50 High-Flow Cervical Carotid Artery to Middle Cerebral Artery Bypass

Michael J. Lang, Joshua S. Catapano, and Michael T. Lawton

Since superficial temporal artery (STA) to middle cerebral artery (MCA) extracranial-intracranial (EC-IC) bypass was introduced by Yasargil in the 1960s, bypass surgery has undergone substantial evolution.1 This first-generation construction is still the workhorse of bypass surgery, but it is not without limitations. In particular, the flow available in most STA donor arteries was found to be insufficient to meet the demand required by the internal carotid artery (ICA) or M1 territories. To meet this challenge, Thoralf Sundt and others developed interpositional high-flow EC-IC bypass for treatment of intracranial aneurysms, skull base tumors, and ischemic conditions.² Interpositional EC-IC bypass more than doubles the flow available from the STA, directly drawing robust flow from the cervical carotid, and has come to define the second generation of bypass surgery.

More recently, the role of high-flow EC-IC bypass has been diminished by innovation in both endovascular and bypass techniques. Flow diversion has become the first-line therapy for symptomatic cavernous ICA aneurysms and is an excellent treatment option for many large or giant paraclinoid ICA aneurysms, which used to be common indications for high-flow bypass.³ Angioplasty and stenting of both cervical and intracranial occlusive disease have similarly decreased the indications for bypass in ischemic disease. On the microsurgical forefront, third-generation intracranial-intracranial (IC-IC) bypass and fourth-generation novel bypass configurations have eliminated the need for EC donor sites in many cases.^{4 ,​ 5} Finally, the results of the EC-IC Bypass Trial and Carotid Occlusion Surgery Study (COSS) have diminished enthusiasm for bypass surgery in general at many centers.^{6 ,​ 7} Despite these advances, high-flow EC-IC bypass remains an essential weapon in the armamentarium of cerebrovascular neurosurgeons.

50.2 Indications and Preoperative Management

In the experience of the senior author (Michael T. Lawton), aneurysms accounted for the overwhelming majority (83.6%) of cases requiring EC-IC interpositional bypass in the anterior circulation, with the remainder equally split between ischemic and tumor pathologies. High-flow revascularization should be considered when definitive treatment of a given pathology requires deliberate or potential occlusion of a large-caliber artery. Conversely, MCA bifurcation or postbifurcation aneurysms that require revascularization of a single M2 division can often be treated with single- or double-barrel STA-MCA bypass (or intermediate-flow procedures, such as interpositional bypasses using the internal maxillary artery as a donor).⁸ This anatomic approach to donor selection has been challenged by some authors, who have advocated the use of noninvasive blood flow imaging techniques or intraoperative flow probe monitoring to quantify the demand required of a bypass.⁹ Although there is an obvious logic to these approaches, an anatomical simplification arrives at a similar result in most cases without the need for bulky probes.

Preoperative assessment should include multidisciplinary evaluation. For aneurysms, endovascular evaluation should confirm that flow diversion or coil embolization is viable treatment option. For complex aneurysms, dedicated digital subtraction angiography is preferable to CT angiography in delineating the relationships of parent, branch, and perforator anatomy.¹⁰ Balloon test occlusion (BTO) with hypotensive challenge is important to determine the necessity of bypass versus Hunterian ligation. Patients who pass a BTO may be candidates for proximal Hunterian ligation. Those who have rapid onset of ischemic symptoms are considered for high-flow bypass, whereas those with mild symptoms during the hypotensive period may require only low-flow bypass to accommodate hemodynamic demand. However, the results should be interpreted with caution, because false-negative rates of 15% have been reported in some series of BTO.¹¹

Ischemic pathology should be approached in consultation with a vascular neurologist to determine the appropriateness of surgical versus medical treatment. Catheter angiography is generally recommended to evaluate steno-occlusive lesions and endogenous collateral supply, particularly given the relative rarity with which chronic cerebrovascular insufficiency requires high-flow bypass. Noninvasive perfusion imaging can help to identify ischemic territory at risk, and we tend to favor CT perfusion over magnetic resonance–based techniques, particularly with supplemental acetazolamide challenge.¹²

Skull base tumors should be approached cautiously and in close consultation with medical and radiation oncologists for treatment alternatives. The vast majority of skull base tumors are benign lesions, with slow, noninvasive compression of the ICA. These features generally favor surgical debulking followed by observation or radiation, and ischemic deficits in such situations are rare. Malignant tumors with direct invasion of the carotid (as well as rare angioinvasive infections recalcitrant to medical management or limited surgical debridement) may be candidates for treatment with high-flow bypass if survival and clinical outcome can be improved with aggressive resection. However, the oncological benefit of this approach remains unclear, and this approach is associated with a high degree of morbidity and mortality.¹³ In our experience, patients who require high-flow bypass for treatment of skull base tumors are treated in a staged fashion, so that bypass patency and tolerance to occlusion can be confirmed.

Our preference is to discontinue antiplatelet therapy for at least 5 days before the planned bypass surgery. Unnecessary oozing into the surgical field can prolong clamp times or result in intracerebral hemorrhage during tissue manipulation or retraction. A regimen of 325 mg of aspirin per day is routinely initiated after immediate postoperative CT angiography is obtained, and this regimen is typically continued indefinitely.

Interpositional EC-IC bypass ( Fig. 50.1a–c), which involves three surgical incisions and two anastomoses, increases surgical complexity compared with low-flow EC-IC bypass. The prototypical operation for high-flow EC-IC bypass in our practice is external carotid artery (ECA) to M2 MCA bypass with a radial artery graft (RAG) (i.e., ECA-RAG-M2 MCA). This chapter will consider this approach in detail, although variations in donor or recipient anatomy or graft length, flow, or availability may require different techniques.

Fig. 50.1 (a) Illustration of an interpositional extracranial-intracranial (EC-IC) high-flow bypass. (b) Illustration of EC-IC high-flow bypass showing the skull, temporalis muscle, and burr hole. (c) Illustration of an interpositional EC-IC high-flow bypass for a giant internal carotid artery aneurysm that is clip trapped. (Used with permission from Barrow Neurological Institute, Phoenix, Arizona.)

50.4 Interposition Graft

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