Effects of extracranial carotid stenosis on intracranial blood flow.

BACKGROUND AND PURPOSE The hemodynamic effects of extracranial carotid stenosis on intracranial blood flow are not well characterized. We sought to determine the impact of degree of stenosis, stenosis length, and residual lumen on intracranial blood flow in patients with extracranial carotid stenosis. METHODS Carotid stenosis patients who had undergone both vessel flow rate measurements using quantitative magnetic resonance angiography and digital subtraction angiography were examined. The impact of the anatomic measurements of the stenosis relative to ipsilateral internal carotid artery (ICA) flow and ipsilateral-to-contralateral middle cerebral artery (MCA) flow ratio were assessed. RESULTS Forty-four patients (mean age, 67 years; 64% symptomatic) were included. Higher percentage stenosis and smaller residual lumen were associated with a significant decrease in ICA flow (P<0.01 and 0.04, respectively). On multivariate analysis, percentage stenosis remained as the primary predictor of ICA flow (P<0.001). MCA flow ratio was not significantly associated with percentage stenosis, stenosis length, or residual lumen. However, mean MCA flow ratio was significantly lower in symptomatic compared with asymptomatic patients (0.92 versus 1.22; P=0.001). In contrast, mean ICA flow ratio was similar among these 2 groups (0.55 versus 0.55; P=0.99). CONCLUSIONS Percentage stenosis and residual lumen are significantly associated with ICA but not MCA flow, suggesting that local hemodynamic effects of carotid stenosis do not translate directly to distal vasculature, because intracranial flows can be maintained through collaterals. The lower MCA flow ratio in symptomatic patients highlights the potential importance of distal hemodynamics in symptomatic presentation.