Introduction

Extracranial-intracranial (EC-IC) bypass is used for flow replacement in the treatment of complex cerebral aneurysms or tumors that require vessel sacrifice, and for flow augmentation in the treatment of cerebral ischemia. EC-IC bypass, however, is a technically demanding surgery that can be fraught with the potential for several complications, including donor vessel injury, intraoperative bypass occlusion, postoperative bypass occlusion, and hemorrhage. The overall morbidity and mortality rates in our own previously published series were 0% mortality and 4% morbidity, which are comparable to other reports of 0.6% to 4.3% mortality and 2.0% to 4.0% morbidity. Our overall bypass patency rates for anterior and posterior circulation bypass are 90% and 83%, respectively, and are also similar to the patency rates documented by large clinical series. Here we discuss the complications that can occur with cerebral bypass surgery as well as the associated preventive measures and management strategies.

Anatomic Insights

Superficial Temporal Artery

The superficial temporal artery (STA) to middle cerebral artery (MCA) bypass is considered the workhorse of cerebral revascularization. Indeed, the STA is an in situ native donor that has good flow-carrying capacity and can frequently be sufficient for flow replacement as well as flow augmentation. Consequently, harvesting the STA is a technique that should be honed by cerebrovascular neurosurgeons.

The STA crosses the root of the zygoma, and then above the zygoma it divides into an anterior (frontal) and a posterior (parietal) branch ( Fig. 12.1 ). The vessel runs parallel to the superficial temporal vein and in between the subcutaneous fat and temporalis muscle fascia. At the level of the zygoma, the diameter of the STA is approximately 3 mm. The anterior and posterior branches are often similar in size, with a diameter measuring approximately 1.5 to 2 mm, although one branch can be dominant.

Diagnostic cerebral angiogram, right external carotid artery injection, lateral projection, showing the course of the superficial temporal artery around the root of the zygoma (large arrow) and its division into an anterior (small single arrow) and a posterior branch (small double arrow).

Doppler ultrasound is routinely used to map both branches of the STA after the patient’s head has been placed in pins, because pinning can pull the skin and distort prior markings. Additionally, dissection of the STA is performed under loupe or preferably microscope magnification with the surgeon and assistant seated. The skin incision is made with Colorado microneedle-tip monopolar cautery (Stryker Corp., Kalamazoo, MI) at a low setting of 8, allowing hemostasis of the skin edges while preventing skin edge necrosis. Once subcutaneous tissue is encountered, a blunt-tip curved hemostat is used to dissect down to the STA. Once the vessel is visualized, the hemostat is used to dissect proximally in the loose areolar plane above the vessel, and then the Colorado tip is used to open the skin to the tip of the hemostat as sequential dissection is performed proximally and distally along the STA.

Prevention

Complication avoidance is key to performing a successful cerebral bypass. Cerebral bypass surgery is highly dependent on careful attention to technique at every stage of the operation, from donor vessel dissection to skin closure. Adherence to a stereotyped step-by-step approach to this operation, with decision-making regarding donor selection and bypass patency guided by intraoperative flow measurements, which our group described previously, can result in consistent technical success.

Management of STA Injury

The STA can be injured during harvesting or during the craniotomy after dissection of the STA. STA injury, however, can be avoided by using microscope or loupe magnification, maintaining meticulous hemostasis, and using a round burr instead of a perforator drill bit.

Injury to the superficial temporal vein rather than the artery should be recognized and managed appropriately because the STA may be inadvertently coagulated when attempting to control venous bleeding. Video 12.1 demonstrates a maneuver that can be applied to obtain venous hemostasis without risking injury to the STA. Once the vein is coagulated, it is important to cut any coagulated portion that might be draped over the STA and subsequently restrict flow through the donor vessel.

Injury to the STA itself can be ameliorated by using an amputated donor if the length remains suitable, using the uninjured anterior or posterior STA branch, or choosing a different donor vessel.

Management of Intraoperative Bypass Occlusion

The best management of intraoperative bypass occlusion is a preventive and preemptive strategy. Indeed, multiple preoperative and intraoperative steps can be taken to ensure patency of the cerebral bypass. First, patients are instructed to take aspirin (325 mg) the day before surgery and are continued on aspirin postoperatively. Additionally, the STA is wrapped in a papaverine-soaked cottonoid during the craniotomy to protect it and to prevent or treat spasm caused by manipulation of the donor. Before the anastomosis is performed, the STA is flushed with heparinized saline through its cut end. Interposition grafts are distended before implantation to reduce the risk of later vasospasm, which can be particularly prevalent with radial artery grafts. Care must be taken during tunneling of interposition grafts to avoid twisting and kinking; these grafts must be cut to a length that avoids tension on the anastomosis. At the same time, it must be taken into consideration that the elongation that occurs once the graft is distended with blood flow can lead to kinking of overly lengthy grafts. Attention to correct orientation of the vein graft with respect to its valves is also imperative.

It is also important to critically assess the suitability of a donor or recipient vessel before undertaking the bypass. More specifically, iatrogenic injury to the donor during dissection or atheromatous changes within the STA should be appreciated. Video 12.2 shows an example of atheroma in the STA and demonstrates a technique to manage this problem by going more proximally on the STA. At the same time, the chosen recipient vessel should have a large enough diameter to allow adequate outflow to the graft. Our group previously outlined the use of a quantitative microvascular ultrasonic flow probe (Charbel Micro-Flowprobe; Transonics Systems, Inc., Ithaca, NY) and described the use of CFI to detect potential problems with the donor or recipient vessels. Briefly, the technique entails measurement of the “cut flow” of the STA—that is, the maximal flow-carrying capacity after dissection and cutting the vessel open. Once the anastomosis has been completed, the flow in the donor STA is remeasured, and this bypass flow is compared with the cut flow to provide the CFI. When performed for flow augmentation, an index close to 1.0 indicates a highly successful bypass because it indicates that the donor graft is carrying its full capacity. A low index can indicate a problem with either the donor, the recipient, or the anastomosis that requires attention. For flow replacement bypass, cut flow measurement indicates the carrying capacity of the STA and its suitability as a donor that can replace the flow measured in the vessel to be sacrificed. After graft placement, whether the STA or interposition graft, measurement of the bypass flow indicates whether the graft has been successful in replacing the necessary flow to the revascularized territory.

Most importantly, intraoperative bypass occlusion must be recognized. Although visual inspection of the anastomosis should be done, the presence of pulsation can actually be misleading. Intraoperative blood flow measurements, then, allow for a simple and accurate method to quantitatively assess bypass patency. If the bypass is found to be occluded intraoperatively, thrombectomy may be performed, and the anastomosis can be repeated, or a new anastomotic site can be chosen ( Video 12.3 ).