STA-MCA Bypass

49 STA-MCA Bypass

Renan M. Lovato, Carlos Eduardo Prata Fernandes Ferrarez, Luis A. B. Borba, and Jean G. de Oliveira

Abstract

The superficial temporal artery (STA) to middle cerebral artery (MCA) bypass was described in the 1960s and since then it has been performed worldwide for a variety of cerebrovascular diseases. It is usually performed for flow augmentation but in some select cases it can provide a flow replacement treatment.

The main indication for this procedure is the ischemic cerebral diseases, including atherosclerotic diseases, where it was widely used in the past but currently restricted to properly selected cases. STA-MCA bypass remains the gold standard treatment for moyamoya disease and is also indicated for complex intracranial aneurysm treatment.

STA-MCA bypass is a challenging procedure due to the need for microsurgical skills for this operation, but most of its steps can be learned with extensive laboratory training. In this chapter, the authors describe historical landmarks of the cerebral revascularization with a focus on STA-MCA bypass, its indications, and the various steps of this type of microsurgical brain revascularization.

Keywords: vascular neurosurgery, cerebral revascularization, bypass surgery, low-flow bypass, moyamoya, microsurgery

49.1 Introduction

Cerebral revascularization might be used for augmentation or complete replacement of the blood flow to the brain in one of the arteries that supply the intracranial compartment.1 Indications include treatment of chronic ischemia for moyamoya disease,² ^, ³ ^, ⁴ ^, ⁵ and some patients with arteriosclerotic disease who might benefit from this surgery, although this indication is currently controversial.⁶ ^, ⁷ Superficial temporal artery (STA)-middle cerebral artery (MCA) bypass can be useful for the treatment of complex vascular lesions such as fusiform or dolichoectactic aneurysms, or in some skull base tumors that may require an artery to be sacrificed.¹

49.2 Historical Landmarks

The first arterial repair was performed by Hallowell in 1759 with the correction of a brachial artery lesion using silk sutures.⁸ More than a century later, in 1877, the Russian Military surgeon Nicholas Eck published a study documenting the lateral anastomosis of the portal vein and inferior vena cava in canines, showing that it was possible to join blood vessels together.⁹ Other pioneers in these surgical procedures include Alexander Jassinowsky who in 1889 described a technique of arterial sutures,¹⁰ and Robert Abbe who in 1893 used glass tubes to reconnect arteries in dogs and cats.¹¹

The first successful end-to-end anastomosis in a human was used to repair a femoral artery in 1896 by John Murphy. This method replaced the arterial ligature that was the standard treatment for those lesions until then.¹³

However, the breakthrough work that showed that vascular anastomosis was possible and could have a long-term good outcome was done by the French physician Alexis Carrel in his first paper in 1902, describing the triangulation method of sutures. He then moved to the United States, where he collaborated with Charles Guthrie, who helped him achieve good results by perfecting the technique and materials used in those procedures. They evolved experimental surgery with venous interposition of grafted and organ transplants in animal models. In 1912, Alexis Carrel was awarded the Nobel Prize for medicine in recognition of his work on vascular suture and transplantation of blood vessels and organs.¹² ^, ¹³ ^, ¹⁴

In 1922, a Swedish otologist Carl Olof Nylén introduced the use of the microscope in surgeries, helping to inaugurate a new chapter with the possibility of operating on tiny blood vessels.¹⁵ ^, ¹⁶ The first use of this technology in neurosurgery was by Theodore Kurze in 1957, for the removal of a facial nerve schwannoma in a 5-year-old patient.¹⁷ The first vascular microneurosurgery was performed in 1960 by Raymond Donaghy and Julius Jacobson, with their resident Martin Flanagan; it was an MCA embolectomy.¹⁸

The first attempt of cerebral revascularization occurred in 1962 when Lawrence Pool and Gordon Potts used a synthetic graft to create a shunt between the STA and the anterior cerebral artery. However, the postoperative angiography showed occlusion of the tube.¹⁹ In 1963, Woringer and Kunlin used a saphenous vein to perform the first extracranial-intracranial (EC-IC) bypass between the common carotid artery and the internal carotid artery, but the patient did not survive, even though the graft was patent in the autopsy.²⁰

In October of 1965, Gazi Yasargil began training with Donaghy, initially using rabbits to perform grafts bypass, both end-to-end and end-to-side anastomosis.²¹ ^, ²² The initial technical difficulties of operating in the intracranial arteries were overcome with the use of 9–0 suture and bipolar coagulation. Yasargil then proceeded to attempt intracranial to intracranial bypass, and due to technical difficulties and thrombosis of some grafts, he then decided to directly join the STA to the MCA. In 1966, he had already performed 30 end-to-side STA-MCA anastomosis in dogs.²² ^, ²³

