Surgical Revascularization of the Posterior Circulation

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27 Surgical Revascularization of the Posterior Circulation

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.24 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.

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.

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.

  Treatment Total number
Aneurysms    
Basilar tip aneurysms M2 MCA to P2 PCA with microsurgical clipping (2), ECA to P2 PCA with distal basilar occlusion (1) 3
Midbasilar aneurysms V3 VA to P2 PCA (SVG or RAG) with distal occlusion of basilar artery 5
Vertebral artery aneurysms V3 VA to PCA with occlusion of vertebral artery 4
AICA aneurysms AICA-PICA in situ side-to-side anastomosis 1
PICA aneurysms OC-PICA anastomosis CCA to PICA
RAG with occlusion of PICA (1), PICA to PICA (1)
10
Ischemia    
Vertebral dissections V3 VA to P2 PCA (SVG or RAG), with occlusion of vertebral artery 3
Tumors    
Chordoma V3 VA to P3 PCA (SVG or RAG) with gross total resection tumor along with encased portion of vertebral artery 5
Giant cell tumor (foramen magnum) SVG interposition graft for vertebral artery with gross total resection of tumor 4
Meningioma (foramen magnum) Vertebral artery resection and anastomosis, and gross total resection of tumor 7

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.

Case examples

Patient 1

This 72-year-old woman presented with a subarachnoid hemorrhage (Hunt and Hess 4) secondary to a ruptured basilar tip aneurysm and was initially treated by endovascular coiling. She had various strokes during the hospitalization, and sustained permanent right third nerve palsy, but recovered to be independent for all daily living activities. Follow-up angiograms showed aneurysm recurrence due to coil compaction with the entire aneurysm mass measuring 21 × 19 × 25 mm, causing significant brainstem compression. She had four additional coiling procedures over the previous 5-year follow-up period (see Figures 27–1, 27–2A and 27–2B). Her examination indicated a right third nerve palsy, memory difficulties, and mild gait ataxia, and no other deficits. Her functional status was graded as modified Rankin score (mRS) 2.

During the clipping procedure, with a frontotemporal craniotomy and orbitozygotomy approach because of the coil mass, it was felt that the clip blades might slide onto the right PCA, or both PCAs during the clipping procedure. A RAG bypass surgery was performed from the right middle cerebral artery to the right PCA-P2 segment, under propofol-induced burst suppression, moderate induced hypertension, and mild anticoagulation. Following the bypass, the aneurysm could be clipped, albeit with significant narrowing of the right PCA by the clip due to the “funnel effect” caused by the coils.

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