Pterional Craniotomy for Exposure of Contralateral Aneurysms

Patient Selection

21.2.1 Multiple Intracranial Aneurysms

The majority of patients with multiple aneurysms harbor 2 aneurysms, although as many as 13 aneurysms have been described in a single patient. Approximately half of multiple aneurysms occur on different sides. ¹^, ⁶ Bilateral carotid-ophthalmic segment aneurysms, usually in females, are one of the most common situations of multiple aneurysms.

In appropriately selected patients for surgical treatment of intracranial aneurysms, a common objective for treatment of multiple supratentorial aneurysms is to treat the ruptured or symptomatic aneurysm first via an ipsilateral pterional craniotomy as well as other ipsilateral aneurysms, except in cases of ruptured aneurysms where exposure of additional aneurysms would be difficult. Typically, one returns at a subsequent time to clip the any remaining contralateral aneurysms via a second pterional craniotomy. This is based on the central tenet of intracranial aneurysm surgery of obtaining the requisite proximal vessel control during microdissection.

Using the contralateral approach, aneurysms of the precommunicating segment of the anterior cerebral artery and the ICA bifurcation are observed in a straight line and can be clipped with relative ease with minimal risk of injury to the perforating arteries. Aneurysms arising from the posterior communicating or anterior choroidal arteries point posterior and lateral, making repair from the contralateral side difficult. MCA aneurysms, particularly when the M1 segment is short, can also be explored from the contralateral side. ⁷ Avoidance of a separate, second craniotomy to treat contralateral, incidentally detected aneurysms is desirable because it avoids a waiting period for treatment of a known unruptured aneurysm, as well as additive risks, recovery time and costs of a separate craniotomy or embolization procedure, general anesthesia, and hospital stay. ³^, ⁴^, ⁸ Disadvantages of approaching unruptured aneurysms from the contralateral side include increased depth of exposure, lack of easily accessible proximal control, and need for additional dissection and brain retraction in some cases. Lack of proximal control, exposure, and brain retraction are issues in the setting of acute hemorrhage when the brain is swollen and anatomy is obscured by blood.

Currently, the other option for incidental aneurysms with appropriate anatomical characteristics is endovascular coil embolization instead of a second craniotomy. It is important to consider the development of endovascular techniques as an additional tool, especially for those who present with subarachnoid hemorrhage and contralateral aneurysms. Endovascular coiling may be chosen whenever a contralateral approach seems too risky.

21.2.2 Carotid-Ophthalmic Segment Aneurysms

Carotid-ophthalmic segment aneurysms frequently arise from the medial wall of the ICA and are often bilateral. This renders a subset of cases of nongiant aneurysms to be particularly well suited for a contralateral approach ( Fig. 21.1). Optimal results from microsurgical treatment of carotid-ophthalmic segment aneurysms necessitate understanding of the anatomical relationship of these aneurysms to the distal dural ring at the roof of the cavernous sinus, anterior clinoid process including the optic strut, falciform ligament, optic nerve, and chiasm, origins of the ophthalmic and superior hypophyseal arteries, and pituitary stalk. ⁶^, ⁷ The limited working space of the paraclinoidal area makes avoidance of trauma to the optic nerve and chiasm during microdissection of paramount importance. Complete division of the falciform fold is essential to permit safe mobilization of the optic nerve. Intradural drilling is usually unnecessary for small aneurysms, and if partial removal of the tuberculum sellae is required, this is more easily accomplished than the removal of the anterior clinoid process and the optic strut that is necessary in ipsilateral approaches. ⁵ Size, location, morphology, and dome projection of carotid-ophthalmic aneurysms should be carefully evaluated for optimal patient selection. ⁹ Saccular aneurysms, smaller than 10 to 15 mm, that displace the optic nerve superolaterally and the optic chiasm superiorly, arising from the medial or superomedial wall of the ICA, are suitable candidates to be approached from a contralateral approach ( Table 21.1). Complex aneurysms (giant, fusiform, bilobulate, the presence of wall irregularities and calcifications) are preferentially approached via an ipsilateral craniotomy. ⁵^, ⁶^, ⁷

Fig. 21.1 Exposure of contralateral aneurysms at the: A, ophthalmic region; B, posterior communicating segment; C, carotid termination; and D, middle cerebral artery, and their relationships to the triangular subarachnoid spaces.

