Splanchnic venous anatomy parallels the arterial system to drain into the portal venous system. The splenic and superior mesenteric veins join to form the portal vein. Hepatic venous blood is drained by the right, middle and left hepatic veins. Portal–systemic connections are multiple and important when considering portal hypertension.
Physiologic intestinal function relies on adequate perfusion and oxygenation of the microvascular splanchnic circulation. Splanchnic blood flow is estimated as 300 to 1,200 mL/min, or about 10% to 35% of the cardiac output, with approximately 70% of this blood flow supplying the mucosa and submucosa.1,2 Various intrinsic and extrinsic autoregulatory mechanisms including tissue metabolites, myogenic mechanisms, and the autonomic nervous system ensure adequate gut circulation through both vasoconstriction and relaxation of arterial smooth muscle. The degree of visceral artery dilation and constriction determines the relatively large fluctuations in splanchnic blood flow during fasting and postprandial states.
Duplex ultrasound demonstrates moderate to high arterial resistance in the SMA, with low diastolic flow and slight flow reversal during fasting states. Figure 91-2 demonstrates duplex ultrasonography of normal aorta and SMA. Vessel diameter, flow velocity, calculated volumetric blood flow, spectral analysis, and response to physiologic stimuli allow for precise assessment of the visceral circulation. In the postprandial period (30 to 90 minutes), low-resistance signals are noted throughout systole and diastole, indicative of dilated splanchnic arteriolar beds. Flow reversal does not occur. In contrast, low arterial resistance signals are noted in the celiac artery circulation regardless of feeding, likely due to the influence of the low resistance hepatic vascular bed.
2 Intestinal ischemia may result from presplanchnic conditions that decrease total mesenteric blood flow (i.e., heart failure and hypovolemia), splanchnic conditions that decrease regional blood flow through the mesenteric circulation (i.e., thromboembolism) or postsplanchnic venous congestion (i.e., mesenteric vein thrombosis [MVT] and cirrhosis). Regardless of the mechanism, impaired perfusion of the intestine results in hypoxia-associated sequelae that include cytokine release, free radial production, microcirculatory damage, and bacterial translocation. Mucosal barrier function is compromised secondary to physical disruption of the mucosa, a change in normal intestinal microflora resulting from treatment with broad spectrum antibiotics, and impairment of host immune defenses.3 Intestinal ischemia may rapidly progress to tissue necrosis causing severe metabolic derangements and ultimately multiple organ dysfunction and death. In addition to ischemia-related bowel injury, splanchnic reperfusion exaggerates injury patterns. Reperfusion injury is associated with increased microvascular permeability, increased epithelial permeability with leaking of fluid and molecules into the bowel lumen, and decreased intestinal blood flow.
ACUTE MESENTERIC ISCHEMIA
“Occlusion of the mesenteric vessels is apt to be regarded as one of those conditions of which … the diagnosis is impossible, the prognosis hopeless and the treatment almost useless”.4 AMI remains an uncommon event but carries catastrophic potential as illustrated by AJ Cokkinis’ statement above from 1926. Klass is credited with the first successful restoration of arterial blood supply in an attempt to salvage AMI in 1951 by SMA embolectomy.5,6 While clear medical and technologic progress has been made in the interim, delayed diagnosis and treatment continues to impede improvements in contemporary patient morbidity and mortality rates that approach 80%.6,7
AMI accounts for <1 in every 1,000 hospital admissions, presents most commonly during the seventh and eighth decades of life, and epidemiologic study suggests that women are affected with three times the frequency of males, at least in part attributed to relative female longevity.6 A Swedish epidemiologic study cites an AMI incidence of 12.9/100,000 people years (as diagnosed by autopsy or operation).8
3 The most common causes of AMI are referenced above and include: embolization (40% to 50%), arterial thrombosis (25% to 30%), nonocclusive mesenteric ischemia (NOMI) (15% to 20%), and mesenteric venous thrombosis (5% to 15%).6,8 Additional causes may include visceral malperfusion associated with aortic dissection, mesenteric arterial dissection, and trauma. Historic mortality rates for AMI are as high as 93%, although more contemporary series suggest in-hospital mortality rates of 17% to 62% and a systematic review of 45 observational studies cites a mean mortality rate of 71.6% for 3,692 patients with AMI.8–13 While there is no validated prediction model for mortality in this clinical setting, authors have shown with logistic regression models that age, duration of symptoms, age-adjusted comorbidity, specific EKG markers, and shock index may be associated with higher mortality.12,14,15
Patients with AMI complain predominantly of abdominal pain followed by nausea, vomiting, and diarrhea. Pain is often described as “out of proportion” to the clinical examination as tenderness to palpation may be minimal until transmural necrosis of the intestine develops. Patients can be tachycardic and demonstrate melena or heme positive stools. Fever, guarding, and rebound are typically late findings associated with bowel infarction.
