatic complex. The orbitozygomatic osteotomy consists of six cuts through the root of the (1) zygoma, (2 and 3) malar eminence, (4) lateral orbital roof, and (5 and 6) lateral orbit. C, The resulting osteotomy is mobilized as one piece to be reinserted during closure.

(With permission from Barrow Neurological Institute, Phoenix.)

The zygoma and orbital rim are ensheathed in two layers of the temporalis fascia that can be elevated through an interfascial dissection that peels apart the layers or by a subfascial dissection. The subfascial dissection does not disturb the frontalis nerve, which runs superficial to the outer fascial layer because the inner layer is cut as it passes under the zygoma to expose the bone. The periorbita is continuous with the outer layer of temporal fascia. It is carefully stripped from the orbit to help contain the periorbital fat during the orbital osteotomies. When the orbitozygomatic unit is exposed, the temporalis muscle is mobilized inferiorly, a frontotemporal craniotomy is made, and the dura of the frontal lobe is elevated. The pterion does not need to be removed when the orbit is detached.

The orbitozygomatic unit is released by six osteotomies (Fig. 365-11) made with a reciprocating saw. First the zygomatic root is cut. The temporomandibular joint is avoided. A fixation plate is secured to the zygoma and registered to improve repair cosmesis. The second and third cuts are across the zygomatic bone; from the inferolateral margin halfway across to the lateral orbital rim, and then from the inferior orbital fissure to the same end point. The resulting V in the zygomatic bone allows the fragment to be secured into position when replaced. Fourth, the medial orbital roof, just lateral to the supraorbital notch is cut. The fifth cut crosses the posterior orbital roof, approximately 2.5 cm posterior to the frontal bone inner table (to preserve the orbital roof), and finishes laterally in the sphenoid ridge and pterion. The final cut crosses the lateral orbital wall to connect the previous cut with the inferior orbital fissure. The orbitozygomatic unit then is removed as a single piece, and additional bone is removed over the orbital apex around the superior orbital fissure. A dural flap based over the orbit is tented forward with tacking sutures to depress the globe gently. The intradural part of this approach is similar to a pterional approach. To widen the view, the choroidal fissure is opened and the temporal pole mobilized laterally.

Extended Orbitozygomatic Approach

The subtemporal, pterional, and orbitozygomatic approaches provide different trajectories to the basilar bifurcation. The optimal angle to expose and occlude a BB aneurysm often is determined intraoperatively rather than from presurgical imaging. A combination of approaches therefore provides the greatest flexibility. The posteroinferior cranial exposure therefore can be extended to augment the orbitozygomatic approach. This extended orbitozygomatic approach is similar to the combined subtemporal and pterion approach (the “half-and-half” or “one-and-a-half” approach) and provides multiple views of the BB aneurysm within a 90-degree window of exposure.

Inner Skull Base Obstacles

Several structures, including parts of the skull base (e.g., posterior clinoid process, dorsum sellae, clivus), neurovascular structures, (e.g., ICA, second and third nerves) and dural structures (e.g., tentorium and cavernous sinus), may limit access to the upper basilar artery. It is important to access the BA below the SCA to gain proximal control. Consequently, when the basilar bifurcation is low-lying, the posterior clinoid process may need to be drilled off, or the tentorium mobilized inferolaterally with a tacking suture or incised to provide greater access into the posterior fossa. The tentorium can be incised behind the fourth nerve (transtentorial-retrocavernous) through a subtemporal approach or between the third and fourth nerves (transtentorial-transcavernous) with a pretemporal trajectory. There are two transclinoidal approaches to very low-lying BB aneurysms: inferolaterally through the cavernous sinus (transclinoidal-transcavernous) or inferomedially through the dorsum and upper clivus outside the cavernous sinus (transclinoidal-extracavernous). Finally, the approach also may be augmented through a medial petrosectomy in Kawase’s triangle (see later discussion).

