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Radiological case of the month: Paritosh C. Khanna, MD; Suleman A. Merchant, MD; Anagha R. Joshi, MD.

CASE SUMMARY

A 19-year-old woman presented with a 2-month history of dull pain in the abdomen, especially in the right iliac fossa (RIF) and the right hypochondrium (RHC), and a 1-month history of abdominal distension and low-grade pyrexia. She had been admitted 1 month previously, at which time an abdominal paracentesis revealed lymphocytic predominance, no acid-fast bacilli, and negative cultures. The patient had been in contact with her sister, who had been diagnosed with an intracranial tuberculoma 4 years earlier. A 6-month empiric course of antituberculous therapy was subsequently begun, but the patient was noncompliant. At her most recent visit, the abdominal examination revealed a 15-x 15-cm palpable lump that was tender, firm, and mobile in the RHC and RIF.

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IMAGING FINDINGS

A current chest radiograph revealed a left pleural effusion, and a small bowel follow-through was inconclusive (neither is shown). Ultrasound (US) examination (ATL HDI 5000 SonoCT[TM], Philips Medical Systems, Bothell, WA; Figures 1 and 2) of the abdomen revealed generalized free peritoneal and pelvic fluid with a few internal echoes and multiple flimsy, mobile adhesions that displayed after-movement with abdominal pressure. The greater omentum was thick and cakelike with a mixed echogenic area subjacent to a thickened parietal peritoneum. Mesenteric thickening and hypoechoic, mesenteric lymphadenopathy were also noted. The ovaries and other pelvic organs were normal, and the peritoneal surfaces were clear. Multidetector computed tomography (MDCT; Siemens Volume Zoom, Siemens Corp., Siemenstrasse, Forchheim, Germany) of the lower chest and abdomen (Figure 3) confirmed the US findings and also revealed multiple enlarged, mildly enhancing mediastinal and azygoesophageal lymph nodes. The "omental cake" was seen to moderately enhance on contrast-enhanced axial and multiplanar sections. Bowel wall thickening was also noted on computed tomography (CT), which resulted in wide separation of opacified small bowel loops.

The patient underwent a CT-guided biopsy of this clumped mass of greater omentum. Histopathology (Figure 4) revealed multiple caseating granulomas and Langhan's giant cells, which were suggestive of tuberculosis (TB).

DIAGNOSIS

Omental cake from tuberculosis

DISCUSSION

There are many causes of the omental cake, which not infrequently occurs secondary to intraperitoneal tumor spread. Other, although less common, causes of this finding include inflammatory conditions such as tuberculosis, Crohn's disease, phlegmonous pancreatitis, granulomatous enterocolitis, benign disease (such as desmoid fibroma, extramedullary hematopoiesis, and hemoperitoneum), and malignant disease entities. While an omental cake is classically associated with ovarian carcinoma, it may be seen with carcinomatosis and lymphomatosis, mesothelioma, primary peritoneal serous carcinoma, tubal carcinoma, and, rarely, cholangiocarcinoma. (1-3)

The incidence of tuberculous peritonitis ranges from 0.1 to 0.7% among all cases of TB. It can occur at any age and has a slight female predilection. Different subtypes include the wet-ascitic, fibrotic-fixed, and dryplastic types. The wet type is the most common (97%). Tuberculous peritonitis results from reactivation of latent tuberculous foci in the peritoneum or from the hematogenous spread from primary foci in the lungs, which usually heal completely. (1) Concurrent pulmonary TB is uncommon, since peritoneal seeding may occur years prior to presentation. (4) In our patient, a pleural effusion was noted, despite the absence of pulmonary parenchymal stigmata. Peritoneal TB has also been described in patients on continuous ambulatory peritoneal dialysis probably because their chronic renal failure and local intraperitoneal immune derangements predispose them to increased risk. (5)

As in this case, symptoms generally include an insidiously developing ascitic abdominal swelling with pain, fever, weight loss, and night sweats. Ascites, abdominal tenderness, and a palpable abdominal mass are the most common findings on physical examination. These signs are usually nonspecific and may also be suggestive of an occult malignancy or cirrhosis. (1)

Laboratory findings include a positive purified protein derivative test, anemia, raised erythrocyte sedimentation rate and C-reactive protein levels, and hypoalbuminemia. The ascitic fluid is exudative with lymphocytosis, with a low incidence of acid-fast bacilli and positive cultures. Raised adenosine deaminase (>30 U/L eliminates a false-positive test resulting from cirrhosis or malignancy) and CA-125 levels (1,6) are also noted. The latter is also a feature of ovarian epithelial tumors, some benign conditions, and other nonovarian malignancies and may initially present diagnostic difficulties. (1)

Diffuse peritoneal infiltration, which is designated omental cake, first presents in most cases as ascites. The principal CT findings of the omental cake are the presence of abnormal and thickened omentum and ascites. CT also shows enlarged mesenteric and peripancreatic nodes, with a low-density caseated center and a high-density rim, a finding highly suggestive of TB. (1,7,8) Thickened and adherent small bowel loops, splenic calcification, and a stellate pattern of the mesentery are also often seen. The latter represents the results of an infiltrating process, benign or malignant, that causes thickening and rigidity of the mesentery. (8) These findings are nonspecific in isolation. (1,7)

Imaging features in our patient were septations in the ascitic fluid, infiltration and thickening of the omentum (resulting in the omental-cake sign), smooth peritoneal thickening, bowel wall thickening, and left-sided mild-to-moderate pleural effusion with mediastinal lymphadenopathy. CT scan and US are complementary to each other. Ultrasound is superior to CT in revealing multiple, fine, complete or incomplete and mobile septations, whereas CT can reveal involvement of the mesentery or omentum and peritoneum, and thickening of the bowel wall better than US can. Although CT has a larger field of view than conventional US, real-time compound imaging or panoramic imaging, which is available with modern US scanners, can be used (Figure 1B).

