Prevalence and Characteristics of Transsphincteric Anal Fistulas Unclassifiable by the Sumikoshi Classification: A Single-center Retrospective Study

Objectives: The Sumikoshi classification for anal fistulas is widely used in Japan; however, it does not include a category for transsphincteric fistulas. Therefore, low transsphincteric fistulas were included in type IIL (low intersphincteric) for convenience; however, high transsphincteric fistulas have not been properly classified. We defined high transsphincteric fistulas as type IIT and investigated their prevalence and clinical characteristics. Methods: Consecutive patients who underwent fistula surgery at our hospital were included. The operative and endoanal ultrasonography records were retrospectively reviewed, and the following cases were reclassified as type IIT: cases documented as transsphincteric fistulas or cases with written records and/or illustrations indicating that the fistula tract penetrated the upper two-thirds of the external anal sphincter. Results: Of the 1,069 eligible patients, 895 (83.7%) had type II (intersphincteric) fistulas. Among the type II subtypes, type IIL was the most common with 771 (86.1%) patients, whereas type IIT accounted for 54 (6.0%) patients. The direction of the primary opening was more posterior (62.2%) in patients with type II fistulas other than type IIT, but it was more anterolateral (55.6%) in patients with type IIT fistulas. Patients with type IIT fistulas were more likely to undergo sphincter-sparing surgery than patients with other type II fistulas (37.0 vs. 3.7%, p<0.001). Conclusions: Type IIT is not rare (6.0%) and should be treated as a complex fistula because of the greater involvement of the external anal sphincter. Surgeons may benefit by including type IIT as a new type II subclass in the Sumikoshi classification system.