The entire gastrointestinal tract is derived from a common dorsal mesentery. During development, as the mesentery fuses with the retroperitoneum, the remaining segments become the small bowel mesentery, transverse mesocolon, and sigmoid mesocolon. These mesenteries serve as the primary pathways to and from the bowel for arterial, venous, lymphatic, and neural structures.

The small bowel mesentery originates as the root of the mesentery, located at the fourth part of the duodenum and posterior border of the pancreas. It courses along the medial border of the jejunum and ileum in a fan-like projection. The transverse mesocolon connects the transverse colon to the retroperitoneum, and the sigmoid mesocolon connects the sigmoid flexure to the inferior retroperitoneum and pelvis.

Acute Mesenteric Lymphadenitis

Acute mesenteric lymphadenitis (AML) is the marked focal inflammation of mesenteric lymph nodes and has been associated with viral illness and other infectious processes.^1,2 AML most commonly occurs in children, with an equal distribution in males and females. The most common presenting symptoms are abdominal pain, mild fever, nausea, and occasionally vomiting. Both laboratory and systemic symptoms are less severe in comparison to those seen with acute appendicitis.³

Given the young age at presentation, most children undergo ultrasound rather than computed tomography (CT) for diagnosis. Diagnostic criteria by either ultrasound or CT include clusters of enlarged (>5-8 mm) hypervascular mesenteric lymph nodes with a normal appendix.^1,3 While imaging findings are often suggestive, true diagnosis is often made at the time of negative appendectomy, when mesenteric lymphadenopathy is noted.^1,4

When AML is suspected, one can proceed with supportive care, as the disease is self-limiting and typically resolves without invasive measures.^1,3,4 Given that the inciting agent is typically viral, there is no indication for antimicrobial agents.

Mesenteric Panniculitis/Sclerosing Mesenteritis

Mesenteric panniculitis was first described in 1927 by Jura as an infiltration of plasma and polymorphonucleated cells into the mesentery.⁵ The condition, now more vigorously investigated, has many names throughout the literature including sclerosing mesenteritis, retractile mesenteritis, and mesenteric lipodystrophy. The disease process of marked mesenteric inflammation and fibrosis more commonly affects the small bowel mesentery and can be asymptomatic or present as diffuse, nonspecific abdominal pain.^6-8 Most cases present in the sixth decade, with a slight male predominance.^8,9 The etiology is not well understood and has been related to various sources including autoimmune, infectious, ischemic, and traumatic causes.^8-10 In one study, 35% of patients with sclerosing mesenteritis were noted to have a history of abdominal surgery, and therefore, surgical trauma may be a predisposing factor.⁶

The inflammation and fibrosis originate at the root of the mesentery and proceed to involve varying amounts of the mesentery. Histologically, the disease is classified into 3 stages, based on the relative proportions of fatty and fibrotic changes.⁶ Mesenteric lipodystrophy has fatty predominance and is characterized by replacement of mesenteric fat by a layer of foamy macrophages. Mesenteric panniculitis, composed of both fatty and fibrotic changes, is marked by plasma and polymorphonuclear leukocytes as well as foreign body giant cells and foamy macrophages. Finally, sclerosing mesenteritis, or retractile mesenteritis, is marked by fibrotic changes with collagen deposition, fibrosis, and inflammations.^7,11

This process is most frequently diagnosed by CT scan.⁸ Findings often include an adiposidic mass, which often encases the mesenteric vessels and displaces but does not directly invade the bowel (Fig. 18-1).¹² Additional imaging with magnetic resonance imaging (MRI) may help with diagnosis, but this will not ultimately change management. Positron emission tomography (PET) imaging may be helpful in distinguishing mesenteric panniculitis from lymphoma or metastatic disease, as mesenteric panniculitis will have minimal PET avidity. While the majority of these cases can be diagnosed by imaging findings, if there is suspicion for malignancy or if surgical intervention is undertaken due to symptoms or obstruction, biopsy is warranted.

Mesenteric panniculitis is a self-limiting disease, and therefore, if it is asymptomatic, it requires no intervention. Alternatively, for the >20% of patients who do report a debilitating course due to chronic abdominal pain, intervention is primarily medical and focuses on decreasing inflammation and fibrosis with a variety of therapeutic agents including corticosteroids, cyclophosphamide, or colchicine.^7,8,10 For patients who present with symptoms of obstruction and are refractory to medical management, surgical intervention may be warranted and should focus on relieving the area of obstruction with segmental resection of the involved mesentery and potentially segmental small bowel resection. If a patient does require surgical management, this should be followed by medical interventions, because surgery alone does not address the underlying problem.

