eP412 - Primary Exploration of Surgical Management for a New Anal Fistula Type: Horseshoe Fistula of Inner External Sphincter Complex

Purpose/Background: The existing infection within the external anal sphincter (EAS) has not previously been reported. This study describes a new type of anal fistula, the horseshoe fistula of inner external sphincter complex, and outlines its surgical management.Methods/Interventions: This retrospective case series includes six patients with horseshoe fistulas of inner external sphincter complex surgically corrected by the same surgeon between 2018 and 2023. The modified transanal opening of intersphincteric space (MTROPIS) was performed with draining towards the lumen of the anal canal and rectum with or without placement of loose setons for counter drainage. The magnetic resonance imaging (MRI) features, fistula healing and Wexner fecal incontinence scores were recorded.Results/Outcomes: Six patients with horseshoe fistula of inner external sphincter complex underwent the MTROPIS procedure, with a mean follow-up period of 30.5 ± 29.3 weeks. All patients had a clear inter-sphincteric space on the MRI imaging, with only 16.7% (1/6) involving an ischiorectal fossa. No patients had fistula tracts spreading to the supralevator and deep postanal space. All patients had completely healed by eight weeks of follow up postoperatively. No patients experienced fecal incontinence, with two patients developing mild flatus and liquid seepage postoperatively (Wexner score=3).Conclusion/

Discussion: The horseshoe fistula of inner external sphincter complex is a new type of anorectal fistula. It is based on the hypotheses that the infection originate from the inner external sphincter complex other than from intersphincteric space in virtue of cryptoglandular theory. MTROPIS procedure was safe and effective with good anorectal function.Figurea-c A patient with half-a-circle horseshoe fistula of inner external sphincter complex treated with MTROPIS (Distinguishing from the traditional TROPIS, a full-thickness incision of IAS was done, with a curvilinear incision made along external sphincter fiber to maintain fluent drainage.). d-f A patient with the horseshoe fistula of inner external sphincter complex involving circumferential EAS performed with MTROPIS. The inner external sphincter (yellow arrow) infections were marked. g Schematic diagram for horseshoe fistula of inner external sphincter complex.