Introduction

Mesenteric panniculitis is an uncommon inflammatory disorder of the fatty tissue of the bowel mesentery. It has been reported under several names, such as mesenteric lipodystrophy, mesenteric Weber-Christian disease, fibrosing mesenteritis, sclerosing mesenteritis and retractile mesenteritis [1–4], resulting in considerable confusion. This varied terminology to some extent reflects the pathological spectrum of what is now considered to be one single disease [5], characterized by a chronic non-specific inflammatory process in the mesentery that in rare instances may lead to fibrosis and retraction. If inflammation predominates over fibrosis, the process is known as mesenteric panniculitis; when fibrosis and retraction predominate, the terms fibrosing mesenteritis, retractile mesenteritis or sclerosing mesenteritis are more commonly used [6].

Mesenteric panniculitis is supposed to be very rare, with approximately 250 cases reported in the literature [1]. However, due to the increased use of abdominal diagnostic imaging, mesenteric panniculitis is diagnosed more often today, with a recently reported prevalence of 0.6% of all patients undergoing an abdominal computed tomography (CT) for various indications [1].

The goal of this pictorial review is to illustrate the features of mesenteric panniculitis at ultrasonography (US) and CT examination and to discuss its imaging-based differential diagnosis, while providing background information about the disorder.

Pathogenesis

Histopathologically, mesenteric panniculitis is characterized by infiltration of the mesenteric fat by inflammatory cells, mainly consisting of lymphocytes and fat-laden macrophages (Fig. 1) [4]. Along with the inflammation, a mixture of fat necrosis and fibrosis may be present in the mesentery. The exact cause of the inflammation remains unclear, and whether mesenteric panniculitis occurs independently or in association with other disorders has been a subject of discussion [1, 2, 6, 7]. A variety of possible causative factors have been proposed, such as autoimmune disorders, ischemia and prior abdominal surgery. In addition, it has also been suggested that mesenteric panniculitis might represent a paraneoplastic response [1, 8, 9].

Fig. 1
figure 1

Microscopic section of histological specimen of mesenteric panniculitis (H and E, ×200), showing infiltration of the mesenteric adipose tissue by lymphocytes and macrophages

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This latter possible association with a concomitant malignancy has recently been highlighted in a study by Daskalogiannaki [1], reporting the presence of a coexisting abdominal or distal malignancy in 69% of patients with CT features of mesenteric panniculitis. In that study, however, a patient-selection bias seems likely, as 53% of all patients undergoing an abdominal CT in their hospital had a malignancy. In our own experience, comprising a group of 24 patients with mesenteric panniculitis, the prevalence of malignancy does not appear to be any different from the general population of patients undergoing US or CT for all various indications in our community hospitals.

Clinical characteristics

Mesenteric panniculitis mostly occurs in middle or late adulthood, showing a male predominance [1, 7]. The clinical manifestations may be related to the inflammation or to mass-effect, but the disorder may also be entirely asymptomatic [1]. Presenting symptoms may vary, but commonly include non-specific abdominal pain. At physical examination, a palpable abdominal mass may be present that can lead to the clinical misdiagnosis of a presumed aortic aneurysm because of transducted aortic pulsations [10]. Laboratory findings are often within the normal range, but may demonstrate mild leucocytosis and elevation of the erythocyte sedimentation rate. These findings are non-specific and preclude a correct clinical diagnosis, and before the advent of modern diagnostic imaging, mesenteric panniculitis was diagnosed exclusively as an unexpected finding at exploratory laparotomy or autopsy [1, 7].

The disorder itself has generally been reported as self-limiting, though symptoms and radiological abnormalities may persist up to months or years [4]. In advanced or progressive cases, treatment with immunosuppressants has been advocated; however, progression from panniculitis to fibrosis has only rarely been documented, and usually no specific treatment is required [4].

In view of the remaining uncertainty concerning the previously mentioned possible paraneoplastic etiology, it has been recommended to perform close follow-up studies in patients with mesenteric panniculitis, both clinically and with US or CT, to search for any hidden malignancy [11].

Diagnosis

A definite diagnosis of mesenteric panniculitis can be made only by pathologic analysis. However, the incidental benign and often asymptomatic nature of mesenteric panniculitis usually does not justify biopsy. In these cases, the diagnosis may be suggested by characteristic imaging features from the radiological literature [1, 3, 6, 11, 12] from pathologically proven cases.

US features

Sonographic findings in mesenteric panniculitis are often quite subtle and easily may be overlooked. The main US feature suggesting its diagnosis is poorly defined hyperechoic change of the mesenteric fat (Fig. 2), with a marked decrease in mesenteric compressibility (Fig. 3) [12]. These findings are non-specific and may also be seen in various other conditions with mesenteric involvement, such as mesenteric lipomatous tumors [13]. CT is therefore always recommended to analyze any US-found mesenteric abnormalities.

