Cerebral Vascular Insufficiency: Direct & In-direct By-pass
Cerebral artery bypass is utilized for mainly 4 categories: complex aneurysms that are not amenable to clipping or coiling, atherosclerotic narrowing of vessels leading to too little blood flow to the brain, Moyamoya disease, and tumor-invaded important vessels where blood flow to the brain needs to be maintained.
By-pass can be Direct or In-direct. Direct by-pass also known as extracranial-intracranial (EC-IC) bypass can provide blood flow to the anterior circulation of the brain (Carotid circulation) or the posterior circulation (Vetebrobasilar Circulation) of the brain.
Direct by-pass can be low-flow or high flow.
Low-flow Direct By-pass
This, most commonly involves direct anastomosis of the superficial temporal artery (STA) to the middle cerebral artery (MCA), a major branch of the anterior (Carotid) circulation of the brain. The STA is a pedicled arterial graft that provides low-flow (15-25 ml/min) and has 95% graft patency. This provides immediate improvement of circulation to the brain. Occipital to PICA is another type of direct low-flow by-pass bringing blood to the posterior (Vertebrobasilar) circulation of the brain.
High-flow Direct By-Pass
Radial artery graft, a moderate to high flow graft (40-70 ml/min) is easy to harvest form the arm, and has a lumen adaptable to the lumen of M2 (A branch of the Middle Cerebral artery) and P1 (A Brain of the Posterior Cerebral Artery) but carrying a risk for vasospasm. Saphenous vein graft form the leg, is a high flow graft (70-140 ml/min) that is easily accessible and has long length but has risk of thrombosis and distal anastomosis due to flow mismatch and turbulence and lower overall graft patency rates. Radial artery grafts have > 90% graft patency at 5 years, while saphenous vein grafts only have 82% patency at 5 years.
Indirect revascularization, includes encephaloduroarteriosynangiosis (EDAS) wherein the STA is laid on the surface of the brain without a direct anastomosis and encephalomyosynangioisis (EMS) which involves laying a section of the temporalis muscle on the surface of the brain. The principal point is that these procedures stimulate processes that enhance formation of new blood vessels on and into the brain.
STA-MCA bypass is the intervention of choice for Moyamoya disease but has been shown to have no significant benefit over medical management for carotid artery occlusion.