Subtemporal Approach to Basilar Tip Aneurysms

Introduction: Management of basilar bifurcation aneurysms remains one of the most difficult tasks in neurosurgery. In spite of the increased use of endovascular methods, total obliteration of the base of the aneurysm by clip remains the most enduring cure. The art of the proper use of the subtemporal approach in surgery of basilar bifurcation aneurysms lies in getting quick access to the tentorial edge without excessive compression of the temporal lobe. A clear understanding of the three-dimensional relationship between the parent artery, aneurysm, and skull base landmarks is of paramount importance.

A classic subtemporal approach has been used historically for rhizotomy of the trigeminal nerve. Professor Drake developed the anterior subtemporal approach while working out the approaches for the various segments of the basilar artery in the postmortem room in 1958. This approach across the floor of the middle cranial fossa and tentorial edge into the mouth of the incisura in front of the midbrain seemed to provide the most direct and widest exposure of the interpeduncular region. Drake and Peerless used an anterior subtemporal approach for the surgical treatment of 1250 cases in their never-to-be-repeated series of 1767 patients with vertebrobasilar artery aneurysms (VBAAs) between 1959 and 1992. Eighty percent (80%) of the 1234 patients with basilar tip aneurysms were treated by the subtemporal approach and the frontotemporal approach was only indicated in the presence of multiple aneurysms on the anterior circulation. Much of the merit of an approach is a matter of continued use and familiarity with the anatomy. The subtemporal approach was adopted by the author (J.H.) during the 1980s and refined later during collaboration with Drake and Peerless.

Imaging: DSA is still the present gold standard for imaging of posterior circulation aneurysms. However, multislice helical CTA is the primary imaging modality in our center. CTA is a noninvasive imaging technique with comparable sensitivity and specificity to DSA in aneurysms larger than 2 mm. Furthermore, CTA has the distinct advantages of providing good visualization of the relationship of the aneurysm to neighboring vasculature and bony structures, demonstrating calcifications in the walls of arteries and the aneurysm neck, and quickly reconstructing 3D images. Two-dimensional reconstructed oblique images allow the surgeon to evaluate detailed anatomic relationships.

Neuroanesthesiologic Principles: Readers are referred to a general review of our neuroanesthesiologic principles in aneurysm surgery.

CSF Drainage: Spinal drainage is routinely used in the subtemporal approach. Drainage of 50 to 100 mL of CSF prior to craniotomy offers us a slack brain even in the presence of acute SAH in good grade patients.

Approach and Craniotomy: The patient is placed in park bench position, with the head above the cardiac level. The right side is usually preferred unless the projection or complexity of the aneurysm, scarring from earlier operations, severe anatomical rotation of the basilar bifurcation, a left oculomotor palsy, a left-sided blindness or a right hemiparesis, dictates an approach under the dominant temporal lobe. A linear incision perpendicular to the zygomatic arch 1 cm in front of the ear is carried to the level of the zygomatic arch. In order to preserve the upper branch of the facial nerve, we retract the lower part of the incision downward (toward the zygomatic arch) with a fish hook. The next step is to split the fascia and temporal muscle along its fibers upright to the arch. The muscle with its fascia is dissected posteriorly from the temporal root of the zygoma. Anteriorly, both leaves of the temporal fascia are separated from the arch carefully for a centimeter or so, usually with blunt dissection so as not to injure the nerve. This allows wider spreading of the temporal muscle with a curved retractor and fish hooks. A small craniotomy (3 × 3 cm) is done through a single burr hole placed on its upper margin. Extreme care is taken not to injure the dura, which may be attached, especially at the lower border of the craniotomy. A part of the zygomatic arch can be resected with a diamond drill if needed. Subtemporal craniotomy can be easily converted to a pterional or posterior temporal bone flap. However, this has been seldom necessary. The dura is opened in a V shape with its base downwards. Dural edges are elevated by multiple stitches, extended over craniotomy dressings. Epidural oozing can be prevented by fibrin glue. From this point on, all surgery is performed under the operating microscope, including the skin closure.

