An elderly man with amaurosis fugax has carotid duplex showing a severely diseased internal carotid artery with a narrow residual lumen and a peak systolic velocity of only 85 cm/s, much lower than expected for the degree of narrowing. Distal ICA branches show low-resistance damped flow. What does this pattern indicate?

• A Mild ICA stenosis because PSV is below 125 cm/s
• B ICA dissection with normal haemodynamics
• C Near-occlusion (critical stenosis with collapsed distal lumen), where PSV criteria fail ✓
• D Technical error from incorrect angle correction

Explanation

In near-occlusion the residual lumen is so tight that flow volume falls, so PSV paradoxically drops and standard velocity thresholds underestimate severity. The combination of a visibly tight lumen, low velocities and damped distal waveforms defines near-occlusion, which behaves like critical stenosis. Option A is the classic trap: applying the 125 cm/s cut-off blindly here would misclassify a surgical-grade lesion as mild.

Reference: Rumack's Diagnostic Ultrasound, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.