Fig. 2
figure 2

Sonographic appearance of mesenteric panniculitis. The mesenteric fat contains poorly defined hyperechoic changes (arrowheads), enveloping the mesenteric vessels

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Fig. 3
figure 3

US examination in a patient with mesenteric panniculitis a without and b with compression, showing marked decrease in compressibility of the mesenteric fat

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CT features

On CT, the hallmark of mesenteric panniculitis is increased density of the mesenteric fatty tissue (approximately−40 to −60 HU) as compared to the attenuation values of normal retroperitoneal or subcutaneous fat (−100 to −160 HU). This hyperattenuating fat surrounds the mesenteric vessels, but does not displace them (Fig. 4). The mesenteric lesion, however, may show some regional mass-effect by displacing locally small bowel loops (Fig. 5). The mass is most frequently located at the left side, corresponding to the jejunal mesentery [1]. Several additional CT features have been reported that may be valuable clues for the diagnosis of mesenteric panniculitis: the fat-ring sign, a tumoral pseudocapsule and soft-tissue nodules.

Fig. 4
figure 4

The hallmark CT finding in mesenteric panniculitis. Contrast-enhanced CT scan shows increased density of the mesenteric fat, displaying higher attenuation than normal retroperitoneal or subcutaneous fat, surrounding but not displacing the mesenteric vessels

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Fig. 5
figure 5

A 79-year-old woman with a presumed clinical misdiagnosis of an aortic aneurysm, due to transducted aortic pulsations. Contrast-enhanced CT scan shows mesenteric panniculitis with displacement of small bowel loops (arrowheads): mass-effect

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Fat-ring sign

The “fat-ring” sign, or “fatty halo,” consists of low-density fat surrounding vessels and nodules within the mesenteric mass (Fig. 6). This “halo” in fact represents preservation of normal fat density, corresponding to unaffected noninflamed fat interposed between vessels or nodules and inflammatory cells at histopathology [2].

Fig. 6
figure 6

Fat-ring sign in mesenteric panniculitis. a Non-contrast and b contrast-enhanced CT images of two different middle-aged patients with non-specific abdominal pain. The mesenteric fat contains increased density, but the fat surrounding the mesenteric vessels has been preserved (arrowheads). This has been named the fat-ring sign, or “fatty halo,” and may also involve mesenteric lymph nodes (see Fig. 8)

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The presence of the fat-ring sign has been reported in 75–85% of patients with mesenteric panniculitis [1, 3]. This finding, however, is non-specific, and its presence has also been reported incidentally in patients with non-Hodgkin’s lymphoma in whom chemotherapy treatment has led to shrinkage of the mesenteric lymphadenopathy, leaving a fine haziness throughout the mesenteric fat [14].

Tumoral pseudocapsule

A tumoral pseudocapsule consists of a peripheral band with soft-tissue attenuation limiting the inflammatory mesenteric mass from the surrounding normal fat (Fig. 7). The thickness of this dense stripe usually does not exceed 3 mm [1]. The presence of a tumoral pseudocapsule has been reported in 50–59% of patients with mesenteric panniculitis [1, 3]. A lipomatous tumor such as a lipoma or liposarcoma may also be well-defined by a similar dense rim, but these lesions will often show some mass-effect on the mesenteric vessels in contrast to mesenteric panniculitis.

Fig. 7
figure 7

Tumoral pseudocapsule in mesenteric panniculitis. Non-contrast CT examination in two different patients: one with vague abdominal pain and one asymptomatic patient. The slightly hyperattenuating mesenteric fat is bordered by a thin hyperdense rim (arrowheads) anteriorly and posteriorly a or anteriorly only b, forming a so-called “pseudocapsule”

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Soft-tissue nodules

In approximately 80% of the patients with mesenteric panniculitis, small soft-tissue nodules can be found scattered within the hyperattenuating mesenteric mass (Fig. 8) [1]. These nodules, thought to correspond to lymph nodes, are usually less than 5 mm in diameter. Mesenteric lymph nodes with an axial diameter larger than 10 mm are atypical for mesenteric panniculitis, and in these cases a lymph node biopsy or fine-needle aspiration (FNA) must be considered to exclude malignancy [3].

