The pathology of SM is usually limited to the mesentery of the small intestine and involves the root or a segment of the mesentery [15, 42]. The lesions are described as yellow/gray and hard with gritty consistency [15]. Typically, the lesions appear as diffuse thickening of the mesentery or as a single rubbery nodular mass or multiple masses [15, 42]. In addition, extensive scarring and shortening of the mesentery in addition to thickening may be observed [15, 42]. In rare cases, the omentum, mesoappendix, mesocolon, and large bowel mesentery may be involved [13, 15, 42]. The inflammatory process may extend to the retroperitoneum and involve the inferior vena cava, pancreas, duodenum, hepatic peduncle and bladder [1, 15, 42, 51].

The microscopic picture is that of a mild to moderate infiltration of the fat by macrophages with an abundant foamy cytoplasm [13, 15, 42]. The macrophages are distributed in thin and broad interconnecting bands. Focal collections of lymphocytes are seen, usually adjacent to small vessels and frequently without follicle formation with fewer plasma cells and scattered eosinophils [13, 15, 42]. Focal or multifocal venulitis and (obliterative) phlebitis may be observed, predominantly affecting small- to medium-sized venous channels and in rare cases large veins [6]. Polymorphonuclear leukocytes are uncommon. The mesenteric inflammatory process may progress to include necrosis, fibrosis and calcification [13, 15, 42]. A zone of lipid-laden macrophages oriented about a central lymphoid aggregate or lymph node with an interposed zone of normal fat may be seen, the so-called halo-effect [9]. In patients with cavitation, areas of amorph material containing cholesterol were present [13]. Depending on the predominant histologic appearance, it was thought that SM presented in three distinct and sequential histologic stages with accordingly appropriate and different naming of the disease: (1) the presence of lipid-laden foamy macrophages infiltrating the mesenteric fat, mesenteric lipodystrophy; (2) predominant chronic inflammatory infiltrate, mesenteric panniculitis; and (3) prominent fibrosis with scant inflammation and fat necrosis, retractile mesenteritis/mesenteric fibrosis/sclerosing mesenteritis [1, 13, 15, 42]. However, the diagnostic groups all share the presence of fibrosis, chronic inflammation and fat necrosis and, in addition, have common demographic and clinical characteristics [15]. Upon statistical analysis of the three major histologic components, the amount of fibrosis was found to be the main feature of the different stages. Hence, the term “sclerosing mesenteritis” was proposed as the most accurate naming of the disease in the majority of cases [15].

Immunohistochemical staining typically shows a mixed population of CD3-positive T cells and CD19/CD20-positive B cells. Keratin, S-100, bcl-2, CD117/c-kit immunostain and T-cell receptor gene rearrangement studies are all negative [1]. MDM-2 immunohistochemistry may differentiate SM from well-differentiated liposarcoma, negative MDM-2 immunoexpression essentially ruling out the latter [62]. The connection between SM and IgG4-RD has not been studied extensively [1, 4, 6, 38, 40]. Abundant tissue infiltration of IgG4-positive plasma cells was observed in 4 of 12 SM cases (33 %) [1]. In a pathologic study of tissue material from 9 SM patients, IgG4-reactive plasma cells ranged in number from 0 to >100 per hpf, in 4 cases IgG4-positivity of plasma cells being between 11 and 20 per hpf and in 2 cases >100 per hpf [6]. The ratio of IgG4-positive/IgG-positive plasma cells varied from 3 to >100 per hpf in 6 cases studied, in 3 of these 6 cases (50 %) being ≥60/hpf [6]. In a 82-year-old woman with SM [40], microscopic examination showed abundant stromal fibrosis and obstructive phlebitis. Numerous IgG4-positive plasma cells were observed with a IgG4/IgG ratio of 76 %. IgG4 serum level, examined post-surgery, was high. In another case report of a 53-year-old man with extensive SM, storiform fibrosis, obliterative phlebitis and infiltration of many IgG4-positive plasma cells was observed [38]. The IgG4/IgG ratio amounted to 64 %, and the ratio of forkhead box protein 3 (Foxp3)-positive/CD4-positive cells was elevated (13 %) [38]. Foxp3 + cells are typically observed in auto-immune pancreatitis (AIP), the most prominent manifestation of IgG4-RD and are a good marker of CD4 + CD25 + regulatory T cells, which are thought to participate in the pathogenesis of the IgG4 reaction in AIP [64, 65]. These combined findings suggest that in a subset of patients, SM may be a manifestation of IgG4-RD.

Most cases of SM are asymptomatic and incidentally detected on abdominal CT examination. CT features vary from subtle increased attenuation (attenuation values of −40 to −60 Hounsfield Units [HU]) of the mesentery to a more solitary well-defined soft tissue mass at the root of the small bowel mesentery [7, 10, 24, 29]. There is engulfment of superior mesenteric vessels and displacement of the bowel loops without infiltration (Fig. 11.1a,b). Small lymph nodules (short axis <10 mm) are often seen within the region of mesenteric fat stranding. Typically, the mesenteric vessels and soft tissue nodes show a “fat-ring” sign (Fig. 11.1c), referring to preservation of fat nearest to the mesenteric vessels and nodes [24, 56, 61]. A “tumoral pseudocapsule” (Fig. 11.1d), a dense stripe in the peripheral region differentiating normal mesentery from the inflammatory process is also suggestive for SM [10, 48]. Although uncommon, calcifications may be seen [23, 34, 59]. Table 11.2 shows the prevalence of the main CT features of SM in two large studies [10, 59].

Ultrasound may reveal a well-defined homogeneous hyperechoic (fatty) mass at the mesenteric root with in most cases a clear interface between the normal fat and the inflammatory fat in SM [46]. Ultrasound findings may be subtle, easily missed, and findings are nonspecific and may be seen in other conditions involving the mesentery [58]. Magnetic resonance imaging (MRI) findings are similar to the CT features. On MRI, a mesenteric mass is seen with intermediate signal intensity on T1-weighted images and with slightly higher signal intensity on T2-weighted images [27]. ¹⁸Fluorodeoxyglucose-(FDG) positron emission tomography (PET) has been proven useful mainly for the differentiation between SM (not FDG-avid) and malignant mesenteric involvement (FDG-avid), especially in patients with tumoral or lymphomatous involvement of the mesentery. A negative PET scan has a high diagnostic accuracy in excluding lymphomatous or tumoral involvement of the mesentery [66].