Laboratory abnormalities may include hemoconcentration, leukocytosis, lactic acidosis, and elevated anion gap, amylase, aspartate aminotransferase, and lactate dehydrogenase. While a reliable biomarker for AMI remains elusive, D-dimer offers 96% to 100% sensitivity as an exclusionary test for early AMI with low specificity.16,17 Intestinal fatty acid binding globulin (I-FABP), α-glutathione S-transferase, and D-lactate are currently promising plasma markers.16,18 I-FABP and α-glutathione S-transferase are located in the small bowel mucosa while D-lactate originates from bacteria in the intestinal lumen as a normal product of bacterial fermentation. These markers leak into the blood stream during early ischemia from damaged enterocytes and may offer increased specificity for small bowel ischemia.
Figure 91-2. Mesenteric duplex ultrasound of a normal aorta and SMA that demonstrates moderate to high arterial resistance in the SMA, with low diastolic flow and slight flow reversal during fasting states. AO, aorta; SMA, superior mesenteric artery; PSV, peak systolic velocity; EDV, end-diastolic velocity; RI, resistive index; OR, origin.
Diagnostic work-up often starts with a plain abdominal radiograph which may reveal ileus and vascular calcifications; bowel wall edema (i.e., thumbprinting), pneumatosis (Fig. 91-3), pneumobilia, and pneumoperitoneum may become evident with advanced ischemia and infarction, respectively. High-quality computed tomography angiography (CTA) has become the gold standard for imaging AMI given near-universal availability and its ability to assess for bowel perfusion while excluding alternate sources of abdominal pain. Additionally, “biphasic” CT offers arterial and delayed phase imaging that permits visualization of the portal venous system in addition to arterial stenosis, occlusions, and bowel wall features that support the diagnosis of AMI with a negative and positive predictive value reported up to 96% and 100%, respectively.19,20 Duplex ultrasonography, magnetic resonance angiography (MRA), diagnostic peritoneal lavage, and diagnostic laparoscopy may serve as adjuncts for diagnosis but secondary to intrinsic limitations should not be considered first line. Finally, the role for arteriography remains important and will be described further as it allows for simultaneous therapeutic measures.
Mesenteric emboli most commonly affect the SMA given its oblique orientation off the aorta. Most emboli are cardiac in origin; less common embolic sources may include proximal aortic aneurysm, atrial myxoma, and paradoxical embolus secondary to venous thromboembolism with cardiac shunting. Patient history will likely reveal atrial fibrillation, myocardial infarction, congestive heart failure or cardiomyopathy, rheumatic heart disease or ventricular aneurysm. Most emboli will spare the SMA origin and lodge distal to the middle colic artery and early jejunal branches. This pattern of ischemia, in the absence of well-formed collaterals, results in a classic pattern of ischemia that compromises the bulk of small bowel and ascending colon while sparing the proximal jejunum and distal transverse colon (Fig. 91-4).
Figure 91-3. Abdominal radiograph demonstrating diffuse ileus and intestinal pneumatosis (arrow) in a patient with acute mesenteric ischemia.
Figure 91-4. Pattern of embolic mesenteric ischemia – SMA emboli typically spare the middle colic artery (arrow) and proximal jejunal branches (bracket) resulting in an ischemic pattern that spares the jejunum and distal transverse colon.
Acute in situ arterial thrombosis typically coincides with pre-existing atherosclerotic disease. Up to 20% to 50% of patients will describe antecedent postprandial abdominal pain and weight loss consistent with CMI.2,6,9 Clinical presentation may be more subacute given the presence of preformed collaterals that are typically absent in cases of embolic AMI. Patient history will likely reveal common cardiovascular risk factors. Figure 91-5 demonstrates the highly calcified aorta of a patient presenting with AMI and chronic celiac artery occlusion, SMA thrombosis (encircled), and IMA occlusion.
MANAGEMENT OF AMI
4 Patients with AMI warrant immediate medical management that includes the establishment of adequate intravenous (IV) access and hemodynamic monitoring, systemic anticoagulation with heparin, fluid resuscitation, correction of electrolyte abnormalities, and administration of broad-spectrum IV antibiotics. Further vasoconstrictive insult with vasopressors should be avoided if possible. Patients with AMI and evidence of threatened bowel viability warrant immediate surgical exploration. Frankly necrotic or perforated bowel should be resected and left in discontinuity to limit contamination while definitive intestinal management should never delay revascularization.
Figure 91-5. CT imaging demonstrating in situ SMA thrombosis in the setting of highly calcified aortoiliac occlusive disease and chronic celiac and IMA occlusions presenting as acute mesenteric ischemia.
