Often an incidental finding, a mucocoele of the appendix is the macroscopic description of a dilated, mucin-filled appendix. Mucocoeles can have a number of underlying pathologies. In this instance, the appendiceal lumen was obstructed by malignancy, and appendiceal distension occurred secondary to inspissated mucus. Other causes for obstruction included endometriosis, inflammatory stricture, faecolith or rarely, a focus of mucin-secreting hyperplastic epithelium. Rarely, such a mucocoele may rupture, spilling otherwise innocuous mucus into the peritoneal cavity.

Mucocoeles may otherwise be due to mucin-secreting tumours, including mucinous cystadenoma and mucinous cystadenocarcinoma, which replace appendiceal mucosa. These cause appendiceal rupture more frequently, leading to free mucus and mucinous collections in the peritoneal cavity, that may continue to undergo cellular proliferation, filling the abdomen with tenacious, semisolid mucin known as pseudomyxoma peritoneii.