Superficial Temporal Artery to Middle Cerebral Artery Bypass

Patient Selection

The two main indications for extracranial to intracranial bypass are flow replacement for treatment of complex aneurysms or tumors, which requires vessel sacrifice, and flow augmentation for treatment of cerebral ischemia in those demonstrating misery perfusion.

Sacrifice of the parent vessel may be required for definitive management of giant or complex aneurysms or to allow complete excision of skull-base tumors. ⁸^, ⁹^, ¹⁰^, ¹¹ Although carotid sacrifice may be tolerated, stroke can occur in up to 30% of patients. Endovascular balloon test occlusion combined with hypotensive challenge and neurological, electrophysiological, and blood flow testing can be used to select patients who will not tolerate carotid sacrifice and who may benefit from surgical revascularization. ²^, ³ Fusiform or unclippable aneurysms of distal vessels, such as the MCA or its branches, would typically require revascularization because collaterals to such terminal vessels are inadequate. ²^, ³

Revascularization for patients with ischemic disease is controversial and case selection is individualized. Our indications for STA–MCA bypass are failure of maximal medical therapy (adequate antiplatelet or anticoagulant regimen, combined with modification of vascular risk factors), symptoms (stroke or transient ischemic attacks) concordant with radiographic findings, compromised cerebrovascular reserve as demonstrated by multimodal magnetic resonance imaging (MRI) using quantitative MR angiography performed with the noninvasive optimal vessel analysis software (VasSol, Inc., Chicago, IL) ¹² before and after acetazolamide challenge, and functional MRI with regional BOLD sequences using a multivessel territory task paradigm, as well as global BOLD before and after a CO₂ challenge. ²^, ³^, ¹³ Direct STA–MCA bypass is also an option in the treatment of symptomatic adult moyamoya disease. ⁷

36.3 Preoperative Preparation

In addition to the disease-specific workup that pertains to the indication for the STA–MCA bypass, a catheter angiogram is obtained to delineate the intracranial lesion. ⁴ Selective external carotid injections evaluate the caliber and course of the STA branches on the affected side. If there is concern regarding the adequacy of the STA, alternative bypass strategies using interposition grafts (saphenous vein or radial artery) anastomosed to the STA trunk or the cervical carotid can be entertained. Duplex mapping of the saphenous veins is useful to reveal the size of the veins and their course. If radial artery harvesting is contemplated, an Allen’s test should be performed while monitoring the oxygen saturation on the thumb.

Patients take 325 mg acetylsalicylic acid the night prior to the surgery. ⁴ If they are on warfarin, it is stopped and they are started on heparin, which is withheld 6 hours prior to surgery because antiplatelets are administered. Arterial line and central venous access are routinely obtained. Perioperative antibiotics are administered.

For ischemia cases, normovolemia, normocapnia, and normotension are maintained throughout the surgery. ⁴ For aneurysms, cerebrospinal fluid drainage via a lumbar drain can be used for brain relaxation to avoid the need for intravenous diuretics (furosemide), hyperosmolar agents (mannitol), or hyperventilation. Scalp electrodes for electroencephalographic monitoring are placed outside the surgical field. This allows for induction of burst suppression during temporary vessel occlusion. During temporary vessel occlusion mean arterial blood pressure is also raised, with the goal of achieving values 25% above baseline.

36.4 Operative Procedure (Video 36.1 and 36.2)

36.4.1 Positioning

The head is fixed in the lateral position with a four-pin Sugita head holder ( ▶ Fig. 36.1). A shoulder roll may be needed in patients with restriction in cervical lateral rotation. If intraoperative angiography is desired, a radiolucent frame (Mizuko America, Inc., Beverly, MA) is used and the right groin is prepared and draped for placement of a femoral angiography catheter. In the case of saphenous vein or radial artery harvesting, the appropriate lower (based upon size and length) or upper limb (the nondominant forearm) is also prepared and draped. For the lower limb, generally the thigh vein is used. For radial artery, the area of the ventral forearm extending from the hand to the antecubital fossa is prepared.

The scalp is shaved and a Doppler is used to map the STA starting at the level of the zygoma, as well as both anterior and posterior branches of the vessel ( ▶ Fig. 36.1).

Fig. 36.1 Positioning for a right superficial temporal artery (STA)–middle cerebral artery bypass. The posterior and anterior branches of the STA are traced with Doppler and marked on the scalp as shown.

