STA-MCA Microanastomosis: Surgical Technique

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8 STA-MCA Microanastomosis

Surgical Technique

Nadia Khan, Luca Regli

Introduction

The purpose of extracranial to intracranial bypass is always to improve the cerebral blood flow (CBF) by diverting blood flow from the extracranial carotid artery circulation to the intracranial cerebral circulation. There are two main indications for cerebral revascularization procedures: (1) flow augmentation to increase CBF in patients with chronic compromised CBF (chronic cerebral ischemia), and (2) flow preservation to maintain CBF in patients undergoing acute vessel sacrifice (complex aneurysms, skull base tumors). Revascularization procedures can be divided into direct (connecting the donor and recipient vessel directly by microanastomosis) and indirect techniques (laying vascularized tissue in contact with the brain to develop delayed collateralization).

The first successful direct extracranial-intracranial (EC-IC) bypass surgery was performed by Yasargil in 1967.1 Since then, many operative techniques for direct cerebral revascularization have been described (see chapters 7 through 14). The most commonly used EC-IC procedure to revascularize the anterior circulation is the superficial temporay artery (STA)–middle cerebral artery (MCA) bypass between one branch of the STA and a cortical MCA branch (M4). It has the advantage of being a safe, simple, and readily available technique, and the disadvantage of supplying low flow rates. In this chapter we will describe the technical aspects of the STA-MCA bypass as well as the pitfalls and lessons learned by the authors based on their longstanding experience.

Perioperative considerations and surgical technique

The preoperative as well as the postoperative period and special anesthetic considerations are important general steps of the STA-MCA procedures. The surgical technique of the STA-MCA bypass can be divided into four fundamental steps:

1. Preparation of the donor STA vessel
2. Localization and performance of the craniotomy
3. Preparation of the recipient vessel and completion of the microanastomosis
4. Closure

Preoperative Period

Patients to receive cerebral revascularization must have accurate preoperative systemic assessment especially for cardiovascular risk factors to optimize their medical condition and reduce operative and postoperative morbidity and mortality. Patients presenting with cerebral ischemic disorders frequently have systemic atherosclerotic disease.

Antiplatelet treatment with low-dose anesthetic is begun three days preoperatively and continued daily thereafter in order to optimize microanastomosis patency rates (>95%). Variability in individual response to antiplatelet therapy exists, and 4% to 20% of patients are resistant to low-dose treatment.2,3

The STA has two main branches, parietal and frontal, either of which can be used as the donor vessel. The choice is for the branch with the largest diameter as the amount of flow is proportionate to the radius at the power of 4 (Poiseuille’s law). The availability of an adequate STA branch is checked on preoperative imaging (six-vessel intra-arterial angiography or computed tomography angiography [CTA]). In patients with chronic ischemia, preoperative cerebrovascular hemodynamic assessment is essential for proper indication.

Anesthesia and Cerebral Protection

Patients are operated under general anesthesia. All the anesthetic interventions that decrease cerebral perfusion pressure (CPP) must be strictly avoided. Therefore hyperventilation is not used and patients are operated under normocapnia (35 to 40 mm Hg). Similarly, hypotension must be avoided and slight (10% to 20%) elevation of mean arterial pressure is aimed at during cross-clamping. There is no clear evidence in the literature that inducing brain protection via decreased metabolism changes outcome, and we do not recommend it for the standard STA-MCA. However, hyperthermia must be avoided.

Surgical Technique

Preparation of the STA

The patient is placed supine and the head is fixed in a three-point Mayfield fixation frame. The head should be elevated above the heart, turned to the contralateral side, keeping the operative field horizontal to the floor. Depending on the suppleness of the cervical spine, a shoulder roll may or may not be required.

The STA and its branches are mapped with a Doppler ultrasound probe and serve as a guideline for the skin incision (Figure 8–1). Using the operative microscope, a linear skin incision is made directly over the course of the parietal branch of the STA all the way to the zygoma and an 8-cm length of the STA is dissected out. In the event that the frontal branch of the STA is to be used, a curvilinear frontotemporal skin flap behind the hairline allows for a clean dissection of the frontal branch from the underside of the scalp flap. This can be tedious since the dissection from under the flap may require additional skin retraction and dissection through the fat plane.

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Figure 8–1 The two major STA branches, parietal and frontal. Dotted line indicates line of surgical incision for dissection.

The STA is then dissected out from the galeal tissue, ensuring that a small cuff (2 to 3 mm) of soft tissue surrounding it remains. Bipolar electrocoagulation of the arterial side branches is performed at a distance of several millimeters to avoid thermal injury to the STA. The isolated STA must be long enough so that no traction occurs during the bypass. The distal end of the artery can then be clipped and divided after temporary clamping at the proximal end, or left intact until after the craniotomy and just prior to the anastomosis (Figure 8–2). After the STA is divided, meticulous care must be taken to irrigate it with heparinized saline and then place it protected in moist gauze at the proximal end of the incision.

