Introduction
Aneurysms of the mesenteric circulation are a rare but potentially fatal pathology. Therefore, a thorough understanding of the natural history and treatment options available is critical to the vascular surgeon potentially faced with treating this entity. Successful treatment with standard open surgical techniques was first described in the 1950s. Since that time, a major paradigm shift toward endovascular treatment has occurred not unlike many vascular pathologies we commonly encounter. Furthermore, the quality of cross-sectional imaging has improved dramatically, which has led to more frequent incidental detection of mesenteric aneurysms. At the same time, technologic advances have made an increasing number of devices available for the treatment of mesenteric aneurysms. This chapter will cover the epidemiology of mesenteric aneurysm including incidence and standard treatment indications. We will focus primarily on contemporary endovascular treatment techniques, descriptions of currently available devices, and how to avoid common complications.
Background
Visceral artery aneurysms are defined as aneurysms involving the celiac artery, superior mesenteric artery, inferior mesenteric artery, or any branch of these arteries. Renal artery aneurysms are classified separately and discussed in the following section. Estimates of the true incidence of visceral artery aneurysms vary from as low as 0.1% to 2 % in adults. The proportionate distribution of visceral aneurysms is more clearly understood than the overall incidence and is as follows: 60% splenic artery, 20% hepatic artery, 5.5% superior mesenteric artery, 4% celiac artery, 4% gastric artery, jejunal, ilial, and colic arteries 3%, pancreaticoduodenal artery 2%, gastroduodenal artery 1.5%, and inferior mesenteric artery <1%. The traditional definition of aneurysms applies with aneurysm defined as 1.5 times the diameter of the normal nondilated vessel. Visceral aneurysms include a large proportion of degenerative fusiform aneurysms, pseudoaneurysms, and saccular aneurysms. Underlying etiologies are varied as well and include, atherosclerosis, medial degeneration, collagen vascular disease, fibromuscular dysplasia, infectious etiologies, inflammatory etiologies, vasculitis, iatrogenic injury, and trauma. In previous decades it was more common for mesenteric aneurysms to present ruptured, with associated mortality as high as 70%. Exact rates of mesenteric aneurysm rupture are still not well established, but with improved imaging quality more incidental nonrupture aneurysms are being discovered.
Congruent with the global trend in vascular surgery there is a shift towards endovascular treatment of mesenteric aneurysms as a first line option. This is secondary to advances in technology and an increasing array of devices available to practitioners. Standard open surgical techniques can certainly still be applied to intact or ruptured mesenteric aneurysms, but retrospective data have demonstrated equivalent survival between open and endovascular treatment for intact aneurysms and decreased morbidity with endovascular repair. Additionally, in the ruptured cohort endovascular treatment was shown to have significantly lower postoperative mortality (7.4% versus 28.6%). Therefore, the trend toward treatment with endovascular techniques is likely to continue. This chapter will describe available methods for each anatomic location, as well as generally accepted indications.
Indications
Treatment indications vary by anatomic location. For splenic aneurysms, indications are diameter greater than 2 cm, symptomatic, ruptured, pseudoaneurysm, pregnancy, child-bearing age, orthotopic liver transplant, and portal hypertension. Hepatic artery aneurysms are the next most common mesenteric aneurysm and treatment indications are ruptured, symptomatic, pseudoaneurysm, multiple aneurysms, polyarteritis nodosa, other inflammatory conditions, and size greater than 2 cm. Superior mesenteric aneurysms are less common, but all SMA aneurysms should be repaired in appropriate surgical candidates. Similarly, all celiac artery aneurysms, gastric, gastroepiploic, pancreaticoduodenal, gastroduodenal, inferior mesenteric, jejunal, ileal, and colic artery aneurysms should be repaired unless the patient is deemed too high risk with respect to comorbidities. Finally, mesenteric venous aneurysms can generally be safely observed unless they become symptomatic.