36.4.2 Skin Incision

The skin incision generally overlies the STA trunk just anterior to the tragus in the region of the zygoma and extends along the course of the posterior branch in a linear fashion. ⁴ In the case of a bypass for ischemia, a linear incision of this nature is adequate. If the posterior branch is inadequate, an incision could be placed directly over the anterior branch instead, but frequently this course will carry the incision onto the forehead. In such cases it might be preferable to create a semicircular incision behind the hairline ( ▶ Fig. 36.2) and expose the anterior branch from the undersurface of the skin flap. This is also used when a posterior branch is initially exposed but appears to be of poor quality or caliber—the linear incision is merely curved forward and converted into a skin flap, which allows dissection of the anterior branch. In the case of bypass for aneurysm, the linear incision over the posterior branch of the STA is curved forward in a semicircular fashion resembling the standard pterional incision, to allow access for the required craniotomy.

Fig. 36.2 Steps in performing the initial exposure for a superficial temporal artery–middle cerebral artery (STA–MCA) bypass. (a) The STA trunk and branch are exposed and dissected free with their surrounding cuff of tissue. (b) The temporalis muscle is divided and retracted anteriorly or in a T-shaped/cruciate fashion, maintaining the STA in continuity. (c) Burr holes are placed and the craniotomy flap is elevated, a pterional flap for aneurysm exposure or more limited craniotomy for access to cortical vessels in ischemia cases as shown. The dura is opened and flapped anteriorly or in a cruciate fashion.

36.4.3 Superficial Temporal Artery Dissection

The initial incision through the epidermis and dermis is made with a Colorado microneedle tip monopolar cautery (Stryker Leibinger, Kalamazoo, MI), set on 8, along the midpoint of the projected course of the STA branch. We perform this under loupe or microscopic magnification with the surgeon and assistant seated. Opening the skin in this fashion limits bleeding from the skin edges, whereas the low cautery setting helps to prevent skin edge necrosis or poor wound healing. Once subcutaneous tissue is encountered, a blunt tip, fine curved snap is used to dissect down to the STA. Once the vessel is visualized the snap is used to dissect proximally in the loose areolar plane above the vessel. The Colorado tip is then used to open the skin to the tip of the snap as sequential dissection is performed, until the main trunk of the STA is reached. This procedure is continued distally.

The goal is to dissect about 8 to 10 cm of STA. Once exposed, Bovie electrocautery with a coated shaft at a setting of 25 to 30 is used to divide the tissue around the STA and allow it to be lifted from the underlying temporalis muscle fascia. The dissected STA and its surrounding tissue are then wrapped in a papaverine-soaked cottonoid to alleviate spasm induced by the mechanical manipulation of the vessel. ⁴

36.4.4 Craniotomy

Self-retaining fishhook retractors are placed at the skin edges with the STA within its protective cottonoid reflected to one side or the other. Bovie electrocautery is used to incise the temporalis fascia and muscle. For aneurysms, the muscle is cut along the line of the scalp incision and reflected anteriorly with the skin flap ( ▶ Fig. 36.2). For ischemia, the muscle can be opened symmetrically under the linear incision in a T-shape or cruciate fashion, with each quadrant of muscle retracted anteriorly or posteriorly with hooks.

For aneurysms, a standard frontotemporal pterional bone flap is elevated with removal of the sphenoid ridge extradurally. For ischemia, a burr hole is placed with a Midas (Medtronic, Inc., Fort Worth, TX) round drill bit below the proximal and distal aspect of the course of the vessel and a circular craniotomy created with the Midas (Medtronic) B1 drill bit ( ▶ Fig. 36.2). Thin strips of Gelfoam (Pfizer, Inc., New York, NY) or Surgicel (Johnson and Johnson, Somerville, NJ) are packed under the bone edges and the dura is tacked extensively around the margins of the craniotomy to stop and prevent epidural bleeding.

The dura is opened as a semicircular flap, which is retracted anteriorly ( ▶ Fig. 36.2, ▶ Fig. 36.3) or alternatively in a cruciate fashion with additional cuts placed to form multiple triangular flaps, which are tacked backward, exposing the cortex. ⁴ In moyamoya disease, dural arteries should be preserved because these are potential sources of collateralization.