IMPORTANCE The step after conservative treatment of periappendicular abscess arouses controversy, ranging from recommendations to abandon interval appendectomy based on low recurrence rates of the precipitating diagnosis to performing routine interval appendectomy owing to novel findings of increased neoplasm risk at interval appendectomy. To our knowledge, there are no randomized clinical trials with sufficient patient numbers comparing these treatments. OBJECTIVE To compare interval appendectomy and follow-up with magnetic resonance imaging after initial successful nonoperative treatment of periappendicular abscess. DESIGN, SETTING, AND PARTICIPANTS The Peri-Appendicitis Acuta randomized clinical trial was a multicenter, noninferiority trial conducted in 5 hospitals in Finland. All patients between age 18 and 60 years with periappendicular abscess diagnosed by computed tomography and successful initial nonoperative treatment from January 2013 to April 2016 were included. Data analysis occurred from April 2016 to September 2017. INTERVENTIONS Patients were randomized either to interval appendectomy or follow-up with magnetic resonance imaging; all patients underwent colonoscopy. MAIN OUTCOMES AND MEASURES The primary end point was treatment success, defined as an absence of postoperative morbidity in the appendectomy group and appendicitis recurrence in the follow-up group. Secondary predefined end points included neoplasm incidence, inflammatory bowel disease, length of hospital stay, and days of sick leave. RESULTS A total of 60 patients were included (36 men [60%]; median [interquartile range] age: interval appendectomy group, 49 [18-60] years; follow-up group, 47 [22-61] years). An interim analysis in April 2016 showed a high rate of neoplasm (10 of 60 [17%]), with all neoplasms in patients older than 40 years. The trial was prematurely terminated owing to ethical concerns. Two more neoplasms were diagnosed after study termination, resulting in an overall neoplasm incidence of 20% (12 of 60). On study termination, the overall morbidity rate of interval appendectomy was 10% (3 of 30), and 10 of the patients in the follow-up group (33%) had undergone appendectomy. CONCLUSIONS AND RELEVANCE The neoplasm rate after periappendicular abscess in this small study population was high, especially in patients older than 40 years. If this considerable rate of neoplasms after periappendicular abscess is validated by future studies, it would argue for routine interval appendectomy in this setting.

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Purpose Acute appendicitis may present as uncomplicated and complicated and these disease forms differ both epidemiologically and clinically. Complicated acute appendicitis has traditionally been defined as an appendicitis complicated by perforation or a periappendicular abscess, and an appendicolith represents a predisposing factor of complicated disease. There are histopathological differences between uncomplicated acute appendicitis and the previously established traditional forms of complicated acute appendicitis, but to our knowledge, the histopathological differences between uncomplicated acute appendicitis and complicated acute appendicitis presenting with an appendicolith have not yet been reported. The study purpose was to assess these differences with two prospective patient cohorts: (1) computed tomography (CT) confirmed uncomplicated acute appendicitis patients enrolled in the surgical treatment arm of the randomized APPAC trial comparing appendectomy with antibiotics for the treatment of uncomplicated acute appendicitis and (2) patients with CT-verified acute appendicitis presenting with an appendicolith excluded from the APPAC trial. Methods The following histopathological parameters were assessed: appendiceal diameter, depth of inflammation, micro-abscesses, density of eosinophils, and neutrophils in appendiceal wall and surface epithelium degeneration. Results Using multivariable logistic regression models adjusted for age, gender, and symptom duration, statistically significant differences were detected in the depth of inflammation ≤ 2.8 mm (adjusted OR 2.18 (95%CI: 1.29-3.71, p = 0.004), microabscesses (adjusted OR 2.16 (95%CI: 1.22-3.83, p = 0.008), the number of eosinophils and neutrophils ≥ 150/mm 2 (adjusted OR 0.97 (95%CI: 0.95-0.99, p = 0.013), adjusted OR 3.04 (95%CI: 1.82-5.09, p < 0.001, respectively). Conclusions These results corroborate the known clinical association of an appendicolith to complicated acute appendicitis.

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