With common carotid artery occlusion there is usually no flow in the distal vessels. However, flow to the ipsilateral internal common carotid artery may be maintained by collateral vessels. Nine of 1100 patients referred for carotid duplex sonography were found to have occluded common carotid arteries. Five of these patients had collateral circulation with reconstitution of flow via the external carotid artery. The color Doppler sonograms of three representative patients are presented. There was antegrade flow in the internal carotid artery with retrograde flow in the external carotid artery.

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