Endoscopic Manifestation of Disease Progression from Low-Grade Appendiceal Mucinous Neoplasm to Pseudomyxoma Peritonei: A Three-Year Follow-Up Case Report

Appendiceal mucinous neoplasm is a rare tumor of the appendix with the potential to progress to pseudomyxoma peritonei (PMP), often misdiagnosed as acute appendicitis or appendiceal abscess. Here, we present a case of an older female with low-grade appendiceal mucinous neoplasm (LAMN) type 2, manifesting as chronic abdominal pain, distension and elevated carcinoembryonic antigen levels over 3 years. Despite typical imaging findings, the patient was initially misdiagnosed with chronic appendicitis, leading to the development of an appendiceal abscess and extensive peritoneal metastases. The patient underwent three colonoscopies during hospitalization, revealing atypical lesions differing from the usual LAMN presentation near the appendiceal orifice. This case underscores the diverse endoscopic appearances of LAMN type 2 and emphasizes the importance of early recognition and treatment to prevent PMP progression, highlighting the crucial role of endoscopic physicians in appendix-related diseases.