Dissection toward Aneurysm: The exposed surface of the brain is covered with Oxycel and large wet cottonoids. The temporal lobe is retracted first slightly downward, then across the floor of the middle fossa, and after that upward to the tentorial edge. The operative route is created like taking the temporal lobe in your retracting hand and pulling upward to see the tentorial edge. One to two self-retraining retractors are applied subtemporally. Broad spatulas or instruments divide the pressure on broader areas with less injury to the temporal lobe. Utmost care is taken not to injure the veins, but sometimes one or two small bridging veins to the floor of the middle fossa have to be severed. In order to prevent retraction injury, other veins on either side can be freed a few millimeters from the arachnoid. The junction of the vein of Labbé and the transverse sinus is the most critical point at which to prevent major hemorrhagic infarction. After visualization of the tentorial edge, the uncus is elevated by the retractor tip to expose the opening to the interpeduncular cistern. As the third cranial nerve is usually elevated with the uncus it is possible to work below the third nerve to clip the aneurysm. In the case of a high basilar bifurcation or a giant sac, it may be necessary to separate the third nerve from the uncus by sharp dissection of the dissecting arachnoid bands holding it. To have better access to the interpeduncular fossa, the tentorial edge may be retracted by placing a small aneurysm clip with one blade through a small incision created by sharp bipolar forceps in the middle cranial fossa, and the other blade at the free margin of edge. Special care is needed to avoid the insertion point and intradural course of the fourth nerve under the tentorium. Bleeding may be heavy and is stopped by the use of fibrin glue. This maneuver may displace the tentorial edge for 1 cm or so. The fourth nerve is then dissected free from its arachnoid adhesions, and can be tucked below a cottonoid under the tentorial edge. In order to have better access to the basilar artery for temporary clipping, and in low-lying aneurysms, the tentorium may be divided by sharp incision and coagulation and free margins fixed with one or two small clip(s). The transmastoid-transpetrosal presigmoid approach, through a completely divided tentorium, is reserved for complex and large aneurysms with low-placed necks, as well for basilar trunk aneurysms. Sharp dissection of the arachnoid is performed using microscissors. Dissection is started above the fourth nerve and the superior cerebellar artery and below the third nerve on the side of the midbrain, and carried forward anterior to the third nerve visualizing also the carotid and posterior communicating artery. The third nerve tolerates little manipulation, but its potential for postoperative recovery in the long term (weeks or months) is nearly 100%. Even in the presence of complete paralysis in the early postoperative period, complete recovery is observed in 91% of cases where no paresis existed before the operation. Usually, there is a small gap between the peduncle and the dorsum sellae laterally. Furthermore, the brainstem can be crowded against the clivus, hiding the interpeduncular fossa. Depending on the extent and interval from SAH, the interpeduncular fossa may be filled with fresh or disintegrating clot or otherwise with clear CSF. In long-standing SAH, dense thick arachnoid bands may be present. Painstaking arachnoid dissection and CSF drainage is mandatory to get enough space. The peduncle can be retracted with instruments and or cottonoids to see the basilar artery and the base of the aneurysm. The posterior clinoid is not removed as the work in this position is lateral (posterior) to it. P1 is always above the third nerve and the posterior communicating artery joins it; the SCA is always below. To avoid heavy retraction, it is our practice to follow the superior cerebellar artery back under the third nerve to the basilar artery, sucking away clots to expose its origin just below that of P1. The lower aspect of P1 then followed outward will disappear underneath the third nerve. The position of the lateral aspect of the neck and waist of the aneurysm is now known as being just medial to P1 and usually partially covered laterally and posteriorly by the P1 perforators. The posterior communicating artery should be carefully preserved in case some injury to P1 occurs or if it becomes necessary to include P1 in the clip. Furthermore, the communicating artery gives rise to important diencephalic branches (anterior thalamoperforating arteries), the integrity of which might be compromised by its occlusion. The same rules are valid during dissection and exposure of contralateral P1 and SCA.

