| Stage |
Characteristics |
| I |
Intersphincteric fistula not involving the external anal sphincter fibers |
| IA |
Simple linear, non-branching intersphincteric tract |
| IB |
Intersphincteric tract with at least one of the following: Horseshoe tract, secondary extensions and associated abscess cavities, anterior fistula in female patients, history of previous surgery for anal fistula |
| II |
Low transsphincteric fistula involving less than 30% of external anal sphincter fibers |
| IIA |
Simple linear, non-branching low transsphincteric tract |
| IIB |
Low transsphincteric tract with at least one of the following: Horseshoe tract, secondary extensions and associated abscess cavities, anterior fistula in female patients, history of previous surgery for anal fistula |
| III |
High transsphincteric fistula involving more than 30% of external anal sphincter fibers |
| IIIA |
Simple linear non-branching high transsphincteric tract |
| IIIB |
High transsphincteric tract with at least one of the following: Horseshoe tract, secondary extensions and associated abscess cavities, anterior fistula in female patients, history of previous surgery for anal fistula |
| IV |
Unusual types of fistula: suprasshincteric and extrasphincteric |
