Background: A previous report proposed ultrasonography-based classification as a promising means of predicting pediatric spontaneously resolving appendicitis. The present study investigated the long-term prognosis of supportive care for low-grade appendicitis identified by ultrasonography, defined as an appendix with a smooth submucosal layer irrespective of blood flow or an appendix with an irregular layer and increased blood flow. Methods: The present, retrospective cohort study enrolled patients under 16 years of age with acute appendicitis at a children's hospital between October 2010 and September 2016. The inclusion criteria were ultrasonography findings showing an appendix with (1) full visualization, (2) a diameter ≥6 mm, (3) a smooth submucosal layer or an irregular layer with increased blood flow, and (4) no appendiceal mass, abscess, or perforation. The exclusion criteria were: (1) a history of acute appendicitis, (2) antibiotic administration within 72 hours before diagnosis, and (3) antibiotic administration or surgery before supportive care. The primary outcome was the event-free duration, defined as a period of supportive care alone with no additional intervention or recurrence of appendicitis. Results: One hundred and eighty-two patients were enrolled. The median Alvarado score was 7 (interquartile range, 6-8), and the median follow-up duration in event-free cases was 1,922 days (interquartile range, 1,347–2,614 days). The event-free rate was 75.0%, 67.0%, and 62.5%, at 1, 2, and 5 years, respectively. Conclusion: The long-term, event-free rate exceeded 60% in patients with low-grade appendicitis defined by ultrasonography who received neither surgery nor antibiotic treatment. Most recurrences occurred within 2 years of the initial diagnosis.
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