On October 30, 1967, Gazi Yasargil performed the first successful STA-MCA bypass in a human patient with Marfan syndrome with complete occlusion of the MCA, starting a new chapter in vascular microneurosurgery.²² ^, ²⁴ In 1972, he used this surgery to treat a patient with moyamoya disease for the first time,²⁵ and it still is the treatment of choice for this disease.² ^, ³ ^, ⁵

49.3 Indications

Classically, the bypasses are divided into low-flow (or standard flow) bypass when a pedicled donor graft is used, such as the STA or occipital artery, and the flow rates range between 60 and 100 mL/minute; intermediate-flow bypass, in which usually a radial artery interposition is sutured between the external carotid artery to the MCA; and the high-flow bypass, using a saphenous vein interposition graft, with flows rates ranging between 100 and 200 mL/minute.¹ ^, ²⁶ ^, ²⁷ ^, ²⁸

For the moyamoya disease, the choice of the donor is for a flow augmentation with a pedicled in situ donor, usually the STA, or sometimes the occipital artery, with or without the association of an indirect revascularization technique.2 ^, ³ ^, ⁵ ^, ²⁹

For aneurysm surgeries, when there is a need for flow replacement, the most commonly used method for selection of the graft is based on the balloon test occlusion (BTO) result, which estimates the amount of blood flow required to be provided throughout the graft.¹ The simple way to do that would be to choose a high-flow bypass for patients whose the BTO is negative, and the use of a native donor for a low flow in patients who fail hypotensive challenge.¹ ^, ³⁰ Another way of doing that is by using a parenchymal thermal diffusion probe to monitor the cerebral blood flow,³¹ and the cerebrovascular reserve capacity (CVRC) is quantified following acetazolamide application. The flow required through the bypass to avoid ischemia is then estimated depending on: transient deficits, perfusion response, and degree of CVRC impairment during occlusion.¹ ^, ²⁶ ^, ³⁰

Intraoperative blood flow measurement, using the cut flow of the donor graft to reflect the potential of the vessel, might change this strategy, and in some cases, the STA-MCA anastomosis can provide more than 100 mL/minute of flow. Therefore, the division of high-flow and low-flow bypass is less relevant when this method is used.²⁸ ^, ³²

Flow augmentation for chronic ischemia is well established for moyamoya disease, decreasing the 10-year stroke risk from 19.6 to 9.4%.³³ For atherosclerosis, this benefit is less clear. The Carotid Occlusion Surgery Study (COSS) did not show any benefit of bypass over the best medical therapy, mainly due to a high preoperative stroke rate of up to 15% and a short follow-up of only 2 years.⁶ If the follow-up continued for up to 5 years, the study would have adequate power to demonstrate significant benefit of surgery, presuming an ongoing steady stroke rate in both groups.⁷ Case series of high-volume centers have shown that STA-MCA can be performed with a much lower risk than found in COSS, ranging from 3 to 7%.^1, ² ^, ³⁴ ^, ³⁵ A category of patients with refractory symptoms, despite optimal medical therapy, and severely diminished hemodynamic reserve will benefit from the surgery if it can be performed with low perioperative morbidity.⁷ ^, ³⁶

49.4 Surgery

49.4.1 Preoperative Preparation

The patient is given aspirin 325 mg administered orally preoperatively for 1 week. If the procedure is performed as an emergency, the patient is given 325 mg of aspirin nasogastrically or 650 mg of rectal aspirin. In patients with hypercholesterolemia, we also administer statins preoperatively and postoperatively, as these may have an effect on graft patency in the long term. A prophylactic antibiotic is administered intravenously 1 hour before the incision and continued for 24 hours postoperatively.

49.4.2 Anaesthesia and Monitoring

Total intravenous anesthesia is used to allow the monitoring of motor evoked potentials. Brain relaxation is achieved with normal measures. Care must be taken regarding normocapnia in patients with ischemia.

Intraoperative neurophysiological monitoring is performed including an electroencephalogram, somatosensory evoked potentials, brainstem evoked responses, and motor evoked potentials.

Patients are placed in burst suppression using propofol to protect the brain during temporary arterial occlusion for bypass,³⁷ but care must be taken to avoid hypotension during such burst suppression. Although sometimes necessary, vasopressors must be given cautiously to avoid vasoconstriction on the STA. The blood pressure is elevated by 20% above the normal value a few minutes before and maintained on those levels during the occlusion time. These maneuvers reduce brain metabolism and increase collateral blood flow, reducing the risk of ischemic damage from the temporary vascular occlusion.

