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 by 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 lesions:
- mucosal hyperplasia (most common 11)
- mucinous neoplasms
- appendiceal carcinoid
- adjacent cecal tumor obstructing the appendiceal orifice
- myxoglobulosis: seen as with multiple small intraluminal globules which can calcify and produce 1-10 mm mobile calcifications
If a contrast examination is performed, there is usually non-filling or partial-filling of the appendix. Where there is a large mucocoele, 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.
- typically seen as a well-circumscribed, low-attenuation, spherical or tubular mass contiguous with the base of the cecum
- the finding of 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
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 ileo-colic intussusception 9
Differentiating benign (non-neoplastic mucocoele 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.