Sclerosing encapsulating peritonitis (SEP), also referred as “chronica fibrosa encapsulata,” “icing sugar bowel” or “fibroplastic peritonitis” in the literature, is defined as a chronic inflammatory condition often of unknown etiology in which the small intestines are encased in a dense fibrocollagenous membrane (1, 2). It is believed that this pathology results from low grade or subclinical peritonitis that eventually progressed to multiple adhesions and membrane formation, with sclerosing granulation or fibrous tissue encapsulation, and sometimes distortion of the viscera (1, 2).

This pathological entity is uncommon in dogs and rare in cats (3–16). Only few cases are published and beside three case series published in the 90's (3–6), the most recent cases enriching the veterinary literature are single case reports (8–16). In humans, SEP most commonly manifests as recurrent acute, subacute or chronic intestinal obstruction. Clinical presentation of SEP in animals is variable and rather nonspecific: most cases present with chronic peritoneal effusion with or without digestive signs, including vomiting, diarrhea, anorexia, and abdominal pain (4–7, 9–16). Some animals diagnosed with SEP are reported with a history of abdominal foreign bodies or chronic bacterial infections but most of the time the disease seems to be idiopathic (17). Therefore the designation primary idiopathic SEP as opposed to secondary SEP has been suggested (1, 2).

Because imaging is a routine part of the work-up of cases with abdominal symptoms, the objective of this retrospective study is to provide a detailed description of the most common imaging findings of SEP determined from a case series of dogs and cats having a final diagnosis of SEP at surgery.

Case records from the Small Animal Veterinary Clinic of the University of Liège from 2014 to 2021 were reviewed. Animals with a final diagnosis of SEP based on macroscopic findings and/or histopathological analysis of biopsies at surgery were included in the retrospective study and their data (signalment, history, clinical findings, imaging reports and diagnostic images) reviewed. All included animals had undergone a comprehensive clinical examination and a diagnostic imaging investigation, including at least abdominal ultrasonography, with or without abdominal radiography or computed tomography (CT).

Ultrasound examination was performed in dorsal recumbency on awake animals using a fixed ultrasound machine equipped with high frequency (7.5–13 MHz) linear and/or curvilinear probes¹, ², ³.

Radiographs included at least two orthogonal views: a latero-lateral projection and a ventro-dorsal projection. All radiographs were performed on awake animals maintained by human operators using a computed or direct radiography system 150 kV/800 mA⁴, ⁵.

Abdominal CT angiography was performed in sternal recumbency, under general anesthesia after a transient period of hyperventilation to induce apnea and avoid motion artifacts. The animal was premedicated using butorphanol (0.3–0.4 mg/kg). Anesthesia was induced using propofol and followed by endotracheal intubation. Anesthesia was maintained using isoflurane gas and 100% oxygen. CT images were acquired using a multi-slice system with 16 or 64 slice detector⁶. Acquisition parameters used for the 16-multislice CT scanner were as follows: tube voltage 120 kV, reference tube current 200 mA, and pitch factor 0.8. Scan tube current was modulated by automatic exposure control (Care Dose, Siemens Medical Solutions, International). Image data sets were reconstructed using parameters of 200–300 mm field of view, 512 × 512 matrix, 1.5 mm slice thickness, and Br-20f reconstruction algorithm (window level 40 and window width 300) with filter back projection. For the 64-multislice CT scanner, acquisition parameters used were as follows: tube voltage 100 kV, reference tube current 260 mA, and pitch factor 0.6. Scan tube current was modulated by automatic exposure control (Care Dose, Siemens Medical Solutions, International). Image data sets were reconstructed using parameters of 100–200 mm field of view, 512 × 512 matrix, 0.75 mm slice thickness and Br-46 reconstruction algorithm (window level 88 and window width 404) with iterative reconstruction. Post-contrast images were acquired during the arterial, portal and venous phases after injection of 2 ml/kg of iodinated contrast medium (iohexol⁷ or iopromide⁸) with an automated injector.

This case series describes imaging findings in seven dogs and two cats with confirmed SEP. Most animals included were either juveniles or young adults (<3 years old). Previous reports show a wide age distribution demonstrating that juvenile (9, 12, 14, 16) as well as mature (7, 8, 10, 11, 13, 15) animals may be affected. Intact females were overrepresented (5/9) in contrary to previous reports which do not suggest any sex or neutered status influence (7, 8, 10–12, 15, 16).

