Radiology Resident Case of the Week
Etiology/Pathophysiology
Mucoceles of the appendix is rare, appearing in 0.2-0.3% of surgical appendectomy specimens (1). They are pathologically divided into 4 categories (2). A very rare type is secondary to occlusion of the lumen from post-inflammatory scarring, progeric atrophy, congenital obstruction of Gerlach's valve or extramural compression. This type leads to atrophic mucosa. All other types are classified into a spectrum from mucous hyperplasia to mucinous cystadenoma to mucinous cystadenocarcinoma depending on the pathology of the mucosa. Whatever the cause, obstruction of the lumen and accumulation of yellow mucous within the appendiceal lumen result.
Pathology:
Miscellaneous
About 25% of mucoceles are from mucosal hyperplasia (3). These typically have minimal distention. Mucinous cystadenomas, which account for about 60% of mucoceles, are more markedly distended. However, they are typically asymptomatic, and found incidentally as in our case. Mucoceles up to 40 x 24 x 20 cm have been reported (4). About 20% have extra-appendiceal extrusion of mucus. If no cells are present in this peritoneal mucous, the prognosis is excellent. Mucinous cystadenocarcinomas (10-15% of cases) are more likely symptomatic--this diagnosis is made by either neoplastic glands invading the wall or by the presence of cells in the peritoneal mucous. It is thought by some that pseudomyxoma peritonei is a complication of only mucinous cystadenocarcinoma (5, 6). However, other authors believe this can complicate either benign or malignant mucoceles (7,8) although pseudomyxoma peritonei from the former would carry a better prognosis.
Mucinous cystadenocarcinomas are extremely rare (benign:malignant about 10:1), but are believed to arise in cystadenomas. And there is a high correlation of synchronous or metachronous colorectal adenomas and carcinomas (up to 20% in two series). There have also been reports of associations with gastrointestinal tract, breast, ovary and kidney tumors. It is thought that only mucinous cystadenocarcinomas lead to pseudomyxoma peritonei.
Imaging
Characteristic Ultrasound Findings:
There is typically excellent through-transmission and posterior wall enhancement (9). When the wall is calcified, posterior acoustic shadowing may occur, but often cannot be appreciated (10). The wall thickness varies, but if the wall is greater than 6mm, one should also consider uncomplicated acute appendicitis. The internal features vary from anechoic to hyperechoic, and may be dependent. Internal septations, polypoid lesions extending into the lumen and irregular shapes seem to be associated with the malignant variety, although some papillary processes may be seen in mucinous cystadenomas.
Characteristic CT findings:
Typically it is a low-attenuation (0-40 H) smooth or lobulated mass. The more complex and irregularly shaped mucoceles tend to be mucinous cystadenocarcinomas (7). They may have simple or multiple cystic components and some solid elements. These may even demonstrate infiltration into adjecent organs such as colon, bladder and ureter. Attenuation is near that of water. Curvilinear or punctate calcification in the lesion is strongly suggestive of mucinous cystadenoma (yellow arrow), and this is often not seen on plain film. Amorphous calcifications may be seen in the malignant type (7). This is from chronic inflammatory precess incited by the irritating mucous. Vortical folds (red arrow), mimmicking intussusception, have also been described. A pitfall is that fluid filled terminal ileum may resemble a mucocele, so delayed scanning may be warranted in some cases.
Characteristic BE fingings:
A smooth filling defect at the tip of the cecum is classic. Partial filling of the appendix has been reported only twice (11,12). The "volcano sign" described endoscopically is seen as a "vortical fold" pattern on barium studies, which is the concentric ring appearance of the cecal mucosal folds as they approach the obstructed appendiceal orifice. This may give the false appearance of intussusception, which only rarely complicates mucocele. The finding of an appendiceal mucocele should prompt a search for an associated tumor--6-fold increased incidence of colon adenocarcinoma (13), and there may be an association with mucin-secreting tumors of the ovary (14).
DDX
From US, must consider:
Fluid filled small bowel
Fluid in small or large bowel diverticulum
Appendiceal/Diverticular abscess
Mesenteric Cyst
Seroma
From CT, must consider:
Unopacified small bowel
Appendiceal abscess
Small bowel/ Colonic diverticulum
From BE, must consider:
Appendiceal carcinoid
Inverted appendiceal stump
Normal ileocecal valve
Lipoma
Adenocarcinoma of Cecum
Stool
Conclusion
The findings in this case are typical of a mucocele of the appendix. Although the mucinous cystadenoma type is usually asymptomatic, as in this case, these findings should allow the radiologist to suggest this as a diagnosis. Recognition of this entity is important in asymptomatic patients since it may be malignant or become malignant. It may also lead to pseudomyxoma peritonei, volvulus or intusseception, and may become infected. In the symptomatic patient, it is important to differentiate this from other entities for surgical planning.
Key references
1) Collins DC. 71,00 human appendix specimens: A final report summarizing forty years' study. Am J Protocol 14:265, 1963.
2) Higa E, Rosai J, Pizzimbono C, et al. Mucosal hyperplasia, mucinous cystadenoma, and mucinous cystadenocarcinoma of the appendix: A re-evaluation of appendiceal "mucocele." Cancer 1973:139:392-400
3) Mitros, Atlas of GI Pathology
4) Wallis, J.W. et al, Giant Mucocele of the Appendix, AJR, 165, October1995
5) Gibbs, NM, Nucinous cystadenoma and cystadenocarcinoma of the vermiform appendix with particular reference to mucocele and pseudomyxoma peritonei, J Clin Pathol 1973:32:1525-1541
6) Dachman A, Lichtenstein J, Friedman A. Mucocele of the appendix and pseudomyxoma peritonei. AJR 1985;144:923-929
7) Balthazar, Emil J, Computed Tomography of the Abnormal appendix, Journal of Computer Assisted Tomography, 12(4):595-601, July/August 1988
8) Skaane, Per, Ruud, Tom E., Haffner, Jon, Ultrasonographic Features of Mucocele of the Appendix, J Clin Ultrasound 16:584-587, October 1988
9) Madwed D, Mindelzun R, and Jeffrey RB Jr. Mucocele of the Appendix: Imaging Findings. AJR 159:69-72, July 1992
11) Bahia JO, Wilson MH. Mucocele of the Appendix Presenting as an Adnexal Mass. J Clin Ultrasound 17:62-66, January 1989.
12) Horgan JG, Chow PP, Richter JO, et al: CT and sonography in the recognition of mucocele of the appendix. AJR 143:959, 1984
13) Wolff M, ahmed N. Epithelial neoplasms of the vermiform appendix. II. Cystadenoma, papillary adenoma, and adenomatous polyps of the appendix. Cancer 1976; 37:2511-2522
14) Young RH, Gilks CB, Scully RE. Mucinous tumors of the appendix associated with mucinous tumors of the ovary and pseudomyxoma peritonei. Am J Surg Pathol 1991; 15:415-429
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