Chapter 148
Renovascular Disease
Aneurysms and Arteriovenous Fistulae
Keith D. Calligaro, Matthew J. Dougherty
Renal artery aneurysms and renal arteriovenous fistulae (AVFs) are rare entities, but they are encountered frequently enough that vascular surgeons need to be well acquainted with the natural history, diagnosis, and management of these lesions. Endovascular interventions represent the newest advances in the treatment of some of these lesions. Aneurysms and AVFs are discussed separately because they rarely occur concomitantly, and their clinical course and treatment differ.
Renal Artery Aneurysms
Epidemiology
Even in referral centers, few vascular surgeons have extensive experience with the clinical management of renal artery aneurysms.1,2 Autopsy studies have revealed an incidence of 0.01% to 0.09%, which is probably an underestimation, because renal artery aneurysms may be small, intrarenal, or not specifically sought.1,3 In two angiography studies, renal artery aneurysms were documented in 0.73% (7-965) to 0.97% (83-8525) of arteriograms; in a more recent computed tomographic (CT) angiography study, the incidence was 0.7% (6-862).4–6 Conversely, these reports might overestimate the prevalence of these lesions. If renal artery aneurysms were present in almost 1% of patients undergoing abdominal aortography, vascular surgeons would be expected to have far greater experience diagnosing and treating these lesions than has been reported to date. At Pennsylvania Hospital (Philadelphia, Penn), we documented renal artery aneurysms in only 0.12% (1-845) of abdominal aortograms. Renal artery aneurysms are bilateral in about 10% of cases.1,5 If fibrodysplastic cases are omitted, there is an equal incidence in males and females.1,2,7
Because of the lack of controlled data, controversy persists regarding the indications for repair of asymptomatic renal artery aneurysms. The optimal method of repair is also controversial. Types of renal artery aneurysms, their clinical manifestations, indications for repair, and techniques of both traditional surgical and newer endovascular interventions are reviewed here.
Pathogenesis
Types of renal artery aneurysms include true (saccular and fusiform), false, dissecting, and intrarenal aneurysms.
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
Clinical Manifestations and Diagnosis
Most renal artery aneurysms are asymptomatic and are found on imaging studies, such as arteriography, ultrasonography, or CT performed to investigate other intra-abdominal pathology.2,13,23 Magnetic resonance angiography (MRA) can also delineate renal artery aneurysms.99 Clinical manifestations of renal artery aneurysms include rupture, hypertension, pain, and hematuria. In one series, only 11 of 32 patients (34%) who underwent surgery for renal artery aneurysms presented with symptoms.13
The most dreaded complication of renal artery aneurysm is rupture. Patients with this complication present with manifestations similar to those of other intra-abdominal arterial ruptures, including syncope, abdominal, or flank pain, abdominal distention, and possibly, a pulsatile mass. Occasionally, an intact renal artery aneurysm presents with abdominal or flank pain, discomfort, or fullness—symptoms that are presumed to reflect acute aneurysmal expansion. Intuitively, a stable renal artery aneurysm should not cause pain or symptoms, similar to a stable abdominal aortic aneurysm, so the clinician should hesitate to attribute chronic abdominal pain to a stable, noninflammatory renal artery aneurysm.
Renal artery aneurysms may be associated with severe hypertension. Macroaneurysms were found in 2.5% of arteriograms performed for the evaluation of hypertension.10 Renal artery aneurysms may cause renovascular hypertension by distal embolization with segmental hypoperfusion, and renin-mediated vasoconstriction and fluid retention. Compression of an adjacent renal artery branch or luminal stenosis due to extensive thrombus may also lead to renin-mediated hypertension. Frequently, significant renal artery stenosis causes a post-stenotic fusiform aneurysm, and the renal artery stenosis is responsible for the hypertension. Saccular and intrarenal aneurysms are much less likely to be associated with hypertension. Nonetheless, caution should be exercised in attributing the cause of hypertension to a renal artery aneurysm, especially if intervention is being considered. Just as renal artery stenosis without an aneurysm is currently less likely being treated with intervention and more likely treated with medical management, the clinician should hesitate to recommend endovascular or surgical intervention to treat hypertension potentially due to a renal artery aneurysm. Many renal artery aneurysms may have been unjustifiably treated in the past with ill-advised over-aggressive intervention.
Patients with renal artery aneurysms caused by dissection may present with severe flank pain, hematuria, or acute hypertension, although most dissections are asymptomatic. An intravenous pyelogram may reveal nonfunction or diminished function of the involved kidney, but this is rarely the first test ordered, unless urolithiasis is considered a likely cause of the symptoms. CT angiography or MRA is essential to detect dissection.
Intrarenal aneurysms may rupture into calices.3 In addition to pain, microscopic or gross hematuria may occur. Similarly, renal artery aneurysms may rarely cause obstruction of the collecting system. Although main renal artery aneurysms may be large, they are usually not near enough to the caliceal system to cause obstruction. Intrarenal aneurysms tend to be too small to cause significant collecting duct obstruction. However, a 9-cm renal artery aneurysm has been documented to cause hydronephrosis.12
Indications for Intervention
Indications to repair a renal artery aneurysm are related to the risk of rupture, hypertension, acute dissection, and other clinical symptoms.
Rupture and Prevention of Rupture
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
Treatment: Medical, Endovascular, Surgical
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.