SMA thromboembolectomy is the standard technique for embolic disease. Figure 91-6 illustrates anterior exposure of the SMA at the base of the colonic mesentery. Typically venous tributaries, small lymphatics, and autonomic nerve fibers require ligation to facilitate arterial exposure. The inferior pancreatic border may require mobilization for more proximal SMA exposure. The SMA is exposed and controlled proximal to the middle colic artery; the middle colic artery and jejunal branches are similarly controlled. After a therapeutic level of systemic anticoagulation is confirmed a transverse arteriotomy is created sharply (Fig. 91-7). The proximal SMA is vented; in the absence of robust and pulsatile antegrade flow a balloon thromboembolectomy should be performed with a 3-Fr or 4-Fr catheter. Distal vasculature is fragile and thromboembolectomy is best accomplished with a smaller catheter (i.e., 2 Fr or 3 Fr). During balloon thromboembolectomy the balloon should be inflated with heparinized saline (not air) as the catheter is withdrawn. The same person should control the balloon inflation that is controlling catheter removal. Finally, consider flushing regional heparin or thrombolytic agent directly into the distal SMA and branches. Balloon thromboembolectomy should be repeated until the balloon is withdrawn free of thrombus burden. Following the successful clearance of all macroscopic thrombus the arteriotomy can be repaired primarily with simple sutures of 5-0 or 6-0 monofilament suture placed in an interrupted fashion. Vein patch angioplasty should be considered over primary repair for diminutive vessels.
In cases of SMA occlusive disease with in situ thrombosis, bypass is typically required for successful revascularization. In cases of such acuity, typically single-vessel reconstruction (SMA) is all that is required. While there are multiple options for graft orientation and conduit, a retrograde bypass off the right common iliac artery oriented in a “lazy-C” configuration is most often favored (Fig. 91-8). The lateral portion of SMA is exposed cephalad to the fourth portion of the duodenum. This exposure requires opening of the peritoneum adjacent to the duodenum with full exposure of the terminal aorta and iliac arteries. Synthetic graft (i.e., 6- or 8-mm externally supported polytetrafluoroethylene) is typically favored over autogenous conduit given its favorable size match, ready availability, and resistance to kinking. Saphenous vein graft may be considered for cases of gross enteric contamination. The terminal aorta or left common iliac artery may also be used for inflow. Antegrade bypass off the supraceliac aorta may be considered in cases of prohibitive anatomy, but this exposure adds additional technical complexity and operative time combined with the physiologic stress of aortic cross-clamping.
Figure 91-6. SMA exposure at the base of the colonic mesentery (SMA encircled proximal to the middle colic artery).
Figure 91-7. Illustration of balloon SMA thromboembolectomy through a transverse arteriotomy.
Figure 91-8. Retrograde bypass off the right common iliac artery oriented in a ‘lazy-C’ configuration.
While surgery has remained the standard of care for AMI over previous decades, a hybrid procedure for SMA thrombosis has become widely embraced for its efficiency and decreased invasiveness for mesenteric revascularization. This approach negates the need for aortic cross-clamping, prosthetic conduit, and difficulty with arterial quality. Retrograde open mesenteric stenting (ROMS) was first described by Milner et al. in 2006 and further popularized by Wyers et al. in a 2007 case series.21,22 The infracolic SMA is exposed at the base of the transverse colonic mesentery as previously described for retrograde cannulation following local patch angioplasty with vein or bovine pericardium. This hybrid approach allows for simultaneous assessment and treatment of nonviable intestine with mesenteric revascularization by way of SMA stenting. The largest and most contemporary series on ROMS reports a technical success rate of 93% and primary-assisted patency rates of 91% at 12 months.23
Regardless of the approach to successful revascularization, intestinal viability must be assessed; this decision is typically deferred for 20 to 30 minutes following reperfusion and aided by Doppler interrogation of the mesenteric arcade, observation of serosal color, and the presence of peristalsis. Adjuncts also include the use of fluorescein with a modified Wood’s lamp or perfusion fluorometer.6 Nonviable bowel is resected and in cases of unclear viability the intestine may be left in discontinuity, the abdomen left open, and plans for a second-look laparotomy at 24 to 48 hours secured for definitive intestinal management.
Exclusive endovascular treatments for AMI are increasingly reported despite historic concerns that this approach increases time to revascularization, prohibits assessment of bowel viability, and that endovascular failure might delay traditional revascularization thereby worsening patient outcomes. Successful cases of percutaneous mechanical thrombectomy, aspiration thrombectomy, and intra-arterial thrombolysis, with or without adjuvant angioplasty and stenting, have been reported in patients without evidence of advanced bowel ischemia.24–29 The largest single center experience with endovascular therapy for AMI reported a success rate of 87%, with failures most often requiring surgical embolectomy.30 These authors compared outcomes following open and endovascular therapies for AMI and identified a reduction in need for laparotomy, length of bowel resected, and morbidity including acute renal failure and pulmonary failure in the endovascular group. Additionally, successful endovascular therapy was associated with improved mortality (36% vs. 50%) and those endovascular failures succumbed to similar mortality rates as the open surgical group. Most recently a National Inpatient Sample (NIS) review identified a significant increase in the utilization of endovascular therapy for AMI and reported that mortality, hospital length of stay, need for bowel resection, and need for postoperative parenteral nutrition were reduced in those patients receiving endovascular therapy in comparison to traditional open surgical revascularization.31
Nonatherosclerotic Mesenteric Arterial Occlusive Disease
Intestinal ischemia may result from iatrogenic injury (i.e., arterial dissection or thromboembolism during endovascular techniques), aortic dissection resulting in visceral malperfusion, trauma, inflammatory arteritides, and fibromuscular dysplasia (FMD). Indeed, these patients warrant an individualized approach to treatment. In cases of aortic dissection branch revascularization or aortic fenestration is essential. Traumatic injuries often require urgent surgical repair or revascularization. Inflammatory arteritides are typically managed medically (i.e., immunosuppression) with surgical bowel resection reserved for nonviability.
