A mucocele is an appendiceal mass characterized by hypersecretion of mucus contained within the appendix causing dilatation of the lumen.

Mucoceles of the appendix are caused by excessive mucous production secondary to retention cyst (simple) (18%), mucosal hyperplasia (20%), mucinous cystadenoma (52% to 84%), or mucinous cystadenocarcinoma (10% to 20%). They occur in males twice as frequently as females and most often in the fifth or sixth decade. The incidence of mucocele of the appendix is 0.2% and 0.3% of all appendectomies.

Mucocele of the appendix is an elongated tubular mass with a very specific onion-skin texture of the internal structure, giving the appearance of echogenic layers. There is no internal blood flow, and Doppler signal is confined to the outer walls of the mass. Although the mass is usually seen using transvaginal ultrasound, it is also usually visible using a transabdominal approach in the right lower quadrant. The dreaded complication of mucocele rupture or leak is pseudomyxoma peritonei, which appears as diffuse gelatinous ascites.

Most cases are incidental findings and often misdiagnosed. In women, it is commonly diagnosed as an ovarian or tubal mass. The correct preoperative diagnosis based on imaging studies is made in only 15% to 29% of cases.