Introduction

Cerebral revascularization has had a tumultuous history with the nadir being just after the publication of the Extracranial/Intracranial Bypass Study Group (1985)¹ Currently, cerebral revascularization has gained momentum for specific disease processes: basal occlusive disease (MMD), giant or fusiform aneurysms unamenable to clip ligation or endovascular technology, and symptomatic intracranial and/or extracranial atherosclerotic occlusive disease. In 1963, Woringer and Kunlin performed the first external to intracranial internal carotid artery bypass utilizing a saphenous vein graft.² Yasargil is credited with advancement of cerebral revascularization techniques with the incorporation of microinstruments, bipolar technology, as well as the microscope.³ At present, conduits available for bypass include scalp branches such as the superficial temporal artery (STA) and occipital artery (OA), and interposition grafts such as reverse saphenous vein (SV) and radial artery (RA). More recently, intracranial vessels such as the anterior temporal artery, the posterior inferior cerebellar artery (PICA) or vessels in close proximity to each other such as the pericallosal (A2–4) segments of the anterior cerebral artery (ACA) have been used to allow for in situ bypass.

While controversy still exists regarding cerebral revascularization for occlusive disease—and indeed at the time of this writing the Carotid Occlusion Surgery Study (COSS) is still ongoing—its role in the treatment of giant/fusiform aneurysms is well documented.^4,5 In some instances, it is simply impossible to exclude an aneurismal segment of vessel without sacrificing the parent vessel. In addition, dissection of a large aneurysm may require a prolonged period of temporary flow arrest, which would require a cerebral bypass for flow augmentation and prevention of ischemic complications. In such cases, cerebral bypass provides temporary or permanent flow arrest of the parent vessel harboring the aneurysm. Current bypass techniques utilize temporary occlusion of the recipient and donor vessels; however, the ELANA (excimer-laser assisted nonocclusive anastomosis) technique popularized in Europe will allow for cerebral bypass without temporary occlusion.⁶

Aneurysms in the anterior circulation unamenable for simple clip ligation or coiling procedures include giant aneurysms, fusiform aneurysms, and dissecting aneurysms. While clip reconstruction could be performed in some, the possibility of parent or branch vessel occlusion or injury and the time required for surgical dissection lend way to utilization of a cerebral bypass so that the aneurismal segment may be safely sacrificed. Coiling of giant aneurysms is generally not practical because (1) it is rarely a durable form of treatment, and (2) endovascular treatment of distal ACA segment aneurysms is not feasible at present if a stent is required, as stents in use today are not approved for vessels <2.5 mm. However, cerebral bypass may be utilized with coil embolization to deconstruct a vessel in order to minimize cerebral retraction.⁷

Methods

There are multiple options for bypasses within the anterior circulation, and these include both IC-IC and EC-IC bypasses. The workhorse bypasses include in situ pericallosal side-to-side grafts and interposition grafts using arterial or venous donor vessels as follows: graft to distal A1, A1 to contralateral A1, A1 to ipsilateral or contralateral A2, A2 to A2, or A3 to A3.⁸

An anastomosis performed on any portion of the ACA is difficult as there is great depth in addition to a small working area. Yokoh et al. performed autopsy based studies to better define bypass techniques within the anterior circulation.⁹ A1 mobility is approximately 3 mm unless the perforators are sacrificed purposefully to increase mobility. In comparison to the A1 segment, the A2 and A3 segments have greater mobility and their surgical exposure affords a wider access to these pericallosal vessels, thus rendering them somewhat less difficult for cerebral bypass. When bypassing in the region of the A2 and A3, the recurrent artery of Heubner (RAH), orbitofrontal artery (OfA), and frontopolar (FpA) arteries may need to be reimplanted utilizing an end-to-side technique. In their study, Yokoh et al. showed that each of the perforating vessels mentioned above measured 1 mm in diameter except the RAH, which averaged at 0.7 mm.

End-to-end anastomosis is possible between the A1, A2, or A3 segments if the cut surfaces are <5 mm apart.⁹ Distal ACA side-to-side anastomosis protects the pericallosal distribution from ischemic injury and can be performed with approximately 1 cm of width space.^9,10 Grafts utilizing RA, STA, and or vein can be used to bypass to the distal ACA in order to prevent ischemia with parent vessel occlusion.

Anterior circulation bypass scenarios

Preoperative imaging and surgical planning

Preoperative imaging is a crucial part of surgical planning. The patient’s cerebral vasculature is studied utilizing both two- (2D) and three-dimensional (3D) cerebral angiography. Evaluation of the arterial, capillary, and venous phases with 2D angiography provides hemodynamic information, which is unavailable in static 3D renderings. Hemodynamic assessment of 2D angiography, for example, may demonstrate that a giant and/or partially thrombosed aneurysms may produce sufficient mass effect to slow flow in surrounding branches, and even displace supply to smaller perforating vessels.

Three-dimensional rotational angiography is superior to 2D angiography for assessment of anatomic relationships and is critical as a “virtual” surgical planning tool. Three-dimensional angiography allows for simulation of the vasculature as it would appear within the in vivo scenario.

Magnetic resonance imaging (MRI) is obtained preoperatively not only to comprehensively assess the anatomical abnormality for the purposes of surgical planning, but also to assess the adjacent brain that is at risk for ischemia. MRI evidence of recent ischemia (by diffusion weighted imaging) in brain adjacent to a partially thrombosed aneurysm is a crucial finding. It should alert the surgeon as to a risk of hemorrhagic conversion, and the necessity for careful consideration of operative timing relative to the likely age of the ischemia.