Temporary Clipping: Systemic hypotension, down to a mean arterial pressure (MAP) of 40 to 50 mmHg, has been widely recognized to reduce the tension and fragility of the aneurysm wall. However, temporary occlusion of the parent artery can do the same. In our practice, temporary clipping is used in nearly all of the basilar bifurcation aneurysms. However, this may not be applicable to highly calcified, fragile vessels. Temporary occlusion of the parent artery softens the dome; facilitates dissection, coagulation, or clipping; and reduces the risk of intraoperative rupture. A segment of the basilar artery below the SCA and free of perforators for 2 or 3 millimeters should be always exposed for placement of a temporary clip. Due to highly variable collateral circulation to the basilar bifurcation complex, the safe time for temporary occlusion is unknown. Our experience indicates that 3-to 4-minute intervals are safe for any artery, and much dissection can be done in that time. Mean total occlusion time in small and large basilar bifurcation aneurysms has been 11 minutes with three to four applications of the temporary clip. There were no undue complications in the cases with a total application time of more than 30 minutes. For complex and large or giant aneurysms of basilar bifurcation, particularly with thick posterior communicating arteries, one or both of them are temporarily clipped also. It is more convenient to leave the temporary basilar clip in place while removing and replacing the clip on a posterior communicating artery to provide the intervals for reflow.

Dissection and Clipping of the Aneurysm: Dissection technique varies according to height of basilar bifurcation and projection of the aneurysm dome: forward (seldom, may be attached to clivus when low), upward (most common), or backward (most difficult due to vicinity of the perforators). Sharp and blunt dissection in the anterior aspect of the basilar artery will expose the origin of ipsilateral P1 at the bulging of the anterior aspect of the aneurysm base. Visualization of the contralateral P1 depends on the size and the projection of the dome. The site of origin of P1s will determine the angle and length of the clip to be applied. Perforators arising from P1s and also the course of the opposite third nerve are the other important factors. The origin of the SCA may not be mistaken for P1, or else inadvertent occlusion of the basilar bifurcation will occur.

Thalamoperforating arteries arising from the posterior wall of the basilar trunk and their involvement to the dome and base is the major challenge in the treatment of this group of aneurysms. Perforators arising from P1 near its origin course obliquely upward and backward. Gentle forward retraction of the dome with sucker tip facilitates their visualization. Temporary occlusion of the parent artery is mandatory for dissection and releasing them from the dome. We use a small curved dissector to clear and separate any perforators clinging behind the base. Occasionally, the base of the aneurysm can be displaced forward enough to see the origin of the opposite P1 and the root of the opposite third nerve across the interpeduncular fossa.

Adequate dissection, proper size of clips, and painstaking and careful checking that both blades are placed properly up to their tips are required to preserve adjacent arteries. The length of the clip should be selected properly according to radiological data. The blade of a single occluding clip should be 1.5 times longer than the width of the base. Longer clips may cause obliteration at the origin of opposite P1 or its perforators.

Optimal obliteration of the aneurysm is achieved with the blades applied across the neck in parallel with the parent bifurcation, reducing also the risk of P1 kinking. This ideal placement is more likely to occur with the subtemporal exposure. More perpendicular application of the clip often leaves tags of neck in front and behind (“dog ears”), as the sides of the neck are approximated. These neck remnants may grow with time. A low-profile clip applier should be used so as not to obscure vision during application. The design of the fenestrated clip in 1969 by Drake was another important step in clipping the basilar bifurcation aneurysm. P1, adjacent perforators, and even the third nerve can be included safely to the aperture while the blades are occluding the base.

Intraoperative Verification of Clipping: As for any aneurysm, immediate inspection is done mandatorily to determine that each P1 is open and no perforators are caught or kinked by the blades. Rotating the clip handle forward usually exposes the posterior blade and looking just above the blade will determine whether or not any perforators emerge from underneath it. If so the clip must be removed and reapplied as many times as is necessary for perfect placement. We routinely use micro-Doppler to check the patency of the branches after clipping but surprise occlusions may still be seen in postoperative angiography. Noninvasive ICG infrared angiography is very promising in our hands. It helps orientation during dissection, and it visualizes wall thickness and plaques, perforating arteries, and incomplete neck occlusion. ICG angiography will reduce the need for invasive angiography for intraoperative clipping control. Intraoperative DSA using C-arm should be preserved for special cases. Local papaverin is applied after clipping of the aneurysm. The wound is closed to the last stitch under the microscope.

Conclusion: The main advantages of this approach are the ability to dissect and preserve the perforators from the posterior wall of the aneurysm. The microneurosurgeon is able to visualize the application of the clip to the aneurysm in a fast and simple way and regardless of its size, height, direction, or multilocularity. The inner third of the tent can be divided for very low necks and placement of a temporary basilar artery clip, and there is no necessity to open the cavernous sinus or to remove the posterior clinoid or inner petrous apex.