**Table 21.1** Correlation between triangular spaces and aneurysm subtype approached by a contralateral pterional craniotomy ³
Triangular space	Aneurysm type
The interoptic space	Carotid-ophthalmic and superior hypophyseal artery aneurysms projecting medially Posterior communicating and anterior choroidal segment aneurysms projecting inferiorly
The opticocarotid space	Posterior communicating and anterior choroidal artery aneurysms projecting posterolaterally
The supracarotid space	Internal carotid artery bifurcation aneurysms projecting superiorly
Middle cerebral artery space	Middle cerebral artery bifurcation aneurysms projecting superiorly or inferiorly

Since the most common route for contralateral clipping of those aneurysms is through the interoptic space, patients who present with short interoptic space and/or a prefixed chiasm may not be ideal candidates for this approach. Preoperative evaluation with 3D reconstruction computed tomography scans and analysis of the distance between the optic nerves as well as distance between the ICAs at the level of the tuberculum may be helpful. ⁹^, ¹⁰

Relative contraindications to the contralateral approach include a prefixed optic chiasm, giant aneurysms, large ruptured aneurysms, and aneurysms that originate laterally or displace the optic nerve superomedially.

21.3 Operative Procedure

The exposure of contralateral aneurysms begins with the patient positioned for a standard pterional craniotomy. To minimize brain retraction, maximize microscopic illumination for anatomical visualization, and enlarge the space for working corridors to the contralateral vessels, meticulous extension of the fronto-orbital exposure is recommended. This involves removal of the inner table of the medial and inferior frontal bone at the anteromedial extent of the craniotomy and flattening of the irregular surface of the ipsilateral orbital roof and sphenoid wing with a high-speed drill. The greater and lesser medial sphenoid wings adjacent to the sylvian fissure are drilled flat to connect the lateral extent of the anterior cranial fossa with the anterior extent of the temporal fossa. ⁵^, ⁶

After the dura is opened and tented flat against the sphenoid wing, the microscope is brought in to the field and sharp arachnoid microdissection is initiated with entry into the sylvian cistern at the region of the frontal operculum. Wide opening of the sylvian fissure diminishes the need for frontal lobe retraction and provides a broad corridor for dissection of the basal cisterns. Arachnoid dissection continues into the ipsilateral opticocarotid cistern. Complete exposure of the optic apparatus is necessary for orientation. The release of cerebrospinal fluid (CSF) from the basal cisterns will allow brain relaxation. In cases of thick subarachnoid clot in the basal cisterns in which brain relaxation is anticipated to be inadequate via CSF release from opening of the basal cisterns, a lumbar subarachnoid catheter or intraventricular catheter can be used. After the ipsilateral optic nerve is released from the frontal lobe, the chiasmatic cistern is opened. Further brain relaxation may be accomplished by release of CSF from the interpeduncular cistern with opening of the membrane of Liliequist and opening of the ventricular system by fenestration of the lamina terminalis ( Fig. 21.2). Continued contralateral dissection then begins with opening of the contralateral optic and carotid cisterns ( Fig. 21.3).

Fig. 21.2 Opening of the lamina terminalis with use of a sickle knife or microsurgical bipolar allows further drainage of CSF, reduces the need of retraction over the brain parenchyma, and improves the exposure of contralateral vascular structures.

Fig. 21.3 Exposure of the contralateral carotid bifurcation, A1, and M1 branches, after extensive opening of the basal cisterns and lamina terminalis.

21.3.1 Contralateral Exposure of Carotid-Ophthalmic Segment Aneurysms

Complete exposure of the optic chiasm is followed by dissection of the interoptic space. This triangular space that is bordered by the planum sphenoidale anteriorly and optic nerves bilaterally is dissected to gain access to the contralateral ophthalmic artery origin and superior hypophyseal artery origins that are medial. Prior to dissection of the medial wall of the contralateral carotid artery from which the contralateral ophthalmic segment aneurysm arises, mobilization of the contralateral optic nerve is necessary to diminish traumatic nerve injury during manipulation. This is accomplished by longitudinal incision of the contralateral falciform ligament at the center of its circumference as far as the roof of the optic canal ⁶^, ⁷ ( Fig. 21.4). Rarely, further untethering of the contralateral optic nerve is possible with unroofing of the contralateral optic canal with a high-speed drill and small diamond burr. Generous irrigation is performed while drilling close to the optic nerve in order to avoid thermal injury of the nerve. After such maneuvers, mobilization of the contralateral optic nerve laterally enables visualization of the medial wall of the paraclinoid ICA. It is through this interoptic space that the neck of small carotid-ophthalmic and superior hypophyseal artery aneurysms that project medially may be defined for suitable clip placement ( Fig. 21.5 and Fig. 21.6).

Fig. 21.4 Opening of the contralateral falciform ligament is carefully performed with a sickle knife in order to achieve further mobilization of the contralateral optic nerve and exposure of ophthalmic and superior hypophyseal aneurysms.

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