Since its introduction in 1978, percutaneous transluminal renal angioplasty (PTRA) has emerged as a highly effective technique for the correction of renal artery stenoses. Renal angioplasty has notable physiologic, psychological, and economic advantages over other treatment modalities, and it should now be considered the therapy of choice for renovascular hypertension.

The indications of renal angioplasty are as follows (Kidney and Deutsch 1996):

From a technical point of view, different modalities to perform endovascular treatment of renal artery stenosis are available as follows:

Regarding evolution of material employed in this type of intervention, the “trend” is to use materials derived from coronary intervention experience, and significant improvements have been introduced for diagnostic catheters, guidewires, PTA catheters, and stents.

Diagnostic Catheters: in the past, 4 or 5 French pre-shaped diagnostic catheters (Cobra, Renal Double Curve, Simmons, etc.) were used to catheterize a stenotic renal artery, and in association with hydrophilic guidewires, the stenosis was negotiated (Fig. 4a). Frequently, catheters were advanced beyond the stenosis and hydrophilic guidewires were changed with stiff Teflon-coated stainless steel wires (such as 0.035″ Rosen Heavy Duty), and standard profile PTA balloon catheters and premounted stents were advanced over this wire. In the last years, many centers have adopted the use of guiding catheters specifically designed for renal interventions. These guiding catheters, derived from coronary intervention experience, are larger in caliber (6–8 Fr) and have a large lumen. Usually they give a superior support compared to conventional diagnostic catheters and are advanced close to the ostium of the renal artery. At this point, the stenosis is negotiated with a 0.014″ or 0.018″ guidewire avoiding fragmentation of the atheromatous plaque. Then low-profile balloon catheter or premounted stents are advanced (Fig. 4b). This technique has been also referred to as the “do not touch technique” because it avoids, as much as possible, passages of materials and devices at the level of the atheromatous plaque.

Guidewires: while negotiation of renal stenosis was done with 0.035″ guidewire in the past, several cath labs adopt the rule to negotiate stenosis with 0.014″ or 0.018″ guidewires in the last years, in order to decrease trauma to arterial plaques and therefore reduce distal embolization. Both stainless steel and nitinol guidewires are used, generally with a short soft tip, and are still able to give a support to the advancement of low-profile PTA balloon catheters or stents.

Balloon Catheters: for many years, standard balloon catheters were employed for the dilation of renal artery stenosis. These catheters have the following characteristics:

In the last years, many centers have adopted the use of low-profile PTA balloon catheters derived from coronary interventions. These differ significantly from standard PTA catheters for the following reasons:

Stents: also in the field of stents, significant evolutions have been introduced, moving from balloon expandable stent to be crimped over standard PTA balloon catheter to premounted balloon expandable stent on standard PTA catheter to the very last generation of low-profile premounted stent over rapid exchange or monorail balloon catheter. These last types of materials such as microguidewires and low-profile balloon catheters reduce the risk of traumatism to the atheromatous plaques responsible for distal embolization. Although in the past some authors treated renal artery stenosis with self-expandable stent (Raynaud et al. 1994), there is a general agreement that balloon expandable stents are much more preferable because their deployment is more precise and these stents have superior radial force. Until few years ago, only stainless steel stents were available, while in the recent years, new materials such as chromo-cobalt alloys have been introduced. This alloy has similar strength of stainless steel, but inferior weight and superior long-term results are expected, even if not proven currently. Few data are available on the use of drug-eluting stents in the treatment of renal artery stenosis. One recent multicenter trial in which a sirolimus-eluting stent was compared to a bare-metal stent shows that the angiographic outcome at 6 months did not reveal any significant difference between the two stents. Renal artery stenting with both stents significantly improved blood pressure. The paper concludes that only further studies with larger patient population and longer angiographic follow-up will be able to determine if there is a significant benefit of drug-eluting stents in treating ostial renal artery stenosis (Zahringer et al. 2007). Recently a paper (Lookstein et al. 2009) described a case of a patient who, after three failed revascularizations to keep the renal artery patent, underwent a sirolimus-eluting stent implantation which is still patent at the 24-month follow-up. Stents are mounted over noncompliant or low-compliant balloon catheters. In the first case, manufacturers offer stents with intermediate diameters such as 4.5, 5.5, and 6.5 mm, while in the case of stents premounted on low-compliant balloons, no intermediate measures are available due to the possibility of overdilation of these types of balloons. Generally, the lengths of the stents are 10, 12, 15, 18, and up to 24 mm. In addition, stents with a “flaring effect” are also available. This option allows the possibility to oversize the proximal diameter of the stent at the level of the ostium.

