Imaging description

A 10-year-old boy was discovered to be hypertensive. At ultrasound (US) imaging, as in this child (Fig. 35.1a, b), unilateral renal artery stenosis may demonstrate asymmetry of renal size, with atrophy of the affected and compensatory hypertrophy of the contralateral kidney (Fig. 35.1a, b). Doppler imaging within the kidney typically demonstrates absence of an early systolic velocity peak, a slow systolic upstroke in velocity (long acceleration time), and in consequence a delayed peak systolic velocity with increased diastolic flow, known as the “tardus and parvus” pattern. This finding is seen in the segmental intrarenal vessels downstream from the site of stenosis. The effect on peak systolic flow velocity is variable, depending on where you interrogate; normally peak velocity drops progressively along the course of the renal artery towards the kidney. However, unless flow is critically obstructed, flow velocity is elevated at the stenosis (typically peak velocity in excess of 200 cm/s), and spectral widening due to turbulence is present just distal to the stenosis (Fig. 35.1 c–e). Published criteria for the diagnosis of renal artery stenosis also include elevated peak systolic velocity in the renal artery compared to the adjacent aorta (> 3.5 cm/s) and excessive difference in resistive index, which is calculated as (peak systolic velocity–end diastolic velocity)/peak systolic velocity between the two kidneys. The resistive index and resistive index ratio was normal in this patient, reflecting the difficulty in the practical application of this variable. In general, while US is a reasonable screening study for renal artery stenosis, US and Doppler imaging may be normal, especially with milder degrees of stenosis. At CTA (Fig. 35.1f, g) and MRA imaging, which was not obtained in this patient, the location and nature of the arterial stenosis are seen to advantage. Catheter angiography is usually necessary only when interventional/endovascular treatment is going to be attempted. The focal nature of the stenosis, lack of aneurysm formation, and responsiveness to angioplasty (Fig. 35.1) is most consistent with the perimedial fibroplasia type of fibromuscular dysplasia (FMD). Other common angiographic findings of FMD are depicted in a different patient in Figure 35.2.