Transpetrosal Approaches

These lateral approaches provide proximal and distal control of the basilar artery but are best suited for small aneurysms because the aneurysm dome often is between the surgeon and the aneurysm neck, or cranial nerves limit instrument maneuverability. Transpetrosal approaches expose the basilar trunk from a lateral trajectory through presigmoid corridors in the petrous bone categorized into three variations based on an increasing extent of resected bone: retrolabyrinthine, translabyrinthine, and transcochlear (Fig. 365-12). The retrolabyrinthine approach removes temporal bone between the semicircular canals anteriorly and the posterior fossa dura on the posterior aspect of the temporal bone.²⁸ The semicircular canals are skeletonized, but not violated. In the translabyrinthine approach, exposure is increased further anteriorly to the internal auditory canal (IAC) and the semicircular canals are removed (i.e., hearing is sacrificed).²⁹ The seventh nerve is left in its bony sheath to protect it. The petrous bone is almost completely removed in the transcochlear approach. The facial nerve canal is opened, the nerve transposed posteriorly to access the cochlea that then is removed.³⁰

FIGURE 365-12 Transpetrosal approaches. A, The retrolabyrinthine approach (crossed lines) preserves the middle ear structures. The semicircular canals are sacrificed in the translabyrinthine approach (diagonal lines) (i.e., hearing is sacrificed). The cochlea is removed and the facial nerve transposed during the transcochlear approach (vertical area) and provides the most exposure to the midclivus and brainstem. B, The head is positioned laterally and a C-shaped skin incision is used (same for all 3 approaches).

(With permission from Barrow Neurological Institute, Phoenix.)

Retrolabyrinthine Approach

This approach may be used for smaller basilar trunk aneurysms. The patient is positioned supine with a shoulder roll to reduce neck rotation, and the head is positioned in a Mayfield head holder with the midline parallel to the floor and inclined slightly downward. The head is flexed slightly and the shoulder taped caudally to improve access. The mastoid bone becomes the highest point in the surgical field. The skin is incised 1 cm anterior to the tragus and 2 cm above the zygoma, curving gently around the ear to the mastoid tip (see Fig. 116-8B), and the scalp flap is retracted anteriorly.

Bone work starts on the temporal line that marks the inferior temporalis muscle insertion and the middle fossa floor and is completed with a mastoidectomy that also removes bone over the sinodural angle and sigmoid sinus. Posterior retraction of the sinus and presigmoid dura then exposes the middle fossa dura and sinodural angle between them. The superior petrosal sinus lies deep to the sinodural angle and represents the posterior-superior margin of the petrous bone. The mastoid antrum is dissected to expose the horizontal semicircular canal that leads to other anatomic landmarks (external genu of the seventh nerve medially and inferiorly, the posterior semicircular canal posteriorly, and the epitympanum and superior semicircular canal anteriorly). The posterior and superior semicircular canals are skeletonized. Resection of the temporal bone above and below the otic capsule exposes the medial dura of the middle fossa floor, the superior petrosal sinus, and the jugular bulb. When the exposed dural surface is opened, there is access to the cerebellar pontine angle (CPA). The access, however, is limited but may be modified by ligation and division of the sigmoid sinus.

Translabyrinthine Approach

The translabyrinthine exposure extends farther anterior than the retrolabyrinthine approach but still is best suited to small aneurysms. The initial steps of this approach are the same as the retrolabyrinthine approach. Then, all three semicircular canals are drilled away to expose the posterior half to two thirds of the IAC. The medial walls of the vestibule and the superior semicircular canal ampulla represent the lateral wall of the IAC fundus; the IAC is exposed with a small amount of bone removal at this location. The seventh nerve is found anterosuperiorly in the IAC and is separated from the superior vestibular nerve posterior and superior by a shelf of bone (Bill’s bar). Seventh nerve monitoring helps to protect the nerve during drilling.

Transcochlear Approach

Among the transpetrosal approaches, the transcochlear approach provides the greatest exposure (almost the entire petrous bone is resected) and a wide triangular corridor is opened that leads directly to the basilar trunk. With this approach even large aneurysms can be exposed adequately. The transcochlear approach is a forward extension of the translabyrinthine approach that unsheathes and mobilizes the seventh nerve, removes the cochlea, and opens the CPA to expose the anterolateral brainstem, clivus, and basilar trunk.