Mesenteric Vein Thrombosis
Splanchnic vein thrombosis is defined by thrombosis of the portal venous system which includes the superior mesenteric, inferior mesenteric, splenic, and portal veins. The sequelae of such may include bowel or splenic infarction and chronic portal hypertension. MVT is a less common form of intestinal ischemia that may present in a more subacute fashion. While AMI symptoms are similar to those aforementioned, patients with MVT may also complain of days to weeks of a prodrome that includes crampy abdominal pain, distension, nausea, and malaise. Whether thrombosis originates in the small or large splanchnic veins, intestinal infarction requires the involvement of venous arcades and vasa recta resulting in occlusion of venous return, venous engorgement of the bowel wall, cyanosis, and mucosal ischemia that can progress to transmural infarction.32 Idiopathic MVT represents a minority of cases as more than 50% of patients have at least one predisposing risk factor. Risk factors for MVT include heritable and acquired thrombophilia, hypercoagulable states resulting from systemic disorders (including malignancy and heparin-induced thrombocytopenia), and local intra-abdominal processes (i.e., pancreatitis or trauma). A high index of suspicion is necessary for diagnosis given the nonspecific nature of presenting symptoms, signs, and laboratory studies.32 CT imaging with portal phase venous contrast (CT venogram) is the imaging modality of choice. Findings suggestive of MVT include a sharply defined, enhancing venous wall with central low attenuation.32,33 MRA, angiography, and laparoscopy are useful adjuncts. Emergent laparotomy should be considered for signs of advanced intestinal ischemia; intraoperatively blood-tinged ascites is likely to be encountered and the bowel will appear dusky, thick, and rubbery. Systemic anticoagulation is the mainstay of treatment as it reduces mortality, decreases recurrent thrombosis rates from approximately 30% to <5%, and decreases mortality associated with recurrent thrombosis.32,34–37 Additionally, with anticoagulation most patients will demonstrate partial or complete venous recanalization with time. In the absence of a defined risk factor, patients with MVT should undergo a work-up for thrombophilia. A finite duration of anticoagulation is recommended for those with a reversible provoking risk factor with cessation of anticoagulation encouraged after 3 months; remaining cases of idiopathic MVT or persistent risk warrant indefinite anticoagulation.38 Historically open venous thrombectomy was employed for advanced cases of MVT at the time of laparotomy. More recently various systemic and percutaneous thrombolytic techniques have been applied to cases of large vessel disease; however, no large or well-controlled trials exist to guide recommendations for such over anticoagulation alone.32,39–42
Nonocclusive Mesenteric Ischemia
NOMI results from diffuse mesenteric vasospasm. This rare form of intestinal ischemia most often affects critically ill patients with hemodynamic instability. Risk factors include decreased cardiac output (i.e., heart failure or arrhythmia), hemodialysis, shock, vasoactive medications (i.e., vasopressors or digitalis), and drug abuse (i.e., cocaine). Angiography is required for diagnosis and will demonstrate diffuse vasoconstriction and nonopacification of branch vessels (Fig. 91-9). Treatment centers on improving mesenteric perfusion by optimizing volume status, limiting vasoactive medications and correcting contributing comorbidities. Intra-arterial infusion of vasodilators (i.e., papaverine or nitroglycerin) or IV infusion of glucagon or prostaglandin E2, which selectively increases splanchnic blood flow, have been advocated in treating this form of intestinal ischemia without robust large or well-controlled trials to support treatment guidelines.43–45
Figure 91-9. Nonocclusive mesenteric ischemia – mesenteric arteriogram revealing diffuse vasoconstriction and nonopacification of SMA branch vessels.
CHRONIC MESENTERIC ISCHEMIA
Symptoms of CMI are often referred to as “intestinal angina.” Ostial stenosis secondary to arteriosclerosis is the most common etiology and patients typically share risk factors of overt atherosclerosis and long-standing tobacco abuse. Additional causes of CMI may include arterial FMD, vasculitis, aortic coarctation, and connective tissue disorders. In light of aforementioned mesenteric collaterals, the responsible anatomic lesions of CMI typically affect at least two of three splanchnic arteries, although isolated lesions of the SMA distal to the middle colic branch may yield intestinal angina by excluding collateralization.2 Patients classically present with postprandial abdominal pain that occurs within 30 minutes of eating and may last up to 4 hours postprandial. Consistent postprandial symptoms typically result in the modification of eating habits that may range from the consumption of smaller meals to “food phobia” that subsequently results in weight loss and malnutrition.
