Potential clinical implications of direction of a trans-sphincteric anal fistula track

Background: The longitudinal direction of a trans-sphincteric anal fistula track through the anal sphincter complex may have implications regarding fistulotomy. Methods: The angle of the track of trans-sphincteric fistulas relative to the longitudinal axis of the anal canal was measured before operation by means of magnetic resonance imaging (MRI) in 46 patients. This was compared with the findings at operation. Results: The track passed cranially as well as laterally at an acute angle (less than 90degrees) in 23 patients while it passed either transversely or caudally at an obtuse angle (90degrees or more) in the remaining 23. The internal opening was significantly higher in relation to the dentate line (above in eight patients, at the dentate line in 14 and below in one patient) when the track was acute than when it was obtuse (above in one, at the dentate line in 17 and below in five patients) (P = 0.004). The fistula track crossed the sphincter at a median angle of 35degrees, 95degrees and 132degrees from internal openings sited above, at and below dentate line level respectively (P = 0.002). Conclusion: Fistula tracks passed cranially and laterally through the sphincter complex in half of these patients, and were most acutely angled on MRI when internal openings were situated above the dentate line. Preoperative MRI might alert surgeons to the potential hazard of fistulotomy being more extensive than anticipated from simple palpation of the level of the internal opening.