Endovascular Treatment Options
Regardless of the treatment technique used, the therapeutic goal is the exclusion of the aneurysm from systemic circulation, thereby eliminating rupture risk or symptoms if present. A variety of embolic materials can be utilized either to exclude the aneurysm from circulation or to be placed directly in the aneurysm sac to induce thrombosis. The materials available are various types of coils, liquid embolic agents (such as glue or thrombin), and vascular plugs. Coils are the most commonly used and are a permanent agent that can be made of either stainless steel or platinum. Coils are available in an array of lengths, diameters, and shapes. Coils are also available in a fibered variety with increased thrombogenicity. Additionally, coils are classified by delivery mechanism into pushable (where they are advanced precisely to the desired location by advancing a wire behind the coil), injectable (where the coil is advanced quickly by injecting fluid through the catheter in a burst), or detachable (where the coils are connected to a guidewire that must be disconnected prior to final placement, adding an element of precision and safety). The profile of coils ranges from 0.035′′ for large and medium sized vessels to 0.018′′ microcoils that can be advanced through a microcatheter into small vessels with sizes available as small as 2 mm. Pushable coils include the Nestor and Tornado coils (Cook Medical, Bloomington, IN), which are available in both 0.035′′ and 0.018′′ sizes. Interlock coils (Boston Scientific, Marlborough, MA) are of the detachable variety and also come in both 0.035′′ and 0.018′′ sizes. Our preferred microcatheters include the Renegade (Boston Scientific, Marlborough, MA) Highflow and the Direxion. Coils may be used in one of several different techniques. Sac packing is a frequently used technique that refers to the aneurysm itself and only the aneurysm being filled with coils, thereby inducing thrombosis. This technique is useful for pseudoaneurysms where preservation of the end artery is needed, leading to thrombosis of only the pseudoaneurysm and not any other segment of the parent vessel. This can be technically challenging to perform, and the sac packing technique can be used in conjunction with a bare-metal stent covering the neck of the aneurysm, with the coils deployed through a microcatheter placed between the interstices of the stent. This technique is referred to as stent-assisted coiling and has the advantage of preventing coils from being inadvertently placed outside the desired area. It is useful for wide-necked aneurysms where sac packing may otherwise lead to nontarget vessel embolization. A similar technique is referred to as balloon remodeling, originally described for the treatment of intracerebral aneurysms. In this technique, a compliant angioplasty balloon the size of the parent vessel is deployed and the pseudoaneurysm is cannulated with a microcatheter between the balloon and the vessel wall. When performing this technique, we place the microcatheter in the aneurysm prior to balloon inflation, yet have the balloon catheter in place prior to cannulation with the microcatheter. We recommend inflation to no more than 4 atm and use of an 8-French sheath, minimum. For the 0.035′′ platform and 6-French sheath for the 0.018′′ platform. Additionally, we recommend weight-based anticoagulation with heparin. With the balloon in place, coils are deployed in the aneurysm and the balloon is deflated once the sac is filled with coils. This provides the same benefit as the stent-assisted coiling technique but permits use of a temporary balloon occlusion rather than permanently deploying a stent. The coils should be 20%–30% greater than the diameter of the artery. The potential complications of coil embolization are nontarget embolization in which coils end up in a nondesired location. Rupture of the pseudoaneurysm during vigorous coil deployment may also occur, as may failure of the aneurysm to thrombose. The latter is particularly true in patients with abnormal coagulation parameters. Embolization may also be achieved with other agents such as glues (N-butyl cyanoacrylate), which form a cast when coming in contact with blood. When using this type of glue, a hydrophilic microcatheter should be used to decrease the rate of polymerization in the catheter. Additionally, lyophilized thrombin can be used, which triggers the coagulation cascade and results in the formation of a fibrin plug in the desired location. N-butyl cyanoacrylate may result in polymerization within the catheter, which must be flushed with 5% dextrose before and after administration and both agents may also result in nontarget embolization. Additionally, if the glue occludes the catheter, access will be lost and will need to be the vessel reselected. It is therefore common to utilize the balloon remodeling technique with these agents. Vascular plugs such as the Amplatzer (St. Jude Medical, Minnesota) is an additional option. It is a three-dimensional nitinol mesh, which is well suited to occlude the vessel leading to the aneurysmal segment. They are attached to a delivery device, which must be unscrewed after deployment in the proper location. The plugs should be oversized (30%–50%) and are a good option for medium-sized vessels. They are highly effective and precise in their deployment mechanism. More recently, the MVP Micro Vascular Plug System (Medtronic, Bloomington, Minnesota) has become available, allowing exclusion of small and medium diameter vessels using a 5-French catheter. Vascular plugs or coils may also be used with the so-called “sandwich technique”. In this technique the vessel proximal and distal to the aneurysmal segment is occluded with either plugs or coils rather than simply filling the sac itself. This technique is effective but must be limited to situations where occlusion of the parent vessel will not cause significant clinical sequelae. This technique is especially successful in the treatment of large splenic artery aneurysms.
Covered stents are also an option in the treatment of mesenteric aneurysms. This technique involves placement of a stent graft across the aneurysm in the parent vessel, excluding the aneurysm from circulation, but preserving the patency of the vessel. This requires the use of a longer sheath and advancement into the mesenteric artery for support in delivering the covered stent. It is recommended to have at least 1 cm proximal and distal to the aneurysm to achieve an adequate seal. Self-expanding and balloon-expandable varieties are available depending on the anatomic location. Covered stents come in sizes as small as 5 mm and require a minimum sheath size of 6 French. Therefore, this technique can work well in some of the larger mesenteric aneurysms but is not an option for smaller branch vessel aneurysms ( Fig. 28.1 and Fig. 28.2 ).
