Splanchnic and Renal Artery Aneurysms

Splenic Artery Aneurysms

Splenic artery aneurysms account for 60% of all reported splanchnic artery aneurysms. The frequency of these aneurysms in the general population approaches the 0.78% incidence noted in nearly 3600 consecutive abdominal arteriographic studies performed for reasons other than suspected aneurysmal disease. Historically, women appeared nearly fourfold more likely than men to have these aneurysms, although in more contemporary times this female predilection appears to be less.

Four distinct, preexisting conditions are suspected to contribute to the development of splenic artery aneurysms. First is medial fibrodysplasia, a subset of fibromuscular dysplasia (FMD), with its derangements and thinning of the vessel's medial smooth muscle, which is more commonly recognized as a cause of hypertension secondary to its renal artery involvement. Approximately 2% of patients exhibiting this form of renal artery disease have splenic artery aneurysms. Blood pressure elevations in these patients may also contribute to aneurysm development. Coexistence of renal artery medial fibrodysplasia and splenic artery aneurysms has been identified only in women.

Second are the effects of excessive splenic blood flow and the increase in reproductive hormones accompanying pregnancy. Both cause changes in elastin and other matrix proteins within the splenic artery wall. In past decades, approximately 40% of women harboring these aneurysms had completed six or more pregnancies. In contemporary times, fewer children are being born to women in Western societies, and the frequency of multiparous individuals exhibiting these aneurysms has declined to approximately 10%.

Third, portal hypertension with splenomegaly is associated with splenic artery macroaneurysms in 10% to 15% of patients. The resistance encountered by the effluent from the spleen through the portosystemic venous channels of these patients is low compared with the relative high resistance of the liver's hepatic sinusoids. This results in higher velocities in splenic artery blood flow, dilation of the artery, and the evolution of aneurysms. Increased estrogen activity associated with advanced cirrhosis may also affect arterial walls and contribute to aneurysmal development. This accounts for the increased presence of these aneurysms in patients subjected to orthotopic liver transplantation. Aneurysms are more common in female than male liver transplant recipients, and they appear to be directly related to the severity of the patient's portal hypertension.

Fourth, inflammatory disease, such as chronic pancreatitis with pseudocyst erosion into an adjacent splenic artery, is a less common but important cause of these aneurysms. In contrast, microaneurysms within the splenic parenchyma, most often associated with a generalized inflammatory vasculitis such as polyarteritis nodosa, are of less clinical relevance than extraparenchymal macroaneurysms.

Penetrating trauma and iatrogenic injury following abdominal operations are well-recognized causes of splenic artery pseudoaneurysms. Blunt injury is a less frequently encountered etiology, although small intraparenchymal aneurysms may commonly accompany the arterial disruptions within a fractured spleen.

Most splenic artery aneurysms also exhibit elements of arteriosclerosis, although this is considered a secondary event rather than an etiologic process ( Fig. 46.1 ). Nevertheless, arteriosclerotic-mediated inflammation may contribute to further aneurysmal degeneration.

Splenic artery aneurysms associated with arterial fibrodysplasia and those associated with the flow and hormonal changes of pregnancy as well as portal hypertension evolve in arteries that often exhibit considerable tortuosity ( Fig. 46.2 ). Discontinuities exist in the internal elastic lamina of normal vessels at branchings, and subsequent alterations in elastic tissue likely favor aneurysmal changes at these sites. These are true aneurysms, not false aneurysms. Aneurysms unrelated to portal hypertension are multiple 20% of the time. Among patients with portal hypertension and cirrhosis undergoing liver transplantation, multiple aneurysms are even more common. In contrast, false aneurysms associated with pancreatitis usually involve the main splenic artery and tend to be solitary ( Fig. 46.3 ).

