Basilar Artery Bypass Strategy

Bypasses to the basilar artery are the most difficult to perform. The basilar artery is entombed at the geographic center of the head, guarded by the petrous bone laterally and the clivus anteriorly. It is reached through long operative corridors and small windows, and if constrained surgical freedom and obscured views were not frustrating enough, the basilar artery is embedded between the cranial nerves and the brainstem, which react to even the most delicate touch. Anastomosing arteries at the depth of the basilar artery is analogous to mountain climbing at extreme altitudes. That which is comfortable on worn-down trails in hospitable conditions at modest elevations becomes arduous on rugged trails in hostile conditions at high elevations. Just as thin air and frigid temperatures make each step an intense effort, so too tight spaces and inadequate views can make each bite a struggle at an 8-cm anastomotic depth. The perspective needed to discern tissue layers and gauge bites is lost. When just getting the needle to the right spot for the next bite is difficult, driving the needle through its arc can feel impossibly awkward. Trying conditions lead to technical errors that traumatize tissue and threaten the anastomosis. Basilar bypasses are indicated infrequently because the basilar artery is richly collateralized with two vertebral arteries proximally and two posterior communicating arteries distally, but when needed, these bypasses are part of the most challenging treatment solutions for some of the most complex pathologies that we face.

Basilar Artery Aneurysms

The algorithm for basilar aneurysm bypass is based on the location of the basilar aneurysm relative to the basilar apex, classifying aneurysms into three groups: (1) pre-quadrifurcation or basilar trunk; (2) quadrifurcation; and (3) post-quadrifurcation or distal PCA (P2 to P3 segments) and distal SCA (s2 to s3 segments).

Basilar Quadrifurcation (Apex) Aneurysms

Direct surgical clipping offers an excellent method of closing a basilar quadrifurcation aneurysm neck completely while preserving branch arteries and perforators. However, basilar quadrifurcation aneurysms that are giant in size, thrombotic, serpentine, dolichoectatic with branch arteries originating from side walls, recurrent after coiling, or recurrent after wrapping or encasement may be unclippable. I have encountered all of these complexities at the basilar apex, and a bypass strategy provided an effective alternative to clipping. The strategy for basilar quadrifurcation aneurysms is so much simpler than for basilar trunk aneurysms or bifurcation aneurysms elsewhere. The strategy is simply proximal basilar artery occlusion just proximal to the quadrifurcation in the perforator-free zone below the origin of the SCAs, with or without an M2 MCA-RAG-P2 PCA or STA-PCA bypass (Fig. 23.1). The critical issue is the presence and size of the PCoAs. These communicating arteries may be visible on a standard angiographic exam, but when absent, provocative maneuvers may be needed to evaluate their caliber and capacity. The Matas test is a vertebral artery angiogram performed with compression of the carotid artery in the neck to elicit forward flow in the PCoA.

When the PCoA is present and larger than 2 mm in diameter, the basilar trunk can usually be occluded safely, but BTO of the basilar trunk is needed to assess the adequacy of the PCoAs and patient tolerance, with and without hypotensive challenge, as described with cavernous ICA aneurysms (see Chapter 21). The BTO with hypotensive challenge categorizes patients into three groups: (1) pass-pass; (2) pass-fail; and (3) fail. Most patients with large PCoAs pass both the basilar BTO and the hypotensive challenge (group 1), require no bypass, and can undergo proximal aneurysm occlusion safely (first choice). However, patients with large PCoAs may unexpectedly fail their provocative testing. Patients who pass the BTO but fail the hypotensive challenge (group 2) require a simple EC-IC bypass (STA-PCA or STA-SCA bypass) that protects them during periods of decreased cerebral blood flow (second choice). Patients who fail the BTO (group 3) require an IC-IC interpositional bypass (M2 MCA-RAG-P2 PCA bypass) capable of replacing basilar flow (third choice).

The basilar trunk can be occluded safely, with either direct clipping or endovascular coiling. Surgery is usually preferred because it offers a “point” occlusion of the basilar trunk under direct visualization that ensures perforator preservation. Endovascular coiling spares the patient an open surgical procedure, but occludes a segment of the basilar trunk that might harbor vital perforators. Furthermore, digital subtraction angiography lacks the resolution to visualize these perforators intraprocedurally, which compromises the safety of this technique.

When the PCoAs are absent and smaller than 2 mm in diameter, a “surgical PCoA” must be created before the basilar trunk can be occluded safely, and BTO is not needed. The best option for revascularizing the basilar apex is the M2 MCA-RAG-P2 PCA bypass (first choice) (Fig. 23.2), although patients with a sizable STA may be revascularized safely with a STA-P2 PCA bypass (second choice). When in doubt, I prefer the IC-IC bypass because, even though it requires two anastomoses, the calibers of the MCA and PCA are matched, and the M2 MCA-RAG-P2 PCA bypass provides robust flow that meets the needs of all the quadrifurcation’s branch arteries. After the STA travels across the middle fossa to the recipient artery, its caliber may be small and inadequate to supply such a critical territory. The MCA is a natural donor artery because the recipient PCA is exposed through the Sylvian fissure anyway, providing simultaneous MCA access. The basilar aneurysm can be proximally occluded after the bypass because the point of occlusion is also accessed simultaneously through the Sylvian fissure. Direct clip occlusion of the parent artery spares the basilar perforators because they can be visualized and avoided. Endovascular occlusion of the basilar artery would not spare the patient an open procedure and has the disadvantages of segmental occlusion, but it can check the patency of the bypass and clinical tolerance of basilar occlusion before permanent basilar occlusion is performed. Nonetheless, I still favor simultaneous clip occlusion because ICG videoangiography confirms bypass patency, and immediate basilar occlusion also places demand on the bypass to keep it open.