Postprocedural imaging has been used for follow-up and to assess the effectiveness of percutaneous revascularization, clinical criteria, and laboratory findings together. Serial Doppler ultrasound (US) is a readily available technique and able to diagnose residual stenosis or restenosis (Akan et al. 2003). Anyway, Doppler US is known to be challenging; even for the evaluation of native RAs owing to patient-related factors, its diagnostic performance in patients with implanted stents is additionally hampered by metal-induced waveform distortion (Sharafuddin et al. 2001; Parenti et al. 2008; Rocha-Singh et al. 2008). Magnetic resonance angiography after stent placement is limited by the ferromagnetic artifacts caused by most stents (Tello et al. 1998). More recently, technical advances with multidetector CT have pointed out with prospective studies (Steinwender et al. 2009) how CT angiography can provide excellent noninvasive technique to detect and evaluate intra-stent restenosis, in comparison with invasive DSA.

Distal embolic protection devices: this device was initially designed for carotid endovascular interventions as carotid artery stenting. It is a sort of microguidewire which mounts on the distal part a conic-shaped filter made of a membranous material such as polyurethane or nylon or a windsock-like nitinol basket, 2–3 cm distal to a floppy tip (Fig. 5). Distal protection devices work by interrupting or filtering blood flow in the internal carotid artery. Its use in renal interventions is still under investigation. Some centers have evaluated this tool on a significant number of cases (Henry et al. 2003). The most important characteristics are the ability to cross stenotic lesions and the “capture capability.” Generally, they are able to entrap embolic debris from medium to large size, generally particles more than 100 μm in long diameter. The aim of distal filter is to reduce embolization during the different phases of the maneuver. Some limitations in this filter exist due to the fact that they are not specifically designed for renal arteries, but for carotid arteries. The main limitation is that the “landing zone” of the filter may end distally to the main division of the main renal trunk and so only part of the kidney parenchyma is protected by the risk of debris embolization.

Arterial Embolization: renal conditions eligible to be occluded by means of an embolization procedure of a renal artery consist mainly of kidney tumors (both benign such as angiomyolipomas (AML) or malignant such as advanced renal cell carcinomas); the other common setting of embolic therapy is renal trauma.

From a technical point of view, this procedure needs confidence with the use of embolic materials such as:

Renal denervation (RDN) is a new minimally invasive procedure for the treatment of refractory hypertension, based on the use of an endovascular catheter using radiofrequency (RF) ablation aimed at treating resistant hypertension. By applying RF pulses to the renal arteries, the nerves in the vascular wall (adventitia layer) can be denervated. This causes reduction of renal sympathetic afferent and efferent activity and blood pressure can be decreased (Esler et al. 2010). Early data from international clinical trials are promising, demonstrating average blood pressure reduction of approximately 30 mmHg at 3-year follow-up in patients with treatment-resistant hypertension (Symplicity HTN-1 2011; Esler et al. 2010).

From a historical point of view, before pharmacological management of hypertension, surgical sympathectomy was a recognized treatment for hypertension (Doumas and Douma 2009). This was often successful in reducing blood pressure, but unpredictable side effects of the procedure were poorly tolerated. Side effects included orthostatic hypotension, palpitations, anhidrosis, intestinal disturbances, loss of ejaculation, thoracic duct injuries, and atelectasis (Doumas et al. 2010).

Modern antihypertensive pharmacological interventions have improved the control of hypertension, but an estimated 30 % of hypertension cases are resistant. Resistant hypertension is defined as blood pressure above the target (140/90 mmHg) despite concomitant use of three or more antihypertensives – one of which should be a diuretic.

