How to Diagnose Fistula in Ano
The diagnosis of fistula in ano is primarily clinical, based on a focused history and complete physical examination including digital rectal examination, with imaging reserved for complex cases, recurrent disease, or suspected inflammatory bowel disease. 1, 2
Clinical Diagnosis
History and Physical Examination
Collect a focused medical history looking specifically for: 1, 2
- Recurrent perianal pain, swelling, or discharge
- History of prior perianal abscess (approximately one-third of anorectal abscesses lead to fistula formation) 2
- Symptoms suggesting inflammatory bowel disease (diarrhea, weight loss, abdominal pain) - mandatory to exclude Crohn's disease, which occurs in one-third of patients with anorectal abscess 1, 2
- Duration of symptoms (chronic presentation with recurrence strongly suggests fistula) 2
Perform a complete digital rectal examination to identify: 1, 2
Do NOT probe for occult fistulas during examination, as this risks creating iatrogenic fistula tracts 2
Key Clinical Features
The classic triad includes: 2, 3
- Internal opening at the dentate line
- Fistula tract (may be palpable as cord-like structure)
- External opening with intermittent or chronic drainage
Mandatory Laboratory Testing
- Check serum glucose, hemoglobin A1c, and urine ketones to identify undetected diabetes mellitus 1, 2
- If systemic infection suspected: complete blood count, serum creatinine, inflammatory markers (C-reactive protein, procalcitonin, lactates) 1
When Imaging Is Indicated
Imaging is NOT routinely needed for typical fistula in ano - most can be diagnosed clinically. 4 However, imaging is specifically indicated for: 1, 2, 4
Absolute Indications for Imaging
- Recurrent fistula after prior surgery 2, 4
- Suspected complex fistula (high intersphincteric, transsphincteric, suprasphincteric, or extrasphincteric) 2, 5
- Suspected inflammatory bowel disease (particularly Crohn's disease) 2, 6
- Non-healing wound after drainage 4
- Atypical presentation (lateral location, multiple fistulas, or associated mass) 6
- Preoperative surgical planning for high intersphincteric fistulas 2
Imaging Modality Selection
MRI pelvis without and with IV contrast is the preferred imaging modality when imaging is indicated, due to superior soft tissue resolution and higher detection rates for complex fistulas and secondary extensions. 1, 2, 4, 5
- MRI advantages: Superior for detecting fistula tracts, internal openings, secondary extensions, and occult abscesses; markedly diminishes recurrence rates when used preoperatively 5
- CT with IV contrast: Alternative when MRI unavailable or contraindicated, but has poorer spatial resolution and lower sensitivity (77%) for perirectal abscesses 4
- Endoanal ultrasound: Equivalent to MRI for surgical planning in experienced hands 2
Critical Pitfalls to Avoid
- Do not order imaging for simple, straightforward fistulas - this delays definitive treatment without changing management 4
- Do not miss Crohn's disease - always obtain focused history for IBD symptoms in recurrent cases, as surgical success rates are markedly reduced in Crohn's patients 2, 7
- Do not assume all lateral or multiple fissures/fistulas are benign - these atypical presentations require investigation to exclude malignancy, tuberculosis, or sexually transmitted infections 6
- Do not proceed with surgery for complex fistulas without preoperative imaging - inadequate assessment leads to recurrence rates up to 44% 2, 4
Differential Diagnosis Considerations
When evaluating suspected fistula, consider: 1, 6
- Anorectal abscess (acute infection with fever, swelling, leukocytosis - absence of these suggests chronic fistula) 2
- Anal fissure (linear tear, typically posterior midline)
- Crohn's disease (approximately 11% of colovesical/colovaginal fistulas are caused by malignancy) 1
- Malignancy (carcinoma can arise in chronic fistulas or present with fistula formation) 1
- Hidradenitis suppurativa (multiple sinus tracts in perianal region)