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[1] and to reduce the incidence of unidentified sepsis, which is the leading cause of fistula recurrence [2].

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% [3]

• Helpful for chronic fistulae with an external opening distant from the anus

Endoanal ultrasound

• Operator dependent• Highly accurate at identifying the internal

opening [4]

• 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 [4].

• More accurate predictor of patient outcome than surgical findings at EUA[5].

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[6]

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

Grade 1 Simple Intersphincteric Fistula

Grade 2 Intersphincteric track with secondary track or abscess

Grade 3 Trans-sphincteric Fistula

Grade 4 Trans-sphincteric Fistula With Abscess or Secondary

Track

Grade 5 Supralevator and Translevator Disease

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

Results13

29

421

25

8

Grade 0Grade 1Grade 2Grade 3Grade 4Grade 5

% 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%).

Grade 1- Intersphincteric fistula

Grade 2- Intersphincteric fistula with collection

Grade 3- Trans-sphincteric fistula

Grade 4- Trans-sphincteric fistula with secondary track

Grade 5- Translevator disease

Summary

• MR is a valuable modality in the assessment of peri-anal fistula

• Accurately identifies disease complexity

References

• 1: 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

• 2: Bartram C, Buchanan G. Imaging anal fistula. Radiol Clin N Am 41 (2003) 443-457

• 3: Kuijpers HC, Schulpern T. Fistulography for fistula-in-ano: is it useful? Dis Colon Rectum 1985;28:103-4

• 4: Buchanan GN, Halligan S, Bartram CI et al. Clinical Examination, Endosonography, and MR Imaging in Preoperative Assessment of Fistula in Ano: Comparison with Outcome-based Reference Standard. Radiology 2004; 233:674-681

• 5: Spencer JA, Chapple K, Wilson D et al. Outcome After Surgery for Perianal Fistula: Predictive Value of MR Imaging. AJR 1998; 171:403-406

• 6: Horsthius K, Stoker J. MRI of perianal crohn’s disease. AJR 2004; 183:1309-1315

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