Several devices have been approved for this procedure. The first one is the Symplicity™ renal denervation system (Fig. 6a), produced by Medtronic (formerly Ardian), and consists of an endovascular catheter (6 Fr), RF generator, dispersive electrodes, foot switch, and power cable. The Symplicity device currently has over 5 years of clinical experience and 3 years of follow-up data. The Symplicity catheter is introduced using standard interventional technique via the femoral artery and is positioned in the renal artery under fluoroscopic guidance. The catheter is a 5 Fr compatible, single-use RF probe and it features a monopolar platinum electrode at the distal tip of the catheter that is used in conjunction with a standard dispersive electrode. The catheter is torquable, such that the user can easily rotate the catheter to treat different segments of the vessel. The treatment involves the delivery of relatively low-power and precisely focused RF bursts of approximately 8 W through the wall of the renal artery to disrupt the surrounding renal nerves lying in the adventitia. Multiple ablations must be done (6–8 for each renal artery) and the entire procedure may take from 40 to 60 min (Fig. 6b).

There are a number of other devices that are currently being evaluated, but these different technologies have limited clinical data and do not have long-term data. Some of these should have the advantage to do multiple ablations reducing the procedure time. In addition to Medtronic’s Symplicity system, other renal denervation devices have a CE mark. These include St. Jude Medical’s EnligHTN™ system, Vessix’s V2™ renal denervation system, Covidien’s OneShot™ system, and ReCor’s PARADISE™ system. Currently, no renal denervation device has FDA approval.

From the diagnostic point, the only role left for renal phlebography is the sampling of blood from the renal veins in patients with renal hypertension to clarify which kidney is responsible for an increased production of the renin hormone. In these cases no particular imaging documentation of veins is required, unless anatomic significant variants are found. It is, however, important to obtain blood samples from each renal vein and from the inferior vena cava above and below the renal veins. Samples need not be simultaneous; however, it is better if they are closely spaced temporally. Renin levels from one kidney, which are >1.5 times of the contralateral kidney, are the indicative of renovascular hypertension. An increased renin value in the suprarenal inferior vena cava compared to the infrarenal vena cava is also suggestive of renovascular hypertension.

In the majority of patients, the main renal arteries originate between the upper margin of L1 and the lower margin of L2 vertebrae, and frequently the origin of the right renal artery is higher than the left renal artery. The ostium of both renal arteries is located laterally; however, the ostium of the right renal artery tends to be located on the ventral aspect of the abdominal aorta, and as previously said and well shown on CT studies (Verschuyl et al. 1997a), there is a significant variation in its position.

The ostium of the left renal artery more frequently originates on the lateral aspect of the abdominal aorta; however, in a significant number of cases, it tends to be located on the dorsal wall of the aorta. The right renal artery in the first 1–2 cm has an anterior course and then turns posteriorly with a craniocaudal obliquity. The left renal artery has a more linear and horizontal course. The caliber of the artery in the proximal part generally is 6 mm. The main renal artery divides into segmental arteries near the renal hilum (Kadir and Brothers 1991). The first division is typically the posterior branch, which arises just before the renal hilum and passes posterior to the renal pelvis to supply a large portion of the blood flow to the posterior portion of the kidney. The main renal artery then continues before dividing into four anterior branches at the renal hilum: apical, upper, middle, and lower anterior segmental arteries. The apical and lower anterior segmental arteries supply the anterior and posterior surfaces of the upper and lower renal poles, respectively; the upper and middle segmental arteries supply the remainder of the anterior surface. The segmental arteries then course through the renal sinus and branch into the lobar arteries. Further divisions include the interlobar, arcuate, and interlobular arteries (Fig. 7).

Renal artery variations are divided into two groups: early division and extrarenal arteries. The branching of the main renal arteries into segmental branches more proximally than the renal hilus level is termed as early division (Fig. 8), while the “extrarenal artery” is divided into two groups: hilar (accessory) and polar (aberrant) arteries. The hilar arteries enter the kidneys from the hilum with the main renal artery, whereas the polar arteries enter the kidneys directly from the capsule outside the hilum. The origin of these extra renal arteries is quite variable, and the majority of them arise from the abdominal aorta over or below the main trunk. Infrequently, they may arise from the common iliac arteries. Renal artery origins arising above the superior mesenteric artery are extremely rare. Congenital anomalies of renal position and conformation are often associated with aberrant location of renal artery origins and supernumerary vessels. In particular, horseshoe kidney has a 100 % incidence of multiple renal arteries (Fig. 9). The renal pelvis and proximal ureters are supplied by small branches of the interlobular, arcuate, and distal main renal arteries. The middle portion of the ureters is supplied by the gonadal arteries (Fig. 7). The distal ureters are supplied by terminal branches of the internal iliac arteries, most notably the cystic artery.

