Analysis of Deep Posterior Anal Fistulas by Magnetic Resonance Imaging: Site of Primary Abscess and Extension Patterns According to the Primary Abscess Depth

Objectives: The aim of this study was to use magnetic resonance imaging (MRI) to elucidate the site and depth of the primary abscesses associated with deep posterior anal fistulas and their extension patterns. Methods: We analyzed 176 consecutive patients with deep posterior anal fistulas and classified the fistulas according to whether the MRI-detected site of the primary abscess was at a superficial or a deep external anal sphincter (EAS) level. Results: The distance between the anal center and the primary abscess center was significantly shorter than the length of the EAS and radius at an angle of 45°. In addition, deep posterior anal fistulas with primary abscesses located at the deep EAS level penetrated the EAS significantly more laterally and made external openings at a significantly more lateral site than when the primary abscess was located at a superficial EAS level. Conclusions: Primary abscesses associated with deep posterior anal fistulas are located in the posterior intersphincteric space or in the EAS muscle itself, not in Courtney's space, as had previously been claimed.