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CT  A F N: E A   M D D

Jorge Ahualli

Abstract Resumen

Abdominal fat necrosis may cause pain, mimic findings of acute ab - La necrosis grasa abdominal puede causar dolor, simular un abdo - domen, or be asymptomatic and accompany other pathophysiologic men agudo o ser asintomática y acompañarse de otros procesos fi - processes. Common processes that are present in fat necrosis include siopatológicos. Alteraciones que comúnmente se relacionan a torsion of an epiploic appendage (a self-limited inflammation of the necrosis grasa abdominal incluyen la torsión de un apéndice epi - appendices epiploicae), infarction of the greater omentum (a he - ploico (un proceso inflamatorio autolimitado de los apéndices epi - morrhagic infarction resulting from vascular compromise), encap - ploicos), infarto del omento mayor (un infarto hemorrágico sulated fat necrosis (traumatic or ischemic insult that causes fat resultante del compromiso vascular del omento), necrosis grasa en - degeneration), fat saponification and pancreatitis and heterotopic capsulada (afección traumática o isquémica que causa degenera - ossification in surgical incisions of the abdomen (represent a subtype ción grasa), saponificación grasa y pancreatitis y osificación of traumatic myositis ossificans in which osseous, cartilaginous, and, heterotópica vinculada a incisiones quirúrgicas del abdomen (un occasionally, myelogenous elements forms within a surgical inci - subtipo de miositis osificante traumática en la que elementos óseos, sion). cartilaginosos y ocasionalmente mielógenos se forman en una in - cisión quirúrgica).

Key words: necrosis, fat, abdominal. Palabras claves: necrosis, grasa, abdomen.

Epiploic appendagitis

Epiploic appendices: Normal Imaging Aspect and Anatomy Epiploic appendices constitute small invaginations of the visceral peritoneum containing fat and small blood vessels. They start in the serous surface of the colon adjacent to teniae coli (from the cecum to the recto - sigmoid junction) and they are more abundant in the left colon and cecum (1,2) and not present in the rec - tum. Since they are mixed with the surrounding peri - colic fat they are not usually identified with computed tomography except when they are surrounded by li - quid (Fig 1) (3-6).

Datos de contacto: Jorge Ahualli. Centro Radiológico Luis Méndez Collado. Recibido:  de Agosto de  / Aceptado:  de Octubre de  San Miguel de Tucumán. Tucumán. - Argentina. Recieved: August ,  / Accepted: October ,  e-mail: [email protected]

 Revista Argentina de Diagnóstico por Imágenes Epiploic Appendagitis and its Main Differential Diagnosis Ahualli, J.

Fig. : Epiploic appendices. a-b A) Macroscopic image showing an epiploic appendix of the sigmoid colon with many adipose content and vascular structures inside (Arrow). B) CT of a patient with ascites due to peritoneal carcinomatosis where epiploic appendices of the sig - moid colon can be identified as tubular structures with fat density (Arrows).

Epiploic Appendagitis: Due to the absence of pathognomonic characte - Pathophysiology and Clinical Aspects ristics, it is often difficult to diagnose EA clinically, Epiploic appendices can spin on their axis or have so imaging studies are become essential, not only spontaneous central venous thrombosis. Both pro - to determine the diagnosis but also to establish the cesses determine vascular occlusion with inflamma - most appropriate treatment (3,12,15). tion and eventually acute ischemic infarct of the affected epiploic appendix, (1) recently recognized condition, (7) called epiploic appendagitis (this is Epiploic Appendagitis: Tomographic Aspect the preferred term as opposed to epiploic appen - In CT, infarcted epiploic appendix is presented as dicitis to avoid confusion with acute cecal appen - a small (1-4 cm), oval, and pericolonic focal area dicitis) (4, 8-10). Even though this process may with fat density (-40 to -120 Hounsfield units), ge - occur in a spontaneous way, extreme exercise has nerally surrounded by a thin (1-3 mm) dense ring been reported as a predisposing factor (11-13). (sign of the hyperdense ring) that represents the The age in which the process becomes present visceral peritoneum covering the epiploic appen - varies from 12 to 82 years of age, with a maximum dice, inflamed generally in relation to sigmoid incidence in the fifty-year-old people, more com - colon (Fig 2) (16,17). monly in obese people and women (3, 12). Typi - With less frequency, a dense area, not well defi - cally, Patients suffer from acute abdominal pain (not ned, rounded or lineal (Dot sign or Central Line migrating, it intensifies with coughing and without sign), can be identified inside the lesion, correspon - abdominal defense), generally situated at the left ding to central thrombosed vessels, to hemorrhage iliac fossa (more frequently in relation to sigmoid, areas or fibrosis (Fig 3). Although the presence of then ascendant, descendant and not frequently in these signs is useful for the diagnosis, its absence the transverse) associated with slight abdominal dis - does not exclude EA diagnoses (4,7,12,13,16,18-20). tension (3,7,11,14) and patients can suffer from a Less frequent findings include: a greater increase low-grade fever (3, 15). Appetite and abdominal in the density of the pericolonic adipose tissue than function are not generally modified, and nausea the wall thickening of the adjacent colon (due to and vomiting are not frequent (13,15). In lab analy - associated inflammatory changes) (Fig 4); extrinsic ses, the quantity of white blood cells is normal or compression of the adjacent colonic wall and fascial slightly increased of white blood cells (11). and secondary parietal peritoneum thickening se -