Curvilinear or signet-ring dystrophic calcifications in the left upper quadrant on radiographs may often be the first evidence of a splenic artery aneurysm. However, in contemporary practice, these aneurysms are recognized most often during imaging undertaken for other disease states. Conventional arteriography, ultrasonography, computed tomography (CT), CT arteriography, and magnetic resonance arteriography (MRA) are useful not only in recognizing these lesions but are often helpful in identifying ruptured or leaking aneurysms.

Abdominal discomfort has been reported in approximately 20% of these patients, although left-upper-quadrant or epigastric pain directly attributable to an intact splenic artery aneurysm is rare. In contrast, a 1996 review of 83 cases, which included many single individuals, reported that 46% of patients had abdominal pain and 25% were in shock. The disparity in these publications is attributed to the fact that reviews often summarize spectacular individual case reports that are unlikely to be representative of the usual clinical course in the general population.

True rupture of a splenic artery aneurysm may cause bleeding that is initially contained within the lesser sac. Eventually free hemorrhage into the peritoneal cavity occurs, causing vascular collapse. This “double rupture” phenomenon is often referred to in discussions of splenic artery aneurysms, but its occurrence is exceedingly uncommon. Pseudoaneurysms due to pseudocyst erosion into the adjacent splenic artery and subsequently into the stomach or pancreatic ductal system are often a source of gastrointestinal hemorrhage. Arteriovenous fistula formation is a rare complication of splenic artery aneurysm rupture into the adjacent splenic vein. This is a recognized cause of left-sided portal hypertension and gastrointestinal bleeding from esophageal varices.

The risk of splenic artery aneurysmal rupture is related to its cause. Rupture of bland asymptomatic aneurysms has been reported to be approximately 2%. However, reported rupture rates may be overstated. In fact, a recent review of 168 patients with these aneurysms reported a rupture rate of zero among those not subjected to surgical therapy. Contrary to earlier misconceptions, rupture is just as likely to affect a calcified aneurysm in a normotensive elderly patient as in a hypertensive young patient with a noncalcified aneurysm. Giant aneurysms (> 5 cm in diameter) have a reported overt and covert rupture rate totaling 30%. In the past, the mortality accompanying rupture of a bland splenic artery aneurysm in a nonpregnant patient has been reported to be 25%.

Bland aneurysms in liver transplant recipients appear to be nearly twice as likely to rupture as those in other patients. Rupture is most likely to occur early in the period after transplantation. The mortality following rupture of a splenic artery aneurysm in a liver transplant patient exceeds 50%. Nevertheless, aneurysmal rupture in this setting is uncommon. Inflammatory pancreatitis-related splenic artery aneurysmal rupture occurs more often than rupture in bland aneurysms, affecting approximately 5% of aneurysms and resulting in death in 40% of cases.

Nearly 95% of reported aneurysms recognized during pregnancy have ruptured. However, this figure may be misleading, since few splenic artery aneurysms actually rupture during pregnancy. It is hypothesized that although women may develop splenic artery aneurysms during the course of repeated pregnancies, they are rarely diagnosed unless they rupture. Nevertheless, aneurysmal rupture during pregnancy does represent a serious threat to the health of the mother and fetus. Two-thirds of reported ruptures have occurred during the third trimester and nearly one-fifth rupture at parturition or in the early postpartum period. Overall reported maternal and fetal mortality in these cases approaches 75% and 95%, respectively. Rupture in pregnancy, when recognized early enough to allow for operative intervention, still carries a reported maternal mortality of 22% and fetal mortality of 16%.

It would seem reasonable to consider elective operative intervention for asymptomatic (bland) splenic artery aneurysms greater than 2 cm in diameter, when the risk of operative death is less than 0.5%. The latter figure represents the product of the 25% open operative mortality and the 2% rupture rate of bland aneurysms. However, in more contemporary times, the mortality rate accompanying operation for rupture may be less. Therefore any elective open surgical or endovascular intervention should be undertaken only when the operative risk is exceedingly low. In certain elderly patients the cost-effectiveness of an intervention is minimal and a conservative nonoperative approach seems prudent.

