MR imaging classification of perianal fistulas and its implications for patient management

Until recently, imaging had a limited role in the preoperative assessment o f perianal fistulas. Magnetic resonance (MR) imaging has been shown to demo nstrate accurately the anatomy of the perianal region. In addition to showi ng the anal sphincter mechanism, MR imaging clearly shows the relationship of fistulas to the pelvic diaphragm (levator plate) and the ischiorectal fo ssae. This relationship has important implications for surgical management and outcome and has been classified into five MR imaging-based grades. If t he ischioanal and ischiorectal fossae are unaffected, disease is likely con fined to the sphincter complex (simple intersphincteric fistulization, grad e 1 or 2), and outcome following simple surgical management is favorable. I nvolvement of the ischioanal or ischiorectal fossa by a fistulous track or abscess indicates complex disease related to trans-sphincteric or suprasphi ncteric disease (grade 3 or 4). Correspondingly more complex surgery may be required that may threaten continence or may require colostomy to allow he aling. If the track traverses the levator plate, a translevator fistula (gr ade 5) is present, and a source of pelvic sepsis should be sought.