A femoral artery sheath may be inserted for intraoperative angiography, which is often (but not always) performed. Currently, we always use an intraoperative qualitative Doppler probe (Sugita Doppler probe; Mizuho micro-Doppler; Mizuho America Inc., Beverly, MA) and quantitative Doppler flow measurements (BLF21A laser Doppler flowmeter; Transonic Systems, Inc.). Although we have not found the intraoperative flow measurement to approximate postoperative measurements, the presence of good volume flow through the graft (expectations of flow are based on graft size and recipient’s vessel) obviates the need for intraoperative angiography.

Even though microvascular Doppler is being used for a long time in cerebrovascular surgery, it has its own limitations. Though intraoperative digital subtraction angiography (DSA) is the gold standard technique, it needs additional professional experience and the table position needs to be changed. Moreover, it is a time-consuming procedure, relatively expensive, and has radiation exposure in the operating suite also. Consequently, indocyanine green (ICG) video angiography is becoming the most commonly used method for blood flow analysis in many neurosurgical centers, because it is a simple, safe, rapid, and reliable method; besides, it can provide real-time images with good quality and high spatial resolution, with relatively low cost.³⁸

49.4.3 The Technique

The patient is placed in a supine position and the head is rotated 70 degrees to the opposite side. The angiogram is carefully reviewed; at least, one STA branch must be large enough to be used. The temporoparietal branch is usually preferred over the frontal branch, but the frontal branch can be used if it is the larger of the two. The STA is exposed by a direct cut-down technique. The course of the vessel is traced by Doppler ultrasound and marked on the scalp. Working under the operating microscope, dissection may be started distally or proximally. The STA lies superficial to the galea initially, and then penetrates the galea to lie just deep to it about 1 cm superior to the zygoma. A small cuff of connective tissue is left around the artery. This maneuver prevents direct manipulation of the STA preventing vasospasm and kinking of the artery after the bypass is completed. Small muscular branches are coagulated with bipolar cautery and divided. The vessel is left in situ until the bypass procedure. The artery is wrapped in a piece of a surgical glove and covered with papaverine soaked Gelfoam to allow vasodilation of the donor vessel and protection during craniotomy.

A straight incision is made on the temporalis muscle, on the same direction of the skin to facilitate muscle dissection. Surgical hooks may be used to retract the muscle and skin. A small frontotemporal craniotomy is performed using a single burr hole on the inferior part of the craniotomy, which will serve as a pathway for the STA to pass from extracranial to the intracranial compartment.

After craniotomy is completed, judicious homeostasis must be obtained using bone wax and elevation of the dural edges to prevent epidural bleeding.

The dura mater is opened in cruciate fashion and dural edges elevated. A middle cerebral branch in the distal sylvian fissure (M3 branch), usually the largest temporal or parietal cortical branch, commonly free of perforators, is used for anastomosis. Ideally, the recipient vessel has to be 1.5 mm in diameter but can be as narrow as 1.0 mm. The recipient vessel is dissected free of its arachnoidal covering, and a small rubber dam is placed under the artery. Small papaverine-soaked Gelfoam is placed over the prepared recipient artery. The distal end of the prepared branch of the STA is cut and the 1 cm terminal of the STA is cleaned of its adventitial covering. It is cannulated with a blunt needle and irrigated with heparinized saline. The temporary clip on the STA is opened and then closed, so as to fill the segment of the artery with heparinized saline. An oblique arteriotomy with fish-mouthing of the STA is done.

The suturing is initiated by placing the first two sutures: one on the heel and one on the toe of the anastomosis. After that, the back wall and front wall can be sutured using both continuous running technique or interrupted 9–0 or 10–0 (preferably) sutures, according to the surgeon’s preference. Before the last suture is tied, the lumen is flushed with heparinized saline and then the suture is tightened. The temporary clips on the MCA branch (distal followed by proximal) are released first, followed by the temporary clip on the STA. Small leaks are usually stopped with small pieces of Surgicel or Gelfoam. All steps are represented in schematic and surgical pictures in Fig. 49.1, Fig. 49.2, and Fig. 49.3.

The flow through the STA is checked with a micro-Doppler probe or with a perivascular probe to measure the flow in real time.32 ^, ³⁹ Another method that might be used is the intraoperative video angiography with either ICG⁴⁰ or fluorescein⁴¹ (Fig. 49.4).

An alternative to the M3 vessel is an M4 cortical branch in the temporal or parietal region. The optimal branch for this type of anastomosis can be selected on the basis of a preoperative computed tomography (CT) angiogram. A double bypass may also be created using both branches of the STA, one into the frontoparietal branch and another into a temporal branch.