Percutaneous transcatheter embolization using various agents—including differing types of particulate matter, coils, and glue (acrylate derivatives)—is preferred over open operative interventions in the treatment of most splenic artery aneurysms ( Fig. 46.4 ). ^a

a References

Endovascular interventions may be particularly useful in high-risk patients with liver cirrhosis. Nevertheless, recanalization, splenic infarction, delayed rupture of the spleen, inconsistent obliteration of the aneurysm, as well as migration and erosion with stricture of adjacent viscera are all recognized complications of endovascular therapy. Thus close follow-up of endovascular-treated patients becomes mandatory. Stent-graft and multilayer stent exclusion of an aneurysm with maintenance of splenic artery flow may avoid certain complications of embolic interventions.

Splenectomy was the most commonly reported open surgical intervention undertaken for the treatment of bland splenic artery aneurysms prior to the introduction of endovascular therapies. Given the immunologic benefits of splenic preservation, even in the aged, simple ligature obliteration or excision of these aneurysms unaccompanied by a splenectomy is preferred when an open surgical procedure is required. In select cases, a laparoscopic or robotic approach is favored.

Inflammatory splenic artery aneurysms embedded in the pancreas may require distal pancreatectomy. Large pseudoaneurysms associated with pseudocyst erosion into the adjacent artery may be treated initially by incising the aneurysmal sac and ligating the entering and exiting vessels. Pancreatic resection in the former cases depends on the degree of associated pancreatic inflammation and the general condition of the patient.

Hepatic Artery Aneurysms

Hepatic artery aneurysms account for 20% of all reported splanchnic artery aneurysms and are most often discovered during the sixth decade of life. Men are affected twice as often as women, although gender differences appear to be less in recent times. More than one-third of patients with these aneurysms have other splanchnic artery aneurysms.

Blunt liver injuries have led to a marked increase in the number of reported hepatic artery aneurysms, being recognized as small pseudoaneurysms on CT studies performed for major abdominal trauma. Such traumatic etiologies are a likely reason that almost half of the reported hepatic artery aneurysms affect the intrahepatic arterial branches. Penetrating trauma and percutaneous and therapeutic hepatobiliary injuries are well recognized additional causes of hepatic artery pseudoaneurysms.

Connective tissue arteriopathies, such as periarteritis nodosa, have also been incriminated as a cause of occasional macroaneurysms involving the hepatic arteries. Mycotic aneurysms associated with sepsis, often related to intravenous drug abuse and endocarditis, are relatively uncommon in contemporary practice.

Hepatic artery aneurysms are usually solitary; they are extrahepatic in nearly 80% of cases and intrahepatic in 20% ( Fig. 46.5 ). Arteriosclerosis is considered a secondary event rather than an initiating cause of hepatic artery aneurysms.

Most hepatic artery aneurysms are asymptomatic and are discovered incidentally during imaging for other illnesses. A minority of aneurysms are symptomatic, characteristically causing right-upper-quadrant and epigastric pain. Acute expansion of hepatic artery aneurysms can cause severe abdominal discomfort, similar to that of pancreatitis. Large aneurysms may cause obstructive jaundice.

The incidence of hepatic artery aneurysmal rupture remains ill-defined, but the reported rupture rate in contemporary times approaches 25%. However, bleeding occurred in more than half of the aneurysms reported from experiences having a predominance of iatrogenic intrahepatic aneurysms.

Bleeding from ruptured hepatic artery aneurysms occurs equally into the biliary tract and into the peritoneal cavity. In the case of the former, hemobilia is often evident, manifested by biliary colic, hematemesis, and jaundice. Chronic gastrointestinal hemorrhage is a very uncommon but recognized sequela of this form of rupture. Intraperitoneal bleeding is usually associated with inflammatory pseudoaneurysms. Multiple nonarteriosclerotic aneurysms have an apparent greater risk of eventual rupture. The reported mortality rate attending aneurysmal rupture approaches 40%.