The external auditory canal is transected and oversewn in two layers and the initial procedure is then the same as the translabyrinthine approach. The seventh nerve is skeletonized from its entrance into the IAC to its exit from the stylomastoid foramen. The chorda tympani is sectioned inferiorly at its origin from the descending portion of the seventh nerve to allow the facial recess to be extended inferiorly to the hypotympanum and retrofacial area. The greater superficial petrosal nerve (GSPN) is sectioned anteriorly at its origin from the geniculate ganglion to help free the seventh nerve that is transposed posteriorly after it is dissected from its bony canal. The cochlea then is drilled starting with the promontory that houses the basal turn of the cochlea. Bone is removed forward to the septum between the basal turn and the petrous ICA. The ICA and internal jugular vein leave the carotid sheath and enter the skull base near each other. The jugulocarotid septum, a ridge of bone that separates the ICA as it turns anteriorly from the jugular vein, which turns posteriorly, is removed to expose the jugular bulb. During this exposure, care is needed to avoid injury to the ninth, tenth, and eleventh nerves in the jugular foramen. When drilling is completed, the entire temporal bone is gone. The superior petrosal sinus, from the sinodural angle laterally to Meckel’s cave medially, forms the superior boundary of the exposure. The inferior petrosal sinus and jugular bulb are the inferior border. Bone removal extends medially to the clivus and anteriorly to the ICA and temporomandibular joint periosteum. The bone surrounding the ICA can be removed superiorly to the middle fossa floor. The maneuvers added to the retrolabyrinthine and translabyrinthine approaches (division of the sigmoid sinus or tentorium) are seldom necessary to enhance this approach for basilar trunk aneurysms.

Combined Supratentorial and Infratentorial Approach

The various transpetrosal approaches sometimes are inadequate for large or giant basilar trunk aneurysms because the surgical corridor is confined by the residual petrous bone anteriorly, the tentorium superiorly, and the sigmoid sinus posteriorly. The exposure then can be enhanced through division of the tentorium, and posterior mobilization of the sigmoid sinus^29,30 that is designed to compensate for less presigmoid exposure. The petrosectomy then becomes the cornerstone of a combination approach that relaxes the superior and posterior barriers. The two critical additions are: (1) a supratentorial and infratentorial craniotomy that crosses the transverse sinus, and (2) division of the tentorium that provides communication between the supratentorial and infratentorial compartments. Very little brain retraction is needed then to expose the medial petrous and clival regions and associated neurovascular structures. The combined approach works with any of the petrosectomy variations but typically is used with a retrolabyrinthine approach instead of a transcochlear approach when it is desired to preserve seventh and eighth nerve function.

When the temporal bone drilling is complete, an edge of middle fossa dura above the petrosectomy defect and an edge of posterior fossa dura behind the sigmoid sinus are exposed. These serve as bur holes for a subtemporal-suboccipital craniotomy that crosses the transverse sinus. Once the bone flap is removed, a large dural surface is exposed, and the transverse, sigmoid, and superior petrosal sinuses are visible (Fig. 365-13). The brain is relaxed and the dura is incised anteriorly over the temporal lobe and curves posterior and inferior to the superior petrosal sinus below, where it enters the sigmoid sinus. A second dural incision is made inferiorly anterior to the sigmoid sinus, curving up to the superior petrosal sinus. The superior petrosal sinus is divided; the vein of Labbé is preserved when the dura is opened. Although rarely needed, the sigmoid sinus can be sacrificed when the contralateral transverse and sigmoid sinuses are patent and communicate with the ipsilateral sinuses. A test occlusion can be made to measure sigmoid sinus pressure after the superior petrosal sinus is divided; pressure should not increase by more than 10 mm Hg. If these angiographic and hemodynamic criteria are met, the sigmoid sinus can be divided below its confluence with the superior petrosal sinus. The vein of Labbé consistently enters the transverse sinus above this junction and so will drain contralaterally. Next, the tentorium is incised medially to the tentorial hiatus and posterior to the fourth nerve to connect the supratentorial and infratentorial compartments. The posterior temporal lobe is elevated without traction on the vein of Labbé, which is tethered to the transverse sinus. This provides wide exposure along the skull base from the foramen magnum to the dorsum sellae, with little need for brain retraction; the petrous region, clivus, brainstem, cranial nerves, and posterior circulation vessels now are seen easily.

FIGURE 365-13 Combined approach. The combined supratentorial and infratentorial approach is a combination of the subtemporal and transpetrosal approaches. A, Temporal bone is removed. From this a craniotomy is made that crosses the transverse sinus and exposes the dura of the temporal lobe and cerebellum. The dura is incised across the superior petrosal sinus. B, The tentorium is divided from the transverse-sigmoid junction to the incisura to expose the cranial nerves, brainstem, clivus, and basilar artery.

(With permission from Barrow Neurological Institute, Phoenix.)

