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.)

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