Mesenteric Cysts

Although the majority of mesenteric cysts are congenital, only 60% present during childhood, with up to 40% presenting during adulthood, most commonly in the fourth decade.^13-15 The overall incidence is estimated to be <1/100,000, and etiology varies from failure of mesenteric fusion to lymphatic malformation to trauma.¹⁰ The majority of mesenteric cysts are asymptomatic, but patients can present with either a mobile abdominal mass or pain.¹⁶ Abdominal pain typically arises secondary to rupture, torsion, or mass effect. On physical exam, palpable masses are frequently mobile in a single lateral direction (left to right or right to left), known as the Tillaux sign, and are not freely mobile, as is seen with omental cysts.

Ultrasound or CT is adequate for evaluating cystic lesions and delineating any solid components or septations.^14,15 The majority of lesions are located within the small bowel mesentery, are unilocular, and do not have a solid component. Identification of a solid component should raise suspicion for malignancy, which only occurs in approximately 3% of mesenteric cystic masses, and is typically sarcoma or rarely adenocarcinoma.¹⁵

More than 50% of the cystic mesenteric masses are lymphangiomas.¹¹ The majority of lymphangiomas present within the head, neck, or axilla, whereas small bowel mesenteric lymphangiomas account for <1% of all cases.¹⁷ Lymphangiomas are classically solitary masses that develop due to congenital malformation of the lymphatic vessels, causing failure of the lymphatic channels to drain in the lymphatic system.¹⁷

Treatment of any mesenteric cyst requires surgical excision if the patient is symptomatic or malignancy has not been ruled out, with preference for a laparoscopic approach. Cysts should be excised in total because enucleation, marsupialization, internal or external aspirations, and drainage are associated with a high risk of recurrence.^10,15

Mesenteric Tumors

MESENTERIC MESOTHELIOMA

Mesothelioma is the uncontrolled proliferation of mesothelial cells found in the serosal lining of the pleura, pericardium, peritoneal cavity, and mesentery.⁹ Mesenteric mesothelioma accounts for 25% of all mesotheliomas and 6% to 10% of malignant mesotheliomas.^18,19 Among malignant disease, diffuse malignant peritoneal mesothelioma is the second most common site, with the most common site being pleural.^19,20 Malignant mesotheliomas can be classified as epithelioid, sarcomatoid, or mixed, with epithelioid type being the most common

Mesothelioma was poorly understood until its association with asbestos was reported by Wagner in 1960.²¹ The incidence of all mesothelioma, including mesenteric, has been rising since 1970, with the rate in the United States continuing to rise through the 1990s and plateauing shortly thereafter.¹⁹ The Surveillance, Epidemiology, and End Results database estimates that approximately 250 new cases of mesenteric mesothelioma are diagnosed annually.^19,22 Among all mesothelioma, mesenteric primaries account for 11% of diagnoses in men and 45% of all diagnoses in women.²³ This sex disparity has been debated in the literature, with the predominate theory indicating that the higher rate of female mesenteric primaries is due to the association of mesenteric mesothelioma with nonoccupation exposures, which often result in a higher dose and longer duration of exposure.^22,23

Malignant mesenteric mesothelioma typically presents with abdominal pain and increased abdominal distention secondary to accumulation of both tumor and ascites¹⁹ (Fig. 18-2). CT imaging demonstrates diffuse involvement of the peritoneal surfaces, innumerable nodules within both the abdomen and pelvis, and ascites.^24,25 Local invasion into the bowel and solid viscera can occur in advanced disease, and metastases are usually to regional lymph nodes. Diagnosis requires either a CT-guided or laparoscopic biopsy, as diagnostic paracentesis has repeatedly been shown to have low diagnostic yield.¹⁹

