Subtemporal and Pretemporal Approaches for Basilar and Posterior Cerebral Artery neurysms

Patient Selection

Aneurysms at the distal third of the basilar artery including the basilar artery bifurcation and the origin of the superior cerebellar artery from the basilar artery and less commonly aneurysms that originate from the proximal posterior cerebral artery are most commonly repaired by endovascular methods. Nevertheless, in cases of aneurysms with wide necks and/or incorporation of the origins of the posterior cerebral arteries into the aneurysm, open neurosurgical clipping may be the best option.

The surgical approaches to the upper basilar artery include pterional and its extended variations (orbitozygomatic and supraorbital, which facilitate exposure of a high basilar bifurcation), temporopolar, pretemporal, subtemporal, or various combined approaches ( ▶ Fig. 13.1). Selection of approach depends on several factors including the relationship of the basilar bifurcation to the posterior clinoid processes, direction of orientation of the aneurysm and relationship to the midline, rupture status, presence of additional aneurysms, and size of the posterior communicating arteries. From the pterional direction, aneurysms where the neck is lower than about halfway from the posterior clinoids to the floor of the sella will be difficult to expose; additional exposure usually would be needed to see the basilar artery in order to achieve proximal control.

Fig. 13.1 (a, b) A virtual pyramid construction representing the suprasellar structures helps conceptualize the various viewing angles into this space using a subtemporal exposure. The different windows between the anatomical structures can be used through different angles in a more anterior or more posterior approach.

The pterional approach was popularized by Yasargil. ¹^, ² It provides a slightly oblique and straight downward view of the anterolateral aspect of the basilar bifurcation that is seen through some combination of the space between the internal carotid artery and optic nerve, between the internal carotid artery and third nerve, above the carotid bifurcation or by opening the tentorium cerebelli lateral to the third nerve. ³^, ⁴ Both precommunicating (P1) segments of the posterior cerebral arteries and the origins of the superior cerebellar arteries can be seen. The difficulties are for anteriorly projecting aneurysms where one potentially exposes the dome of the aneurysm first and difficulty seeing the perforators coming off the basilar tip behind the aneurysm. ²^, ³ The temporopolar approach of Sano is a pterional craniotomy with a more extensive exposure of the temporal lobe. ²^, ⁵^, ⁶^, ⁷^, ⁸ The temporal pole is retracted posterolaterally to expose the space between the third nerve and the free edge of the tentorium, creating an anterolateral view of the interpeduncular fossa.

This chapter describes the subtemporal and pretemporal approaches. The pretemporal approach combines the advantages of these approaches in one craniotomy. ⁹^, ¹⁰^, ¹¹ This approach is based on extended resection of the sphenoid and temporal bones, wide opening of the basal cisterns, and detachment from the frontal lobe to the temporal lobe. It combines the multiple angles of view offered by the pterional and subtemporal approaches ( ▶ Fig. 13.2, ▶ Fig. 13.3, ▶ Fig. 13.4, ▶ Fig. 13.5, ▶ Fig. 13.6). The subtemporal approach was first described by Gillingham in 1958 and then popularized by Drake ( ▶ Fig. 13.7, ▶ Fig. 13.8, ▶ Fig. 13.9). The subtemporal approach is used for basilar bifurcation aneurysms as well as for some aneurysms arising from the superior cerebellar and proximal posterior cerebral arteries. It is most favorable in patients with a low-seated basilar bifurcation in relation to the posterior clinoid process. A prefixed chiasm or low-positioned hypothalamic structures also make the subtemporal approach preferable to others. The true advantage of this approach is the excellent exposure and direct visualization of the back of the aneurysm and thus the vital perforators of the basilar tip and precommunicating (P1) segments of the posterior cerebral arteries. This makes it particularly useful for posteriorly directed aneurysms. When the approach is from the side, the direction of the clip blades tends to be parallel to the basilar bifurcation so there is a low risk of kinking the P1 segments. Limitations include the fact that the contralateral neurovascular structures are less easily seen and controlled. Hence, careful preoperative planning to determine the side of the craniotomy is mandatory. Also, because temporal lobe retraction is required, brain relaxation has to be achievable, so the approach may be more difficult than other approaches in the setting of acute subarachnoid hemorrhage (SAH).

Fig. 13.2 (a) Pretemporal craniotomy showing that the greater and lesser wing of the sphenoid, the squamous part of the temporal bone, and the orbital roof are drilled out. (b) Pretemporal craniotomy showing subtemporal extradural view.

Fig. 13.3 Pretemporal craniotomy exposure allows for microsurgical transsylvian, temporopolar, and subtemporal approaches.

Fig. 13.4 The pretemporal approach allows a large exposure of the basal cisterns and the visualization of the carotid artery and its bifurcation, the optic nerve, and the third nerve.

Fig. 13.5 The interpeduncular cistern can be accessed between the internal carotid artery and the optic nerve, the internal carotid artery and the third nerve, from above the carotid bifurcation, and by retracting or opening the tentorium cerebelli lateral to the third nerve.

Fig. 13.6 The anterolateral route shows the interpeduncular cistern through the space between the carotid artery and the third nerve, exposing the basilar artery.

Fig. 13.7 Subtemporal route showing the posterior cerebral artery.

Fig. 13.8 Cadaveric specimen showing subtemporal view; the posterior communicating artery connects with the posterior cerebral artery. The superior cerebellar artery is below the third nerve and the posterior cerebral artery is above the third nerve.

