Cerebrovascular
Two questions drive most items
Nearly every cerebrovascular item asks one of two things: which vessel am I looking at, and how tight is the narrowing? Answer the first before you grade the second.
Telling ICA from ECA
The ICA is usually larger, sits posterolateral, has no cervical branches, and shows a low-resistance waveform with continuous forward diastolic flow because it supplies the brain. The ECA is smaller, anteromedial, gives off branches (superior thyroid first), and shows a high-resistance, triphasic pattern with early diastolic reversal. A temporal tap or jaw movement confirms the ECA; the ICA does not respond to either. Trap: a severely stenotic ICA can lose diastolic flow and mimic the ECA, so find the bifurcation and confirm anatomy before you trust the waveform.
Grading ICA stenosis
Widely used consensus criteria combine four parameters: peak systolic velocity (PSV), end-diastolic velocity (EDV), the ICA/CCA PSV ratio, and plaque plus spectral broadening. Approximate thresholds: normal, PSV under 125 cm/s and ratio under 2.0; 50 to 69 percent, PSV 125 to 230, EDV 40 to 100, ratio 2.0 to 4.0; 70 percent or greater, PSV above 230, EDV above 100, ratio above 4.0. Sample the CCA from the mid-to-distal segment, well below the bulb, or the ratio becomes meaningless. Near-occlusion may show a string sign or paradoxically low velocities; do not call it occluded on velocity alone.
Worked example
ICA PSV 260 cm/s, EDV 110 cm/s, distal CCA PSV 60 cm/s. Ratio = 260 divided by 60 = 4.3. With PSV above 230, EDV above 100 and ratio above 4.0, this meets criteria for a 70 percent or greater diameter-reducing stenosis.