True Aneurysms

More than 90% of true renal artery aneurysms are extraparenchymal.^2,8–10 The peak incidence is in patients between the ages of 40 and 60 years. Stanley² and Stanley et al¹⁰ have suggested that true aneurysms are probably due to either atherosclerosis or a congenital defect. Although arteriosclerotic changes have been identified in most aneurysms in patients with multiple lesions, this is not a uniform finding, suggesting that arteriosclerosis may not be the most important factor in the genesis of renal artery aneurysms. These aneurysms are more likely due to a congenital medial degenerative process with weakness of the elastic lamina.^7,10 Lesions typically occur at the primary or secondary renal artery bifurcations and are rarely confined only to the main trunk of the renal artery. As discussed later, this finding makes surgical repair challenging.

Approximately 75% of true renal artery aneurysms are saccular. This type of renal artery aneurysm is usually less than 5 cm in diameter,¹¹ although some as large as 9 cm have been reported.^9,12,13 Saccular aneurysms occur almost invariably at the main renal artery bifurcation.¹⁴ Fusiform aneurysms are usually associated with atherosclerosis or are a result of a post-stenotic dilatation distal to a hemodynamically significant renal artery stenosis, the latter of which results from atherosclerosis or fibromuscular disease.^7,8,14,15 Fusiform aneurysms are generally less than 2 cm in diameter and usually affect the main renal artery trunk.⁸

Arterial fibrodysplasia is often a direct contributor to the development of an aneurysm.^2,10 Medial fibroplasia is typically associated with multiple stenoses and post-stenotic dilatation of the distal two thirds of the renal artery. Renal artery aneurysms in association with fibromuscular dysplasia are generally only a few millimeters in diameter. The typical angiographic appearance of a renal artery involved with medial fibroplasia is a “string of beads.” Larger aneurysms can also occur, however, and in one study, renal artery macroaneurysms were found in 9.2% of adults with fibromuscular dysplasia.¹⁰

A rare cause of renal artery aneurysms is Ehlers-Danlos’ syndrome. This disorder is associated with extreme arterial fragility and spontaneous rupture, and is discussed in Chapter 78.¹⁶

False Aneurysms (Pseudoaneurysms)

False aneurysms of the renal artery arise from blunt or penetrating trauma, and occasionally from iatrogenic causes such as renal artery catheterization or after nephrectomy. They represent contained ruptures of the renal artery, with only inflammatory and fibrous tissue encasing the leak.

Dissections

Spontaneous dissections confined to the renal artery that do not arise from the adjacent aorta are rare; however, primary dissections causing pseudoaneurysms affect the renal arteries more than any other peripheral artery.^2,14,17–20 Poutasse¹⁴ and Stanley et al¹⁰ reported that 14 of 57 cases of renal artery aneurysms were due to spontaneous dissection. An intimal defect of the renal artery due to atherosclerosis is probably the underlying cause of spontaneous renal artery dissection causing aneurysms, along with dysplastic renovascular disease and trauma.⁸ The incidence of dissection in patients with fibrodysplastic renal arteries ranges from 0.5% to 9.0%.^2,17 Dissection often extends into the branches of the renal artery and may pose particularly challenging reconstruction problems.

Traumatic renal artery dissection can occur secondary to blunt abdominal trauma or catheter-induced injury. Blunt trauma accounts for the higher prevalence of dissection in men and is more likely to result in right-sided injuries, possibly because of ptosis-related physical stresses affecting the renal pedicle.² Blunt trauma can cause renal artery dissection by either severe stretching of the artery, with fracture of the intima, or compression of the artery against the vertebra. Renal artery dissection caused by guide wires or catheters can occur, but is rare, having been observed in only 4 of 2200 selective renal artery arteriograms.¹⁷

Intrarenal Aneurysms

Less than 10% of renal artery aneurysms are intraparenchymal.^9,10 Intrarenal aneurysms are usually multiple and may be congenital, are associated with collagen vascular disease, or are posttraumatic. They may be associated with AVFs, possibly as a result of spontaneous closure of a fistula. Intrarenal aneurysms can occur with polyarteritis nodosa and are usually in the renal cortex.^21,22

Rupture of a renal artery aneurysm is an indication for emergency intervention, as it is for virtually any arterial aneurysm. Probably less than 3% of renal artery aneurysms rupture.^2,10 This complication is associated with a mortality rate of approximately 10% in men and nonpregnant women.^2,10,24,25 In a hemodynamically stable patient, an emergent CT scan may reveal the pathology and allow the surgeon to plan the operative repair. However, if an elderly patient presents to the emergency department with abdominal pain, a tender, distended abdomen, and severe hypotension that does not respond to fluid resuscitation, emergency exploration for a presumptive ruptured abdominal aneurysm may be indicated.