Cerebral bypass technique

Following administration of general anesthesia, patients are placed in the supine position with a shoulder roll, and are secured in three-point fixation utilizing radiolucent pins and a Mayfield head-holder (Integra, Plainsboro, NJ). The radiolucent head holder allows us to perform intraoperative angiography without substantial streak artifact. Both the left and right groins are prepped routinely in case cerebral angiography is required. For interhemispheric fissure (IHF) approaches we prefer that the patient’s head be turned 90 degrees so that the IHF is parallel to the floor, with the vertex tipped up 30 to 45 degrees with the operative side down. This both obviates the need for brain retractors (by using gravity) and allows the surgeon to work with instruments ergonomically in a more natural working plane.

For a bypass involving the A1 segment, a pterional craniotomy with orbital osteotomy is performed in order to allow for better visualization within the chiasmatic region. If an A2 or A3 anastomosis is to be performed, then a bifrontal craniotomy is performed for access to the anterior interhemispheric region. This approach may also be combined with the pterional approach. After a segment relatively free of perforating vessels is selected, the vessel to be bypassed is dissected clear of arachnoid adhesions, which allows further freedom to manipulate the recipient vessel. The case is performed with electrophysiological monitoring and the patient is placed under burst suppression with a short-acting barbiturate such as pentothal (Hospira Inc., Lake Forest, IL). The mean arterial pressure is maintained greater than 90 mm Hg to maximize collateral flow during temporary clip ligation.

Patients are treated preoperatively with antiplatelet therapy, using aspirin 325 mg daily for a week. After a wide cranial exposure, burst suppression by EEG, normocarbia, and mild hypothermia have been obtained, a piece of durapair (TEI Biosciences, Boston, MA) is placed under the recipient vessel to provide better visualization. A self-retaining suction (size 4, French) is placed within the field as to prevent cerebrospinal fluid/blood accumulation. After exposure of the recipient vessel and harvest of the donor, but before temporary clipping, heparin (Baxter International, Deerfield, IL) boluses are given (1000 units at a time) to maintain an activated clotting time (ACT) near 200.

Both the recipient and donor vessels are highlighted with a marking pen in the region to be anastomosed to enhance visualization. End-to-end or end-to-side anastomoses are performed with 8-0 to 10-0 nylon suture (Deme Tech, Miami, FL) in an interrupted fashion, while side-to-side anastomosis are performed in a running fashion. When the anastomosis is nearly complete, we use the technique described by Dacey, which employs unclipping of a temporarily clipped side branch on the donor vessel to back-bleed and rid the anastomosis of any air or debris.¹² Indocyanine green (ICG) angiography is then performed to evaluate the patency of the bypass. Tamponade with fibrillary collagen (avitene) rather than bipolar cautery should be used to obtain hemostasis of any slow oozing at the anastomosis. Rarely since the advent of ICG angiography do we perform intraoperative catheter cerebral angiography.

Before final wound closure, the ACT is checked, and the anticoagulation is partially reversed with protamine 1 mg for every 10 seconds above 200 seconds. The protamine dose rarely exceeds 5 mg, should never exceed 50 mg, and should be given over at least 10 minutes, as otherwise patients can develop an enhanced anticoagulation effect as well as extreme hypotension. In the immediate postoperative period, patients are maintained on aspirin therapy with systolic blood pressure normally maintained >120 mm Hg. Follow-up cerebral angiography is performed postoperatively and at 12 months to evaluate for patency and maturation.

Illustrative case

A 32-year-old male presented to our institution with a prior history of a left ICA terminus aneurysm that had been treated by Drake 22 years ago. At the age of 10 years, the patient had undergone craniotomy for clip ligation of the left ICA terminus aneurysm along with a cerebral bypass. Further evaluation had revealed the bypass to be thrombosed as the left cervical ICA was occluded.

After a motor vehicle accident in 2004, the patient underwent evaluation with a computer tomographic angiography (CTA) of the head that revealed continued filling of the left ICA terminus aneurysm as well as development of a large anterior communicating (ACOM) aneurysm. These findings were corroborated by a diagnostic cerebral angiogram (Figure 24–1). A 10-month follow-up angiogram revealed enlargement of the ACOM aneurysm as well as persistence of the left ICA terminus aneurysm. The patient underwent coil embolization of the left ICA terminus aneurysm without any complications. Eight months later the ICA terminus aneurysm had significant recurrence as well as further enlargement of the ACOM aneurysm.

Figure 24–1 Right ICA injection in the anteroposterior projection showing a fusiform aneurysm of the ACOM as well as a recurrence of the left ICA terminus aneurysm.

With the presence of an enlarging ACOM aneurysm and recurrent ICA terminus aneurysm, the decision was made to proceed with clip ligation and cerebral bypass. Since the left cerebral circulation was dependent on the ACOM complex, an ECA to M3 segment bypass was performed utilizing a radial artery graft (Figure 24–2). Twenty-four hours after the procedure, cerebral angiography revealed a thrombosed left ICA terminus aneurysm.

Figure 24–2 Left CCA injection in the anteroposterior projection revealing a patent ECA-M3 bypass

Since the ACOM aneurysm was a fusiform aneurysm, clip ligation of the neck was not possible; instead, the patient required cerebral bypass and trapping of the aneurysm. First an A3-A3 side-to-side anastomosis was performed and then a radial graft was utilized to perform a right-M1 to left-A3 bypass (Figure 24–3). The right A1 was clipped distally and the A2s were not clipped to allow backfilling of perforating vessels arising from the A2. Subsequent angiography revealed thrombosis of the left ICA terminus and ACOM aneurysms. The patient tolerated these three separate procedures and made a remarkable recovery, returning to his daily life.

Figure 24–3 Right ICA injection in the anteroposterior projection revealing a patent M1 to left A3 bypass.