Ultrasound images Renal artery stenosis (RAS)

Stenosis of the renal artery is due to atrerosclerotic disease in the vast majority of patients, or to fibromuscular dysplasia of the arterial wall in the younger, generally female patient. RAS may cause hypertension and may eventually cause renal failure. It is frequently bilateral, and is responsible for up to 15% of patients who require long-term dialysis. It is associated with aortic aneurysm, neurofibromatosis or can be traumatic in origin.
Stenosis generally affects the main vessel at its origin and involves the aorta (ostial) or occurs within 1 cm of its origin (non-ostial). It can occur in both native and transplanted organs. It is frequently bilateral.
If the stenosis is long-standing and/or severe, thekidney is likely to be small. Loss of renal mass is associated with a stenosis of 60% or greater. However, the ultrasound appearances are often normal with milder grades of RAS. Ultrasound has traditionally had a limited role to play in the diagnosis of RAS and digital subtraction angiography is still considered the gold standard, although magnetic resonance angiography (MRA) is now regarded as the first-line imaging modality of choice. However, colour and spectral Doppler techniques have greatly enhanced the usefulness of ultrasound, particularly in experienced hands.
At the site of a stenosis, an increase in peak systolic velocity may be found (greater than 1.5–1.8 m/s) with poststenotic turbulence. Although, it is often not technically possible confidently to examine and sample the whole main renal artery and thus make a definitive diagnosis, it nevertheless remains the best Doppler technique for diagnosis.
In addition, the intrarenal vessels may show changes on colour or power Doppler which are indicative of a downstream stenosis. Within the kidney, the perfusion may appear subjectively reduced in the number of vessels and velocity of flow and it may be necessary for the operator to use a low PRF value to detect blood flow. This is very subjective and variable. The spectral waveforms of arteries distal to the stenosis also reflect changes which suggest a proximal stenosis; the normally fast systolic upstroke is replaced by a delayed parvus tardus pattern (Fig. ultrasound images below), making the waveform less pulsatile with a rounded envelope This type of waveform can be appreciated subjectively, but quantitative measurements may be used to support the diagnosis. The acceleration time (AT) or acceleration index (AI) is the most common; a normal AT is < 0.07s, and a normal AI > 3 m/s.
image
ultrasound images Renal artery stenosis. The kidney is
small, with subjectively reduced perfusion on colour
Doppler. The spectrum displays the parvus tardus pattern
image
The arteriogram in case (A) confirms a stenosis (arrow).

The actual value of these indices, however, does not reflect the severity of stenosis; unfortunately stenoses of < 70–80% narrowing do not normally demonstrate the parvus tardus effect (although these tend to be less clinically significant) and these spectral phenomena may be obscured altogether if the vessels are rigid and severely diseased or if a good collateral circulation has developed. In such cases the Doppler result is falsely negative and the operator should bear this in mind when attempting to exclude RAS. Renal artery occlusion may occur as a result of further progression of the same disease process which causes stenosis. Doppler will confirm the lack of renal perfusion. The kidney is likely to be small as a result of gradually deteriorating arterial perfusion.
Ultrasound images Stenosis of the main renal artery is amenable to percutaneous angioplasty and/or stenting, which can effect a cure or more realistically stabilize or slow disease progression. A postangioplasty ultrasound scan can confirm vessel patency, and may play a role in monitoring the patient for disease recurrence. For those with deteriorating function, for whom percutaneous techniques have failed, renal failure will ultimately necessitate dialysis.
Renal transplant is a viable option, particularly for those who have been treated in the long term.

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