Appendiceal mucoceles occur when there is an abnormal accumulation of mucin causing abnormal distention of the vermiform appendix due to various neoplastic or non-neoplastic causes.
The reported prevalence at appendectomy is 0.2-0.3%. They are thought to typically present in middle-aged individuals, particularly considering the epidemiology of the mucinous neoplasms. Though carcinoid tumor is the most common primary appendiceal neoplasm in surgical pathology series, mucoceles due to neoplasms are the most common appendiceal tumors detected on imaging.7
The term mucocele is simply a macroscopic description of an appendix that is grossly distended by mucus 7,12. They may be caused by either benign or malignant lesions, categorized into four histologic types:
- mucus retention cyst (due to obstruction most commonly by a fecalith)
- mucosal hyperplasia (analogous to a hyperplastic colonic polyp)
- mucinous cystadenoma of appendix (most common11)
- mucinous cystadenocarcinoma of appendix
- myxoglobulosis: a rare mucocele variant seen with multiple small intraluminal globules which can calcify and produce 1-10 mm mobile calcifications
It can be characterized by a right iliac fossa mass with peripheral calcifications 12.
If a contrast examination is performed, there is usually non-filling or partial-filling of the appendix. Where there is a large mucocele, the associated mass effect can cause the indentation or lateral displacement of the cecum.
Typically cystic mass with variable internal echogenicity 8. The presence of an "onion sign" (sonographic layering within a cystic mass) is considered a highly suggestive feature 2,6. Acoustic shadowing may be present due to the mural calcifications 12.
They are typically seen as a well-circumscribed, low-attenuation, spherical or tubular mass contiguous with the base of the cecum.
- curvilinear mural calcification suggests the diagnosis but is seen in less than 50% of cases
- intra-luminal bubbles of gas or a gas-fluid level within a mucocele indicate the presence of superinfection, which can occur in both benign and malignant mucoceles
- mural nodularity and irregular wall thickening are suggestive of a malignant process 12
When identifying a mucocele on CT, search for extraluminal mucin is mandatory, which are low-attenuated deposits commonly seen in certain locations 12:
- periappendiceal space
- peritoneal cavity
- at the surface of abdominal viscera, including ovaries and bowel
Seen as a rounded right iliac fossa mass and the typical signal characteristics include:
- T1: depending on the mucin concentration, the signal may be variable, from hypointense to isointense 9
- T2: hyperintense
Treatment and prognosis
Treatment is usually surgical.
- rupture: may lead to pseudomyxoma peritonei (mucinous ascites) if the underlying cause is neoplastic 11,12
- can act as a lead point and result in an ileocolic intussusception 9
Differentiating benign (non-neoplastic mucocele and mucinous cystadenoma) and malignant (mucinous cystadenocarcinoma) appendiceal lesions can be difficult. Wang et al. 10 found a statistically significant difference in wall irregularity and soft-tissue thickening between malignant and benign cases.