Appendiceal mucoceles occur when there is an abnormal accumulation of mucin causing abnormal distention of the appendix. They are due to either nonneoplastic process, such as luminal obstruction, or mucin-secreting epithelial tumors.
The reported prevalence at appendectomy is 0.2-0.3%. They are thought to typically present in the middle-aged individuals, particularly considering the epidemiology of the mucinous neoplasms.
The term mucocele is simply a macroscopic description of an appendix that is grossly distended by mucus 7,12. They result from the chronic appendiceal obstruction that may be caused by either benign or malignant lesion:
- mucosal hyperplasia (most common 11)
- mucinous neoplasms
- appendiceal carcinoid
- adjacent cecal tumor obstructing the appendiceal orifice
- 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 an air-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 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.