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condary to inflammatory condition (Fig 5) (3- the last necrosis the one that most frequently pre - 8,12,16,21,22). sents peripheral calcifications (or eggshell) (Fig 9) With less frequency, those findings can be seen (23). With less frequency, they can present central in relation to ascendant and descendant colon (FI - calcification surrounded by a peripheral ring of fi - GURE 6), being CT an essential diagnosis tool for brous tissue (24). Cases of epiploic appendices as the differentiation of this process from other abdo - sequela have been reported. They were calcified, minal acute conditions (for example, acute appen - avulsioned and present in the abdominal cavity as dicitis or acute peridiverticulitis). Cases of EA have lost intraperitoneal bodies (3,15,21,25). These intra - even been reported in eventration or hernial sac peritoneal bodies can even be attached to structures with intestinal content (Fig 7) (3,4,15). or organs (more commonly to the inferior surface In other occasions, the inflammatory and ische - of the spleen); in these cases, they are called para - mic process involving the epiploic appendix is a sitic epiploic appendices (3). The smooth surface consequence of the direct involvement in another and the calcified consistency of EA help distinguish neighboring inflammatory process (more frequently them from other omental lesions, especially from acute peridiverticulitis, appendicitis, cholecystitis, metastasis (6). etc.); this condition is a secondary epiploic appen - dagitis (8,11,12,21). In these cases, the findings of the cause should be added to the findings above mentioned (Fig 8). As it evolves, the affected epiploic appendix can show several aspects which vary from a focal area with soft tissue density to a focal area with fat den - sity and to residual encapsulated fat necrosis, being

Fig. : a-b Epiploic Appendagitis. CT showing an oval formation, in relation to the sigmoid colon (SC), with fat density corresponding to an inflamed ischemic epiploic appendix. The image is surrounded by a dense halo corresponding to inflamed visceral peritoneum (Arrow).

 Revista Argentina de Diagnóstico por Imágenes Epiploic Appendagitis and its Main Differential Diagnosis Ahualli, J.

Fig. : Dot sign or Central a-b Line sign. Axial view (A) and sagittal multiplanar reconstruction (B) showing an oval image with fat density, surrounded by a peripheral dense ring corresponding to epiploic appendagitis. Note that in - side, there is a dense lineal image co - rresponding to the thrombosed central vessel (Arrow).

Fig. : Associated inflammatory changes. a b A) Ultrasound image of left iliac fossa showing a com - c pressible echogenic oval formation (Arrows). Axial view B) and sagittal multiplanar reconstruction (B) showing a fat density oval image, surrounded by a peripheral dense ring corresponding to epiploic appendix infarction. Note the increase in density of the surrounding adipose tissue due to the presence of inflammatory perilesional changes (Arrows).

Fig. : Fascial thickening. a-b Coronal (A) and sagittal (B) multipla - nar reconstruction of a patient with epiploic appendagitis (*). Note the in - crease in density of the adipose tissue surrounding the epiploic appendix and lateroconal and homolateral fascia thickening (Arrows), also known as "comma sign".

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Fig. : Right epiploic a-b appendagitis. A) Ultrasound transversal image of right iliac fossa showing a non-com - pressible echogenic oval image (arrows). B) CT shows a flame epiploic appendix surrounded by a dense ring correspon - ding to inflammatory visceral perito - neum (Arrow). Its typical appearance determines the differential diagnosis with acute appendicitis.

