Sclerosing mesenteritis associated with ankylosing spondylitis

47-year-old male patient, admitted to the emergency department with diffused abdominal pain for one week, anorexia and weight loss. No fever. During physical examination he presented with a tender and depressible abdomen, nasogastric tube with enteric drainage, and normal inflammatory markers. Supine abdominal X-ray and Computed Tomography (CT) scan were performed.

Abdominal X-ray showed a distended small bowel with air-fluid levels (Fig. 1).

CT scan before and after intravenous administration of iodinated contrast depicted distended small bowel loops, mainly affecting the ileum for up to 10 cm from the ileocaecal valve, where there was an abrupt calibre transition zone due to a stenosis. The ileum distal to the stenosis presented with thickened walls. (Fig. 2)

Segmental resection of the distal ileum and transverse-enterostomy were performed. The histology revealed a fibroinflammatory process of the mesentery, consistent with sclerosing mesenteritis. The patient started therapy with corticosteroids.

Follow-up CT 2 months after surgery depicted maintained distension of several ileal loops due to multifocal stenosis and a heterogeneous densification of the mesentery, with an increased space between the bowel loops, mainly in the right abdominal quadrants (Fig. 3). There was also a superior mesenteric artery fat ring sign. The colic arteries were patent. (Fig. 4)

Bone evaluation showed syndesmophytes in the lumbar spine and bilateral sacroiliitis with ankylosis of the left sacroiliac joint. (Fig. 5)

Sclerosing mesenteritis (SM), also named xanthogranulomatous mesenteritis or retractil liposclerotic mesenteritis, is a rare condition with  benign inflammatory and fibrotic changes and mass forming that affect the mesenteric adipose tissue. [1, 2]

It is more common in men (2:1), presenting in the 7^th decade, usually with abdominal pain, bloating, weight loss, nausea and a palpable abdominal mass (50% of the cases). [1, 2]

Several associations have been proposed, namely autoimmunity, and some cases of ankylosing spondylitis have been reported. Other associations include surgery, trauma, vasculitis, infection or malignancy. [1, 2]

Three pathophysiological stages of the disease have been proposed: first, a macrophageal layer replaces mesenteric adipose tissue (mesenteric lipodystrophy), that leads to the second stage, with proliferation of plasma and polymorphonuclear inflammatory cells (mesenteric paniculitis), and eventually nodular mass retraction due to fibrosis and fat necrosis, the end stage (MS).

Imaging can vary depending on the stage. The inflammatory response leads to a densification of the mesentery fat – the “misty mesentery”. It is not specific and can also occur with other conditions. [3]

The differential diagnosis includes lymphoedema (not common in the absence of intraperitoneal free fluid) or haemorrhage (improbable in the absence of hyperattenuating free fluid). Inflammatory conditions, such as pancreatitis, may mimic MS with mesentery densification, but not mass forming. [3, 4]

The most challenging differentiation is with neoplasia, since both share some imaging findings:

-          The pseudocapsule sign, describing healthy hypodense adipose tissue surrounding the mass lesion, commonly seen in MS, but also common in rare benign and malignant lipomatous tumours.

-          Calcification, a common feature of MS, usually presenting in the centre of the lesion, reflecting a sequela of fat tissue necrosis. It is helpful to exclude lymphoma (unless previously treated), but not to exclude other neoplasias, since it is also seen in adenocarcinomas and in 70% of carcinoid tumours.

-         “Fat ring sign”, reported in 50% of MS patients, representing healthy adipose tissue surrounding a vessel encased by the MS mass; it can also be seen with mesenteric lymphomas.

-          Distribution of nodules in the root of the mesentery, a typical feature of MS, can be helpful in the differentiation with carcinomatosis and mesothelioma that usually have a wider range of distribution.

MS with an underlying inflammatory pathophysiology can be treated with immunosuppressive agents, so prompt recognition is important. Histopathological characterisation should be done to exclude underlying neoplasms. Complications, such as stenosis and intestinal obstruction, may need surgical intervention.

Prognosis depends on the response to medical and surgical treatment, some having a benign course, and other a dismal prognosis.

Written informed patient consent for publication has been obtained.