Until recently, imaging had a limited role in the preoperative assessment of perianal fistulas. Magnetic resonance (MR) imaging has been shown to demonstrate accurately the anatomy of the perianal region. In addition to showing the anal sphincter mechanism, MR imaging clearly shows the relationship of fistulas to the pelvic diaphragm (levator plate) and the ischiorectal fossae. This relationship has important implications for surgical management and outcome and has been classified into five MR imaging-based grades. If the ischioanal and ischiorectal fossae are unaffected, disease is likely confined to the sphincter complex (simple intersphincteric fistulization, grade 1 or 2), and outcome following simple surgical management is favorable. Involvement of the ischioanal or ischiorectal fossa by a fistulous track or abscess indicates complex disease related to trans-sphincteric or suprasphincteric disease (grade 3 or 4). Correspondingly more complex surgery may be required that may threaten continence or may require colostomy to allow healing. If the track traverses the levator plate, a translevator fistula (grade 5) is present, and a source of pelvic sepsis should be sought.

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OBJECTIVE. The purposeof thisstudywasto determineif MR findingsarepredictiveof long-term outcome in a cohort of patients whose initial surgery was performed without access to the findings of MR imaging. SUBJECTS AND METHODS. Forty patientswith surgicallyprovenperianal fistulasunderwent preoperative dynamic contrast-enhanced MR imaging. The MR and surgical find ings were independently recorded on an identical anatomic form. Three patients were subse quently lost to follow-up. The outcome for the remaining 37 patients was determined from surgical review, case notes, and questionnaires. Minimum follow-up period was 14 months (range, 14â€"39 months).Outcomewas determinedby one observerwho was unawareof the initial MR grading and had not been present during surgery. Outcome was considered unsatis factory if further surgery was required. RESULTS. MR imagingwasbetterthansurgical explorationin predictingoutcome(for MR imaging:positivepredictivevalue,73%; negativepredictivevalue,87%; sensitivity, 89%; and specificity, 68%; for surgical exploration: positive predictive value, 57%; negative predictive value, 64%; sensitivity, 73%; and specificity, 47%). MR classification of fistulas was signifi cantlyassociated with outcome(p = .0004), andsurgical classification wasnot significantlyas sociated with outcome (p = .22, chi-square test). Also MR grades differed significantly for patientswith satisfactory and unsatisfactoryoutcomes(p < .001, Mann-Whitney U test).CONCLUSION. MR imaging is valuablein the managementof patientswith perianal fistulas.MR imagingaccuratelyrevealssurgicalanatomyandcanbe usedto makebetterpre dictionsregardingpatientoutcomethansurgicalfindings.

Forty-two patients with a suspected diagnosis of fistula in ano underwent prospective comparison of digital rectal examination, dynamic contrast enhanced magnetic resonance imaging (DCEMRI) and surgical exploration. There were five discordancies: DCEMRI showed an ischiorectal abscess and track with no enteric connection in one patients who at operation was found to have a well epithelialized primary fistula. Four patients with fistulas on DCEMRI had no enteric opening found at surgery and were treated as having sinuses. Long-term follow-up has shown failure to heal in all patients and further surgery confirmed missed fistula. Compared with final outcome measures DCEMRI had a sensitivity of 97 per cent and specificity of 100 per cent in the detection of fistula. DCEMRI also identified more secondary tracks and was more accurate at identifying complex fistulas than either digital rectal examination alone or surgical exploration.

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