Prevention of rupture is the most common indication for intervention in cases of asymptomatic renal artery aneurysms. Traditionally, repair has been recommended for renal artery aneurysms greater than 2 cm in diameter,^8,15 although this recommendation may very likely be over-aggressive, and more conservative guidelines may very well be indicated. The likelihood of rupture of a renal artery aneurysm is controversial because the natural history has not been delineated. Most reports are retrospective reviews of incidentally discovered intact renal artery aneurysms in autopsy series or collections of ruptured aneurysms that lack full details concerning their size and the presence or absence of calcification. Harrow and Sloane²⁶ reported one of the highest rates of rupture of renal artery aneurysms, noting 14 ruptures in 100 cases. In another series of 126 renal artery aneurysms, 6 ruptured.²⁷ Many authorities believe that there are no good data to support the belief that the larger the renal artery aneurysm, the more likely it is to rupture.^1,2,8,10,28

Most other series of asymptomatic renal artery aneurysms in men and nonpregnant women report a much lower incidence of rupture. Only 1 of 62 patients with aneurysms 4 cm in diameter or smaller ruptured after follow-up from 1 to 17 years.²⁹ None of 19 small aneurysms in another series ruptured.²⁴ A group of 21 patients was observed for an average of 3 years without rupture.²³ In another series of 18 patients with renal artery aneurysms less than 2.6 cm, who were followed for 1 to 16 years, none of the aneurysms ruptured.¹ There were no ruptures in a series of 32 patients (who eventually underwent surgery) with renal artery aneurysms that ranged from 0.7 to 9 cm.¹³ Of 83 renal artery aneurysms found on arteriography and followed up without surgery,²⁸ none ruptured or became symptomatic after a mean of 4.3 years.⁵ In a pooled analysis, there were no ruptures in more than 200 renal artery aneurysms observed for up to 17 years.⁷ In a recent large series of 59 renal artery aneurysms, 30 were treated conservatively and followed (all <2.0 cm), whereas 29 were treated (most with ex-vivo repair).³¹ Because of the very low risk of rupture and the relatively high rate of significant postoperative complications (approximately 55%), we recommend that a more conservative approach be used for these cases, and that a 2-cm guideline for intervention is too aggressive and unwarranted. There was an obvious selection bias in the follow-up of many of these aneurysms (i.e., small size), and many of the larger aneurysms were repaired.

Besides size, other factors may play a role in the consideration of elective surgery for asymptomatic renal artery aneurysms. Calcification of the aneurysm has been thought to protect against rupture. Poutasse¹⁵ suggested that a heavily calcified renal artery aneurysm may be less likely to rupture than a noncalcified or minimally calcified one. In a review of cases through 1959, 14 of 100 noncalcified aneurysms ruptured.²⁶ In a more recent series, 15 of 18 ruptured renal artery aneurysms were noncalcified.³⁰ However, in a series of 62 solitary aneurysms less than 4 cm in diameter, one third were not calcified, and only 1 aneurysm in the entire series ruptured in 1 to 17 years of follow-up.²⁵ Because of these conflicting data, some authorities believe that the presence or absence of calcification is not relevant when predicting the risk of rupture.²

Most authorities agree, however, that pregnancy is associated with a significantly increased risk of rupture of a renal artery aneurysm.^2,10,13,32 Pregnancy may increase the risk of rupture because of the hyperdynamic state, with increased blood volume and cardiac output; hormonal influences on the aneurysm; and increased intra-abdominal pressure due to the gravid uterus.^1,10 Cohen and Shamash³² reported 18 cases of rupture during pregnancy. In another series of 18 patients who underwent surgery for renal artery aneurysms, the only two ruptures were in women at childbirth; both of these aneurysms measured only 1 cm in diameter.¹ In a review of 43 ruptured renal artery aneurysms, 81% occurred in women; 21 of the 35 women in this series were younger than 40 years old, and 18 were pregnant. Of the 18 aneurysms of known size, 3 ruptured when they were less than 2 cm.³⁸

Of note, rupture of renal artery aneurysms in pregnancy has been associated with a maternal mortality rate of 55% and a fetal death rate of 85%.^32,33 Risk of renal artery rupture is small, however, even in pregnant women. In a series of 19,600 autopsies of pregnant women, no ruptured renal artery aneurysms were found.²⁷ This report did not indicate the number of unruptured renal artery aneurysms found in this population, so the risk of rupture remains uncertain. Regardless, we agree with others that there are enough data to support an aggressive surgical or endovascular approach for pregnant women with renal artery aneurysms of any size.

