Diagnostic imaging evaluation of renovascular hypertension

Percutaneous revascularization for ischemic nephropathy: the past, present, and future

Occlusion of the renal arteries can threaten the viability of the kidney when severe, in addition to accelerating hypertension and circulatory congestion. Renal artery stenting procedures have evolved from a treatment mainly for renovascular hypertension to a maneuver capable of recovering threatened renal function in patients with “ischemic nephropathy” and improving management of congestive heart failure. Improved catheter design and techniques have reduced, but not eliminated hazards associated with renovascular stenting. Expanded use of endovascular stent grafts to treat abdominal aortic aneurysms has introduced a new indication for renal artery stenting to protect the renal circulation when grafts cross the origins of the renal arteries. Although controversial, prospective randomized trials to evaluate the added benefit of revascularization to current medical therapy for atherosclerotic renal artery stenosis until now have failed to identify major benefits regarding either renal function or blood pressure control. These studies have been limited by selection bias and have been harshly criticized. While studies of tissue oxygenation using blood oxygen level dependent (BOLD) MR establish that kidneys can adapt to reduced blood flow to some degree, more severe occlusive disease leads to cortical hypoxia associated with microvascular rarefication, inflammatory injury and fibrosis. Current research is directed toward identifying pathways of irreversible kidney injury due to vascular occlusion and to increase the potential for renal repair after restoring renal artery patency. The role of nephrologists likely will focus upon recognizing the limits of renal adaptation to vascular disease and identifying kidneys truly at risk for ischemic injury at a time point when renal revascularization can still be of benefit to recovering kidney function.

Blood oxygen level-dependent magnetic resonance imaging identifies cortical hypoxia in severe renovascular disease

Atherosclerotic renal artery stenosis has a range of manifestations depending on the severity of vascular occlusion. The aim of this study was to examine whether exceeding the limits of adaptation to reduced blood flow ultimately leads to tissue hypoxia, as determined by blood oxygen level dependent MRI. We compared 3 groups of hypertensive patients, 24 with essential hypertension, 13 with "moderate" (Doppler velocities 200-384 cm/s), and 17 with "severe" atherosclerotic renal artery stenosis (ARAS; velocities >384 cm/s and loss of functional renal tissue). Cortical and medullary blood flows and volumes were determined by multidetector computed tomography. Poststenotic kidney size and blood flow were reduced with ARAS, and tissue perfusion fell in the most severe lesions. Tissue medullary deoxyhemoglobin, as reflected by R2* values, was higher as compared with the cortex for all of the groups and did not differ between subjects with renal artery lesions and essential hypertension. By contrast, cortical R2* levels were elevated for severe ARAS (21.6±9.4 per second) as compared with either essential hypertension (17.8±2.3 per second; P<0.01) or moderate ARAS (15.7±2.1 per second; P<0.01). Changes in medullary R2* after furosemide administration tended to be blunted in severe ARAS as compared with unaffected (contralateral) kidneys. These results demonstrate that severe vascular occlusion overwhelms the capacity of the kidney to adapt to reduced blood flow, manifest as overt cortical hypoxia as measured by blood oxygen level-dependent MRI. The level of cortical hypoxia is out of proportion to the medulla and may provide a marker to identify irreversible parenchymal injury.

Complementary roles of color-flow duplex imaging and intravascular ultrasound in the diagnosis of renal artery fibromuscular dysplasia: should renal arteriography serve as the "gold standard"?

OBJECTIVES

The purpose of this study was to compare color-flow duplex imaging (CFDI), intravascular ultrasound (IVUS), and renal arteriography in diagnosing renal artery (RA) fibromuscular dysplasia (FMD) and correlating with the hemodynamic response to balloon angioplasty (BA) in patients with drug-resistant hypertension.

BACKGROUND

Renal arteriography is generally regarded as the gold standard for diagnosing RA FMD. The observation that CFDI and IVUS depicted endoluminal abnormalities suggestive of RA FMD in some patients with normal renal arteriograms prompted comparison of these modalities in a consecutive series of patients.

METHODS

Twenty hypertensive patients with CFDI suggestive of RA FMD (mid-to-distal flow derangement and velocity augmentation) underwent renal arteriography, IVUS, and BA, with both immediate and long-term blood pressure (BP) response assessment.

RESULTS

All patients were women, aged 31 to 86 years (mean 62 years). On IVUS, various endoluminal defects (eccentric ridges; fluttering membranes; spiraling folds) were depicted at locations predicted by CFDI and were uniformly identified at sites where arteriography depicted classic evidence of FMD (8 patients). However, similar defects were detected by IVUS when angiography was borderline (7 patients) or normal (5 patients). Balloon angioplasty eliminated (16 patients) or reduced (4 patients) the IVUS findings and lowered systolic BP in all (mean reduction 53 mm Hg, p < 0.0001). This reduction was maintained during follow-up of 4 to 22 (mean 13) months (mean reduction 44 mm Hg, p < 0.0001), independent of baseline angiographic appearance.

CONCLUSIONS

Both CFDI and IVUS depict the blood flow and endoluminal abnormalities of RA FMD. Balloon angioplasty eliminates or improves IVUS findings and produces substantial, sustained BP reduction, an effect that is independent of baseline arteriographic appearance, calling into question the legitimacy of arteriography as the diagnostic gold standard.