Fig. 23.1 Algorithm for aneurysm treatment and bypass strategy for complex basilar quadrifurcation aneurysms.

Fig. 23.2 Summary of bypass options according to three basilar aneurysm locations and seven bypass types. 1^st, 2^nd, 3^rd, and 4^th refer to the preferred choices for bypass options.

For such a daunting aneurysm, this strategy is paradoxically simple. Inviolable thalamoperforators that originate from the P1 PCA segments prevent aneurysm trapping and therefore elaborate vascular reconstructions (such as a double reimplantation bypass) are not needed at the basilar quadrifurcation. Although these aneurysms are not excluded completely from the circulation, flow alteration from a vertical jet into the aneurysm preoperatively to a horizontal stream paralleling the neck postoperatively usually elicits delayed aneurysm thrombosis. Results with this strategy have been dramatically less morbid than those with basilar trunk aneurysms, probably because there is strong distal runoff to the PCA and SCA on the side opposite the bypass and because the perforators originate from the main streamline from ipsi- to contralateral P1 PCA segments. In addition, the transformation of a basilar apex aneurysm from a terminal aneurysm initially into a sidewall aneurysm after bypass/proximal occlusion promotes thrombosis. Aneurysm thrombosis can lead to enlargement, increased midbrain compression, parenchymal edema, and hydrocephalus, but these perioperative deficits are temporary.

Post-Quadrifurcation Basilar Aneurysms

Unlike basilar trunk and quadrifurcation aneurysms that are treated mainly with IC-IC interpositional bypasses, aneurysms arising from the quadrifurcation’s branch arteries are amenable to the full gamut of bypasses, including all of the IC-IC reconstructions. The PCA and SCA course in parallel and can be joined with an in-situ bypass using a side-to-side anastomosis (s1 SCA-P2 PCA in-situ bypass). Their proximity also facilitates reimplantation, and in addition, other arteries such as the anterior temporal artery can be reimplanted as a donor artery for an anterior-to-posterior circulation bypass (see Chapter 16). The caliber of the PCA is typically double that of the SCA, which can limit the utility of the SCA as a donor artery, whether the affected artery is revascularized with an in-situ bypass or reimplantation. The SCA can be further diminished in caliber when it is one of a duplicated pair of SCAs. Reanastomosis is possible, but as with distal ACA segments, the PCA and SCA both have a course with little redundancy or tortuosity, and therefore are difficult to reconnect after an aneurysmal segment has been excised. Interpositional IC-IC bypasses are excellent for PCA and SCA aneurysms. A direct interposition graft between the arterial ends requires two difficult anastomoses to the parent artery, whereas a more indirect interpositional graft from a donor MCA trunk within the transsylvian corridor eliminates one of these deep anastomoses. However, the course of the graft differs from the pretemporal course with the basilar trunk or apex aneurysms, running more subtemporally to reach an efferent artery that is more distally located. The subtemporal exposure of the recipient PCA requires subtemporal retraction and occasionally some resection of the parahippocampal gyrus. Similarly, the subtemporal exposure of the SCA often requires subtemporal retraction and an incision in the tentorium behind the insertion of the trochlear nerve into its dural sleeve, with tacking of the tentorium laterally with a stitch or a clip. Although never performed, the double reimplantation technique can revascularize both the SCA and PCA with aneurysms whose occlusion compromises both of the efferent arteries, again using the M2 MCA as the donor site (M2 MCA-RAG-s2 SCA+P2 PCA).

The spectrum of bypasses for these distal PCA and SCA aneurysms is extensive, but the frequency of their use has been low. Two explanations for this are the rarity of distal PCA and SCA aneurysms and the increased application of endovascular therapies in the posterior circulation. Also, the PCoA eliminates the need to bypass most complex aneurysms involving the P1 PCA. These aneurysms can often be proximally occluded or trapped, with the preserved PCoA supplying the P2 PCA and its distant territories. For more distally located P2 aneurysms and beyond, the rich collateral blood supply to the occipital lobe from the MCA makes many patients tolerant of PCA sacrifice without bypass. It has been estimated from experiences with BTO that 75% of patients tolerate deliberate PCA occlusion without visual field deficits. A similar collateral network exists between the SCA and PICA territories, making SCA sacrifice tolerable in some cases, although much less than with PCA sacrifice. However, the SCA’s small caliber makes it difficult and impractical to perform BTO to evaluate this collateral reserve.

Therefore, the algorithm for post-quadrifurcation aneurysms begins with aneurysm location (Fig. 23.3). P1 segment PCA aneurysms are evaluated by assessing the PCoA and its relationship to the distal neck. P1 PCA aneurysms with a large PCoA that are confined to the P1 segment without thalamoperforators arising from the aneurysm are trapped with no bypass. Those that are confined to the P1 segment with thalamoperforators arising from the aneurysm, or those that extend beyond the P1 segment, are proximally occluded to preserve the PCoA-P1-P2 junction and back-fill these perforators. In contrast, P1 PCA aneurysms with an absent or small (< 2 mm diameter) PCoA are revascularized with either an IC-IC bypass (in-situ or interpositional bypass, or reimplantation; first choice) or an EC-IC bypass (second choice). The aneurysm can then be either trapped or proximally occluded, depending on the thalamoperforators. P2 PCA aneurysms are evaluated with a BTO, selecting those patients who fail the test with visual field deficits during balloon occlusion or with hypotensive challenge for bypass, either an IC-IC bypass (in-situ or interpositional bypass, or reimplantation; first choice) or an EC-IC bypass (second choice). Patients who pass the test are treated with endovascular PCA sacrifice alone.