Introduction

Revascularization of the posterior circulation may be critical in order to prevent a major stroke during the treatment of aneurysms, and certain cases of intractable ischemia and skull base tumors. Aneurysms requiring revascularization generally involve situations not amenable to endovascular surgery such as fusiform or blister aneurysms, origin of vessels from the aneurysm sac, or failure with maximal endovascular interventions. Strokes in the posterior circulation can be devastating, whether they occur due to perforator occlusion, or due to the occlusion of the major arteries. A careful understanding of the anatomy of the patient, the operative approaches, and the techniques of revascularization is needed in order to achieve good outcomes.

Surgical anatomy and collateral circulation

Most patients have two patent vertebral arteries (VAs), and the left one is usually dominant. In some patients, one of the arteries ends in the PICA, or is occluded by atherosclerotic disease, which makes the inflow situation more tenuous. At the upper end, the posterior communicating arteries connect the ICAs with the PCAs; however, the size of these is variable. When there are two posterior communicating arteries (PCOM) that are 1 mm. or more in size, then the patient may safely tolerate basilar artery (BA)¹ or bilateral VA occlusion, but in patients who have less than optimal collaterals (e.g., only one PCOM of ≥1 mm in size), the revascularization must be considered, prior to occlusion. Other variants may include bilateral (or unilateral) fetal PCOMs, fenestrations of the vertebrobasilar junction, persistent trigeminal arteries (with varying degrees of contribution to the upper BA), etc.

In general, younger patients without significant comorbidities such as heart disease, hypertension, or diabetes are able to withstand vascular occlusions better than older patients, who may be smokers and/or have chronic hypertension, diabetes, and hypercholesterolemia. Connective tissue disorders such as Marfan’s disease will also need to be taken into account in planning surgery. In patients about to undergo elective surgery, every attempt should be made to modify risk factors. This includes moderate (<140 to 160 mm systolic pressure) control of the blood pressure, oral statins to lower cholesterol, and smoking cessation. Patients who may undergo revascularization for ischemia should also be worked up for hypercoagulable disorders.

Endovascular alternatives

In patients with aneurysms, or ischemic disease, endovascular treatment assisted by endoluminal stents (stent with coiling, or angioplasty followed by a stent) may be an alternative.^2–4 Based on the patient’s age and other risk factors, this should be discussed with the patient. Centers where the microsurgical treatment is performed should also have the appropriate endovascular expertise, and collaboration.

Anesthesia and neuro monitoring

All patients who undergo general anesthesia for posterior circulation procedures should be managed by a neuroanesthesiologist, with specialized training in bypass procedures. In our center, most such patients are managed under total intravenous anesthesia, to permit the monitoring of somatosensory-evoked potentials (SSEP), and motor-evoked potentials (MEPs). The brainstem-evoked response and some cranial nerve functions (CNs 7, 8, 10, 11, 12) may also be monitored during such procedures.

All patients are maintained on antiplatelet therapy with aspirin 81 mg per oral on the day of surgery, and postoperatively. For all EC-IC bypass grafts, we administer 2000 to 2500 units of heparin intravenously after the clamping of the vessels. During the temporary occlusion of arteries, the patient is placed in burst suppression with propofol. In patients with unruptured aneurysms, ischemia, or basal tumors, blood pressure is raised 20% over the baseline to improve the collateral circulation. If MEP changes during the occlusion, then the BP can be raised further. The BA cannot be clamped longer than 15 minutes without adverse consequences. If prolonged clamping is needed, then the patient should be placed in deep hypothermic circulatory arrest.

Skull base approaches

For operations involving bypass procedures, and for complex posterior circulation aneurysms in general, skull base approaches are essential. The correct approach to use for each case is determined by a careful study of the aneurysm anatomy, along with the landmarks on the skull, and the experience of the surgeon. The most common approaches used are the orbitozygomatic approach, the transpetrosal approach (retrolabyrinthine approach, or partial labyrynthectomy petrous apicectomy approach, and rarely, a total petrosectomy approach), a combined far-lateral retrosigmoid approach, the presigmoid approach (with the division and resuture of the sigmoid sinus in some patients), and the extreme lateral transtubercular approach.⁵ Some of the complex approaches take time to develop, and in elective cases, a staged approach is taken, with the approach being performed on one day, and the definitive operation on another day.