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Figure 8–2 Schematic intraoperative drawing of the prepared STA clipped and ready for anastomosis. Underlying craniotomy with dural opening and visualization of a cortical branch.

Performance of the craniotomy: Location and size

The craniotomy is planned so as to expose at least one adequate recipient cortical MCA branch. Numerous modifications for the identification of the site have been proposed in the literature.4 Considerations by Chater et al.5 to locate the craniotomy are recommended: “[P]erform the craniotomy at a point 6 cm above the external auditory meatus on a line perpendicular to the base of the skull.” To approximate the base of the skull, “a horizontal line from the external auditory canal to the lateral canthus of the eye” is the most suitable as shown by Pena-Tapia et al.4 The temporalis muscle is divided in a linear incision and mobilized in order to expose the region of the planned craniotomy. After placing a temporal burr hole, a 3×3-cm craniotomy is performed. The size of craniotomy varies depending on the indication or need for revascularization. For Moyamoya angiopathy, a larger craniotomy is usually preferable, allowing for a wider selection of suitable cortical vessels as well as for a larger surface area for additional indirect revascularization as needed.

Preparation of the recipient vessel and completion of the microanastomosis

The dural opening can be performed in a Y-fashion to allow for three triangular dural flaps, or the opening can be further extended into several flaps, depending on the location of the preferred cortical M4 branch (Figure 8–3). These are retracted with tag-up sutures, and the cortex is inspected to locate the most suitable M4 recipient artery. The ideal recipient artery is the one located at the cortical surface, has a straight segment without significant side branches, and usually is the largest in diameter (ideally 1.5 mm). Cortical vessel length of 1 to 1.5 cm is adequate. The M4 recipient cortical vessels that can be used include suprasylvian: precentral, central, anterior parietal, posterior parietal, and angular branches; and infrasylvian: anterior temporal, middle temporal, and posterior temporal. With extension of the craniotomy in the anterior and posterior directions, the prefrontal branch anteriorly and the temporo-occipital branch posteriorly can also be visualized.

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Figure 8–3 Schematic intraoperative drawing of prepared STA-MCA anastomosis.

After localizing a recipient cortical artery, the arachnoid over it is opened and dissected with microsurgical instruments. Small side branches (usually not more than two or three) are cauterized and cut. A small rubber high-visibility background is placed under the recipient artery. Papaverine is intermittently instilled over the cortical branch and the predissected STA.

Once preparation to perform the anastomosis is complete, the STA is prepared. The STA is flushed with heparinized saline, and the distance between the donor and recipient artery is measured. It is important to allow for 2 cm of redundancy in order to ease flipping the artery forward and backward while placing the microsutures on both sides of the microanastomosis. The distal end of the STA is cleared of any remaining adventitia over 1 to 2 cm to allow for a clean microanastomosis. The STA is then cut at a sharp angle and fish-mouthed in order to obtain an opening that is at least twice the diameter of the recipient artery. In making the operative field ready for the microanastomosis, it is essential to keep it clean from cerebrospinal fluid (CSF) and blood. Placing a flexible microsuction system is helpful. The recipient cortical branch is then clamped on both ends with atraumatic clips and an arteriotomy is performed in accordance with the size of the distal end of the STA. The arteriotomy is irrigated and rinsed with heparinized saline. Using 10-0 or 11-0 microsutures (depending on the size and fragility of both the donor and recipient vessels), two anchoring sutures (one on the heel and the other on the toe of the donor vessel) are placed followed by 10 to 14 interrupted sutures to complete the microanastomosis. Alternatively, two running sutures can be used. Whether interrupted or continuous sutures are placed depends on surgeon preference and training. Once suturing is complete, flow is restored by removing first the distal clip, followed by the proximal, and finally the clip on the STA. Minor oozing from the anastomosis site can be controlled by placing small hemostatic sponges locally (surgical or tachosyl) without compressing the anastomosis. More brisk bleeding points may need additional microsutures. The anastomosis is tested for patency either with near-infrared indocyanine green fluorescence angiography or an intraoperative quantitative flow meter (Charbel Micro Flowprobe, Transonic).

Surgical closure

The dura is closed leaving a generous durotomy at the point of entry of the STA. The bone flap is repositioned and fixed with titanium plates. Here again sufficient bone opening at the point of entry of the STA is allowed for. The temporalis muscle is then sutured, leaving a generous space for the artery. Wound drain is not used as standard. Subcutaneous 3-0 vicryl and separate skin sutures are applied while ensuring that a puncture or compression of the donor graft does not occur. It can be advantageous at this point in time to mark the point of penetration of the STA on the skin to allow for ease of detection at the time of postoperative Doppler measurements. Light, noncircumferential postoperative head dressing should be applied. Writing “Attention: Bypass” on the wound dressing helps prevent pressure on the graft in a setting where bypass grafts are not performed routinely.