Extended Middle Fossa Approach

The middle fossa approach was developed to remove small, intracanalicular acoustic neuromas and preserve the hearing apparatus.³¹ The approach was adapted to basilar aneurysms by skeletonizing the IAC and removal of the medial petrous apex. It is suitable for select small basilar aneurysms because the bony corridor created is narrow and the fifth, seventh, and eighth nerves can limit the view of the vertebrobasilar junction and reduce working space. This approach has several names, including the extended middle fossa approach, Kawase’s approach, and rhomboid approach.^32,33 The extended middle fossa approach has two differences from other lateral transpetrosal approaches: (1) it reaches the basilar artery from a more anterolateral and superior trajectory in front of the otic capsule and (2) the posterolateral temporal bone is intact.

The key to this approach to the basilar artery is removal of the medial petrous apex or Kawase’s triangle (Fig. 365-14). This is really a quadrangle of bone formed by the IAC posteriorly, the GSPN laterally, the lateral margin of the trigeminal nerve and ganglion anteriorly, and the medial edge of the petrous bone medially.³² The inferior petrosal sinus marks the inferior boundary. For the procedure the patient is positioned supine, with the head positioned as for a subtemporal approach. A question mark or horseshoe-shaped incision is used, and the skin and temporalis muscle flaps are reflected anteriorly. A 5- × 5-cm craniotomy is made in the squamosal portion of the temporal bone, two-thirds anterior and one-third posterior to the external auditory canal. The middle fossa floor is exposed, the dura elevated medially to the petrous ridge, where a self-retaining retractor is placed with its tip over the lip of the ridge. The middle meningeal artery is followed along the dura to the foramen spinosum, where it is coagulated and divided. The GSPN is identified about 1 cm medially; it originates from the seventh nerve geniculate ganglion and runs superficially along the middle fossa floor in an anteromedial direction. Lateral to the GSPN is Glasscock’s triangle, through which the petrous ICA can be exposed as it runs horizontally toward the cavernous sinus.

FIGURE 365-14 Extended middle fossa approach. Right-sided approach, surgeon’s view. Bone is removed from Kawase’s quadrangle (dashed line) to open a corridor to the basilar artery. GSPN, greater superficial petrosal nerve.

(With permission from Barrow Neurological Institute, Phoenix.)

Next, the arcuate eminence that marks the underlying superior semicircular canal is identified. This is a bony prominence along the middle fossa floor that is almost perpendicular to the petrous ridge. The GSPN and arcuate eminence form a 120-degree angle that is bisected by a line that parallels the IAC; drilling medially along this line exposes the IAC. Bone is removed around the porus acusticus and IAC, working medial to lateral. The cochlea (anterior) and the vestibule and ampulla of the superior semicircular canal immediately posterior surround the lateral IAC or fundus. The basal turn of the cochlea is vulnerable when drilling in the angle between the GSPN and the IAC; cochlea perforation causes complete hearing loss. As drilling proceeds laterally, the exposure is narrowed to avoid the cochlea and superior semicircular canal. The dura of the internal auditory canal is exposed laterally to Bill’s bar. The medial petrous apex is removed, drilling between the borders of Kawase’s quadrangle. The horizontal segment of the petrous ICA is identified laterally in Glasscock’s triangle and skeletonized to increase anterior and inferior exposure. Once drilling is complete, the petrous dural exposure extends from the superior to the inferior petrosal sinus. The superior petrosal sinus is coagulated and divided just lateral to the trigeminal ganglion. The posterior dura is opened to enter the posterior fossa. The middle fossa dura is incised laterally, and the tentorium is incised medially to the incisura, just behind the fifth nerve’s entrance in its dural canal. The lower basilar artery can then be exposed between the fifth and seventh nerves.

Midline Suboccipital Approach

This approach is suitable for aneurysms of the distal PICA after it courses around the anterior and lateral medulla, proximal vertebral lesions, or for PICA bypass procedures.³⁴ The midline approach does not provide good exposure of aneurysms located distally on the vertebral artery near the typical PICA origin.

The patient is placed prone, with the head flexed to open the interval between the foramen magnum and the C1 posterior arch. A midline incision is made from just above the inion to C3. The paraspinous muscles are mobilized laterally to expose the occiput, foramen magnum, C1, and C2. Complete exposure of the C2 spinous process improves soft tissue retraction. A midline suboccipital bone flap or craniectomy is performed from the transverse sinus to the foramen magnum, and the posterior C1 arch is removed. Additional lateral bone around the foramen magnum is rongeured off to increase exposure.

