Diagnosis of anal fistula

Anal fistula should be suspected in patients presenting with chronic or recurrent perianal pain, lump, or discharge. (1,2)

Inquire about:

• recurrent abscess or failure of healing at an incision and drainage site
• about previous perianal sepsis, surgery or radiotherapy, trauma (obstetric or otherwise), and other conditions which are associated with anal fistula

Examination:

• inspection of the perianal area for an external opening
  • it may appear as a simple pit in the skin or may be obviously discharging, with or without a surrounding rim of raised granulation tissue
  • or it may appear within the scar of a previous abscess
    
    
• palpation of the perianal area with a lubricated finger
  • a palpable track maybe felt like a cord-like structure beneath the skin
    
    
• digital rectal examination
  • may detect indentation or induration (often described as "a grain of rice")
  • adequate in most patients with a simple fistula (in those with a more complex fistula, it should be interpreted in the light of imaging, particularly MRI
    
    
• examination under anaesthesia
  • allows a thorough assessment of the fistula opening

Imaging

• endoanal ultrasound (3)
  • operator dependent
  • provides anatomical detail of the tracks and the sphincters
  • injection of hydrogen peroxide into fistula tracks improves accuracy
  • has similar sensitivity but less specificity than MRI.
• MRI (4)
  • considered the gold standard
  • indicated in
    • all recurrent fistulas
    • primary fistulas that appear to be complex after examination under anaesthesia or endoanal ultrasound.
    • very sensitive and specific in diagnosing fistulous tracts and characterizing their internal and external openings.(5)
• anal manometry
  • provides objective assessment of sphincter function by measuring the anal canal pressure
  • may have a role in in patients with compromised continence or those at risk (e.g. patients with a history of sphincter surgery or injury)

References:

1. Simpson JA, Banerjea A, Scholefield JH. Management of anal fistula. BMJ. 2012;345:e6705
2. Williams JG et al.The treatment of anal fistula: ACPGBI position statement. Colorectal Dis. 2007;9 Suppl 4:18-50
3. Siddiqui MR et al. A diagnostic accuracy meta-analysis of endoanal ultrasound and MRI for perianal fistula assessment. Dis Colon Rectum. 2012 May;55(5):576-85
4. Konan A et al. The contribution of preoperative MRI to the surgical management of anal fistulas. Diagn Interv Radiol. 2018 Nov;24(6):321-327
5. O'Malley RB et al. Rectal imaging: part 2, Perianal fistula evaluation on pelvic MRI--what the radiologist needs to know. AJR Am J Roentgenol. 2012 Jul;199(1):W43-53