SEP has been referred to be idiopathic or secondary depending on previous conditions that may be identified as potential trigger of peritoneal inflammation (1, 2). Foreign bodies and necrotic neoplastic masses have been reported as potential initiating causes of abdominal inflammation in SEP (8, 10, 16). In the present case series one dog had undergone a previous abdominal surgery, one dog had a perforating foreign body and two had been pregnant prior or at the time of the diagnosis of SEP. Neoplasia was found in three other dogs (one carcinomatosis, one duodenal leyomyosarcoma and one scrotal leydigoma) and necrotic pancreatitis was found in one. These associated medical conditions may have been potential triggers for peritoneal inflammation. One dog had been castrated prior to the diagnosis of SEP (D6) and an ascending infection or systemic inflammation may have occurred.

The diagnostic imaging abnormalities in this case series predominantly involved the intestinal tract and peritoneal space. Intestinal findings most commonly involved plication and corrugation (consistent with the presence of adhesions in the absence of a foreign body) associated with peritoneal effusion and lymphadenomegaly. Sonographically, peritoneal fluid was often echogenic (6/9) and frequently associated with echogenic septae suggesting an inflammatory cause. This hypothesis is in accordance with previous descriptions in case reports (7, 8, 11, 12, 15, 16) and imaging text-books (18, 19). Thickening of the serosal surface and parietal peritoneum is also a sonographic finding described in textbooks (18, 19) but was not clearly seen in this retrospective case series.

Adhesions around and between small intestinal loops sometimes with an encapsulated visceral mass (6/9) was a common surgically finding. This characteristic encapsulation was seen as a gathering of small bowel loops in the central abdomen surrounded by fluid on radiographic, CT and ultrasonographic examinations of respectively 2/3, 3/4 and 5/9 animals. In two animals a mechanical ileus developed after the first onset of symptoms and this is similar to what is described in humans where mechanical ileus is reported as a consequence of adhesions and impaired bowel motility (1, 2).

Four cases had undergone a CT examination. Consistently with findings reported in literature (1, 2, 16), thickening of the serosa or adhesions and peritoneal fat stranding were noticed on CT images. Fat stranding refers to an abnormal increased attenuation in fat because of edema and engorgement of lymphatics (20). In humans, fat stranding has been found to be very sensitive for peritoneal inflammation, being a key CT feature to differentiate uncomplicated from complicated appendicitis. Whereas mild inflammation causes a subtle hazy increased attenuation of the fat (ground-glass like), severe inflammation produces a reticular pattern (21, 22). In the animals where CT was performed linear/reticular fat stranding was present either in the entire peritoneal cavity or focally around the abnormal bowel loops. At contrast CT, two animals had serosal enhancement, and this is consistent with reports in human patients with SEP (2).

While the entire peritoneal cavity was involved in the majority of animals, two cases had focal ultrasound abnormalities with peritoneal effusion and hyperechoic fat being localized around an abnormal corrugated small bowel loop. These animals may either represent an earlier stage of the disease or simply better show the undergoing focal triggering process. However, if previous surgery is reported, animals with focal implication of the abdominal cavity may be difficult or impossible to distinguish from animals suffering of post-operative adhesions and not necessarily progressing to SEP.

The main limitations of this study are its retrospective nature and the small sample size. The complementary examinations were at the discretion of the clinician after consideration of the animal clinical presentation and owner budget. In particular, 5/9 animals did not have CT examination, 6/9 did not have abdominal radiographs and 5/9 did not have histopathological analysis of the peritoneal adhesions and membranes. However, because of the rarity of the disease, especially in cats (5, 6, 14), and its life threatening character a small case series assembling a detailed description of the most common imaging findings of SEP is considered useful to develop a better pre-surgical diagnosis in veterinary patients.

This small case series confirms findings previously reported in isolated SEP cases. As clinical presentation may be nonspecific, diagnostic imaging should be considered to exclude other disorders. Imaging findings of SEP in dogs and cats include abnormal shape and distribution of small bowel loops on radiographs, plicated and/or corrugated small bowel loops with echogenic loculated peritoneal effusion on ultrasound and fat stranding on CT.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

The animal study was reviewed and approved by Comission d'Ethique Animale, Université de Liège. Written informed consent for participation was not obtained from the owners because this is a retrospective study using clinical datas. The Ethical Committee retrospectively approved the study (File number 22-2459) on the 11th of May 2022.

BG and VB contributed to conception and design of the study. BG and CP gathered database. BG organized the database and wrote the first draft of the manuscript. VB contributed to manuscript revision and wrote sections of the manuscript. All authors contributed to manuscript revision, read, and approved submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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