In several anatomic and angiographic studies, it was stated that the rate of bilateral extrarenal arteries is 10–15 % (Kadir and Brothers 1991; Talovic et al. 2004), that the rate of early division in the general population is 15 %, that aberrant renal arteries are observed twice as often as accessory renal arteries and the frequency rate of extrarenal arteries is the same on the right and left side, and that in 12 % of the general population, extrarenal arteries are bilateral. In a recent paper (Ozkan et al. 2006), the frequency rate of early division and bilateral extrarenal arteries is 8 and 5 %, respectively, which is low when compared to other major series. In this series, no statistically significant differences were detected between the frequency rates of ERA on the right and left side, and this finding was compatible with other studies.

The renal arteries are end arteries. In contrast to other districts (i.e., the colonic and hepatic), the intrarenal collateral pathways are poorly developed. In the presence of slowly progressive proximal renal artery stenosis, renal capsular, ureteral, adrenal, and other retroperitoneal arteries may enlarge sufficiently to provide a collateral blood supply to keep kidney perfusion, but with a compromised function. Acute proximal occlusion of a previously normal renal artery results in profound ischemia due to insufficient preexisting collateral supply.

The renal cortex is drained sequentially by the arcuate veins and interlobar veins. The lobar veins join to form the main renal vein. The renal vein usually lies anterior to the renal artery at the renal hilum. The left renal vein is almost three times longer than the right renal vein. The left renal vein averages 6–10 cm in length and normally courses anteriorly between the superior mesenteric artery and the aorta before emptying into the medial aspect of the inferior vena cava. The right renal vein averages 2–4 cm in length and joins the lateral aspect of the inferior vena cava. Unlike the right renal vein, the left renal vein receives several tributaries before joining the inferior vena cava. It receives the left adrenal vein superiorly, the left gonadal vein inferiorly, and a lumbar vein posteriorly.

While anatomic variants of the renal veins are commonly visible in CT, their demonstration during renal phlebography is not so obvious and easy. Multiple renal veins constitute the most common venous variant and are seen in approximately 15–30 % of patients (Kadir and Brothers 1991). Multiple right renal veins occur in up to 30 % of individuals, and sometimes a single vein may divide before joining the inferior vena cava (Beckmann and Abrams 1980).

The most common anomaly of the left renal venous system is the circumaortic renal vein, seen in up to 17 % of patients (Kahn 1973; Ferris 1969). In this anomaly, the left renal vein bifurcates into ventral and dorsal limbs that encircle the abdominal aorta. The posterior limb is usually the smaller of the two, although this is certainly variable. There are two common variants of the circumaortic vein: in the most frequent (approximately 75 % of cases), one renal vein at the renal hilum subsequently divides before entering the inferior vena cava; in the less common variant, two distinct veins originate from the renal hilum (Beckmann and Abrams 1979, 1980). When a circumaortic renal vein is observed, the gonadal vein will typically join the retroaortic limb and the adrenal vein will join the preaortic limb (Kadir and Brothers 1991). A less common venous anomaly is the complete retroaortic renal vein, seen in 3 % of patients. Here, the single left renal vein courses posterior to the aorta and drains into the lower lumbar portion of the inferior vena cava. Alternatively, the retroaortic renal vein can drain into the iliac vein (Satyapal et al. 1999).

Most of the patients with a positive imaging for renal artery stenosis are asymptomatic. At angiography, atherosclerotic plaques are usually localized within 1 cm to the ostium, more often consisting of stenosis at the origin, since less than 10 % of the stenosis are distal than 1 cm. On that basis, the lesions can be classified as ostial stenosis or proximal stenosis, and on the other hand, intrarenal branches are affected on a variable degree. Frequently, plaque formation begins in the aorta wall with progression into the renal artery lumen, inducing the typical appearance of the ostial, usually eccentric, location of the atherosclerotic renal artery stenosis.

The epidemiology of atherosclerotic plaques is focused on male rather than female gender, affecting especially individuals older than 60 years. In a large autopsy series, stenosis producing more than >50 % of reduction in caliber was found in 18 % of those between 65 and 74 years and in 42 % of those older than 75 years (Fauci and Harrison 2009). The quantification of a renal stenosis on angiography is based on the degree of luminal narrowing: some authors consider significant a narrowing superior or equal to 50 %, while other authors tend to set a 70 % degree of stenosis as the threshold (Olin et al. 1995) (Fig. 11

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