5 The progression from minor symptoms to AMI and intestinal infarction is unpredictable; almost half of patients presenting with AMI describe previous symptoms of CMI.46 The incidence of CMI also remains unclear. Population studies suggest that up to 18% of individuals over 65 years of age have asymptomatic radiographic mesenteric stenosis.47 Duplex ultrasonography should be considered the first-line diagnostic modality for CMI. Specifically, high-grade stenosis of the SMA and celiac artery in a fasting state are suggested by a peak systolic velocity of >275 cm/s and 200 cm/s, respectively.48 CTA and MRA support the diagnosis of CMI while offering additional vascular anatomic evaluation such as the presence of aortic aneurysm, calcific burden, and venous anomalies that may impact open reconstruction. Aortography with anteroposterior and lateral views has been commonly cited as the gold standard for diagnosis and allows for simultaneous mesenteric arteriography, manometry, provocative measures (i.e., vasodilator infusion), and intervention (i.e., angioplasty and stenting) as appropriate. This modality is often favored over cross-sectional imaging when an endovascular approach for treatment is being considered over open revascularization. Often patients will present having undergone extensive gastrointestinal work-up (i.e., contrast motility studies, endoscopy, and laparotomy) before the diagnosis of CMI is considered.
MANAGEMENT OF CMI
Treatment goals in the management of CMI include symptomatic relief, restoration of normal weight, and the prevention of bowel infarction. Mesenteric revascularization provides immediate symptom relief in most patients. Classically, open surgical revascularization options consider patient cardiopulmonary reserve to tolerate aortic cross-clamping and aortic and arterial calcific burden to dictate vascular sites for clamping and sewing. Elective revascularization in the lower-risk patient favors two-vessel (i.e., celiac and SMA) antegrade reconstruction when anatomically feasible. Contingent on vessel calcification, the supraceliac aorta through a transperitoneal approach serves as a preferential target for inflow. After entering the lesser sac, division of the left triangular ligament allows for retraction of the left hepatic lobe. With the esophagus retracted toward the patient’s left (confirmed by palpation of the existing nasogastric tube), transection of the diaphragmatic crura exposes the supraceliac aorta. The celiac trunk, common hepatic artery, and splenic artery are exposed at this time. Subsequently with retraction of the transverse mesocolon cephalad, the SMA is exposed as described above at the root of the mesentery. A retropancreatic tunnel is created that courses anterior to the renal vein. The patient is systemically anticoagulated with heparin and the proximal anastomosis can be constructed with partial aortic occlusion. Most often a knitted polyester synthetic graft is utilized for conduit. There are multiple acceptable variations that include the use of two separate aortomesenteric bypass grafts (Fig. 91-10) or the use of a bifurcated graft. It is conventional to utilize a prefabricated bifurcated graft (e.g., 14 × 7 mm) for reconstruction. In this case a short main body is beveled to create the proximal end-to-side aortic anastomosis; the left limb is tunneled through the retropancreatic tunnel and an end-to-side distal anastomosis created next to the SMA; the right limb is subsequently cut to size and beveled to create an end-to-end anastomosis to the celiac artery directly or an end-to-side anastomosis with the common hepatic artery. These authors favor the approach presented in Figure 91-11. A limb of a bifurcated knitted polyester graft is transected off the main body in such a way that there is a phalange created. This limb is specifically utilized for creation of the aorto-SMA bypass, with the phalange incorporated into the proximal aortic anastomosis as shown. The remaining limb is cut to size and beveled accordingly; the proximal anastomosis is constructed end-to-side off the aorto-SMA bypass graft and the distal anastomosis constructed end-to-side to the common hepatic artery. This facilitates a very short celiac limb with a natural lie that may reduce the risk of limb kinking or torsion. Finally, the descending thoracic aorta may serve as an alternative source for inflow when supraceliac aortic anatomy is prohibitive or has previously been utilized for revascularization (Fig. 91-12). For those patients with comorbidity or anatomy (i.e., calcification) that precludes aortic clamping, a retrograde ilio-SMA bypass is favored as described above (Fig. 91-9).
Transaortic endarterectomy is less often utilized in contemporary practice, but may be applicable for those patients with bowel perforation or contamination, hostile abdominal conditions (extensive adhesive disease or abdominal wall hernia), and bulky coral reef aortomesenteric plaque with lower extremity ischemia. A retroperitoneal approach to the perivisceral aorta through a thoracoabdominal incision permits a longitudinal or trapdoor aortotomy, endarterectomy and repair with primary closure or patch aortoplasty as illustrated in Figure 91-13.
Figure 91-11. Two-vessel antegrade aortomesenteric bypass based off the supraceliac aorta using a presewn bifurcated graft.
Figure 91-12. Two-vessel antegrade aortomesenteric bypass based off the supraceliac aorta using two separate grafts.
While historically criticized for high mortality rates (up to 15%), contemporary outcomes following open mesenteric bypass at high-volume centers support mortality rates <4%, likely secondary to technical refinements, improved patient selection and advances in medical, anesthetic, and critical care management.49–56 Open mesenteric revascularization boasts excellent symptom improvement (77% to 100%) with low recurrence rates at 3 to 5 years (0% to 32%) and perioperative morbidity rates approximating 35% to 40%.56,57
Figure 91-13. Endarterectomy of the aorta, celiac and superior mesenteric arteries through a longitudinal trapdoor aortotomy with primary closure.