Essentially, all recent-onset false renal artery aneurysms should be repaired because of the high likelihood of rupture.⁸ In the rare case of a chronic, contained rupture of a small false aneurysm that is found months or years later, in which the pseudoaneurysm has thrombosed, careful follow-up is probably all that is warranted. Similarly, renal artery aneurysms due to fibrodysplastic disease may be associated with a higher risk of rupture because of the thin-walled nature of these aneurysms, although firm data are lacking.⁸ Certainly, renal artery aneurysms in men or in women beyond childbearing age that are less than 2 cm in diameter and associated with fibrodysplastic disease should be studied closely.⁸

In summary, our recommendation concerning the elective repair of asymptomatic renal artery aneurysms in men and in women beyond childbearing age is based on the data just presented and on the well-documented history of other abdominal arterial aneurysms. General guidelines for the repair of asymptomatic abdominal aneurysms include (1) infrarenal aortic aneurysms greater than 5 to 5.5 cm in diameter, (2) common iliac aneurysms greater than 3 cm, and (3) splenic artery aneurysms greater than 3 cm.²² Surgery is recommended for visceral artery aneurysms of any size.³⁴ Although various hemodynamic factors may play a role in other intra-abdominal aneurysms, and despite the relative paucity of data suggesting a high risk of rupture, it seems prudent to recommend the repair of renal artery aneurysms greater than 3 cm in diameter in good-risk patients when there is reasonable certainty that nephrectomy will not be required.^26,28 This guideline remains controversial, and others have taken a more conservative approach, reserving repair for aneurysms greater than 4 cm.³⁵ As previously mentioned, any renal artery aneurysm in women of childbearing age should be repaired.

Hypertension

Although the prevalence of hypertension in patients with renal artery aneurysms is approximately 80% in several series, there is no conclusive evidence that the aneurysms themselves are the direct cause of hypertension, unless there is an associated stenosis or compression of an adjacent artery.^1,10,13 In a series of 39 patients with renal artery aneurysms, 26 had diastolic hypertension, but only 9 (23%) patients had hypertension that was of renovascular origin.¹ In a more recent series of 16 patients with extraparenchymal renal artery aneurysms, 75% had renovascular hypertension.³⁷ The indication for surgical intervention for renovascular hypertension due to renal artery stenosis, secondary to atherosclerosis, continues to be the failure of medical management—namely, diastolic blood pressure greater than 90 to 100 mm Hg despite three antihypertensive medications—and the same criterion should probably be applied when a renal artery aneurysm is present. If the primary indication to repair a renal artery aneurysm is poorly controlled hypertension, we do not hesitate to obtain a nephrology consult to be sure there are firm indications for intervention. Both the stenotic artery and the aneurysm must be repaired. Our current evaluation of these patients relies primarily on the clinical scenario, the exclusion of other causes of secondary hypertension, the documentation of significant renal artery stenosis, and occasionally, the use of captopril renal scans.³⁸

Dissection

Emergent intervention is required for dissections that cause renal artery aneurysms and threaten the viability of the kidney. Nephrectomies are frequently required, however, because of the extensive damage to the renal branch vessels and the limited time available to salvage a previously healthy kidney that cannot tolerate prolonged periods of ischemia. If hypertension is the only manifestation of a chronic dissection, and the hypertension is well controlled by blood pressure medications, or if the patient is asymptomatic and a renal artery dissection is found incidentally (without an associated aneurysm), surgery is probably not justified.¹⁰

Other Clinical Manifestations

If a patient with an intact renal artery aneurysm, as documented by CT or magnetic resonance imaging (MRI), is symptomatic—that is, experiences abdominal or flank pain or fullness—repair may be indicated. Acute onset of symptoms is more worrisome than a patient referred for months of abdominal pain or fullness, unless the renal artery aneurysm is very large. It is difficult to attribute abdominal pain to a 2-cm diameter renal artery aneurysm, and other causes need to be thoroughly investigated. Symptoms may be a harbinger of impending rupture. Embolization to the renal parenchyma may also account for these symptoms.⁹

