Extracranial–Intracranial Bypass Procedures

Introduction

Extracranial–intracranial (EC–IC) bypass procedures have played an important role in the treatment of cerebrovascular disease since their development in the 1960s. They serve two main purposes : (1) flow replacement when managing challenging aneurysms or tumors requiring cerebral vessel sacrifice and (2) flow augmentation to treat cerebral ischemia mainly in the setting of atherosclerotic occlusive disease and moyamoya disease. In both settings, a new conduit for blood flow is established by suturing a graft vessel surgically to the intracranial circulation, thus providing revascularization to the brain. Over the years, the perfection of microsurgical anastomosis techniques in combination with significant advancements in imaging technology have molded the thought process involved in surgical cerebral revascularization using bypass. Moreover, important studies, such as the 1985 EC–IC bypass study as well as the 2011 Carotid Occlusion Surgery Study (COSS), have had significant impact on the use of EC–IC bypass for flow augmentation, particularly in the setting of carotid athero-occlusive disease. In this chapter, we briefly discuss indications, patient selection, hemodynamic assessment techniques, and bypass options pertaining to flow augmentation and flow replacement EC–IC bypass.

EC–IC Bypass for Flow Replacement

The concept of flow replacement is important when planning treatment of complex aneurysms where parent vessel sacrifice is a necessity, or when confronted with skull base tumors engulfing surrounding vasculature. When considering the carotid artery, for example, although a majority of patients may withstand occlusion, up to 30% may suffer a stroke if this vessel is sacrificed. This is primarily due to poor collateral vascular supply. Consequently, preoperative testing prior to carotid sacrifice is mandatory in order to identify those patients who would not safely tolerate carotid occlusion and would therefore need EC–IC bypass to preserve their cerebral blood flow (CBF). Patients at risk are identified using endovascular balloon occlusion testing (BOT), during which the patient’s response to temporary carotid artery occlusion can be studied based on one or a combination of neurological, angiographic, electroencephalographic, and perfusion criteria. If the BOT is failed, flow replacement is needed and revascularization should be performed prior to vessel sacrifice. If the BOT is failed based on clinical criteria, a higher flow bypass is generally needed, while if it is failed based on subclinical perfusion deficits, a lower flow bypass may be adequate. A minority of specialists advocate revascularization in all cases regardless of BOT results, as they are concerned regarding the possibility of false-negative BOT or that younger patients may be at higher risk for development of contralateral aneurysms over time due to increased hemodynamic stress on collateral vessels. The sacrifice of more distal cerebral vessels, such as the middle cerebral artery or anterior cerebral artery branches, typically requires flow replacement prior to sacrifice, since these are end vessels and the collateral flow from the circle of Willis is lacking. Rarely, leptomeningeal collaterals may be adequate to avert major strokes, but it is unlikely that acute sacrifice of a major intracranial vessel can be tolerated without ischemia, and thus warrants bypass preemptively.

When the decision has been made to replace flow, different donor vessels with varying carrying capacities are available for bypass. Typically larger conduits can provide greater flows. Traditionally, when the need for high flow is anticipated, large conduits capable of greater carrying capacities, such as interposition vein grafts (saphenous vein), are employed. For intermediate flow bypass, the radial artery can be used as an interposition graft. For low flow bypass, the superficial temporal artery (STA) or occipital artery can be utilized as donor vessels, although the STA can provide flows as high as 100 mL/min. The use of the STA as a donor graft is most desirable as it requires only one anastomosis between the donor and the cerebral vessel recipient. Use of vein or radial artery grafts requires not only an intracranial anastomosis but also an extracranial anastomosis to the source of blood supply, typically the cervical carotid artery. Thus, additional surgery to harvest the graft and to expose the cervical carotid is needed, which adds complexity and time to the procedure. Additionally, STA grafts have a higher patency and longevity than interposition grafts. Ultimate determination of the choice of graft can be tailored at the time of surgery by measuring the flow in the vessel to be sacrificed, and assessing the flow in the STA; if adequate, the STA can be used preferentially as the simpler option; otherwise, an interposition graft must be performed. Techniques for optimizing graft selection and technical success have been developed based on information from intraoperative blood flow measurements .

In certain situations, intracranial to intracranial bypass rather than extracranial to intracranial bypass can be performed if a suitable intracranial vessel segment is close enough to the vessel that needs to be revascularized, such as the branches of the MCA or distal anterior cerebral artery segments or posterior inferior cerebellar artery segments. In such instances, the vessels are anastomosed to each other in a side-to-side fashion maintaining the flow in the donor vessel while also providing blood flow to the distal territory of the recipient vessel. Such in situ bypasses obviate the need for EC–IC bypass grafts offering some advantages (i.e., no need to rely on the quality of the donor vessel and the additional morbidity of harvesting a graft) and disadvantages (i.e., putting another intracranial vascular territory at risk if the bypass fails).

With the advent of new endovascular techniques, the need for flow replacement bypass surgery to treat complex aneurysms that once were untreatable without that option is decreasing. Indeed, the development of devices such as flow diverters (endovascular stents that form a scaffold within the vessel in order to block flow into an aneurysm) now allow for endovascular options for treatment of some of the technically challenging large and giant aneurysms, which previously required surgery and EC–IC bypass. Nonetheless, EC–IC bypass remains an important strategy for select cases ( Figs. 162.1–162.3 ).