Interval appendicitis shows histological differences from acute appendicitis and may mimic Crohn disease and other forms of granulomatous appendicitis

While patients presenting with clinical signs and symptoms of acute appendicitis (AA) often receive surgical intervention shortly after presentation, certain patients may instead receive non-operative management initially, with appendectomy later. The histology of such interval appendicitis (IA) has only been described in small series. Also, we have noticed a recent increase in the incidence of IA specimens at our institution.We identified appendectomy specimens in our department during 2018 with available haematoxylin and eosin slides and electronic clinical data, and evaluated multiple histological findings. Cases were then divided into AA and IA, based on clinical history (AA if the patient presented to the hospital within 1 week of symptom onset and underwent appendectomy within 48 h; IA if appendectomy was delayed at least 1 week). Changes between groups were compared. The cohort included 165 cases (125 AA, 40 IA). Findings significantly more common in AA included mucosal acute inflammation, mural acute inflammation and acute serositis. Findings significantly more common in IA included Crohn-like mural inflammation, mural fibrosis, goblet cell hyperplasia, granulomas, xanthogranulomatous inflammation, haemosiderin-laden macrophages and granulation tissue. The rate of IA in 2018 (24%) was noticeably higher than in previous years.Acute inflammatory changes are more common in AA but can remain present in IA. Mural fibrosis, serosal adhesions, haemosiderin-laden macrophages and granulation tissue suggest IA. Granulomas and xanthogranulomatous inflammation can also be seen in IA, and Crohn-like mural inflammation is not uncommon. These histological patterns can guide signout and prevent diagnostic errors, particularly when clinical information is unavailable.