When an untreated inflamed appendix progresses to suppuration, the usual course is rupture with abscess formation. Rarely, acute appendicitis resolves by fixation of the appendix to an adjacent viscus or to the parietal peritoneum and thence to the skin, allowing drainage of pus either into the viscus or externally. Unfortunately, when the appendix forms a fistula in this manner, further trouble can be expected.
In a review of the literature in 1957, Kjellman1 defined an appendiceal fistula as "the primary perforation of the appendix to an adjacent hollow viscus or the skin" and excluded fistulas resulting as sequellae of appendicitis treated surgically. Kjellman noted that only eight cases of fistula between the appendix and intestine had been operated upon to that time and reported three more. Several cases found at autopsy were mentioned. The organ most commonly involved by fistula from the appendix is the bladder. Forbes and Rose
Walker LG, Rhame DW, Smith RB. Enteric and Cutaneous Appendiceal Fistulae. Arch Surg. 1969;99(5):585–588. doi:10.1001/archsurg.1969.01340170037009