The treatment and management of malignant mesenteric mesothelioma are not well defined. Historically, this diagnosis is associated with very poor survival, frequently less than 1 year, secondary to progression, solid organ invasion, bowel obstruction, and malnutrition. Multiple therapies have been attempted in this rare disease including surgical debulking and intraperitoneal and intravenous chemotherapy. Although there is no consensus or level 1 evidence due to the lack of randomized controlled trials for this disease, large retrospective reviews have evaluated the impact of both cytoreductive surgery and hyperthermic intraperitoneal chemotherapy (HIPEC) and advocate for a combined approach.^21,25-28 Cytoreductive surgery includes removal of all visible tumor, peritonectomy, greater omentectomy with splenectomy, lesser omentectomy with cholecystectomy, and pelvic peritonectomy with rectosigmoid colonic resection. Completeness of the surgery is graded by the size of the largest remnant nodules (0 = no residual tumor, 1 = <2.5 mm, 2 = between 2.5 mm and 2.5 cm, 3 = >2.5 cm) and is the factor most strongly associated with posttreatment survival. Intraperitoneal chemotherapy regimens vary, but most commonly include cisplatin and/or doxorubicin and can be given intraoperatively at the time of surgery or at an interval after cytoreductive surgery. Studies evaluating the combined approach of cytoreductive surgery with HIPEC suggest there is a significant survival advantage, with one of the largest reviews reporting 81%, 60%, and 47% survival rates at 1, 3, and 5 years, respectively.²⁷ Therefore, in the setting of a diagnosis of mesenteric mesothelioma, referral to a specialized center is highly recommended.

DESMOIDS

Desmoid tumors were first named by Muller in 1938.²⁹ Microscopically, they are composed of collagen, fibroblasts, and bundles of spindle-shaped cells, often with poorly circumscribed margins and no distinguishable capsule.^9,29,30 They are of mesenchymal origin and therefore can arise anywhere in the body. The most common sites include extremities, anterior abdominal wall, and mesentery. Within the United States, the annual incidence is 2.4 to 4.3 per 100,000.¹¹ Mesenteric desmoids account for only 8% of all desmoids tumors and most commonly develop in patients with familial adenomatous polyposis (FAP).^6,9,31

FAP, which is classically associated with colonic polyposis and increased risk for colonic adenocarcinoma, has multiple extracolonic manifestations, including desmoid tumors. Patients with FAP have a 20% lifetime risk of desmoid tumor formation, of which 80% are intra-abdominal, 10% to 15% occur within the abdominal wall, and 5% are extra-abdominal.²⁹ Desmoids associated with FAP are considered to be more aggressive and have a higher risk of mortality to due to local invasion, unlike desmoids of other anatomic locations that have no associated mortality.⁶

Although desmoid tumors can arise within a previous scar or incision, they may also occur sporadically. Mesenteric desmoids are more frequent after abdominal surgery and, in FAP patients, after prophylactic colectomy.²⁹ Desmoids are locally invasive tumors that may invade surrounding structures or cause mass effect but do not metastasize. Therefore, clinical presentation is often due to a palpable abdominal mass or secondary mass effect and compression of intra-abdominal structures including ureters, pelvic vessels, or the bowel.²⁹

Diagnosis is made by either CT or MRI, and need for intervention is based primarily on symptoms. Interestingly, the natural history of intra-abdominal desmoids tumors shows a unique pattern, in which approximately 10% resolve spontaneously, 30% undergo cycles of progression and resolution, 50% stabilize, and 10% progress rapidly.³² Due to this indeterminate course, many recommend serial imaging with CT to characterize an individual tumor’s propensity for growth.^29,32,33

The treatment of desmoid tumors is best handled in the context of a multidisciplinary sarcoma program because there are a number of potential interventions, including surgical, medical, and radiation therapy. In particular, for small intra-abdominal desmoids that are slow growing and/or not in close proximity to vital structures, there is no clear indication for intervention, only serial monitoring with CT or intervention for symptoms alone.^29,32 For large masses or for those causing compressive effect, a multimodal approach is recommended.^30,32,34,35 Surgical resection requires complete en bloc resection of the mass. Because desmoids often have ill-defined borders, positive margins are not uncommon.^36,37 Unlike most sarcomas, re-resection of a positive margin or performing an extensive multiorgan resection for a desmoid is not done. Because mesenteric desmoids have a high recurrence risk and the required operation for complete resection is often highly morbid, medical therapies are now considered the treatment of choice.^9,29,30,32 A number of systemic treatment options are available, all of which have variable success rates and are best handled by an experienced sarcoma medical oncologist. The potentially effective systemic treatment options vary greatly in toxicity and include nontoxic agents (eg, nonsteroidal anti-inflammatory drugs, tamoxifen, hydroxyurea), marginally toxic treatments (eg, methotrexate and vinblastine), and more toxic traditional chemotherapy (eg, doxorubicin and dacarbazine).^30,32,38 There has been recent success with the newer targeted therapies, and currently, the Desmoid Tumor Research Foundation and the National Cancer Institute are running a national trial using sorafenib based on recent success with this agent.³⁹ Although radiation therapy is often used in the treatment of extremity desmoids, it is generally avoided for intra-abdominal or mesenteric desmoids.