Repair of a Ruptured Renal Artery Aneurysm

If emergent surgery is required for a ruptured renal artery aneurysm, a midline approach and supraceliac aortic control are generally required. A sizable juxtarenal hematoma does not allow safe aortic exposure and clamping immediately above the renal arteries. If proximal control of the renal artery itself can be obtained, the supraceliac clamp can then be removed. If the bleeding is quickly controlled, and the patient is clearly hemodynamically stable, and if the proximal and distal renal arteries lend themselves to a relatively quick and straightforward bypass, consideration can be given to reconstruction. In most cases, however, nephrectomy is required because of the instability of the patient, the prolonged ischemia of the kidney, and the technical and time-consuming nature of surgical repair with a bypass.^2,10,14,29 If the aneurysm extends into the renal parenchyma or if a “bench” repair of the kidney is required, the patient is generally best treated by nephrectomy, as long as the contralateral kidney is intact with normal function. It is possible that a stable patient with a ruptured true or false renal artery aneurysm can be treated with newer endovascular techniques. Use of a stent graft may be the preferred treatment for lesions not involving the distal renal branches, as reported by Bloemsma et al.³⁹ Routh et al⁴⁰ reported thrombosis of a leaking saccular aneurysm using Gianturco coils, thrombin, and bucrylate.

Elective Repair of Renal Artery Aneurysm

Open surgical and endovascular repair are options to repair renal artery aneurysms. Even in the case of elective surgery, repair of a renal artery aneurysm is usually more challenging than revascularization for renal artery stenosis. Most renal artery aneurysms extend past the bifurcation of the main renal arteries and frequently extend into the renal parenchyma. Associated renal artery stenosis may need to be repaired in conjunction with the aneurysm. For in-situ repairs of a renal artery aneurysm, the left kidney can be exposed through a retroperitoneal approach with a transverse left supraumbilical incision. The right kidney can be exposed through a transperitoneal approach with a Kocher maneuver to reflect the right colon and duodenum medially or with a subcostal incision.

Several methods have been used to repair renal artery aneurysms. The most straightforward technique for saccular aneurysms involves aneurysmorrhaphy with primary repair or patching. In three combined series of patients who underwent surgical repair of renal artery aneurysms, approximately one third (6 of 18, 3 of 10, and 6 of 23) of the aneurysms were able to be repaired in this manner.^1,9,13 If this technique is not possible, we and others prefer autologous tissue bypasses, such as a saphenous vein bypass, if the graft can be anastomosed to the distal part of the main trunk of the renal artery or to the most proximal branches.^8,13 The most common renal arterial reconstruction is an end-to-side anastomosis of a small renal artery branch to the main renal artery, or a side-to-side anastomosis of two small renal arteries to create a common inflow channel with a single, larger diameter lumen, which can then be anastomosed to the renal artery or vein. Because the small branches of the main renal artery are often involved with the aneurysm, a branched autologous graft is preferred to reconstruct these lesions. The internal iliac artery is an excellent choice in these reconstructions because of its multiple small side branches.^28,41 Alternatively, the saphenous vein also functions well. The proximal anastomosis of the graft is usually the infrarenal aorta. Useful alternative reconstructions include a splenorenal bypass for a left-sided renal artery aneurysm and hepatorenal bypass for a right-sided aneurysm.

If multiple branch vessels are involved, and especially if the cause of the renal artery aneurysm is dissection resulting in a friable vessel, extracorporeal or bench surgery may be required.^42,43 This technique is recommended when renal ischemia is projected to exceed 45 minutes or when exposure of small renal branches is required. Ex-vivo surgery requires nephrectomy, followed by hypothermic perfusion of the kidney with a heparinized renal preservation solution. The kidney can then be autotransplanted to its original bed, as Dean et al⁴⁴ and Crutchley et al⁴⁵ prefer, or to the iliac fossa. For renal autotransplantation into the iliac fossa, a flank incision with a retroperitoneal approach is used for exposure of the kidney, ureter, and iliac artery. Gonadal and adrenal veins are divided to obtain an adequate length of renal vein. If the reconstruction can be safely performed by placing the kidney on the anterior abdominal wall, the ureter does not need to be divided. The procedure is occasionally best performed at a separate table after dividing the ureter and removing the kidney from the operative field. Perfusion is carried out through the main renal artery to preserve the kidney while selected branches are individually repaired and other branches are perfused. The kidney may be perfused with a heparinized crystalloid solution, such as Collins solution or lactated Ringer’s solution with heparin 1000 U/L with 12.5 g of mannitol, while the kidney is wrapped with gauze and placed in a chilled solution at 4° C.^8,40,46 The use of continuous pulsatile perfusion is controversial.⁴⁰ Ex-vivo repair is also discussed in Chapter 145.

