Anal fistula at roof of ischiorectal fossa inside levator-ani muscle (RIFIL): a new highly complex anal fistula diagnosed on MRI

Abdom Radiol (NY). 2021 Dec;46(12):5550-5563. doi: 10.1007/s00261-021-03261-y. Epub 2021 Aug 29.


Background: As experience with anal fistula imaging (MRI) has increased, new pathways of fistula extension have been identified. A recently described pathway is the 'outer-sphincteric space' present between the external anal sphincter and its covering outer fascia. A new type of complex fistula is being described which is present in the outer-sphincteric space and continues superiorly along the lateral border of the external anal sphincter to the infero-lateral surface of the puborectalis and levator-ani. In effect, these outer-sphincteric fistulas are at the roof of the ischiorectal fossa inside the levator muscle (RIFIL). These fistulas are not transsphincteric fistulas as they remain inside the levator muscle and do not enter the ischiorectal fossa.

Methods: The MRI scans of consecutive anal fistula patients operated over the last two years were analyzed retrospectively.

Results: Of 419 operated fistula patients analyzed, 42(10%) had RIFIL and 377 non-RIFIL fistulas. Compared to non-RIFIL fistulas, there were significantly more recurrent, multiple tracts, horseshoe, supralevator, and suprasphincteric fistulas in the RIFIL group. RIFIL fistulas were significantly more complex than non-RIFIL fistulas(85.7% vs 38.5%, p < 0.00001) and the surgery failure rate was also significantly higher in the RIFIL group (30.6%) than in the non-RIFIL fistula (7.2%) group(p = 0.0001).

Conclusion: RIFIL are highly complex fistulas. Proper diagnosis by MRI, surgical access, and subsequent management of these fistulas is quite challenging and they are associated with poor prognosis. Missing their diagnosis would lead to higher recurrence rate. These have not been described previously and were perhaps confused with high transsphincteric infralevator fistulas in ischiorectal fossa.

Keywords: Anal fistula; Fistula-in-ano; Intersphincteric space; Ischiorectal fossa; Levator-ani; MRI.

MeSH terms

  • Anal Canal / diagnostic imaging
  • Humans
  • Magnetic Resonance Imaging
  • Pelvic Floor
  • Rectal Fistula* / diagnostic imaging
  • Rectal Fistula* / surgery
  • Retrospective Studies