Renal artery stenosis diagnostic criteria
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor-In-Chief: Shivam Singla, M.D.[2]
Overview
Several clinical clues aid in the suspicion of ARAS and warrant further investigation. To date, imaging is considered the optimal modality to diagnose ARAS. According to the ACC/AHA guidelines in 2013, Doppler ultrasonography, CT angiography, and MR angiography are all non-invasive techniques to diagnose ARAS. Renal angiography remains the gold standard for diagnosis of ARAS. Nonetheless, it is an invasive procedure that should be reserved to patients who are planning to perform a catheterization procedure and concede to renal angiography or to patients whose non-invasive imaging was equivocal.
Diagnosis
There are numerous tests and procedures involved in the detection of renal artery stenosis. Renal artery stenosis is best diagnosed with MRA(Magnetic resonance Imaging), Doppler ultrasound, Computed tomography, renal scintigraphy, peripheral renin levels, and renal vein sampling. Though these all modalities are used for making the diagnosis but still renal vein sampling, renal scintigraphy are not the first choice for making the diagnosis of renal artery stenosis because of their low sensitivity and specificity which is around 38-40.
The imaging modalities may be considered diagnostic if the following objectives are met:
(1) Anatomic and or Hemodynamic abnormality
(2) Anatomic consequences and complications associated with renal artery stenosis (Post stenotic dilatation of renal artery can be seen with the use of CTA and MRA, shrinkage of renal parenchyma, with kidneys being < 8 cm.
(3) Functional and cellular consequences of renal artery stenosis
(4) Renal impairment criteria related to renovascular disease should be met
Ultrasonography
Ultrasonography is readily available, secure, and inexpensive and consequently is usually the first imaging study used to detect Renal artery stenosis. Usually the results and accuracy is operator dependent and ranges in between 60-90%. This modality helps in the assessment of
- Renal functional reserve
- Renal resistive index.
A renal artery EDV >90cm/s and RRI< 75-80 represents no microvascular disease. The hemodynamic significant abnormality is concluded with the presence of spectral broadening and increased velocity on USG.
Reno aortic velocity ratio > 3.5 corresponds with 60% stenosis and RAPSV (Renal artery peak systolic velocity) greater than 150cm/s corresponds to 50% stenosis where as velocity greater than 180cm/s corresponds to 60% stenosis. According to the recent studies the sensitivity and specificity of ultrasound guided detection of renal artery stenosis is usually 85% and 92% respectively. Severe stenosis is diagnosed on USG with slowed systolic accelerations along with the decreased resistive index.
Quantitative criteria for diagnosing distal stenosis includes early peak systolic acceleration <3m/s2, an acceleration index > 4m/s2, and or greater than 5% difference in RRI between both the kidneys. Because of these waveform are difficult to interpret these criteria's are difficult to interpret.
Computed Tomographic Angiography
The three dimensional assessment of the tissue made the CT scanning as one of the important tool in the diagnosis of Renal artery stenosis.
CT angiography
- contraindicated in patients with contrast allergy as the this procedure modality involves the ionizing radiations and iodinated contrast medium.
- In patients having underlying renal impairment the use of iodinated contrast can lead to the development of contrast induced nephropathy, but it can be prevented with the use of hydration before doing the procedure.
- The sensitivity of this procedure is extremely high with 94% and specificity varies between 60% to 90 %.
- CTA can give the detailed resolution of even small accessory renal arteries.
- It is also the diagnostic modality of choice in patients having limited capacity to hold breath and also in patients having claustrophobia.
- At the same time CTA is having limited diagnostic modality as compared to MRA in detecting clinically significant Renal artery stenosis and also in patients having renal dysfunction
Magnetic Resonance Angiography
Magnetic resonance angiography has a reported sensitivity and specificity of 90% to 100%55,56 and does not require use of iodinated contrast or radiation. Gadolinium-based contrast medium should be avoided in patients with moderate to end-stage renal failure because of the risk of nephrogenic systemic fibrosis. Additionally, MRA should not be used in patients with certain implanted devices (ie, pacemakers, defibrillators, cochlear implants, and spinal cord stimulators) or in claustrophobic patients. Unlike CTA, MRA has no calcification artifact, neither iodinated contrast medium nor radiation is used, and contrast reaction rates are lower.
Angiography
Invasive renal arteriography is helpful in evaluating ARAS. In addition to assessing the severity of ARAS, angiography can detect intrarenal vascular abnormalities and anatomic abnormalities of the kidneys, renal arteries, and aorta. Digital subtraction angiography improves contrast resolution and may decrease the volume of contrast needed to as little as 15 mL. However, because renal angiography is invasive, there are risks associated with arterial puncture and manipulation of the catheter/wire, which can result in arterial trauma, spasm, or thromboembolic phenomenon.58 In patients with renal impairment or contrast allergy, carbon dioxide can be used as a non nephrotoxic contrast agent.
The early work by White et al59 established that there is substantial intra- and interobserver variability in the visual estimation of coronary stenosis, which likely also applies to the visual estimation of ARAS. Therefore, relying solely on angiography to visually estimate the severity of ARAS is suboptimal, and adjunctive tools should be used to determine whether renal ischemia is present.
Translesional pressure gradients can be measured across areas of stenosis to determine hemodynamic significance (if there is doubt) before performing therapeutic procedures such as percutaneous transluminal renal angioplasty (PTRA) or stenting. In a small case series, Mangiacapra et al60 measured translesional pressure gradients using papaverine and dopamine to induce renal hyperemia in 53 consecutive patients before PTRA. They found that patients with the most substantial improvement in hypertension were those with a translesional gradient greater than 20 mm Hg (corresponding to a distal-proximal pressure ratio of 0.79 as the optimal cutoff). De Bruyne et al61 demonstrated that stenosis with a distal to proximal renal artery pressure decrease greater than 10% were associated with increased renin production, suggesting that measurement of translesional pressure gradients might help identify hemodynamically significant ARAS.