Fig. 13.9 The four important steps of patient positioning. After elevation, the head is carefully rotated 80 to 100 degrees to the contralateral side until the zygomatic arch is in a horizontal position. To facilitate this maneuver, the ipsilateral shoulder might be elevated by a cushion. Thereafter, the head is lateroflexed to the contralateral side to support the gravity-related self-retraction of the temporal lobe and allowing an ergonomic working position for the surgeon. The last step is to retroflex the head 15 to 20 degrees to prevent compression of the larynx and the ventilation tube.

13.2 Anatomy

The posterior cerebral artery originates from the basilar apex immediately distal to the origin of the superior cerebellar arteries and runs into the supratentorial space ( ▶ Fig. 13.7 and ▶ Fig. 13.8). The P1 segment of the posterior cerebral artery runs laterally from its origin at the basilar bifurcation, in the interpeduncular fossa, until the junction with the posterior communicating artery. The most common branches arising from this segment are the posterior thalamoperforating arteries, the medial posterior choroidal artery, the branch to the quadrigeminal plate, the long and short circumflex arteries, and the branch to the cerebral peduncle and mesencephalic tegmentum. The P2 segment begins at the junction with the posterior communicating artery and runs through the crural and ambient cistern until the most posterior and lateral edge of the midbrain. This segment might be divided in two portions: the P2A or crural segment, which courses around the cerebral peduncles, and the P2P or ambient segment, which courses lateral to the midbrain. Usually, the hippocampal, anterior temporal, and peduncular perforating branches, and, sometimes, the medial posterior choroidal artery, originate from the P2A segment, whereas middle temporal, posterior temporal, common temporal, thalamogeniculate, and lateral posterior choroidal arteries most frequently arise from P2P.

13.3 Preoperative Preparation

A cranial computed tomography scan with 3D reconstruction is needed to evaluate the direction of projection of the aneurysm, relationship to the posterior clinoid processes, size of the ventricles, and any cerebral atrophy that will give an indication as to how much brain relaxation will be achieved by cerebrospinal fluid drainage. Calcification in the adjacent arteries and aneurysm can be looked for. A magnetic resonance imaging study will show the topographic relationship of the aneurysm to the surrounding structures. A prefixed chiasm or a low-situated hypothalamus can be detected, making the subtemporal approach relatively more favorable. Catheter digital subtraction angiography will demonstrate perforating arteries. An unsubtracted lateral view should be done to determine the relationship of the aneurysm to the posterior clinoid processes.

For basilar artery aneurysm surgery, a full selection of clips, especially fenestrated and bayonetted, is mandatory. Temporary occlusion of the basilar artery may be necessary and can be accomplished by temporary clipping or with endovascular balloon occlusion with a catheter in the basilar artery. Intraoperative angiography is also beneficial to demonstrate the pre- and postclipping situation immediately.

Anticonvulsants are not indicated unless the patient has a seizure before surgery or in the rare case of a patient with epilepsy. Single injections of perioperative antibiotics are given during surgery. We avoid lumbar drainage and use a drain inserted into the temporal horn of the lateral ventricle at surgery if needed for relaxation. The monitoring consists of evoked potentials for aneurysms that project posteriorly into the pons or mesencephalon.

13.4 Subtemporal Approach

13.4.1 Positioning of the Patient

The patient is positioned supine with the head elevated above the heart level to facilitate venous drainage ( ▶ Fig. 13.9). The ipsilateral shoulder should be elevated with a cushion to prevent extensive head and neck rotation that could compress the contralateral jugular vein or carotid artery. The head is fixed in a pin head holder, radiolucent if intraoperative angiography is planned, and rotated 80 to 100 degrees to the contralateral side. The zygomatic arch should be in an almost horizontal position. The next important step is to laterally flex the head 15 to 20 degrees to the contralateral side (toward the floor) to provide an ergonomic working position for the surgeon by compensating for the upward angle of the floor of the middle cranial fossa. This maneuver is also important because it promotes gravity-related as opposed to physical retraction of the temporal lobe. Extension of the head (retroflexion) of about 10 degrees may prevent compression of the larynx and the ventilation tube.

13.4.2 Skin Incision

An epifascial skin incision of around 50 mm is performed in a vertical line from the zygomatic arch superiorly and 10 mm anterior to the external auditory meatus ( ▶ Fig. 13.10). The subcutaneous tissue is dissected, preserving the superficial temporal artery and auriculotemporal nerve. The fascia of the temporal muscle is incised in a Y-shaped fashion ( ▶ Fig. 13.11, ▶ Fig. 13.12, ▶ Fig. 13.13). The basal leaflet is retracted downward over the zygomatic arch to protect the temporal branch of the facial nerve while the remaining leaflets are retracted bilaterally to expose the temporalis muscle. In some cases, the inferior margin of the muscle can be mobilized bluntly upward to uncover the squamous portion of the temporal bone. In patients with thick temporal muscle, a small vertical incision at the posterior margin is necessary. Postoperative problems with mastication, mandible opening, and muscular atrophy can be prevented and the cosmetic outcome improved by minimizing the exposure and muscle dissection.

Fig. 13.10 The right temporal area from the surgeon’s view. For preoperative orientation, the anatomical landmarks of the temporal skull as the lateral orbital rim, the zygomatic arch, the external auditory meatus, and the mastoid process are precisely defined. Note the course of the superficial neurovascular structures of the preauricular temporal region such as the superficial temporal artery and the auriculotemporal nerve in relationship with the skin incision and the size of the craniotomy.

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