Histopathologically, “mucocele” of the appendix may be caused by a variety of benign and malignant causes of obstruction of the lumen which lead to overproduction of mucus. It is subclassified depending on the depth of invasion of the wall, cellular atypia, extra-appendiceal mucin, and the presence of signet ring cells (associated with poor prognosis). Modern classification is divided into nonneoplastic variants including mucous retention cyst and mucosal hyperplasia and neoplastic variants, including mucosal adenoma (confined to mucosa, mild to moderate cytologic atypia and no atypical mitotic figures), low-grade mucinous neoplasm (low-grade atypia, the loss of muscularis mucosae, and or extra-appendiceal cells, mucocele rupture with extra-appendiceal mucin can lead to this) and mucinous cystadenocarcinoma (high-grade cytologic atypia) [Table 1]. Most benign mucoceles are due to mucinous adenoma which is sessile with dilated appendix with circumferential involvement of mucosa, without extra-appendiceal mucin and are asymptomatic with no recurrence risk after complete excision. Nonneoplastic variant results from chronic, long-standing obstruction of the appendiceal lumen by any nonneoplastic process and are called inflammatory, obstructive, simple mucocele, or retention cyst of the appendix. Mucinous neoplasm of the appendix of low malignant potential is one of the most common causes of “pseudomyxoma peritonei” associated with mucinous peritoneal implants and leads to extensive peritoneal disease without associated lymph node, lung, or liver metastases.[4,5]
Table 1: Pathological types of Appendiceal mucocele.
|Mucosal retention cyst
||Low-grade mucinous neoplasm
Radiological features and pathological correlation
The ultrasound is the preliminary diagnostic tool and may be decisive for the differential diagnosis of appendiceal mucocele and acute appendicitis. The appendicular diameters of ≥15 mm are the threshold for diagnosing appendiceal mucocele, (sensitivity of 83% specificity of 92%) versus 6-mm outer diameter for the diagnosis of acute appendicitis.[3,8] At the ultrasound, mucocele appears as an elliptical cystic mass with or without acoustic shadowing from dystrophic mural calcification. A mucocele is usually encapsulated on ultrasound, with variable echogenicity in relation to the quantity and fluidity of the mucous contained. The inner wall can appear irregular due to the presence of debris or epithelial hyperplasia. The lumen of giant mucocele can have echogenic layers surrounded by mucin so-called “onion skin sign,” pathognomonic for mucocele. Lesser dilated portion of appendix can give “drumstick or pear-shaped” appearance. On ultrasound, mucinous ascites show low-level echoes and poorly defined septation. Ultrasound-guided Fine needle aspiration has not been proposed to avoid dissemination of the mucous leading to pseudomyxoma peritonei.
The CT scan is the modality of choice showing near water density dilated appendix to more than 15 mm with or without wall calcification. When imaged with CT, simple mucocele appears as a well-defined water density mass in the right lower quadrant with curvilinear calcification within the wall. The presence of wall calcification supports the diagnosis of mucocele, but the presence of wall calcification cannot exclude benign from malignant mucocele. Atypical imaging features such as heterogeneity of outer wall diameter, calcifications, periappendiceal fat stranding, or intraperitoneal free fluid are not specific in differentiating malignant from benign mucoceles and can be seen with secondary inflammation/infective process. However, concerning features such as irregular walls, soft-tissue mass, ascites, or pseudomyxoma peritonei can differentiate malignant vs benign etiology. The soft-tissue thickening and wall irregularity are highly suggestive of the neoplastic process. Intraluminal air foci or an air-fluid level within a mucocele are characteristics for secondary infection. CT findings of mucinous cystadenoma and mucocele are indistinguishable and appear as encapsulated low-attenuation cyst. At CT, low-grade mucinous neoplasm appears as a markedly distended appendix (>2 cm) containing low-attenuation mucin with wall calcification. It can demonstrate wall thickening, periappendiceal fat stranding and extraappediceal hypodense mucin from tumor infiltration or superimposed appendicitis. Search for extracellular mucin should be done once mucocele is identified in imaging, which may be localized in perieppendiceal or pericecal region, mesentery or omentum or in the dependent areas in peritoneum such as pouch of Douglas, rectovesical pouch, around ovaries or perihepatic space. CT findings of pseudomyxoma peritonei includes mucinous ascites (hyperdense, internal septations with fixed bowel loops versus central free floating bowel loops in simple ascites), peritoneal soft-tissue implants, scalloped appearance on solid organs with or without parenchymal involvement due to mass effect of tumor implants, omental caking, and may have linear or punctate calcifications of the mucinous deposits. Findings of small-bowel or ureteral obstruction or extensive upper abdominal disease associated with poor prognosis. On CT scan, the mucinous cystadenocarcinoma appears as low attenuating mucin filled dilated appendix or solid appendiceal mass, periappendiceal fat stranding/soft-tissue deposits, pseudomyxoma peritonei and intraperitoneal metastasis. At CT, “arrowhead sign” (focal thickening of the cecal wall at appendiceal orifice with luminal contrast appearing as arrowhead), appendiceal wall thickening, peri-appendiceal fat stranding, small-bowel mural thickening, lymphadenopathy, appendicolith, and abscess points toward inflammatory etiology like acute appendicitis. Other differentials include cystic ovarian neoplasm or tubo-ovarian abscess, duplication cyst (tubular cystic structure communicating with bowel), hydrosalpinx (usually symptomatic), mesenteric cyst, and intussusception. Mesenteric cyst is reportedly rare and most often detected incidentally. They can be developmental, acquired, neoplastic or infectious or degenerative. In the second case, there was an incidental cystic structure abutting the tip of appendix which raised the possibility of mucinous deposit with appendicular mucocele. This was resected along with appendectomyand the pathology was a benign squamous lined cyst with underlying fibrous scar consistent with mesenteric inclusion cyst with squamous metaplasia. These findings should be well documented and if possible, the surgeon should be informed and notified of the possibility of carcinoma in 10% of the cases. Final diagnosis can be confirmed by histopathological evaluation of appendectomy specimen.
On magnetic resonance imaging, the features are similar to CT scan of a cystic mass which is hypointense on T1WI, hyperintense on T2WI; however, it is difficult to assess intraluminal air and calcification.
Plain abdominal radiography may show soft-tissue opacity with curvilinear peripheral calcification in the right lower quadrant. On barium enema, there may be a sub-mucosal or extrinsic mass indenting and displacing the medial wall of caecum with filling the defect.
The treatment of AM is en bloc resection with conventional surgery preferred over laparoscopic surgery due to the risk of rupture. When benign mucocele is suggested, simple appendectomy is performed. If malignant imaging features are suspected, then right hemicolectomy is performed along with abdominal exploration due to the association of AM with other mucinous neoplasms of colon and ovary.