Endovascular mesenteric revascularization has increasingly become utilized for chronic mesenteric ischemia. Stenting of the mesenteric vessels is recommended over angioplasty alone for favorable anatomy that includes short segment SMA stenosis (<2 cm) with minimal to moderate calcification or thrombus, although some authors support a role for stenting in the treatment of higher-risk and occluded vessels.58,59 Endovascular revascularization offers success rates approaching 100% with immediate symptom relief. While perioperative morbidity is notably lower in comparison to open surgical revascularization, endovascular interventions are more likely to result in restenosis, recurrent symptoms, and require reintervention.53,60–62 Primary and secondary patency rates approximate 40% and 88%, respectively at 5 years (in comparison to 88% and 97% following open revascularization) emphasizing the importance of patient counseling preoperatively and close surveillance postprocedure.56 Two-vessel (celiac and SMA) stenting does not appear to reduce the risk of recurrent symptoms or reintervention when compared to single-vessel (SMA) stenting and moreover, isolated celiac artery stenting carries a higher risk of recurrence.63 Covered stents appear to be associated with less restenosis, recurrence, and reintervention in comparison to bare metal stents.64
A large review of mesenteric revascularization from the Nationwide Inpatient Sample between 1988 and 2006 demonstrated increased overall revascularization procedures for CMI.65 Additionally, these authors noted that mesenteric angioplasty and stenting not only surpassed open revascularization procedures for CMI in 2002, endovascular interventions more than doubled open procedures by 2005. While endovascular interventions may be associated with decreased mortality, shorter hospital length of stay, and decreased perioperative morbidity, the data are clouded by both the expansion of treatment indications and the fact that a greater risk of restenosis and recurrent symptoms may increase overall interventions. Moreover, these reinterventions for recurrence likely carry a variable risk for in-hospital morbidity and mortality. Regardless, mesenteric stenting has become the first-line therapy for most patients with atherosclerotic CMI and maintains an important role as a temporizing measure to facilitate weight gain in the high-risk malnourished patients (as a bridge to open revascularization) and for those patients with medical comorbidities that would further complicate or prohibit open surgical revascularization.
Median Arcuate Ligament Syndrome
Median arcuate ligament syndrome (MALS) remains a controversial diagnosis, first described in the 1960s.66 Chronic abdominal pain is associated with radiographic evidence of celiac artery compression by the overlying fibrous arch of the MAL and adjacent celiac ganglion. The pathophysiology remains poorly understood and evidence suggests both ischemic and neurogenic etiologies for pain.67 Gastric tonometry has documented evidence of mucosal ischemia in patients prior to treatment.68 The 7% to 21% incidence of celiac artery compression in asymptomatic patients combined with the typical robust mesenteric collaterals and widely patent superior and inferior mesenteric arteries present in these younger individuals favor a neurogenic etiology.69,70 Specifically, compression of the celiac plexus somatic nerves by the MAL may produce abdominal pain that is improved with a celiac ganglionectomy that accompanies ligament release or percutaneous celiac plexus block.
Patients with MALS are typically females in their third decade of life. Symptoms most commonly include postprandial pain, weight loss, and less commonly nausea and diarrhea. Diagnosis is supported by duplex ultrasonography that demonstrates elevated peak systolic velocities through the celiac artery on expiration that normalize or improve with inspiration and reverse flow through the hepatic artery.71 Cross-sectional imaging like CTA and MRA can support the diagnosis as well, and may reveal poststenotic dilation of the celiac artery. Angiography demonstrating dynamic compression or occlusion during peak expiration remains the gold standard for the diagnosis of MALS (Fig. 91-14). Additionally, as MALS remains a diagnosis of exclusion, some authors favor gastric exercise tonometry test and celiac ganglion block to support patient selection.72
Release of the MAL should include complete dissection and skeletonization of both the MAL and surrounding nerve ganglion. This can be accomplished through an open approach whereby the aortic hiatus and celiac artery are exposed through the lesser sac. Additionally, laparoscopic and more recently robotic techniques have been described. The most contemporary literature review and large series suggest immediate postoperative symptoms relief in approximately 80% to 90% of patients.67,68,73–78 Adjunctive procedures for celiac artery revascularization are utilized in a minority of cases and may include celiac angioplasty, celiac artery patch angioplasty, and celiac artery bypass (i.e., aortoceliac bypass). Late symptom recurrence approximates 6% and only Reilly and colleagues have demonstrated a reduction in recurrence following MAL release with celiac artery revascularization in comparison to MAL release alone (24% vs. 44%).67,79 Multiple authors have highlighted the importance of patient selection for treatment suggesting that specific patient criteria may correlate with successful outcomes including: consistent postprandial abdominal pain, young age, female gender, weight loss >20 lb and anatomic features of poststenotic dilation or increased collateral flow.79–81 A reasonable approach may reserve adjunctive revascularization for those patients with objective angiographic pressure or ultrasound velocity gradients following open surgical MAL decompression or those with persistent symptoms following laparoscopic or robotic release.