The dura is opened in a Y-shaped fashion, with care taken to control bleeding from the circular sinus. The cisterna magna arachnoid is opened, and the cervicomedullary junction is exposed. The vertebral arteries can be identified anterior to the dentate ligaments coursing anteromedially. Dissection in the tonsillomedullary fissure exposes the PICA distally along its course. To gain more distal exposure, the interhemispheric fissure can be split between the tonsils.

Far Lateral Approach

The far lateral approach provides wide exposure of the vertebral trunk and anterolateral brainstem and is the most common approach to vertebral trunk aneurysms^35–37 because most are unilateral and extend beyond the region that can be accessed through a midline exposure. Anterior inferior cerebellar artery (AICA), PICA, and some proximal basilar lesions also can be treated with this approach or modifications of it.³⁸ Vertebral artery exposure is improved by shifting the exposure corridor laterally by bone resection in the angle between the lower medulla and cerebellum. This creates a surgical corridor along the vertebral artery axis that requires minimal cerebellar retraction. The outer cranial exposure is enhanced by resection of the C1 posterior arch and the posteroinferior skull base (including the posterolateral foramen magnum, posterior half of the occipital condyle, and jugular tubercle). Synonyms for the far lateral approach include the lateral suboccipital approach, extreme lateral approach, and extreme lateral inferior transcondylar exposure (ELITE). The approach requires operating around and through the lower cranial nerves.

The patient may be placed in a lateral position, modified park bench or three quarters prone position with the lesion side uppermost. Patients with supple necks also may be positioned supine and the head turned. The patient’s superior shoulder is taped to keep the cervical-suboccipital angle open. The goal of positioning is to place the ipsilateral mastoid process at the highest point and maximally open the cervical-suboccipital angle. This puts the clivus perpendicular to the floor to allow the surgeon to look down the axis of the vertebral and basilar arteries and to work between the horizontally arrayed cranial nerves.

A hockey-stick incision is made that begins in the cervical midline over the C5 spinous process, extends cephalad to the inion, courses laterally along the superior nuchal line to the mastoid bone, and finishes inferiorly at the mastoid tip (Fig. 365-15). The midline nuchal ligament is identified to split the paraspinous musculature in this avascular plane. A cut just below the superior nuchal line detaches the paraspinous muscles, which are mobilized inferolaterally to expose the occipital bone and foramen magnum. This also creates a cuff for muscle reattachment during closure and mobilization rather than muscle transection and reduces postoperative pain. Exposure is continued to the C2 spinous process. The vertebral artery as it courses from the transverse foramen of the C1 lateral mass, through the sulcus arteriosus of the C1 vertebral arch, to its dural entry point is identified and protected. The lateral epidural venous plexus often causes bleeding; it is best preserved by blunt dissection.

FIGURE 365-15 Far lateral suboccipital approach. A, Skin incision (dashed line) and craniectomy (dotted line) to expose aneurysms of the vertebral artery and its branches. The foramen magnum rim is removed laterally to the occipital condyle (arrow) once the extracranial vertebral artery is exposed. The C1 posterior arch is removed between the midline and the arterial sulcus of the vertebral artery laterally (hatched area). B, The dura is opened in a gentle curve between the sigmoid and transverse sinuses junction superiorly to the midline just below C1. The dural flap is reflected laterally.

(From Heros RC: Aneurysms of the vertebral artery and its branches. In: Ojemann RG, Ogilvy CS, Crowell RM, et al, eds. Surgical Management of Neurovascular Disease. 3rd ed. Baltimore: Williams & Wilkins; 1995:292-293.)

The goal of bone removal is to expose the angle between the lower medulla and cerebellum and reduce cerebellar retraction. There are three parts to bone removal: a C1 laminotomy, a lateral occipital craniotomy, and a condylectomy. The C1 arch is removed with the drill; a cut is made just lateral to the sulcus arteriosus, and another is made across the contralateral arch. Additional atlantal bone can be removed under the vertebral artery lateral to the transverse foramen and if needed the foramen can be opened dorsally and the artery mobilized. Suboccipital bone is removed unilaterally from the foramen magnum in the midline to as far lateral as possible and then back around to the foramen magnum. The rim of the foramen magnum is rongeured to extend the opening across the midline and laterally toward the condyle. Then the lateral aspect of the foramen magnum, the posteromedial two thirds of the occipital condyle, and the jugular tubercle are removed using a diamond bur. The hypoglossal canal, the condylar emissary vein, or bone removal that exposes the dura as it begins to curve anteromedially define the anterior extent of condylar resection; this allows the dural flap centered on the condyle to be completely flat.

