Management of Visceral Aneurysms

Indications

The decision for intervention has to take into account the size and the natural history of the lesion, the risk of rupture, which is high during pregnancy, and the relative risk of surgical or radiological intervention. For most asymptomatic aneurysms, expectant treatment is acceptable. For large or symptomatic aneurysms or those with a high risk of rupture, endovascular treatment has become the first-line therapy. Treatment of VAPAs is always mandatory because of the high risk of rupture. We divide the discussion between true VAAs and VAPAs because the thresholds are completely different.

For VAPAs due to inflammation or pancreatitis (e.g., splenic, gastroduodenal artery, SMA, hepatic, or even renal aneurysms), trauma, or those occurring after surgery, the thresholds for treatment are very low. Even small aneurysms (2–5 mm) should be treated regardless of diameter, because the risk of rupture for VAPAs is not related to their size. This type of aneurysm may (rarely) spontaneously heal, but in most cases, VAPAs will increase over time and will eventually rupture. We should treat all of these aneurysms immediately after diagnosis, irrespective of their location or origin.

For true VAAs, the treatment threshold is different and depends mainly on their anatomic location. Size thresholds for treatment for various VAAs are mainly derived from limited series of the natural history of untreated VAAs and are not universally accepted. Notwithstanding this, the threshold for most true SAAs is 2 cm in diameter at the largest axis. Even if peripheral thrombus is present, these aneurysms should be treated in cases of an overall diameter larger than 2 cm. Women of childbearing age should be treated regardless of the diameter because the rupture risk increases significantly during pregnancy. One of the vascular complications of portal hypertension, which could occur in cirrhotic patients, is the development of intrasplenic or extrasplenic aneurysms. These lesions should not be treated systematically except in cases of aneurysms > 4 cm in diameter and in extrasplenic locations. In most cases, multiple, diffuse, small aneurysms related to portal hypertension should be left untreated and followed by additional computed tomography (CT) or magnetic resonance imaging examinations. Once the portal hypertension and underlying cirrhotic disease is treated (e.g., via liver transplantation), the aneurysm may spontaneously decrease and completely disappear over time.

Other types of true aneurysms such as gastroduodenal artery aneurysms or those in the pancreaticoduodenal arcades, which can be caused by chronic hyperkinetic flow, should be treated as soon as they are diagnosed because they are at high risk of rupture, even if they are small. When such aneurysms are associated with celiac trunk stenosis, inversion of the flow in the pancreaticoduodenal arcades to revascularize the liver or spleen needs to be preserved during the embolization procedure, which is sometimes a technical challenge. For true hepatic or SMA aneurysms, the threshold for treatment is slightly lower than for splenic aneurysms. In most cases, we treat hepatic or SMA aneurysms when the large axis is >1 to 1.5 cm.

The treatment of renal artery aneurysms is intended to prevent rupture into either the urinary tract or the retroperitoneal space, as well as the development of systemic hypertension or renal failure in cases of intrarenal arteriovenous fistula development. Even small aneurysms can cause changes to intrarenal hemodynamics and systemic hypertension. If hypertension is present or occurs, the aneurysms should be treated endovascularly or surgically, depending on the type (saccular or fusiform) and location. In cases of isolated, nonsymptomatic aneurysms in the renal arteries, the treatment threshold is around 1 to 1.5 cm. For both visceral and renal arteries, extraparenchymal aneurysms take priority over intraparenchymal aneurysms because the risks and severity of major rupture and hemorrhage seem significantly higher for proximal extraparenchymal lesions.

Contraindications

Contraindications to treatment are few and can be subdivided into contraindications to surgical treatment (usually based on patient comorbidity) and contraindications to interventional radiology. Relative contraindications to interventional radiology treatment include allergy to iodinated contrast media, severe renal impairment, and anatomical issues such as occlusion of the feeding vessels, limited access (e.g., occluded femoral/iliac arteries), and challenging local vascular anatomy, which may make transcatheter treatment too difficult to perform. However, in general, most VAAs can be treated by interventional radiologists with sufficient experience using modern equipment.

