As a stenosis becomes more severe, blood flow through it increases in velocity to maintain volume, flow and pressure. But there is a critical point beyond which further increase in stenosis no longer allows sufficient blood to pass through to maintain volumetric flow, and the carotid artery beyond the stenosis begins to decrease in diameter. This is the near occlusion. To maintain a sufficient blood flow in affected area, there is a progressive recruitment of collaterals followed by an activation of cerebral autoregulation with dilatation of resistance vessels. When this process fails to maintain normal cerebral blood flow, oxygen extraction fraction of the affected brain tissue increases to maintain normal cerebral metabolism. Near occlusion has been described as involving 1 to 10% of all severe stenosis, but the potential for stroke from such critical stenosis is less than its appearance would suggest. The optimum management of near-occlusion therefore remains a matter of debate. Although endarterectomy for carotid stenosis of 70-99% was associated with an absolute risk reduction in any stroke or death of 16% in the original randomized trials, the benefit was less in patients with near-occlusion. In 2015, a meta-analysis focused on patients with near-occlusion confirmed only a small benefit of carotid endarterectomy or stenting compared to medical treatment in patients with near occlusion. In patients with near-occlusion and compromised hemodynamics, revascularization should improve cerebral blood flow and consequently prevent ischemic stroke. Nevertheless the effect of improved cerebral hemodynamics after revascularization on prevention of ischemic stroke is uncertain