Phase-contrast magnetic resonance flow quantification in renal arteries: comparison with 133Xenon washout measurements

To assess the accuracy of 2D phase-contrast magnetic resonance (2D PC MR) renal artery flow measurements, data obtained with this technique were compared with those acquired with the 133Xenon-washout procedure. In addition, the 2D PC MR flow data were related to functional renal information as derived from selective arterial and venous renin sampling. In 53 patients suspected of having renovascular hypertension, MR angiography of the renal arteries was performed, followed by a 3-step angiographic procedure: (1) selective venous and arterial renin sampling; (2) assessment of the renal blood flow by means of the 133Xenon washout technique, and (3) conventional renal angiography. After initial assessment, 71 kidneys were left for analysis. The overall prevalence of renovascular disease > or =50% stenosis was 18%. Mean renal blood flow as assessed with the 2D PC MR technique showed a significant correlation with the 133Xenon washout flow measurements, with a Pearson correlation coefficient of 0.69 (2-tailed; P<0.01). PC MR blood flow measurements correlated poorly with the presence and/or severity of renovascular disease on conventional angiography (r=0.1, P=0.36). Likewise, no statistically significant correlation with either renal venous renin levels or the renin ratio could be identified. Measurement of renal artery blood flow with the use of a 2D PC MR technique is technically feasible. However, the mean renal artery blood flow correlates poorly with either the presence of renovascular disease on angiography or with renin levels. Further improvement of this technique is necessary before it can be applied on a larger scale.

Fibromuscular dysplasia

Fibromuscular dysplasia is an uncommon angiopathy that occurs in young to middle-aged, predominately female individuals. The disease consists of a heterogeneous group of histologic changes, which ultimately lead to arterial narrowing. Clinical manifestations reflect the arterial bed involved, most commonly hypertension (renal) and stroke (carotid). Fibromuscular dysplasia is a pathologic diagnosis, but the characteristic changes seen on an angiogram can be used to make the diagnosis in the appropriate clinical setting. This noninflammatory disease is a common mimic of vasculitis. A very limited amount of new literature has been published in the past year about this relatively uncommon condition.

Diagnosis of renovascular disease by extra- and intrarenal Doppler parameters

It is still a matter of debate as to which parameters should be used for noninvasive diagnosis of renovascular disease by renal Doppler sonography (RDS). The accuracy of RDS in the detection of renal artery stenosis (RAS) was tested in 95 consecutive, moderate to severe hypertensive patients (I-II World Health Organization [WHO] stages). Reno-aortic ratio (RAR) for peak systolic velocity (PSV) was also calculated to assist in the diagnosis of significant (>50%) RAS. Paired receiver-operating characteristic (ROC) analysis was plotted for evaluating the relationship between sensitivity and specificity for each parameter. In a subset of 57 kidneys, the influence of blood pressure and age on intraparenchymal parameters was evaluated. Measurements of maximal peak systolic velocity (PSV) at the site of stenosis, RAR for PSV, and minimum acceleration index in the main renal artery showed high accuracy (areas under the ROC curve 0.97, 0.88, and 0.80, respectively). Among intraparenchymal parameters, early systolic acceleration showed the best area under the ROC curve (0.90), but provided a low positive predictive value (29%) and was significantly influenced by blood pressure (multiple r=0.56; p=0.001). Pulsatility and resistive indices were found to be less powerful as absolute values, and both significantly influenced by blood pressure and age (multiple r=0.60 and 0.50; p=0.001, p=0.02, respectively). However, interindividual variance of intrarenal indices should be minimized by calculation of side difference, although this procedure would become misleading or impossible in patients with bilateral RAS or a single kidney, respectively. These results support the use of extraparenchymal parameters for noninvasive detection of RAS, and emphasize that intrarenal parameters cannot be considered as absolute values.

Variations in blood flow waveforms in stenotic renal arteries by 2D phase-contrast cine MRI

Waveform variations in blood flow measurements through stenotic renal arteries have been reported already with echo Doppler studies. We studied these variations with MRI in 14 patients (mean age, 60 years) with suspected renal arterial stenosis (24 patent arteries, four occluded). Flow measurements were successful in 15 arteries and unsuccessful in nine, due to practical limitations. Seven healthy younger volunteers (mean age, 28 years) and five healthy older volunteers (mean age, 58 years) were recruited for comparison purposes. In patients, the severity of stenoses was also assessed by digital subtraction angiography and intraarterially measured transstenotic pressure drops. We found flow patterns to be statistically significantly (P < 0.01) age-related. Younger healthy subjects showed shorter wave duration, higher diastolic flow, and total blood flow per minute. Also, with increasing stenosis severity, the systolic wave became more damped and the systolic wave duration became statistically significantly (P = .03) longer.

MR angiography of the renal arteries

MR angiography (MRA) has shown considerable promise as a noninvasive tool in the evaluation of renal vascular morphology. There are two fundamental approaches to MRA of the renal arteries: time of flight and phase contrast imaging. Recently, three-dimensional, gradient-echo, gadolinium-enhanced MRA using breath-hold techniques also has been introduced. These techniques have made MRA of the renal arteries a very promising, noninvasive tool in the detection of main renal artery stenosis, with sensitivities between 90% and 100%. MRA is used most often in the evaluation of renal arteries of patients with abdominal aortic aneurysms or allergies to iodinated contrast media, or when other noninvasive screening examinations have been inconclusive. One of the most helpful and unique applications of MRA is in patients who have accelerating hypertension and accelerating renal insufficiency. These patients often have preexisting unilateral renal artery stenosis with a new contralateral renal artery stenosis. Conventional angiography is risky in these patients because of possible contrast nephropathy. MRA, in such cases, is very helpful for differentiating between bilateral renal artery stenosis and end-stage nephrosclerosis.