Figure 91-14. Lateral aortography demonstrating (A) external compression (arrow) of the celiac artery on peak inspiration that (B) worsens dynamically (arrow) during peak expiration.
Laparoscopic release of the MAL is increasingly employed for MALS.67,73,77,78 While no procedure-related mortality has been reported for open or laparoscopic ligament release, there is a 9.1% incidence across series of open conversion for hemorrhage that follows laparoscopic release, which heightens morbidity and mortality risk.67 Certainly independent operator learning curve, institutional technical enhancements, and patient selection may strongly affect this result. More recently, safe and effective robotic-assisted MAL release has been reported to offer benefits over laparoscopic release in light of improved visualization, extra degree of motion provided by robotic arms with “wrist movement” and scaled operator movements (including tremor elimination).82
SPLANCHNIC ARTERY ANEURYSMS
Splanchnic artery aneurysms are historically cited as rare and mainly asymptomatic. In the past, most were syphilitic or mycotic, while contemporary visceral aneurysms are largely true aneurysms caused by atherosclerosis or medial degeneration.83 True aneurysms are defined by the involvement of all three arterial wall layers (intima, media, and adventitia) while false aneurysms, or pseudoaneurysms, represent a collection of extravasating blood outside of the artery that remains confined next to the vessel by surrounding tissue. The incidence of splanchnic artery aneurysms is estimated at 0.1% to 2% and autopsy series suggest a prevalence that may approach 10%.84,85 A minority of aneurysms present with life-threatening rupture and while historic-associated mortality rates approached 25% to 70%; more contemporary series suggest mortality rates of 15%, possibly reflecting improvements in perioperative management and the increasing use of endovascular techniques.86 Most splanchnic aneurysms are asymptomatic however, and diagnosed as incidental findings by cross-sectional imaging.87 False aneurysms may carry a higher rate of rupture in comparison to true aneurysms (76% vs. 3% in once series).88
The natural history of these aneurysms remains elusive in light of their rarity and anecdotal, mainly institutional and retrospective, evidence to date. Indications for intervention remain conservative, especially as no significant difference has been identified between the diameters of ruptured and nonruptured aneurysms.88 Additionally, aneurysm calcification and intraluminal thrombus are not associated with rupture risk. Treatment is recommended for symptoms and size >2 cm or aneurysm diameter three times greater than the respective normal artery. Patients with splanchnic aneurysms generally have 10-year survival rates >80% and open surgical treatment provides excellent, durable long-term results with low perioperative mortality and morbidity following elective surgery.86,89,90 While surgical repair has remained the mainstay of treatment, minimally invasive endovascular techniques continue to evolve and are increasingly applied for both elective treatment and that of rupture. Endovascular intervention with local anesthesia offers excellent technical success, negligible morbidity and mortality in elective cases, and brief hospital stays.86,88,90–94 Transcatheter embolization of visceral artery aneurysms offers additional advantages that include precise aneurysm localization, assessment of collateral flow, and negate the need for certain (i.e., intrahepatic) difficult arterial exposures.83 Notably, embolization has become the standard of care for splanchnic artery pseudoaneurysm.92,95 Additionally, endovascular therapy may offer decreased postoperative morbidity and mortality rates in cases of rupture when compared to open surgery (2.7% to 7% vs. 24% to 29%, respectively).86,91
6 Distribution of splanchnic artery aneurysms is as follows: Splenic artery (60%) > hepatic arteries (20%) > superior mesenteric artery (SMA) (5.5%) > celiac artery (4%) and gastric and gastroepiploic arteries (4%) > ileocolic arteries (3%) > pancreaticoduodenal arteries (2%) > GDA (1.5%) > IMA (<1%).84,87 Most patients present in the sixth decade of life.87,88 Symptomatic patients will present with abdominal pain and frank hemorrhagic shock with rupture. Rupture may be intra-abdominal, although erosion into a gastrointestinal lumen may result in sporadic gastrointestinal bleeding (i.e., herald bleed) and erosion into adjacent mesenteric veins results in arteriovenous fistulae. The asymptomatic patient inconsistently reveals a bruit on auscultation and rarely a palpable mass on examination. Each splanchnic aneurysm warrants individual consideration.
