Intermittent drop in blood flow velocity in the internal carotid artery and its branches.

Extracranial Doppler sonography and transcranial Doppler sonography (TCD) allow the assessment and monitoring of hemodynamic and embolic events in cerebrovascular diseases. We describe an unusual hemodynamic phenomenon in a patient with intracranial carotid siphon stenosis and no clinical symptoms of stenosis. TCD examination suggested and angiography confirmed stenosis of the left internal carotid artery siphon. TCD examination revealed a sudden, intermittent drop in blood flow velocity in both the prestenotic and poststenotic segments of the internal carotid artery, whereas cardiac hemodynamic parameters were unaffected. Embolic signals were detected in the poststenotic vessels only. We speculate that such sonographic findings may be caused by intermittent vessel occlusions due to the reversible displacement of an intraluminal thrombus in relation to the cardiac cycle.