Fig. : Epiploic appendagitis a-b in hernial sac. Axial view (A) and coronal multiplanar reconstruction (B) showing a small in - guinal hernia on the left (Arrowhead) containing adipose tissue and an epi - ploic appendix showing inflammatory changes and thickening of the visceral peritoneum (Arrows).

Fig. : Secondary epiploic appendagitis. a b Coronal (A) and sagittal (B) multiplanar reconstruction of a patient with acute appendicitis (*) showing an epiploic appen - c d dix of the ascendant colon, which is inflamed with a central dot sign (Arrow). Axial views (C and D) of another patient sho - wing at least two epiploic appendices, which are inflamed with a peripheral dense halo (Arrows) due to direct inflammatory compromise from associated acute peridiverticulitis.

 Revista Argentina de Diagnóstico por Imágenes Epiploic Appendagitis and its Main Differential Diagnosis Ahualli, J.

Fig. : Developmental changes. a b A) CT showing an oval image, in relation to sig - c moid colon, with fat density and dense ridge (Arrow) corresponding to a sequelar fat necrosis area due to epiploic appendagitis. B) Sagittal multiplanar reconstruction showing an oval image of low density and partial periphe - ral calcification (Arrow), as sequela of epiploic appendagitis. In another patient (C), calcified pe - ripheral deposits are greater (Arrow).

Evolution and Treatment Pathophysiology In most patients, EA constitutes a self-limited pro - Omentum ischemia and posterior infarction consti - cess that concludes spontaneously in 5-7 days (26). tute an infrequent condition due to the great per - Less frequently, acute EA can result in adhesion, in - fusion and collateral vessels present in the testinal obstruction, invagination, peritonitis or in omentum (26). The cause of omental infarction is formation of abscess (3,4). EA does not require sur - not yet clear, although its pathophysiology seems gery and the treatment of it is based on the patient's to be similar to what happened with the epiploic symptoms (4,8,10,12,22); that is why a wrong diag - appendagitis (27). This condition occurs when nosis can lead to unnecessary surgery, medical tre - there is a vascular compromise of the greater omen - atment, or hospitalization (12). tum (generally due to omental torsion, venous in - sufficiency secondary to trauma or spontaneous thrombosis of omental veins) (4,28). Although most cases of omental infarction are idiopathic (15) some Omental infarction risk factors have been reported as a consequence, which include obesity, recent abdominal surgery, Anatomy extreme exercise —marathon runners who are The greater omentum is a large peritoneal fold next thought to have a low blood pressure as a result of to the visceral peritoneum layers of the stomach and a physiological shunt or splenic vessels constric - transverse colon. It contains adipose tissue and blood tion—, congestive heart failure, digital exploration vessels resulting in an essential structure that acts as a and abdominal trauma (4,9,29-32). barrier against dissemination of infections or tumors Primary omental infarction consists in a hemor - (6). rhagic infarcts resulting from a vascular compro - mise (2,33). Some omental infarctions are related to

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a combination of a low arterial flow and venous Images flow that happens in a state of hypercoagulation, In CT, findings associated to omental infarction vary congestive heart failure and vasculitis (34). These from a subtle, focal and slight increase in omentum infarctions often happen on the right side (superior density (in less serious cases) (Fig 10,11) (1,16,26,40) or inferior, in 90% of the cases), as the omentum is to a presence of a mass (generally bigger than 5 cm) bigger and mobile and has a lower blood supply with a fat, heterogeneous, dense density that does not in that area (15,25,28,29,35,36). show enhancement after intravenous contrast injection The secondary omental infarction can present it - (Fig 12) (5,15,16,25,41). Less frequent associated fin - self after a direct lesion because of an abdominal dings consist in reactive intestinal parietal thickening or surgical trauma. The side of the infarction is ge - (the inflammatory process in the omentum is usually nerally located next to the surgical area (1,37). proportionately more severe) (5,16,25,28,33), and in the Omental torsion is a rare cause of OI and it oc - cases of secondary infarction as a consequence of curs when there is a torsion of a portion of the omental torsion, a whirlwind pattern of lineal and con - omentum over itself causing vascular compromise. centric bands can be identified (25) (Fig 13). Not fre - When there is omental torsion, the torsion of the quently, an omental infarction can have a vessels is often visible inside the omentum (1). superimposed infection and resemble a mass lesion or evolve to become an abscess (Fig 14) (1,16).