Fig. 8
figure 8

Soft-tissue nodules in mesenteric panniculitis. Contrast-enhanced CT scan in two different patients, showing soft-tissue nodules corresponding to enlarged lymph nodes, within the inflamed mesenteric fat. Usually these lymph nodes have an axial diameter of less than 5 mm (a, arrowheads). In case b, with nodes larger than 10 mm in diameter (arrows), a lymph node FNA was performed to exclude malignancy. Also note the fat ring sign in a

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Imaging-based differential diagnosis

Alteration in the density of the mesenteric fat on CT has been coined “misty mesentery” by Mindelzun et al. [15], a descriptive imaging finding with an extensive differential diagnosis. Mesenteric panniculitis is solely reserved for idiopathic inflammation leading to a misty mesentery, and its imaging diagnosis can therefore be made only after exclusion of any of the following alternative causes of a misty mesentery [15, 16].

Mesenteric edema

Mesenteric edema may be due to many causes, including heart failure, portal hypertension, mesenteric vascular thrombosis (Fig. 9) and lymphedema. When mesenteric edema is secondary to a systemic disease, it is usually associated with generalized subcutaneous edema and ascites. Ascites is not a feature of mesenteric panniculitis and indicates an alternative diagnosis.

Fig. 9
figure 9

Mesenteric edema due to mesenteric vascular thrombosis. Contrast-enhanced CT scan in a patient with acute abdominal pain shows dense changes in the mesenteric fat, thickening of some small bowel loops and a thrombus in the superior mesenteric vein (arrowhead)

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Inflammation

The typical inflammatory process associated with increased CT density of the mesenteric fat is acute pancreatitis (Fig. 10). These inflammatory changes are usually centered in the peripancreatic region, and levels of amylase in serum and urine are usually increased, enabling the diagnosis.

Fig. 10
figure 10

Mesenteric infiltration secondary to pancreatitis. Contrast-enhanced CT scan shows a “misty mesentery” a with inflammatory changes centered around the head of the pancreas b indicating the correct diagnosis in this patient with increased amylase levels

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Focal inflammations such as appendicitis and colonic diverticulitis may also cause local hyperattenuation of adjacent mesenteric fat on CT, and findings indicating these diagnoses therefore must be carefully ruled out.

Neoplasm

The most common tumor involving the mesentery is non-Hodgkin’s lymphoma. Typically, this causes bulky lymphadenopathy, which can often also be found in the retroperitoneum, indicating the correct diagnosis. Shrinkage of mesenteric lymphadenopathy after chemotherapy may result in residual scarring that may mimic mesenteric panniculitis (Fig. 11). This can be elucidated by reviewing the patient’s prior CT scans.

Fig. 11
figure 11

“Misty mesentery” in a patient with non-Hodgkin’s lymphoma. Contrast-enhanced CT shows increased density of the mesenteric fat (arrowheads) that could mimic mesenteric panniculitis, although its right-sided orientation would be atypical. Previous CT scans (not shown) had shown extensive mesenteric lymphadenopathy, and the findings represent scarring, after chemotherapy has led to shrinkage of these lymph nodes. Note the caudal tip of the enlarged spleen (sp.)

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Primary mesenteric neoplasms, such as a desmoid, mesenteric cyst or lipomatous tumors, may be recognized by their mass-effect on mesenteric vessels [17]. Other tumors—mesothelioma, or metastatic tumors such as pancreatic carcinoma, colon carcinoma, or ovarian carcinoma—may affect the mesentery by soft-tissue tumor deposits, or may cause mesenteric edema by lymphatic obstruction. The correct diagnosis can be made by identification of the primary tumor or detection of extra-mesenteric peritoneal nodules, or by cytological analysis of ascites.

Mesenteric involvement by carcinoid can closely mimic sclerosing mesenteritis (i.e., mesenteritis with predominant fibrosis and retraction), which may both display a desmoplastic reaction and contain calcification [6, 17]. However, a classic “spoke wheel” appearance is virtually pathognomonic for carcinoid tumor (Fig. 12).

Fig. 12
figure 12

Mesenteric involvement of carcinoid tumor. Contrast-enhanced CT scan depicts a mesenteric lesion with calcification that could mimic sclerosing mesenteritis. The classic stellate radiating pattern of the desmoplastic reaction, however, indicates the correct diagnosis

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Hemorrhage

Mesenteric hemorrhage, caused by blood dissecting from mesenteric vessels or from the bowel wall, may be traumatic or spontaneous. A history of trauma, use of anticoagulantia or high-density peritoneal fluid suggests the correct diagnosis.

Conclusion

Mesenteric panniculitis is an inflammatory disorder of the mesentery that is more commonly encountered than before due to the increased use of abdominal US and CT. It may present with non-specific abdominal pain or a palpable abdominal mass, or it may be an incidental imaging finding in asymptomatic patients. A paraneoplastic etiology has been postulated, but the exact cause of mesenteric panniculitis remains uncertain.

Mesenteric panniculitis has some characteristic imaging features that are valuable in suggesting its diagnosis, but before assigning this diagnosis, care must be taken to exclude other causes of a misty mesentery.