Imaging of Anal Fistula Dr Sue Roach
Introduction Pre-operative confirmation of fistula complexity has been shown to facilitate surgical planning of sphincter saving techniques and to reduce the incidence of unidentified sepsis, which is the leading cause of fistula recurrence .
Imaging Objectives • Determine relationship of fistula to sphincter complex • Identify any secondary fistulous tracks
Imaging Modalities • Fistulography • Endoanal ultrasound • Magnetic resonance
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%  • Helpful for chronic fistulae with an external opening distant from the anus
Endoanal ultrasound • Operator dependent • Highly accurate at identifying the internal opening  • Depicts fewer secondary extensions than MR • Difficulty differentiating active track from fibrosis
Magnetic Resonance • Most accurate technique for evaluation of the primary track and any extensions . • More accurate predictor of patient outcome than surgical findings at EUA.
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%)
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
Gadolinium? • Post operative problems • Complex cases such as crohns disease
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
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
Morris J, Spencer JA, Ambrose S. MR Imaging Classification of Perianal Fistulas and Its implications for Patient Management. Radiographics 2000; 20:623-635
Aims • To establish the common MR patterns of idiopathic peri-anal fistulation in Hope Hospital patients.
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
Results % of patients
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%).