EP490 - A Rare Diagnosis Behind Common Symptoms: Appendiceal Mucocele In The Differential

Purpose/Background: Primary appendiceal neoplasms are rare, accounting for approximately 1-2% of all appendectomy specimens. Among these, mucinous neoplasms represent only 1% of this already small cohort, making them an exceptionally uncommon entity. These lesions can be further subdivided into low-grade appendiceal mucinous neoplasms (LAMN) and high-grade appendiceal mucinous neoplasms (HAMN) based on their cellular atypia and architectural features. Given their rarity and often nonspecific presentation, these tumors are sometimes clinically mistaken for acute appendicitis or radiographically mistaken for ovarian mucinous tumors due to overlapping imaging characteristics. A delay in diagnosis frequently leads to significant disease progression, increasing the risk of severe morbidity and potentially fatal outcomes.

Methods/Interventions: We report a case of a 63-year-old female with a history of ulcerative colitis (UC) who presented to the emergency department with lower abdominal pain, initially attributed to an acute colitis flare. Computed tomography revealed segmental colonic wall thickening extending from the cecum to the proximal transverse colon, as well as a 9.6 x 4.7 cm minimally complex cystic lesion which was presumed to originate from the right adnexa on a pelvic ultrasound. The patient was subsequently referred to gynecology; a transvaginal ultrasound was repeated for second opinion, which showed a mobile, avascular, oval shaped mass, distinct from the uterus and right ovary, with an "onion skin" appearance, raising concern for an appendiceal mucocele. Given these findings, she was evaluated by colorectal surgery and underwent a colonoscopy which demonstrated thickened and congested mucosa in the ascending and transverse colon consistent with Mayo 3 colitis. Due to the possibility of an underlying malignancy within the appendiceal mucocele in the setting of the patient's poorly controlled UC, she proceeded to the operating room for a planned robotic assisted single incision (rSILS) right colectomy with end ileostomy creation. Intraoperatively, a large appendiceal mucocele encased by an inflammatory rind was identified alongside active transverse colitis, confirmed grossly and endoscopically.

Results/Outcomes: Final pathology confirmed a LAMN (pTis). The patient recovered well and was discharged home on postoperative day 3 with plans to initiate biologic therapy for her UC in the coming weeks.

Conclusion/Discussion: This case highlights the importance of recognizing the unique imaging features of an appendiceal mucocele, a rare pathology with potential for serious complications. A comprehensive evaluation and multidisciplinary approach allowed for successful management with complete resection by the appropriate surgical service. By sharing this report, we aim to reinforce the awareness of this uncommon condition and highlight the critical role of early diagnosis and intervention in preventing progression to pseudomyxoma peritonei, a highly feared and life-threatening complication associated with delayed recognition.

Appendiceal Mucocele
Figure 1. A) CT scan (coronal view) of appendiceal mucocele measuring 9.6 x 4.7 cm in diameter, B) Transvaginal ultrasound displaying an avascular, oval shaped mass, with an "onion skin" appearance suggestive of an appendiceal mucocele, C) Intraoperative photo demonstrating right colectomy specimen with appendiceal mucocele (instrument tip positioned at the appendiceal orifice), D) High powered histologic view of dilated appendix with a fibrotic, attenuated wall with patchy chronic inflammation. The mucosa and muscularis propria are no longer present, and a flattened, low-grade dysplastic epithelium lines portions of the wall.