Imaging of Anal Fistula

1. Imaging of Anal Fistula Dr Sue Roach

2. Introduction Pre-operative confirmation of fistula complexity has been shown to facilitate surgical planning of sphincter saving techniques[1] and to reduce the incidence of unidentified sepsis, which is the leading cause of fistula recurrence [2].

3. Imaging Objectives • Determine relationship of fistula to sphincter complex • Identify any secondary fistulous tracks

4. Imaging Modalities • Fistulography • Endoanal ultrasound • Magnetic resonance

5. Fistulography • Acute tracks may not have a patent lumen • Difficult to relate the track to the sphincter and levator ani • Shown to be accurate in only 16% [3] • Helpful for chronic fistulae with an external opening distant from the anus

6. Endoanal ultrasound • Operator dependent • Highly accurate at identifying the internal opening [4] • Depicts fewer secondary extensions than MR • Difficulty differentiating active track from fibrosis

7. Magnetic Resonance • Most accurate technique for evaluation of the primary track and any extensions [4]. • More accurate predictor of patient outcome than surgical findings at EUA[5].

8. Beets-Tan RGH, Beets GL, Gerritsen van der Hoop A. et al. Preoperative MR Imaging of Anal Fistulas: Does it Really Help the Surgeon? Radiology 2001; 218:75-84 • Prospective study 56 patients • MR prior to surgery but result witheld from surgeon until end of surgery while patient still anaesthetised • Important additional information in 21%. Benefit greatest in crohns (40%), recurrent fistulas (24%), primary fistulas (8%)

9. Spencer JA, Chapple K, Wilson D et al. Outcome After Surgery for Perianal Fistula: Predictive Value of MR Imaging. AJR 1998; 171:403-406 • Prospective study 48 patients • MR and then surgical exploration blinded to MR • MR categorised 41% complex. Surgery 38%. Only agreed in 8 cases • 19 patients required further surgery. 13 of these considered complex on MR, 9 by surgery • MR better at predicting outcome than surgery

10. Gadolinium? • Post operative problems • Complex cases such as crohns disease[6]

11. Endoanal coil? • Endocoils give superior anatomical resolution of fistula disease within the sphincter • Resolution falls off rapidly outside the sphincter • Complex tracks outside the sphincter are not well seen

15. MR Technique • Phased array pelvic coil • Axial and coronal imaging of the perineum • T1 and short T1 inversion recovery (STIR) images obtained • Additional saggital high resolution T2 images occasionally helpful • IV gadolinium rarely administered

19. Morris J, Spencer JA, Ambrose S. MR Imaging Classification of Perianal Fistulas and Its implications for Patient Management. Radiographics 2000; 20:623-635

20. Grade 1 Simple Intersphincteric Fistula

21. Grade 2 Intersphincteric track with secondary track or abscess

22. Grade 3 Trans-sphincteric Fistula

23. Grade 4 Trans-sphincteric Fistula With Abscess or Secondary Track

24. Grade 5 Supralevator and Translevator Disease

25. Aims • To establish the common MR patterns of idiopathic peri-anal fistulation in Hope Hospital patients.

26. Methods • Retrospective review • 24 consecutive MR scans performed for idiopathic anal fistulation • Scans performed on a 1 Tesla MR scanner with phased array pelvic coil technique

27. Results % of patients

28. Discussion • Majority (50%) of patients with idiopathic peri-anal fistulation have uncomplicated disease • 25% have trans-sphincteric fistulae complicated by secondary tracks or ischiorectal abscess • Supra-levator or trans-levator disease is relatively rare in this patient group (8%).

29. Grade 1- Intersphincteric fistula

38. Grade 2- Intersphincteric fistula with collection

40. Grade 3- Trans-sphincteric fistula