Relevant Anatomy

Thorough knowledge of conventional and variant visceral arterial anatomy is essential, as is recognition of the many collateral pathways that can develop via normal anastomoses in response to visceral artery occlusive or stenotic disease. This rich collateral supply to most of the abdominal viscera means that embolization of major visceral vessels can often be performed without causing organ infarction. However, a lack of appreciation of the extent of this collateral network can result in failure of endovascular therapy.

Abdominal visceral arteries consist of the splanchnic circulation and the renal arteries. The splanchnic circulation includes the celiac trunk and the superior and inferior mesenteric arteries. It is necessary to understand the important collateral pathways for proper management of VAAs and VAPAs. Common collateral pathways are: (1) between the SMA and celiac axis through the anterior and posterior pancreaticoduodenal arcades; (2) between branches of the gastroepiploic, short gastric, and splenic arteries (within the celiac arterial system); and (3) between the right and left hepatic arteries. Variations of arterial anatomy are possible, such as replaced or accessory hepatic arteries that may have a retroportal course (retroportal hepatic artery), a celiacomesenteric trunk, and a middle colic artery arising from celiac trunk. Uncommon variations include the arc of Buhler (persistence of the direct embryological communication between the celiac trunk and SMA) and arc of Barkow (anastomosis of the right and left gastroepiploic arteries). Absence of collateral pathways in the renal arterial system is also an important deciding factor in selecting the embolization technique.

Preprocedural Imaging

Prior imaging is critical to the interventional management of a VAA or VAPA. Ultrasonography, CT, and magnetic resonance imaging are noninvasive imaging techniques that are most commonly employed for the detection and evaluation of VAAs. Invasive digital subtraction angiography (DSA) is reserved for specific situations. Multidetector CT angiography (CTA) is the most commonly used modality and the most sensitive noninvasive modality for the detection of visceral aneurysms. Routinely, CTA should include both arterial and venous phases because some pseudoaneurysms with a narrow neck may not be seen on the arterial phase and may opacify only in the venous phase. CTA demonstrates a well-defined contrast-filled sac with enhancement similar to the adjacent main artery in both arterial and venous phases. Postprocessing with maximum-intensity projection and volume rendering better demonstrate the aneurysm and its origin and improve detection. Depending on the extent of thrombosis, the sac may show low attenuation areas, usually in the periphery. In addition to the detection of an aneurysm, CTA provides a road map for intervention and identifies associated anatomical arterial variations.

Principles of Endovascular Techniques

Covered Stent Placement

Placement of a covered stent (“endograft”) across the neck of an aneurysm involving a large-caliber vessel such as the splenic, hepatic, or superior mesenteric artery is, at first sight, an attractive option because it will completely exclude the aneurysm while maintaining patency of the parent vessel. Embolization of the collaterals before stent placement may be required to avoid any revascularization ( Fig. 41.1 ). However, it is rarely used for the treatment of VAAs and VAPAs for the following reasons:

• 1.

  Insertion of a covered stent usually requires use of a guiding sheath, and it may not be possible to introduce the sheath into a suitable position because of unfavorable anatomy of the visceral artery origin or arterial tortuosity or both. This is especially true of the splenic artery. However, miniaturized coronary covered stents can now be used off-label for this purpose if the caliber of the parent artery is not more than 5 mm.

• 2.

  Manipulation of the large catheters and sheaths used for insertion of covered stents in a diseased vessel close to an aneurysm poses a greater risk of periprocedural rupture than do the smaller catheters generally used for embolization.

• 3.

  Although the neck of a pseudoaneurysm may appear small, it should be remembered that the disease process causing the vessel defect (e.g., pancreatitis) is likely to involve a greater length of arterial wall than is apparent angiographically. In such situations, placement of a balloon-expandable covered stent may be associated with a high risk of arterial rupture.

• 4.

  If the neck of the aneurysm involves a vessel bifurcation, a covered stent may not cover the arterial defect completely and can present a subsequent risk of continued sac perfusion secondary to retrograde filling of the branch not stented.

• 5.

  Covered stents may occlude normal branch vessels arising close to the aneurysm neck, but this is often not a contraindication to their use in visceral vessels because there is usually an excellent collateral supply from adjacent arteries.