Clinical Presentation Evolution and Treatment Omental infarction is often present in obese patients, In most cases, OI is a process that can often favorably especially men and, contrary to what happens with epi - evolve with a medical treatment and with an indication ploic appendagitis, OI affects pediatric patients in 15% of surgery if the symptoms persist of if an abscess de - of cases. Patients usually suffer from abdominal pain velops (40). Therefore and because omental infarction with a subacute evolution in the right inferior quadrant and epiploic appendagitis share the same common de - (in primary cases) or in other areas of the abdomen (in nominator (spontaneous fatty tissue necrosis), a new secondary cases), generally with a slight increase of term has been recently proposed: "Intraperitoneal Fat white blood cells (34). Other gastrointestinal symptoms Focal Infarction" (IFFI), with the objective of reinforcing like vomiting, nausea and fever are not present (36). that its differentiation is not clinically important, since Establishing the pre-surgical clinical diagnosis of this both are generally self-limited pathologies and the han - entity is difficult since it often resembles acute appen - dling of them is generally conservative (1). dicitis, acute cholecystitis or other abdominal alterations (39); therefore, in most cases radiologists make the diagnosis (1).

Fig. : Omental infarction. a-b Axial view (A) and sagittal multiplanar re - construction (B) of a patient suffering pain in the right iliac fossa after extreme physical exercise involving heights. The images show a slight increase in density of the adipose tissue surrounding the as - cendant colon (Arrows) indicating omen - tal infarction. After the administration of anti-inflammatory drugs, the infarction evolved satisfactorily.

 Revista Argentina de Diagnóstico por Imágenes Epiploic Appendagitis and its Main Differential Diagnosis Ahualli, J.

Fig. : Omental infarction. a b A) Sagittal view ultrasound image of right iliac c fossa showing a non-compressible echogenic oval image (Arrows). Axial view (B) and sagittal multiplanar reconstruction (C) showing a slight increase in density of the pericecal adipose tis - sue (Arrows) due to an omental infarction of idiopathic origin.

Fig. : Omental infarction after surgery. a b Axial view (A and B) and sagittal multiplanar re - c construction (C) of a patient who recently un - derwent surgery due to an inguinal hernia and mesh implant. They show an increase in density of the adipose tissue of the greater omentum in right hypochondrium and periumbilical region (Arrows). Note that it shows a greater density than the cases in figures  and . Metallic ele - ments located in the anterior abdominal wall in the right iliac fossa correspond to the clips used to fasten the surgical mesh (Arrow in B).

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Fig. : Associated findings. a-b A) Enhanced CT showing an increase in density of the adipose tissue of the greater omentum in the right hypochondrium due to omental infarction (*). Note the smooth and regular increase of the parietal thickening of the hepatic angle in the colon (Arrow), proportionately smaller when compared to the great inflammatory and ischemic changes in the omentum. B) In another patient, there is an increase of the greater omentum density in left hypochondrium due to omental infarction. Note the "whirlwind" pattern with a dense focal image corresponding to the thrombosed central vessel and indicating omental torsion. This image is a cour - tesy of Dr. Pablo Cikman.

Fig. : Complicated omental infarction. a-b Axial view (A) and coronal multiplanar reconstruction (B) showing a heterogeneous oval image corresponding to omental infarction. Note the peripheral enhancement and the presence of air bubbles inside (Arrow) due to superimposed infection.

 Revista Argentina de Diagnóstico por Imágenes Epiploic Appendagitis and its Main Differential Diagnosis Ahualli, J.

Encapsulated fat necrosis image (of variable size) with a central fatty density and a peripheral dense line, which often conditions a Even though encapsulated fat necrosis can be pre - soft mass effect over neighboring structures (Fig 15) sent in any part of the body, Schmidt-Hermes and (44-46). The capsule surrounding the central fat ne - Loskant have initially described it in the breast in crosis can show a slight enhancement after the contrast 1975 (42). The EFN is thought to be the result of a injection (Fig 15) or be partially calcified (Fig 16) (43). trauma or ischemia that causes fatty degeneration The natural progression of EFN is to maintain its with a posterior organization of the adipose necro - morphological characteristics or decrease in size tic tissue surrounded by a fibrous capsule (43). with time (Fig 17) (1). The CT aspect can be complex; however, it is ge - nerally characterized by the presence of an oval

Fig. : . Encapsulated fat necrosis in a patient with appendectomy  months ago. a-b Axial view (A) and sagittal multiplanar reconstruction (B) showing an oval image with fat density su - rrounded by a thin capsule slightly enhanced (Arrow).

