Levator ani involvement in perianal fistulas: MRI-based insights into complex anatomy

This study aimed to characterize the magnetic resonance imaging (MRI) features of perilevator fistulas involving the levator ani (LA) muscle and to evaluate whether anatomical or systemic factors are associated with LA involvement.

A retrospective review of 1,697 pelvic MRI scans (2011-2025) identified 89 patients with perilevator fistulas. Patients were classified as LA (+) when the tract directly abutted any component of the levator ani and as LA (-) when separated by ischioanal fat. MRI evaluation included fistula type, orifice localization, associated complications, tract diameter, wall thickness, and morphometric features including the anorectal angle, anal canal length, and the thickness of the anal sphincters and intersphincteric groove. Clinical and etiologic data were also compared between groups. Univariable and multivariable logistic regression analyses were performed to identify independent predictors of LA involvement. The discriminative performance of the final model was assessed using ROC curve analysis.

Seventy-one patients (79.8%) had LA (+) and 18 (20.2%) had LA (-) fistulas. One-third of LA (+) tracts lacked an external orifice, ending as blind sinuses in deep pelvic planes. LA (+) fistulas were predominantly transsphincteric and cryptoglandular, whereas LA (-) fistulas were more often extrasphincteric and associated with inflammatory bowel disease. Multivariable analysis demonstrated that fistula type, etiology, and increased thickness of the fistulous wall and external anal sphincter (EAS) were independently associated with LA involvement, yielding an AUC of 0.938.

In perilevator fistulas, transsphincteric configuration and increased thickness of the fistulous wall and EAS represent imaging factors associated with levator ani involvement, likely reflecting upward extension of sepsis along the outer sphincteric space. Accurate MRI mapping is essential for identifying hidden extensions and tailoring the most appropriate surgical management.