METASTATIC DISEASE

Primary disease of the mesentery is rare, with the 2 most common forms discussed above. However, both systemic and metastatic disease can present within the mesentery, either due to symptoms of an abdominal mass or pain or as incidental imaging findings.

Lymphoma is the most common solid neoplasm identified within the mesentery.^9,11 While very few patients present with symptoms of a palpable mass, bulky adenopathy is frequently noted. On CT imaging, mesenteric lymphoma presents as an agglomeration of homogenously enhancing lymphadenopathy that typically does not invade or obstruct the mesenteric vessels or the bowel.¹¹ The most common form of lymphoma to present with mesenteric lymphadenopathy is non-Hodgkin lymphoma.¹¹ Treatment is primarily nonsurgical, and therefore, these cases should be referred to a medical oncologist.

Metastases from other primary cancers are also common within the mesentery. Metastatic spread can occur through direct extension, intraperitoneal seeding, or hematogenous or lymphatic spread.^6,9 The most common primary tumors resulting in mesenteric metastases include ovarian, gastrointestinal, melanoma, breast, pancreatic, and bladder tumors.^6,10 Diagnosis typically includes additional imaging to identify a primary site and may require image-guided or laparoscopic biopsy for tissue diagnosis.

Furthermore, gastrointestinal primaries, including gastrointestinal stromal tumors (GISTs) and carcinoid tumors, may initially present as mesenteric disease due to local extension or invasion. These tumors should be treated as gastrointestinal primary tumors, and therefore, further discussion of diagnosis and management can be found in other chapters.

The greater omentum is a thin fibrofatty apron composed of 2 fused bilayers of peritoneum, with one bilayer originating along the greater curvature of the stomach and a second along the transverse colon. These 2 bilayers fuse and then extend inferiorly and anteriorly over the small bowel and into the pelvis and laterally to the pylorus and gastrosplenic ligament. The lesser omentum is a smaller bilayer of peritoneum that extends from the liver to the lesser curvature of the stomach, including the hepatoduodenal and hepatogastric ligaments.

The omentum has many theoretic functions and was coined the “abdominal policeman” by the British surgeon Rutherford Morison due to its ability to wall off areas of infection and limit its spread.^10,40 The propensity for this policing arises due to the high concentration of tissue factor within the omentum, which assists with activation of coagulation and fibrosis at the sites of inflammation, infection, ischemia, and trauma.¹⁰

Omental Torsion and Infarction

Omental torsion and infarction were first described by Eitel in 1899 and arise when a pedicle of omentum twists along its axis, compromising the vascular supply.⁴¹ Such torsion then progresses from vascular congestion and thrombosis to arterial occlusion and ultimately necrosis of the omentum, with associated extravasation of serosanguinous ascites.^42,43 Cases of omental torsion are rare.⁴⁰ The majority of cases occur in male adults, typically in the fourth to fifth decade.^42,44,45

Given the anatomic pattern of the omental sheet, torsions typically occur in the right or left lower quadrant and therefore may mimic the pain of other intra-abdominal pathologies including appendicitis, diverticulitis, ruptured ovarian cysts, and rarely cholecystitis. Clinical history often reveals localized pain without the presence of gastrointestinal symptoms such as nausea, vomiting, and diarrhea. Physical exam may reveal diffuse abdominal tenderness, mild peritoneal signs, and in some cases, a tender palpable mass. Approximately 50% of patients will have a low-grade fever and mild leukocytosis.⁴³

Torsion can be either primary or secondary. Primary omental torsions arise due to anatomic abnormalities including tongue-like projections, accessory omentum, or anomalous vascular supply, whereas secondary torsions are associated with hernias, cysts, scaring, tumors, or foci of intra-abdominal inflammation.^10,42,43,46 Some reports have demonstrated an increased risk associated with obesity, such that in one study >70% of patients with omental torsion were also obese.⁴⁶ This association is attributed to the increased density and thickening of the omentum acting as a lead point.

Additionally, infarction of the omentum can occur without preceding torsion. Omental infarction can be either idiopathic segmental infarction or secondary infarction. Secondary infarction occurs in the setting of systemic disease including vasculitis, hypercoagulability, or thrombi.⁴⁰ The presentation of isolated infarction or infarction secondary to torsion is similar.

Appropriate radiographic evaluation includes an abdominal and pelvic CT scan with intravenous contrast, which often demonstrates a localized inflammatory mass, with a whirl sign or concentric linear stranding.^41,43 Fat stranding in the area will be disproportionate to any adjacent bowel wall thickening, indicative of a pathologic process centered within the omentum rather than the bowel.⁴²