Fig. : Encapsulated fat a-b necrosis. Axial view (A) and sagittal multipla - nar reconstruction (B) showing an oval image with predominantly fat density corresponding to fat necrosis. Note the presence of high-density images (Arrow) in the capsule corres - ponding to calcifications.

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Fig. : Fat necrosis evolution. a-b A) CT showing an encapsulated fat necrosis in the greater omentum (Arrow). It did not show significant chan - ges in the following  months.

Fat saponification and pancreatitis of hypocalcemia that often occurs in cases of severe pancreatitis (49). After the acute exudate and ascites, Mesenteric and retroperitoneal fat necrosis can cause disseminated fat necrosis nodules can be present in fat saponification because of a pancreatitis, where the retroperitoneum and abdominal cavity. These no - the pancreas releases lipolytic enzymes that digest dules can exhibit the mass effect and delay in enhan - the pancreatic parenchyma and peripancreatic adi - cement, possibly due to low contrast diffusion through pose tissue (46-48). The damaged adipose tissue ac - small capillary in the granulation tissue (Fig 18) tivates macrophages and other inflammatory (45.50). A history or prior imaging studies that show mediators that aggravate the inflammatory response previous episodes of pancreatitis may be necessary (47,48). Since phospholipase and protease affect the to distinguish nodular fat necrosis from peritoneal plasma membrane of fat cells, triglycerides are rele - carcinomatosis (1). ased and hydrosolubilized, producing free fatty acid that mixes to the serum calcium and precipitating as calcium soap. This process is thought to be the cause

Fig. : Fat saponification. a b Axial view (A), coronal (B) and sagittal (C) multiplanar reconstruction c showing a compromised left posterior para-renal space, focal dense ima - ges with a nodular aspect (Arrow), presenting a slight enhancement after intravenous contrast injection, corresponding to areas of fat saponi - fication in this patient with acute pancreatitis.

 Revista Argentina de Diagnóstico por Imágenes Epiploic Appendagitis and its Main Differential Diagnosis Ahualli, J.

Heterotopic ossification in supra-umbilical incision. Sometimes, bone ma - Mesenteric and peritoneal heterotopic ossification rrow can be identified in its interior, although this (also referred to as osseous metaplasia) is an infre - is not a sensible finding (Fig 19) (53). quent condition that can develop after trauma or Heterotopic ossification is quickly developed and repeated surgical procedures in the abdomen. The it has been reported in CT within two weeks after pathogenesis of mesenteric heterotopic ossification the surgery (53). Sequential monitoring with CT can is not clearly understood; however, it has been sta - demonstrate the increase and decrease in size pre - ted that this condition represents a metaplasia of sumably showing that heterotopic ossification is an the submesothelial mesenchymal metaplasia (51). active process of bone formation and resorption. Mesenteric heterotopic ossification has been re - The significance of this condition lies in two facts: ported in patients who have undergone abdominal A) Clinically, the incision scar can cause pain; and surgery once or more (exact prevalence is not well B) radiologically, it can be at times, misinterpreted established although it has been reported to affect as a foreign body or tumor, especially when they 25% of patients with abdominal incisions) due to a increase in size during check-ups (55-57). non-neoplasic condition and they appear with sub - sequent bowel obstruction (52,53). In CT, Mesenteric heterotopic ossification is cha - racterized by multiple ramified lineal structures of high density inside the mesenterium and extending to the peritoneal surfaces (51,54). This mainly oc - curs after abdominal incisions in the vertical middle line (more commonly in the high line in supra- and infra-umbilical incisions) and with more frequency

Fig. : Heterotopic ossification. a Axial view (A), sagittal (B) and coronal (C) b c multiplanar reconstruction showing a li - neal image with calcium density compro - mising the greater omentum in the infra-umbilical region. The surgical history of this patient included an appendectomy,  cesarean interventions and a hysterec - tomy.

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 Revista Argentina de Diagnóstico por Imágenes Epiploic Appendagitis and its Main Differential Diagnosis Ahualli, J.

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