Interventional therapy is recommended for all symptomatic aneurysms and all asymptomatic saccular hepatic aneurysms exceeding 2 cm in diameter. Smaller intrahepatic aneurysms that appear to be expanding and pseudoaneurysms of the extrahepatic arteries warrant earlier treatment.

Percutaneous transcatheter obliteration of hepatic artery aneurysms with balloons, coils, various types of particulate matter, and even thrombin is preferred over an open surgical intervention by many interventionists ( Fig. 46.6 ). Long-term follow-up is required, since transcatheter embolization may be only transiently successful and repeated embolization or eventual surgical therapy may be required in some patients. The most serious complication of endovascular embolization is liver necrosis and death. Endovascular stent-graft exclusion of select aneurysms avoids some of the limitations accompanying embolization alone.

Open surgical treatment of hepatic artery aneurysms—including ligation, aneurysmectomy, or aneurysm exclusion with or without arterial reconstruction—is favored by some. In this setting, if liver blood flow appears compromised on temporary hepatic artery occlusion, a direct vascular reconstruction should be undertaken with a prosthetic or autologous graft. Hepatic ischemia is most likely to accompany exclusion of aneurysms involving the proper hepatic artery. Simple ligation of extrahepatic branches on rare occasions may cause liver necrosis and necessitate hepatic territory resection.

Superior Mesenteric Artery Aneurysms

Aneurysms of the proximal superior mesenteric artery are the third most common splanchnic artery aneurysm, accounting for 5.5% of these lesions ( Fig. 46.7 ). Men are affected nearly twice as often as women. An infectious etiology associated with bacterial endocarditis was more common in the distant past. Nonhemolytic streptococci account for the majority of mycotic lesions encountered in contemporary practice, although these aneurysms are relatively rare. Superior mesenteric artery (SMA) aneurysms may also be related to medial degeneration, spontaneous SMA dissection, periarterial inflammation, and trauma. Arteriosclerosis, when present, is considered a secondary event rather than an etiologic process.

SMA aneurysms are usually recognized during imaging studies for other diseases. The majority of these aneurysms are asymptomatic. When they are symptomatic, there is usually an acute expansion, dissection, or rupture of the aneurysm. Rupture occurred in a little more than one-third of cases recently reported from a large health care system. However, a rupture rate of less than 10% is more representative of contemporary practice; mortality approaches 50% with rupture.

SMA dissections with mural aneurysms ( Fig. 46.8 ) are believed to be more common than simple aneurysms. The unique location of dissecting aneurysms near the origins of the inferior pancreaticoduodenal and middle colic arteries place the intestine of the distal mesenteric circulation at risk for ischemia if these branches become occluded. In such cases a loss of the usual collateral networks from the adjacent celiac and inferior mesenteric arterial circulations may result in bowel ischemia. Occasionally the latter is responsible for postprandial abdominal pain suggestive of intestinal angina. Symptomatic, expanding aneurysms and those greater than 2 cm in diameter warrant interventional therapy.

Endovascular stent-graft placement is an appealing treatment option for selected SMA aneurysms. Although late complications of thrombosis and infection may accompany endovascular stent-graft therapy, the early morbidity and mortality are much less than those accompanying an open surgical procedure. Obliteration of these aneurysms by coils or direct thrombin injection is preferred in high-risk patients with discrete aneurysm necks. This assumes that the distal mesenteric circulation will be preserved by the collateral circulation.

Similarly, open surgical ligation of superior mesenteric artery aneurysms without arterial reconstruction has proved efficacious in certain cases, especially for aneurysms associated with prior arterial obstruction and the development of an adequate collateral circulation to the midgut structures. In fact, ligation has been the most common reported surgical means of managing these lesions. If trial clamping results in bowel ischemia in this setting, then intestinal revascularization is required.