Bypass procedures

Two types of bypasses are used: in situ bypass procedures and bypass procedures using graft vessels for flow diversion. Bypasses using graft vessels such as the radial artery graft (RAG) and SV graft are generally high flow and can be either EC-IC (connecting extracranial vessel to intracranial vessel) or IC-IC (connecting two intracranial vessels). In general, the type of bypass performed depends upon the need (small vessel replacement needs an in situ or low-flow bypass, whereas large vessel replacement will require a high-flow bypass). The availability of donor and recipient vessels, and graft vessels, is also a factor. A bypass can only replace major vessels, and cannot solve perforator problems. In some patients with unruptured aneurysms, a distal (rather than proximal) occlusion of the aneurysm may be used in order to protect the flow through the perforating vessels. EC-IC bypasses include low-flow bypasses such as the superficial temporal to the superior cerebellar artery, and an occipital (OC) to PICA anastomosis (OC-PICA bypass), and high-flow bypasses that encompass RAGs or SV grafts. The choice between the RAG and the SVG is based on the size of the available donor vessel (diameter >0.23 cm for the RAG, and 0.3 cm for the SV), the passage of the Allen test for the RAG, and an adequate length available (at least 20 cm) for the SVG. We make this determination on the basis of a preoperative duplex examination. For RAGs, the use of the pressure distension technique has revolutionized the patency rate, by greatly reducing the incidence of postoperative vasospasm (Figure 27–1).⁶ High-flow bypasses are generally constructed from the ipsilateral VA, or the ECA, to end in the VA, or the PCA. In one case, the bypass was placed directly into the BA, under deep hypothermic circulatory arrest.

Figure 27–1 Pressure distention technique: distention of the radial artery with saline injection while distally occluding the graft with fingers.

(Courtesy of Laligam Sekhar.)

In situ bypasses are technically easier to perform than EC-IC bypasses. In situ bypasses include reimplantation of arteries, short interposition grafts, direct resuture of vessels, and side-to-side anastomosis. Reimplantation of vessels can be done in an end-to-side fashion after the resection of the aneurysm from which the vessel originates. Segmental resection with an interposition graft of similar caliber is performed when an aneurysm involves a segment of a vessel (e.g., fusiform aneurysm). Side-to-side anastomosis in posterior circulation is performed for flow replacement when clipping of aneurysm will result in total occlusion (or critical stenosis) of a small artery. Both the vessels of the anastomosis need to be approximately of the same caliber. Aneurysms involving PICA, AICA, or SCA can be addressed with PICA-PICA to AICA-PICA, or SCA-PICA, anastomosis (Table 27–1).^7,8 Side-to-side anastomosis is a comparatively easier technique to perform for a small-vessel revascularization and generally stays patent (there were no known occlusions in our series) (Figure 27–2).⁷

Table 27–1 Bypass Procedures Performed for Various Conditions in Posterior Circulation.

AICA, anterior inferior cerebellar artery; MCA, middle cerebral artery; PCA, posterior cerebral artery; PICA, posterior inferior cerebellar artery; SCA, superior cerebellar artery; VA, vertebral artery.

Figure 27–2 A, First suture from outside in to inside out. B, After tying the first suture, it is taken to the left wall, and then the suture is run continuously on the posterior wall to reach the other end of the arteriotomy. C, A new suture is placed in the upper corner and tied, which is then tied to the end of the first suture. D, A suture is started from the inferior corner, and both are tied together. Sutures are then tightened and anastomosis completed, after flushing with heparinized saline.

(Courtesy of Laligam Sekhar.)

Follow-up

After a bypass operation, we perform an indocyanine green (ICG) angiogram to check the patency, and the speed of flow. The patient is monitored with Doppler flow probes in the postoperative period, and a cerebral angiogram is performed on the first postoperative day. In some patients, the postoperative angiogram may be performed the same evening, and may be repeated after 3 to 7 days, to verify aneurysmal thrombosis. Patients are maintained on aspirin 81 mg orally for at least 3 months, and subcutaneous heparin 5000 units twice daily for prophylaxis against deep venous thrombosis for about a week. Further postoperative follow-up examinations can be done by Duplex examinations, and CT angiography. We generally obtain a postoperative angiogram at 12 to 18 months.

Case examples