Historical Background
Although renal artery aneurysms located in a distal branch can be sacrificed with little consequence to renal function, centrally located aneurysms posed an initial challenge for the development of successful endovascular treatment options. In 1995 Bui and colleagues described the treatment of a renal artery aneurysm with a stent graft. Although a useful strategy in carefully selected patients, covered stents could not be used for aneurysms located at a bifurcation point, in tortuous vessels, or at more peripheral sites. Indeed, renal artery aneurysms occur predominantly at the renal bifurcation or in the first-order renal artery branches. Moreover, because most renal artery aneurysms are large necked or fusiform, traditional coil embolization is associated with significant potential for coil migration and occlusion of the parent artery. In response to this challenge, stent-assisted embolization, in which coils are delivered to the aneurysm through the struts of a stent in the parent vessel, was first described in 2008 for the treatment of renal aneurysms. Endovascular repair is the predominant treatment for renal artery aneurysms. Although current data suggest that both short- and intermediate-term outcomes of endovascular interventions are excellent, long-term durability remains to be determined.
Indications
Aneurysms of the renal artery and its branches are uncommon, occurring in less than 0.1% of the general population and are often discovered incidentally through computed tomography (CT) imaging. Renal aneurysms may present more frequently among women in the sixth decade of life and may be related to fibromuscular dysplasia, atherosclerosis, trauma, or mycotic infection. Renal aneurysms are also observed with vasculitis, such as Takayasu or Behçet disease and Marfan syndrome or neurofibromatosis. Rupture is the main complication.
Aneurysms larger than 2 cm and all renal aneurysms in women considering future pregnancy or in a solitary kidney are considered appropriate for repair due to the risk of rupture. Although 2 cm has been used as the threshold for treatment, even smaller aneurysms can rupture or may cause renovascular hypertension by direct compression of the main renal artery or branch vessel. Renal aneurysms may also embolize to terminal vessels, causing renal infarcts with pain, or may be associated with an arteriovenous fistula.
Preoperative Preparation
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Patients are symptomatic in fewer than 50% of cases undergoing repair.
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Preoperative imaging is critical in determining the type of endovascular repair that can be performed. Three-dimensional (3D) CT angiography or magnetic resonance angiography can define the relationship of the aneurysm to the surrounding branch vessels or the surrounding renal parenchyma. CT angiography can also reveal the extent of calcification in the aneurysm sac and adjacent arteries. Once occlusion coils or stents have been placed, artifact may limit the ability of CT angiography or magnetic resonance angiography to evaluate aneurysm exclusion ( Fig. 39-1 ).
Figure 39-1
A and B, Axial CT images and coronal reconstruction revealing a saccular left renal artery aneurysm. C and D, Coronal and 3D CT imaging after coil embolization and stenting of the renal artery aneurysm.
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Conventional angiography may provide additional information, such as the association of the renal artery aneurysm with an arteriovenous fistulae, or define entry site tears in patients with renal artery dissection associated with aneurysmal dilatation ( Fig. 39-2 ).
Figure 39-2
A, Aneurysmal dilatation of the left renal artery after arterial dissection. B, The entry tear to the false lumen is revealed by the presence of an intimal defect (arrow).
Pitfalls and Danger Points
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A brachial approach may be needed in severely caudally angulated renal arteries. Many devices needed for treating renal aneurysms require a 7-Fr sheath with open repair of the brachial artery.
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Ipsilateral oblique images are critical for identifying occult ostial stenosis and for ensuring that the aneurysm does not obscure the true length or the neck of the aneurysm.
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Care must be taken when crossing the aneurysm to avoid rupture of the vessel or embolization of mural thrombus.
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Renal artery aneurysms occur predominantly at the renal bifurcation or in the first-order renal artery branches ( Fig. 39-3 ). This precludes placement of a covered stent and requires repair with a combination of coil or glue embolization and stenting.
Figure 39-3
CT angiogram demonstrating a renal artery aneurysm at a distal branch point of the main renal artery.
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Renal artery aneurysms that arise as a poststenotic dilation, often seen in patients with fibromuscular dysplasia, may benefit from stent grafting to address both the stenosis and the aneurysm.
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Peripheral renal artery aneurysms may be treated with distal coil embolization, sacrificing a small amount of renal parenchyma ( Fig. 39-4 ).
Figure 39-4
A, Magnetic resonance angiogram of a distal left renal artery aneurysm. B, Angiogram of the distal renal aneurysm revealing filling of the left renal vein and the presence of an arteriovenous fistula. C, Status post coil embolization of the left renal artery aneurysm with obliteration of the arteriovenous fistula.
Endovascular Strategy
Initial angiography should be performed with a flush catheter at the level of the renal arteries. Avoid placement of the catheter in the visceral segment of the aorta, as the superior mesenteric artery often obscures the origin of the left renal artery. A visceral selective, Cobra 2, or renal double-curved (RDC) catheter can be used to select the renal artery to be treated. In general there are four methods described for the endovascular management of renal artery aneurysms: use of bare stents or stent grafts, coil or glue occlusion of the aneurysm sac, stent-assisted embolization of the aneurysm sac, and occlusion of the inflow and outflow vessels of the portion of the renal artery supplying the aneurysm. If there are multiple aneurysms, or if the aneurysm involves a major branch point, endovascular repair may not be possible.
Endovascular Technique
Techniques used to gain access to the renal arteries are similar to those used for the treatment of renal artery occlusive disease. Retrograde femoral access can be used with either a 6- or 7-Fr, 45- to 55-cm sheath and 5-Fr diagnostic catheter, such as Cobra 2 or a visceral selective catheter, Additionally, a 7- or 8-Fr sheath with a compatible guide catheter may be used (left internal mammary artery or RDC). Retrograde brachial access may provide a better approach to access severely angulated renal arteries. The advent of 0.014- and 0.018-inch guidewires, catheters, and balloons has also allowed easier access to these vessels.
Deployment of a Bare Stent
Renal artery aneurysms best suited for stenting alone are those that arise secondary to dissection. Newer stent technologies that implement multiple braided layers may allow continued perfusion of side branches while changing flow dynamics within the aneurysm sac to increase the propensity of thrombosis.
Deployment of a Stent Graft
Given that most renal aneurysms tend to be saccular as opposed to fusiform, use of a covered stent works well for excluding the aneurysms. An endoleak is not likely given that there are few collaterals. Another advantage of stent grafting is immediate resolution of flow within the aneurysm, as opposed to coil embolization, which may not lead to immediate thrombosis. Nonetheless, of the three available self-expanding stent grafts, Viabahn (W.L. Gore and Associates, Newark, Del.), Fluency (Bard Medical, Covington, Ga.), and Wallgraft (Boston Scientific, Natick, Mass.), and the two balloon-expandable stent grafts, iCast (Atrium Medical, Hudson, N.H.) and Jostent (Abbott Vascular, Abbott Park, Ill.), none is approved in the United States for use in renal arteries ( Table 39-1 ). These stent grafts tend to be bulkier and have less trackability compared with bare metal stents and microcatheters. The smallest inner diameter needed to deliver any of these stent grafts is 6 Fr, with the majority requiring a 7- or 8-Fr sheath. This should be considered if a brachial approach is planned and a direct cutdown should be performed.
