CHAPTER 365 Surgical Approaches to Intracranial Aneurysms
Surgical Anatomy
Detailed surgical anatomy is beyond the scope of this chapter. The reader is referred to scholarly reviews that describe the relevant vascular and surgical anatomy and that of surrounding osseous and neural structures needed for aneurysm surgery by Rhoton (see Chapter 2), de Oliveira, Yasargil, and colleagues.1–5
Preoperative Considerations
Neurodiagnostic Studies
CT Angiography
CT angiography (CTA) uses a rapid injection of iodinated contrast and image acquisition during the arterial phase. During CTA, the tomographic data is collected in a three-dimensional array and then is reconstructed to provide anatomic information about the aneurysm and adjacent vessels. Reconstruction algorithms include maximum intensity projection (MIP), shaded surface display (SSD), volume rendering (VR), and ray-sum projection (RSP). These reconstructions provide a 3-D appearance to the aneurysm and vascular anatomy. Several clinical series demonstrate that CTA reconstruction can provide useful information about aneurysm morphology that may guide management decisions including when to triage patients to surgical or endovascular procedures.6,7 For example CTA is useful when a patient with a large intracranial hemorrhage (ICH) is evaluated and can eliminate the need for conventional DSA in the rapidly deteriorating patient. The information obtained from CTA also can supplement DSA images and provide useful information about the aneurysm lumen, presence of thrombosis, anatomic detail about the aneurysm neck, vascular relationships and bony landmarks, and in some instances may offer an aneurysm perspective that better approximates the surgical approach and view than conventional DSA.
Angiography
Four-vessel angiography in multiple projections remains the “gold standard” for diagnosis and treatment planning. Five features need to be evaluated: (1) the aneurysm’s vessel of origin; (2) aneurysm size, shape, and relationship to parent and adjacent arteries; (3) the presence and location of vasospasm; (4) adjacent vessel displacement because this suggests mass effect (e.g., ICH or partial aneurysm thrombosis [i.e., the aneurysm dimension is larger than that seen on DSA]); and (5) the presence of other aneurysms or vascular abnormalities. When multiple aneurysms are present after a SAH, several radiological features can be used to decide which lesion ruptured (Table 365-1). Spin, rotational, and three-dimensional DSA with appropriate algorithms can display 3-D data in an orientation similar to intraoperative views.8 This is particularly useful for large and complex aneurysms.
If the site of rupture cannot be reliably identified, all aneurysms may need to be treated starting with the one most likely to have ruptured. The role of MRI to identify site of rupture remains ill defined. | |
Exclude extradural aneurysms | |
CT | Focal clot accumulation |
Angiographic features | Focal vasospasm Focal mass effect (perianeurysmal hematoma) Aneurysm change on serial angiogram |
Aneurysm morphology | Active bleeding with contrast extravasation Multilobulated or irregular Larger size Daughter aneurysm (nipple) |
Focal clinical signs |
Anesthesia
There is no standard anesthetic regimen for all aneurysm surgery and the technique should be individualized (see Chapter 21). There are several basic principles9,10 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
Prevent aneurysm rupture | Avoid hypertension Avoid rapid ICP decrease with hyperventilation |
Prevent cerebral ischemia | Maintain adequate cerebral perfusion pressure (70 to 80 mm Hg) Optimize oxygen delivery (Hgb, PaO2) Decrease cerebral metabolic demand (pharmacologic suppression, prevent hyperthermia) |
Maintain euvolemia | Monitor CVP, urine output, blood loss Normal saline replacement |
Optimize surgical exposure | CSF drainage Cranial venous drainage Hyperventilation Osmotic diuretics |
Monitor electrolytes/glucose | Prevent hyperglycemia |
Cardiac stability | Control blood pressure with short-acting agents EKG monitor—arrhythmias |
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.
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).
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.
Anterior Circulation
Pterional Craniotomy
Most anterior circulation aneurysms can be approached through a pterional craniotomy, including aneurysms of the ICA, posterior communicating artery (PcomA), anterior communicating artery (AcomA), and middle cerebral artery (MCA). Specific lesions may require minor modifications (e.g., orbitozygomatic approach for some AcomA aneurysms or anterior clinoidectomy for carotid-opthalmic artery aneurysms). For aneurysms that involve the proximal ICA (i.e., paraclinoid region), exposure of the cervical ICA is recommended for proximal control. Because the pterional approach is frequently used in aneurysm surgery it is discussed in detail (Fig. 365-3).
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.
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.
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.
Posterior Communicating Artery
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.
Anterior Cerebral and Anterior Communicating Artery Aneurysms
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):
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