Postoperative Care

Patients should spend the first night in an intensive or intermediate care bed for close monitoring of neurological and hemodynamic parameters. Blood pressure control is important to prevent hypotension and the risk of graft thrombosis or hypertension and the risk of hyperperfusion. Graft patency can be checked by Doppler ultrasound, while always avoiding compression of the graft. We like to perform an imaging of the bypass (preferably a CTA) between 24 and 48 hours after the surgery as a baseline exam for future follow-up. A six-vessel cerebral angiogram is performed usually at 6 months (Figure 8–4).

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Figure 8–4 Postoperative cerebral angiogram showing lateral external carotid injection demonstrating a patent robust graft with good distal filling of MCA territory.

Special considerations and pitfalls

Intraoperative Tests for Anastomosis Patency

Patency of the anastomosis can be evaluated using a micro-Doppler probe. However, we prefer to use intraoperative quantitative blood-flow measurements with calculation of the cut flow index (CFI). The CFI is calculated by dividing the bypass flow by the cut STA flow (free flow through the cut STA). This index has been shown to be a predictor of bypass patency: CFI > 0.5, permeability of 92%; CFI < 0.5, permeability 50%.6

Near-infrared indocyanine green fluorescence angiography to visualize graft, cortical, and pial filling is also a useful technique, allowing direct visualization of the anastomosis patency and dynamic distribution of flow.7 Fluorescence angiography does not indicate quantitative blood flow, and we recommend using it in combination with a quantitative measurement.

It is important to remember that graft pulsation is not a reliable indicator of graft patency and one should not rely solely on this indicator.

Complication: Microanastomosis Occlusion

Poor anastomosis flow will normally induce subsequent thrombosis and graft occlusion. It is therefore mandatory to identify these situations perioperatively. Graft occlusion can be attributed to two major causes: poor indication and technical errors.

Poor indication refers to patients undergoing a bypass without cerebral hemodynamic compromise. The blood flow in the MCA territory is strong enough to be competitive with the bypass flow. This cannot be corrected perioperatively.

The most frequent technical errors are traumatic handling of the endothelium; inadequate flushing with heparinized saline; misplacement of the microsutures, catching the back wall or irregular stitches; inadequate anastomosis size; twisting or kinking of the donor vessel; and inadequate length of STA resulting in continuous traction and preventing proper suturing of the frontal and back wall. In case of poor flow or occlusion, the microanastomosis has to be inspected and opened and redone to correct the technical error.

Wound Healing

Skin blood flow is reduced because the STA flow has been diverted to the brain. This reduces the natural healing process, sometimes resulting in necrosis, secondary infection, and wound dehiscence. Careful postoperative wound inspection is recommended.

Instrumentation and Training

Choice of microinstruments, microscope, and flow measuring devices all depend on the surgeon’s choice, preference, and experience. There is, however, no doubt that successful microanastomosis requires meticulous microsurgical skills and frequent training of microanastomosis techniques in the laboratory.

Less-Invasive Technique for STA-MCA Bypass

Preoperative identification of the STA and a sizable recipient artery on CTA is easy: the larger STA branch is chosen as the donor vessel. The choice of the recipient vessel is based on the size and superficial cortical location. Using 3-D CTA reconstructions, a small craniotomy (2 to 2.5 cm in diameter) can be ideally located to expose the recipient vessel. Using CTA-based neuronavigation, the craniotomy and a short linear incision (5 cm) over the STA can be planned to overlie the recipient and the donor vessel. The precise localization of the target point allows the procedure to be less invasive.8,9

References

1 Yasargil M.G. Microsurgery Applied to Neurosurgery. Stuttgart: Georg Thieme-Verlag, 1969.

2 Blais N., et al. Response to aspirin in healthy individuals. Cross-comparison of light transmission aggregometry, VerifyNow system, platelet count drop, thromboelastography (TEG) and urinary 11-dehydrothromboxane B(2). Thromb Haemost. 2009;102(2):404-411.

3 Collet J.P., Montalescot G. Platelet function testing and implications for clinical practice. J Cardiovasc Pharmacol Ther. 2009;14(3):157-169.

4 Pena-Tapia P.G., et al. Identification of the optimal cortical target point for extracranial-intracranial bypass surgery in patients with hemodynamic cerebrovascular insufficiency. J Neurosurg. 2008;108(4):655-661.

5 Chater N., et al. Microvascular bypass surgery. Part 1: anatomical studies. J Neurosurg. 1976;44(6):712-714.

6 Amin-Hanjani S., et al. The cut flow index: an intraoperative predictor of the success of extracranial-intracranial bypass for occlusive cerebrovascular disease. Neurosurgery. 2005;56(Suppl 1):75-85.

7 Woitzik J., et al. Intraoperative control of extracranial-intracranial bypass patency by near-infrared indocyanine green videoangiography. J Neurosurg. 2005;102(4):692-698.

8 Coppens J.R., Cantando J.D., Abdulrauf S.I. Minimally invasive superficial temporal artery to middle cerebral artery bypass through an enlarged bur hole: the use of computed tomography angiography neuronavigation in surgical planning. J Neurosurg. 2008;109(3):553-558.

9 Kaku Y., et al. Less invasive technique for EC-IC bypass. Acta Neurochir Suppl. 2008;103:83-86.

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