From the Guidelines
Colonoscopy is not typically indicated for the diagnosis of complex anal fistulas. The primary diagnostic tools for complex anal fistulas include physical examination, digital rectal examination, anoscopy, and imaging studies such as MRI, endoanal ultrasound, or CT scan, as suggested by the WSES-AAST guidelines 1. These imaging modalities provide detailed information about the fistula tract, its relationship to the anal sphincter complex, and any secondary extensions or abscesses.
Key Points to Consider
- Colonoscopy would only be indicated if there is suspicion of inflammatory bowel disease (particularly Crohn's disease) as an underlying cause of the fistula, or if the patient has other symptoms suggesting colonic pathology, as noted in the ACR Appropriateness Criteria for Crohn disease 1.
- In such cases, colonoscopy helps evaluate the colonic and terminal ileal mucosa for signs of inflammation or other abnormalities.
- For most patients with anal fistulas without symptoms suggesting inflammatory bowel disease, proceeding directly to MRI or endoanal ultrasound provides the most useful information for surgical planning without the risks and preparation associated with colonoscopy.
Diagnostic Approach
- The choice of imaging investigation should take into account multiple factors, such as patient’s past medical history, clinical presentation, local availability of resources, and skills, as outlined in the WSES-AAST guidelines 1.
- CT, MRI, and EUS could have a role in the diagnosis of perianal abscesses, with the principal aims of excluding related conditions and correctly determining the regional anatomy and extent of the disease.
- After the resolution of the acute phase, routine imaging after incision and drainage of the anorectal abscess is usually not required, unless there are signs of recurrence, suspected IBD, or evidence of a fistula and/or non-healing wound.
From the Research
Diagnosis of Complex Anal Fistula
- Colonoscopy is a diagnostic tool that can be used to diagnose complex anal fistula, particularly in patients with Crohn's disease 2.
- A study published in 2016 found that colonoscopy is recommended for patients with complex anal fistula who are at high risk of having Crohn's disease 2.
- The study identified several parameters that predict Crohn's disease in patients with perianal fistula, including age, BMI, abdominal pain, classification of fistula, and platelet count 2.
- Another study published in 2018 found that endoscopic ultrasound is more accurate than magnetic resonance imaging in detecting perianal fistulae in patients with Crohn's disease 3.
- Colonoscopy can also be used to assess the extent of Crohn's disease and guide clinical decision-making, particularly when used in combination with other diagnostic tools such as bowel ultrasound and magnetic resonance enterography 4.
Indications for Colonoscopy
- Colonoscopy is indicated for patients with complex anal fistula who have a high risk of having Crohn's disease, particularly those with symptoms such as abdominal pain, weight loss, and bowel obstruction 2, 5.
- Colonoscopy is also indicated for patients with inflammatory bowel disease who require assessment of the extent of their disease and guidance for clinical decision-making 5, 3.
- The use of colonoscopy in the diagnosis and management of complex anal fistula is supported by several studies, which highlight its accuracy and effectiveness in detecting Crohn's disease and guiding treatment decisions 2, 3, 4.