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Role of and Indications for Bypass Surgery After Carotid Occlusion Surgery Study (COSS)?

Bypass surgery falls into 2 distinct categories: flow augmentation and flow preservation. Flow augmentation aims to restore flow to hypoperfused brain territories in patients with steno-occlusive diseases.1 Flow preservation aims to replace the blood flow provided by a major intracranial vessel, the occlusion of which is necessary for treating an underlying disease, such as an aneurysm or a tumor.2,3

Flow augmentation bypass has been critically studied in randomized clinical trials (RCTs),4–6 most recently the Carotid Occlusion Surgery Study (COSS)5 and the Japanese Adult Moyamoya (JAM) trial,6 whereas flow preservation bypass remains a niche procedure because of its rare indication.2,3,7,8

In this review, we aim both to critically summarize the current state of knowledge on the role of cerebral bypass surgery because the publication of COSS and to present possible future directions for surgical cerebral revascularization.

### Bypass Technique

Beyond the underlying disease and the consequent aim that defines the 2 categories of bypass (see above), several other criteria are used to classify bypass constructs. A well-known classification is the distinction into direct versus indirect revascularization procedures or the combination of both.9,10 Direct bypasses consist of direct anastomosis between a donor artery and an intracranial recipient artery. A direct bypass has the advantage of instantly delivering blood flow to the brain.1,2,7 Indirect techniques rely on the overlay of vascularized tissue (ie, muscle, dura, pericranium, omentum) onto the cerebral cortex to promote neoangiogenesis over time and achieve a delayed revascularization.10,11 Combined procedures consist of the combination of direct and indirect techniques in the same surgical session.10

According to the origin of the donor artery, direct bypass is further categorized into extracranial-to-intracranial (EC–IC) versus intracranial-to-intracranial (IC–IC). Furthermore, the …