Splenic Artery Aneurysm
Splenic artery aneurysms account for 60% of all splanchnic artery aneurysms and are the third most common intra-abdominal arterial aneurysm. Their prevalence in the general population is very low approximating 0.16% to 0.8%.96 These aneurysms occur two to four times more frequently in women than men and are associated with multiparity.83,87,90,96,97 Hormonal and local hemodynamic events likely contribute to the evolution of splenic aneurysms. An etiologic classification system has previously been proposed by Stanley and Fry that implicates arterial dysplasia, portal hypertension, local inflammation, and hormonal and hemodynamic events in the pathogenesis of these aneurysms.96 Hypertension is a common comorbidity affecting 50% to 60% of patients with splenic aneurysm, and up to 20% of patients will demonstrate concurrent aneurysms affecting alternate arterial beds.94,95,98 Additional etiologic risk factors for true aneurysms may include dissection, septic emboli, polyarteritis nodosa, systemic lupus erythematosus, and connective tissue disorders like Ehlers–Danlos syndrome, while splenic pseudoaneurysms are typically attributed to chronic pancreatitis and trauma.87 Splenic aneurysms most often arise at branch points off the mid- and distal main splenic artery. Tortuosity of the main splenic artery is common (Fig. 91-15). The majority (80%) of splenic aneurysms are solitary and saccular, although 90% of patients with portal hypertension will demonstrate multiple splenic aneurysms. Those aneurysms measuring >5 cm are often described as “giant lesions” and appear to be more common in older males.99
Splenic aneurysms typically rupture into the lesser sac with limited self-containment. Later hemorrhage into the peritoneal cavity results in cardiovascular collapse and the so-called “double rupture.” The incidence of splenic aneurysm rupture approximates 2% to 5% and is not associated with calcification, blood pressure, or age.84,87,95,96 Increased levels of aneurysmal peripheral calcification are associated with smaller aneurysm size at the time of diagnosis, but appear to have no impact on aneurysm growth.95 Growth rates among surveyed aneurysms (out to 3.1 years) have been calculated as 0.2 mm/yr.95 Rupture-associated mortality is dramatically increased in the peripartum patient (68% to 75% vs. 25%), and associated fetal mortality approaches 100%.96,98,100 Rupture in this setting occurs most commonly during the last trimester.83 While historically a strong association between pregnancy and splenic aneurysm rupture was reported, a contemporary series reporting on >67,000 consecutive live births over 5 years identified no cases of splenic artery aneurysm rupture.98 There is no apparent role for routine screening for splenic aneurysm in the pregnant patient.101 Rupture risk is increased two fold by liver transplantation, the mortality following which is 50%.102,103
Figure 91-15. (A) CT angiogram and (B) splenic arteriogram revealing a tortuous splenic artery with two large aneurysms affecting the main splenic artery; the distal aneurysm (arrow) has extensive calcification and mural thrombus.
Indications for treatment include rupture, symptoms, size >2.0 to 2.5 cm and any size asymptomatic aneurysm at the time of liver transplantation or in women of childbearing age.87,95,104,105 Historically, the open surgical treatment of splenic artery aneurysms included aneurysm resection or splenectomy. Recognizing the immunologic benefits of splenic preservation, arterial ligation, and aneurysm excision is preferred when possible; however, those patients with multiple and perihilar aneurysms may still require splenectomy. Laparoscopic and robotic techniques at ligation, aneurysm resection and splenectomy have become common-place offering negligible risk of open conversion, need for reoperation and mortality.106–108 Splenic vaccinations and education regarding the need for booster vaccinations at regular 5-year intervals should be provided. Perioperative morbidity and mortality are estimated as 9% to 25% and 1% to 5%, respectively, across series.95,96,109–111
More recently, endovascular therapy has been utilized with increasing frequency for both elective and ruptured cases boasting decreased early morbidity, mortality, and hospital length of stay. While only 2% of all splenic aneurysms were managed with endovascular therapies in 1999, this proportion increased to 70% between the years 2010 and 2013.111 Embolization with coils, gelfoam, glue, thrombin, and vascular plugs has been advocated for saccular aneurysms with narrow necks (Fig. 91-16). Larger vessels with complex wide-neck aneurysms may be better approached with covered stents or stent-assisted coil embolization.112 Technical success for endovascular treatment is 93% to 98%, limited primarily by vessel tortuosity that may be overcome with newer, steerable catheters.95,111,113,114 Conversion to open surgery is rarely necessary (<2%) to accomplish successful aneurysm exclusion.111 Postembolization syndrome (PES) complicates 10% to 30% of cases across series and may include left upper quadrant pain, fever, ileus, platelet dysfunction, leukocytosis, and pancreatitis following embolization with and without radiographic evidence of splenic infarction.95,111,114,115 Up to 21% of patients will demonstrate major splenic infarct and may require reintervention for splenic abscess by way of percutaneous drainage or splenectomy.95,113 Portal hypertension appears to increase dramatically the risk of major splenic infarction.113 With late follow-up, endovascular repair is associated with an average regression in aneurysm size of 1.5 mm/yr.95 Late complications may arise at an average rate of 3.7% per year, and there is a 3.2% rate of reintervention per year following endovascular repair.111
Figure 91-16. (A) Celiac arteriogram of a 78-year-old woman with a 4-cm splenic artery aneurysm; (B) Splenic arteriogram demonstrating a catheter in the proximal splenic artery. Vascular occlusion devices (arrows) have been positioned in the main splenic artery outflow (distal to the aneurysm) and inflow (proximal to the aneurysm) with multiple vascular coils embedded within the aneurysm sac.