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January 2011
Endovascular Treatment of Renal Artery Aneurysms
Current data demonstrate the safety of devices and prostheses used for treating RAAs.
By Sanjeeva P. Kalva, MD; Steven Wu, MD; and Zubin Irani, MD
True renal artery aneurysms (RAAs) are uncommon, and their actual prevalence is unknown. Initial autopsy studies revealed an incidence of one per 8,000 to one per 10,000;1,2 however, further careful autopsy studies revealed a prevalence of 9.7%.3 The angiographic prevalence of RAAs varies between 0.3% and 0.7% in the general population;4-6 a higher prevalence of 2.5% and 9.2% was reported in subjects with hypertension and fibromuscular dysplasia, respectively.7,8 The prevalence of false RAAs is unknown, but an increasing incidence has been reported with the advent of minimally invasive kidney procedures such as biopsy, nephrostomy, and percutaneous stone removal.
The most common etiology of true RAAs is fibromuscular dysplasia and atherosclerosis. False aneurysms are usually secondary to iatrogenic and noniatrogenic trauma and infection. True aneurysms usually involve the main renal artery and its bifurcation, whereas traumatic and inflammatory aneurysms tend to involve the intrarenal parenchymal arteries. Based on the location and morphology, RAAs have been classified as saccular, fusiform, aneurysmal dissection, and intrarenal microaneurysms. 9 The size of the aneurysms vary from a few millimeters to 8 cm. Calcification of RAAs may be apparent on plain radiography in 27% to 50%.2,10,11
Symptoms occur in < 50% of patients and include high blood pressure, hematuria, abdominal pain from expansion of the aneurysm, decreasing renal function, and rarely, acute hemodynamic collapse secondary to rupture. The pathophysiology of hypertension in patients with RAAs is not well understood but may be related to localized ischemia secondary to kinking of the renal artery branch, turbulence in the aneurysm, distal embolization from the aneurysm, or coexisting renal artery stenosis. Several studies reported improvement of hypertension after treatment of the aneurysm.12,13 Progressive renal dysfunction may occur secondary to distal embolization of thrombus in the aneurysm. Rupture of the aneurysm is rare. The reported incidence of aneurysm rupture varies from 0% to 10%.12,14,15 The incidence of rupture appears to be high in large aneurysms (> 2 cm) and in pregnant women. The mortality secondary to rupture is high during pregnancy, with a reported maternal mortality of 50% and a fetal mortality of 80%;16-19 however, the mortality in nonpregnant patients is < 10%.12
The indications for treating RAAs include symptomatic aneurysms, large (> 2 cm) or enlarging aneurysms on serial angiography, aneurysms in patients anticipating pregnancy or in pregnant women, ruptured aneurysms, false aneurysms, and aneurysms associated with arteriovenous fistulas or dissection.13 Small (< 2 cm) aneurysms in nonpregnant patients may be followed with serial imaging. Traditionally, RAAs are treated with surgery either through aneurysmorrhaphy, resection and bypass grafting, ex vivo repair, or nephrectomy. Currently, most RAAs are treated via an endovascular approach due to its minimal invasiveness and reduced morbidity.
ENDOVASCULAR THERAPY
Various endovascular therapeutic options exist for
treatment of RAAs. These options fall into two categories—
embolization with metallic or liquid embolic
materials and exclusion through stent grafts. Based on
the treatment options for RAA, Rundback et al proposed
an angiographic classification system.13 Type I RAAs are
saccular and arise from either the main renal artery trunk
or proximally from a large segmental artery. These can be
successfully treated with stent grafts or stent/balloonassisted
coil embolization (Figure 1). Type II RAAs are
fusiform, occur at the main renal artery or proximal segmental
arteries, and are best treated surgically (Figure 2).
