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