MRI of Recurrent Fistula-in-Ano: Is it Different from Treatment-Naïve Fistula-in-Ano and How Does it Correlate with Anal Sphincter Morphology?

 The main aim of this study was to compare magnetic resonance imaging (MRI) findings of recurrent and treatment-naïve fistula-in-ano and to correlate imaging findings with anal sphincter morphology in recurrent fistula-in-ano.  This is a retrospective study of adult patients who underwent MRI for suspected fistula-in-ano in 2018. After excluding patients with alternative diagnosis, patients were stratified into recurrent (  = 103) and treatment-naïve (  = 106) fistula-in-ano groups. Two blinded radiologists reread MRI scans in consensus for fistula characteristics and anal sphincter morphology. We compared imaging features of recurrent and treatment-naïve fistula-in-ano, assessed the incidence of anal sphincter scarring among patients with recurrent fistula-in-ano, and studied its association with fistula features.  Two-hundred nine patients (187 males) with mean age of 40.6 (standard deviation: 12.2) years were included. Trans-sphincteric, inter-sphincteric, extra-sphincteric, and supra-sphincteric fistula-in-ano were seen in 63.6, 33, 2.9, and 0.5%, respectively. There were secondary tracts, supralevator extension, and secondary cause for fistula in 49.3, 12.9, and 14.8%, respectively. There was no difference between the fistula features of recurrent and treatment-naïve fistula-in-ano, except for significantly fewer external openings among recurrent fistula-in-ano (  = 0.005). Among patients with recurrent fistula-in-ano, MRI detected anal sphincter defect/scarring was seen in 53.4% (  = 55) and was significantly associated with posterior fistula-in-ano (  = 0.031), collections and/or supralevator extension (  = 0.010), and secondary tracts (  = 0.015).  Fistula features of recurrent and treatment-naïve patients were mostly similar. There was high incidence (53.4%) of MRI-identified anal sphincter scarring/defect among recurrent fistula-in-ano, which was significantly associated with posterior fistula, collections, supra or translevator extension, and secondary tracts. MRI-identified anal sphincter scarring is very common among patients with recurrent fistula-in-ano and seen in more than half of them.There was significantly higher incidence of sphincter scarring among patients who had posterior fistula, collections, supralevator/translevator extension, and secondary tracts.