Type III RAAs refer to intraparenchymal aneurysms
affecting the small segmental or accessory arteries. These
are best treated with coil embolization with minimal loss
of parenchyma (Figure 3). Successful embolization of the
aneurysm while preserving the distal perfusion of the
normal parenchyma is preferable to prevent kidney dysfunction.
Because the renal arteries are end arteries, it is unlikely that the intraparenchymal aneurysms would reperfuse with embolization of the feeding arteries. This often results in infarction of distal renal parenchyma supplied by the feeding artery. Embolization of intraparenchymal aneurysms can be achieved with regular nondetachable coils, detachable coils (Figure 3),20 N-butyl cyanoacrylate, ethylene vinyl copolymer (Onyx, ev3 Inc., Plymouth, MN),21 or gelfoam. Selective embolization of the intraparenchymal aneurysm without occlusion of the feeding artery can be considered if the feeding artery supplies a large amount of parenchyma or if the parenchymal loss is unacceptable, as in patients with a solitary kidney or renal dysfunction. Techniques that allow preservation of the feeding artery while successfully achieving occlusion of the aneurysm include the use of detachable coils to selectively occlude the aneurysmal sac, stent- or balloon-assisted coil embolization of the aneurysm,22 and stent graft placement across the neck of the aneurysm.23 Exclusion of intraparenchymal aneurysms with stent grafts is often technically challenging due to the small size and tortuosity of the intraparenchymal arteries and rigid stent structure. However, a few case reports indicated successful use of stent grafts for these aneurysms.23
Treatment of the proximal segmental and main renal arteries is best achieved with stent grafts (Figure 1), which allow successful exclusion of the aneurysm while preserving flow through the arteries.13 If the neck of the aneurysm is narrow, selective embolization of the aneurysmal sac can be safely achieved with detachable coils.20 Wide-necked aneurysms are treated either with stent grafts or stentassisted coiling of the aneurysm.24 Three-dimensional computed tomography and three-dimensional angiography are very helpful in assessing the neck and size of the aneurysm.24 However, when the aneurysms involve the bifurcation points of the main or segmental renal arteries, the use of stent grafts may result in occlusion of some of the branches. Small segmental branches can be sacrificed, but if the branch vessels supply a significant portion of the renal parenchyma, alternate techniques may be employed. One of the approaches is selective stenting of the branch vessels with bare-metal stents while embolizing the aneurysm with coils through a microcatheter that is positioned within the aneurysmal sac through the interstices of the stent (Figure 4).25 Another approach is the use of multiple stent grafts across the aneurysm neck into the branch vessels. Often, surgical therapy may be required if the aneurysm is complex and involves multiple branch vessels.
CONCLUSION
Data on the long-term efficacy of endovascular therapy
for RAAs are lacking, but current data support the
high safety of the devices and prosthesis used for treatment.
Short-term data demonstrate successful exclusion
of the aneurysm and improvement of hypertension and
renal dysfunction.26 Further technological advances in
stent graft technology may allow successful use of these
devices for intrarenal aneurysms.
Sanjeeva P. Kalva, MD, is with the Division of Vascular Imaging & Intervention, Department of Imaging at Massachusetts General Hospital, and the Department of Radiology, Harvard Medical School, in Boston, Massachusetts. He has disclosed that he holds no financial interest in any product or manufacturer mentioned herein. Dr. Kalva may be reached at (617) 726-8315; skalva@partners. org.
Steven Wu, MD, is with Division of Vascular Imaging & Intervention, Department of Imaging, and the Division of Interventional Nephrology, Department of Medicine, at Massachusetts General Hospital; and the Department of Medicine, Harvard Medical School in Boston, Massachusetts. He has disclosed that he holds no financial interest in any product or manufacturer mentioned herein.
Zubin Irani, MD, is with the Division of Vascular Imaging & Intervention, Department of Imaging at Massachusetts General Hospital, and the Department of Radiology, Harvard Medical School, in Boston, Massachusetts. He has disclosed that he holds no financial interest in any product or manufacturer mentioned herein.
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