How do you assess for fistulas?

How to Assess for Fistulas

Physical examination should be the first-line assessment method for all fistulas, supplemented immediately by imaging (MRI or endoanal ultrasound) for accurate classification and treatment planning, as clinical examination alone is insufficient for complete evaluation. 1

Initial Clinical Assessment

Physical Examination Components

Physical examination must include inspection, palpation, and auscultation of the suspected fistula site 1:

• For dialysis access fistulae: Assess for adequate maturation using the "Rule of 6s" - minimum 6 mm diameter with discernable margins when tourniquet applied, less than 6 mm deep, blood flow >600 mL/min, evaluated at 6 weeks post-creation 1
• For perianal fistulae: Identify external openings, assess for fluctuation indicating abscess, evaluate sphincter tone, and detect active drainage 1
• Document specific findings: Location of external openings, presence of induration, erythema, purulent drainage, and relationship to anatomical landmarks 1

Critical pitfall: Clinical examination alone has only 75% sensitivity for discriminating simple from complex perianal fistulae, compared to 97% for MRI 2. Never rely on physical examination as the sole diagnostic modality 1.

Imaging Modalities by Fistula Type

Dialysis Access Fistulae

Fistulography with digital subtraction angiography (DSA) is the reference standard for suspected dialysis access dysfunction 1:

• Provides comprehensive visualization from arterial anastomosis to right atrium 1
• Allows simultaneous therapeutic intervention during diagnostic procedure 1
• Should only be performed with intention to intervene on significant findings 1
• Sensitivity for detecting stenosis: 94% with iodinated contrast 1

Duplex Doppler ultrasound serves as non-invasive surveillance tool 1:

• Venous diameter ≥0.4 cm plus flow volume ≥500 mL/min predicts maturation with 95% certainty 1
• Should be performed within first 4 months after access creation 1

Perianal and Gastrointestinal Fistulae

MRI is the optimal imaging technique for perianal fistula classification 1, 2:

• Sensitivity: 87-97% for detecting and classifying fistulae 1, 2
• Positive likelihood ratio of 22.7 for confirming complex disease 2
• Superior to clinical examination (sensitivity 75%) and comparable to endoanal ultrasound (sensitivity 92%) 2
• Should precede examination under anesthesia (EUA) unless immediate drainage of sepsis required 1

Endoanal ultrasound (EUS) is a useful alternative when MRI unavailable 1:

• Accuracy 86-95% for correct classification 1
• Frequency 5-16 MHz allows detailed sphincter visualization 1
• Limitation: Cannot accurately identify ischioanal fossa or supralevator abscesses due to restricted penetration 1

CT abdomen/pelvis with IV contrast for enterovaginal/enterocutaneous fistulae 3:

• Diagnostic sensitivity 76.5% for fistula detection, 94.1% for defining etiology 3
• Water-soluble contrast (not barium) should be placed in bowel to opacify fistulous tract 3

Algorithmic Assessment Approach

Step 1: Determine Fistula Type and Location

• Dialysis access → Physical exam + fistulography if dysfunction suspected 1
• Perianal → Physical exam + MRI (or EUS if MRI unavailable) + endoscopy 1
• Enterovaginal/enterocutaneous → CT with water-soluble contrast or MRI pelvis 3

Step 2: Classify Anatomic Complexity

For perianal fistulae, classification determines surgical approach 1:

• Course relative to sphincter: intersphincteric, transsphincteric (high vs. low), suprasphincteric, extrasphincteric 1
• Presence of secondary tracts or abscesses 1
• Associated proctitis or rectal stricture 1

Combination of modalities increases accuracy to 100%: EUA + MRI or EUA + EUS 1

Step 3: Assess for Complications

• Abscess formation: Defined as confined fluid collection with rim enhancement on MRI or hypo/anechoic area on EUS 1
• Active inflammation: T2 hyperintensity and contrast enhancement on MRI indicates active disease 1
• Stenosis: Particularly relevant for dialysis access (>50% diameter reduction) 1

Step 4: Functional Assessment

For dialysis fistulae 1:

• Blood flow measurement using ultrasound dilution or thermal dilution 1
• Kt/V adequacy monitoring 1
• Access recirculation assessment 1

For gastrointestinal fistulae 3:

• Output volume (high >500 mL/day predicts management difficulty) 3
• Nutritional status assessment 3
• Electrolyte and fluid balance 4

Common Pitfalls to Avoid

• Never perform fistulography without intention to intervene - this violates European Best Practice Guidelines 1
• Do not rely on clinical examination alone for perianal fistulae - imaging is mandatory for accurate classification 1
• Avoid excessive probing during abscess drainage - may create iatrogenic fistulae 5
• Do not attempt surgical repair in presence of active sepsis or undrained abscess - significantly increases failure rates 3
• Never use barium contrast for suspected fistulae - use water-soluble contrast only 3

Special Diagnostic Considerations

Examination under anesthesia (EUA) should be reserved for 1:

• Immediate drainage of suspected abscess when MRI not readily available 1
• Confirmation of MRI/EUS findings before definitive surgical intervention 1
• Placement of setons after imaging-guided classification 1

Endoscopy is essential for perianal Crohn's disease fistulae to 1:

• Determine presence of macroscopic rectal inflammation 1
• Identify anal stenosis 1
• Guide medical treatment planning 1