The dural incision (Fig. 365-15B) curves from the cervical midline, across the circular sinus, and laterally to the craniotomy edge. An inferior cut laterally under C1 mobilizes the flap farther against the bone-opening margin. The cisterna magna is opened under the microscope, and the arachnoid layers are reflected. The proximal vertebral artery is prepared for proximal control just as it penetrates the dura to keep temporary clips, if used, out of the surgical corridor. To do this the dentate ligament is divided. The tonsillomedullary fissure is dissected and the ipsilateral cerebellar tonsil moved away from the medulla to expose the trajectory along the vertebral artery that is dissected from proximal to distal. The PICA also is dissected distal to proximal. These converging lines lead to the PICA origin. The vertebral artery can be followed distally to the vertebrobasilar junction if needed.

Extended Far Lateral Approach

The far lateral exposure can be extended superiorly by occipital bone removal to the junction of the transverse and sigmoid sinuses.³⁹ This retrosigmoid addition enables the CPA to be entered, and large vertebral artery aneurysms and their efferent arteries can be accessed. The trajectories from the far lateral and retrosigmoid approaches are almost perpendicular (i.e., the retrosigmoid extension does not improve exposure but rather provides an additional vantage of anatomy to help clarify the anatomy and operative strategy). The approach is identical to the far lateral technique, but the superolateral edge of the craniotomy is defined by drilling through the mastoid bone to define the transverse-sigmoid junction. The sigmoid sinus is skeletonized to the jugular bulb. The subsequent far lateral bone opening then connects to this exposed dura, and the craniotomy flap is enlarged. The dura is opened with a flap based on the sigmoid sinus. When tacked anteriorly, the flap pulls the sinus forward to open the route into the CPA. The far lateral dural flap then connects with this flap.

Far Lateral Combined Supratentorial and Infratentorial (Combined-Combined) Approach

Some giant aneurysms that involve the vertebro-basilar junction or lower basilar trunk require an exposure that spans the entire length of the posterior fossa from the petrous apex to the foramen magnum. The far lateral approach, when used with the combined supratentorial and infratentorial approach, exposes the entire petroclival region.^40,41 This joins the transpetrosal, subtemporal, and far lateral approaches, and overcomes the limitations of a transpetrosal approach or an extended far lateral approach alone.

The patient is placed in the park bench position, with the head fixed as it would be for a far lateral approach. The hockey-stick incision is enlarged: it begins anterior at the zygoma, courses superior around the ear toward the inion, and ends inferiorly in the midline to about C5. A myocutaneous flap is elevated to expose the lateral temporal bone, mastoid, posterior cranium, and C1 and C2 laminae. A cuff of nuchal fascia is left to reattach the cervical muscles during closure.

Bone removal consists of four parts: a petrosectomy, a C1 laminotomy, a craniotomy, and a condylectomy. A neuro-otologist should first drill the temporal bone. Rotation of the patient’s head can be adjusted during the petrosectomy to bring the head parallel to the floor to facilitate drilling. The arch of C1 is exposed, the vertebral artery is identified, and a C1 laminotomy is performed. The craniotomy cut then connects the midline foramen magnum with the anterior margin of the petrosectomy over the inferior temporal lobe, crossing the transverse sinus just lateral to the torculum. A second cut connects the lateral foramen magnum with the posteroinferior petrosectomy margin, immediately behind the sigmoid sinus. The underlying dural sinuses are dissected from the bone flap that then is removed to expose a large dural surface, with the transverse and sigmoid sinuses in the middle. Finally, the lateral aspect of the foramen magnum, posteromedial two thirds of the occipital condyle, and jugular tubercle are removed.

The dura can be opened either in two flaps anterior and posterior to the sigmoid sinus to preserve it or in a single flap that sacrifices this sinus. The two dural flaps represent the usual openings for the combined approach and for the far lateral approach. The flaps create two windows of exposure on either side of the preserved sigmoid sinus. When the sigmoid sinus is sacrificed and crossed below the sinodural angle, the two incisions are joined to create a single flap that extends from the anterior margin of the craniotomy over the temporal lobe, across the sigmoid sinus, and down to C1. When the tentorium is incised to the hiatus, a large unobstructed opening is created that exposes the anterolateral brainstem from the midbrain to the upper cervical spinal cord. Arachnoid dissection then exposes the second through twelfth nerves, both vertebral arteries, the PICA, the anterior spinal artery, the vertebro-basilar junction, the basilar artery, and the entire length of the clivus.