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