Further, high diagnostic accuracy and safety of image-guided needle biopsy result from the selection of the most appropriate biopsy site. (1) Blind percutaneous peritoneal biopsies have a lower success rate and are not without complications. The clinical relevance of image-guided peritoneal biopsy (IGPB) is undoubtedly clear; it is less invasive than laparoscopy and does not require general anesthesia. The use of US as image guidance gives tremendous flexibility, and the procedure can be performed even at the bedside. Laparoscopic biopsy may be hazardous in the adhesive type of peritoneal TB, and IGPB may be helpful. An algorithmic approach is suggested by doing IGPB or image-guided (particularly CT-guided) biopsy from associated lesions such as an enlarged lymph node, a thickened omentum, an extraperitoneal mass, etc., first, and, if the result is inconclusive, then laparoscopic biopsy may be the next step. (7)

Imaging features that overlap with other disease entities must be accounted for and the differential diagnoses ruled out. Tuberculosis peritonitis is the third most common cause of ascites after hepatic cirrhosis and neoplasms. (2) Diffuse lymphadenopathy may be a helpful sign of lymphatic peritoneal dissemination. (1,3) The most confusing aspect of the radiological pattern of TB peritonitis is the finding of associated adnexal masses. Moreover, the coexistence of TB and an ectopic pregnancy may add to diagnostic difficulties. (2)

Thus, judicious use of imaging improves diagnostic accuracy and clinical management, obviating unnecessary exploratory laparotomies. Although CT can image high-density ascites, peritoneal and mesenteric thickening, and mesenteric lymphadenopathy, it inadequately depicts multiple, thin, interlacing septations, which can be shown only by US. Further, especially in the presence of ascites, US clearly depicts all deep pelvic organs, the superficial peritoneum, and the omentum. As the omental lesions are located in front of the bowel gas and mesenteric fat, they are well delineated by US. Also, associated nodules on markedly thickened omentum can be visualized by scanning with high-frequency transducers. Thickened peritoneum can also be clearly seen. With affliction of the female genital tract, bilateral fallopian tube involvement with low-echogenic material, tubal dilatation with beaded appearance, and tuboovarian masses are striking in the presence of ascites. Endometrial involvement can also sometimes be seen. (2)

CONCLUSION

The omental cake, although an important imaging sign shared by a spectrum of disease entities, is a fairly reliable sign of abdominal TB in the appropriate clinical setting and patient population. Tuberculosis can also be considered when the omental cake occurs in conjunction with other imaging features, such as ascites with adhesions/septations and loculation, peritoneal and mesenteric thickening, bowel wall thickening, lymphadenopathy, and stigmata of TB in the chest. These features may be demonstrated by the combined use of US and CT, which are also useful tools for IGPB.

REFERENCES

(1.) Vardareli E, Kebapci M, Saricam T, et al. Tuberculous peritonitis of the wet ascitic type: Clinical features and diagnostic value of image-guided peritoneal biopsy. Dig Liver Dis. 2004;36:199-204. Comment in: Dig Liver Dis. 2004;36:175-177.

(2.) Yapar EG, Ekici E, Karasahin E, Gokmen O. Sonographic features of tuberculous peritonitis with female genital tract tuberculosis. Ultrasound Obstet Gynecol. 1995;6:121-125.

(3.) Shovman O, Levy Y, Tal S, Shoenfeld Y. Omental cake in a male. Isr Med Assoc J. 2003;5:73-74.

(4.) Levine CD, Patel UJ, Ghanekar D, et al. Benign extraovarian mimics of ovarian cancer. Distinction with imaging studies. Clin Imaging. 1997;21:350-358.

(5.) Quantrill SJ, Woodhead MA, Bell CE, et al. Peritoneal tuberculosis in patients receiving continuous ambulatory peritoneal dialysis. Nephrol Dial Transplant. 2001;16:1024-1027.

(6.) Wu JF, Li HJ, Lee PI, et al. Tuberculous peritonitis mimicking peritonitis carcinomatosis: A case report. Eur J Pediatr. 2003;162: 853-855.

(7.) Dhiman RK. Tuberculous peritonitis: Towards a positive diagnosis. Dig Liv Dis. 2004;36:175-177. Comment on: Dig Liver Dis. 2004; 36:199-204.

(8.) Kim Y, Cho O, Song S, et al. Peritoneal lymphomatosis: CT findings. Abdom Imaging. 1998;23:87-90.

Paritosh C. Khanna, MD; Suleman A. Merchant, MD; Anagha R. Joshi, MD

Prepared by Paritosh C. Khanna, MD, Suleman A. Merchant, MD, and Anagha R. Joshi, MD, Radiology Department, LTMG Hospital, Mumbai, India.
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Author:Khanna, Paritosh C.; Merchant, Suleman A.; Joshi, Anagha R.
Publication:Applied Radiology
Article Type:Case study
Geographic Code:9INDI
Date:May 1, 2007
Words:1597
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