When performed for proper indications by well-trained surgeons, repair of renal artery aneurysms should be associated with low morbidity and mortality.^2,9,46,47 English et al⁴⁸ reported a 1.7% perioperative mortality rate for surgical repair in 62 patients with 72 renal artery aneurysms, with a 4-year patency of 96% and cured or improved hypertension in three quarters of patients. Pfeiffer et al,⁴⁹ from Germany, reported similar excellent long-term surgical results in a series of 94 patients. In another series of 12 patients operated on for renal artery aneurysms, there was no mortality, and only 1 patient required reoperation for ureteral stenosis.⁴⁷ A more recent series of 26 renal artery aneurysms that required reconstruction of first- or second-order branches in 25 cases were repaired primarily with in-situ techniques. Autogenous vein bypasses resulted in a mortality of 0% with long-term patency of 95% at an average follow-up of 99 months (range, 1-300 months).⁵⁰ Ex-vivo repairs have been shown to be safe and effective by Dean et al,⁴⁴ Crutchley et al,⁴⁵ and others.⁴² Murray et al⁴¹ reported a series of 11 patients with renal artery aneurysms successfully treated using ex-vivo repair. In another series of eight aneurysms, all were successfully repaired with the ex-vivo technique, without deaths or complications.⁴⁹ In a review of ex-vivo repairs, postoperative mortality rates ranged from 0% to 9.6%.⁵¹ Use of bifurcated internal iliac artery autografts was also highly successful in a series of 11 patients, most with fibrodysplastic aneurysms, who were treated by in-situ or bench repair.⁴¹ Finally, in a series of 35 repairs of renal artery aneurysms treated by in-situ repair, ex-vivo repair, or nephrectomy, there was no mortality and only one postoperative graft occlusion.¹³ Recent technologic and surgical developments have also allowed less invasive surgical repair of complex renal artery aneurysms, and both laparoscopic and robot-assisted techniques have been described.^52,53 A series of five renal artery aneurysms, albeit small-diameter ones measuring less than 3 cm, were treated with robot-assisted laparoscopic in-situ aneurysmectomy followed by revascularization without complications.⁵⁴

An alternative approach to the treatment of renal artery aneurysms includes the use of endovascular techniques.^28,55–57 Degenerative renal artery aneurysms have been treated with transcatheter embolization with detachable platinum coils that occlude the aneurysms, but maintain renal flow.^56–58 One patient in whom a renal artery aneurysm occurred after percutaneous renal biopsy was also successfully treated by embolization.⁵⁹ In a small series of endovascular repairs, Klein et al⁵⁶ treated 12 renal artery aneurysms using selective endovascular embolization with nondetachable microcoils or Guglielmo’s detachable coils. Eight aneurysms were located in the bifurcation of the main renal artery, two were in the main renal artery, and two were intrarenal. All 12 aneurysms were successfully occluded, with only 2 minor complications. These authors concluded that endovascular treatment of renal artery aneurysms with microcoils is as safe as surgical treatment and less invasiave.⁵⁶ In another series, 13 patients with renal artery aneurysms were successfully treated using a variety of endovascular techniques with combinations of coil embolization, liquid embolization, stenting, and stent grafts with no complications after an average follow-up of 43 months (range, 13-103 months).⁶⁰ A larger series of 18 renal artery aneurysms were also treated with endovascular intervention.⁶¹ Sixteen of the 18 aneurysms were embolized utilizing adjunctive techniques to protect embolizing into the main renal artery. There was a 100% technical success rate, with only one aneurysm showing residual minor flow into the sac and with no long-term complications. Alternatively, several authors have recently reported using ethylene vinyl alcohol copolymer to ablate renal artery aneurysms.^62,63 With this technique, the main renal artery is protected with an angioplasty balloon while the polymer is infused into the aneurysmal sac, achieving controlled exclusion. Flow-diverting stents have been used to treat six renal artery aneurysms by reducing flow in the aneurysm sac and promoting thrombosis while maintaining flow in the main artery and branches.⁶⁴

Over the past 5 years there have also been more numerous reports of endovascular stent graft exclusion as definitive treatment for renal artery aneurysms. Many aneurysms extend to branch vessels, making this approach risky or ill-advised, but it may be ideal for aneurysms of the main renal artery not involving branches. Advances in stent graft technology, with smaller diameter devices and lower profile delivery systems, have made definitive treatment of renal artery aneurysms feasible. A review of the English literature analyzing use of stent grafts to treat renal artery aneurysms up to April 2010 yielded 22 cases with a technical success rate of 91% (20 cases) without any deaths or significant morbidity reported.⁶⁵ Although most published series have been small, results have been excellent, and this approach will undoubtedly become more common as technology improves.^66–70