Endoanal Ultrasound with Hydrogen Peroxide Enhancement for Fistula-in-Ano Mapping
Hydrogen peroxide is injected into the external opening of the fistula during endoanal ultrasound, creating a brightly hyperechoic tract that enhances visualization of the fistula course, connections to abscess cavities, and helps differentiate active tracts from scar tissue 1.
Technical Procedure
Equipment and Patient Positioning
- Use a 360-degree ultrasound probe with frequency range between 2.5 and 16 MHz 1
- Position patient in left lateral decubitus or lithotomy position 1
- Obtain 3-D acquisitions and reconstructions to facilitate coronal plane display of anorectal anatomy 1
Step-by-Step Enhancement Technique
Initial baseline scan: Perform conventional endoanal ultrasound first to identify the three anal planes and baseline anatomy 2
Hydrogen peroxide preparation: Use 3% hydrogen peroxide solution, diluted to approximately 2% concentration 3, 4
Cannulation and injection:
Real-time imaging: Reinsert the endoprobe and visualize the entire echogenic fistula course in real time, including its relationship to:
- Internal and external sphincters
- Levator ani muscle
- Secondary extensions and abscess cavities 3
Imaging Interpretation
What to Look For
- Enhanced fistula tracts: Appear as brightly hyperechoic bands when filled with hydrogen peroxide 1
- Baseline fistula appearance: Hypoechoic or mixed echogenicity bands without enhancement 1
- Abscess cavities: Anechoic or hypoechoic fluid collections 1
- Scar tissue differentiation: Active tracts enhance differently than fibrotic tissue 1
Clinical Performance and Limitations
Diagnostic Accuracy
The technique demonstrates good overall performance with surgical correlation:
- Internal opening identification: 94-96% detection rate 7, 2
- Primary tract classification: 95% accuracy for tract course 7
- Overall concordance with surgery: 95% 5
Important Caveats
The evidence shows hydrogen peroxide enhancement improves conspicuity but does NOT statistically improve diagnostic accuracy compared to standard 3-D endoanal ultrasound alone 1. In a 19-patient study, enhancement improved visualization but did not significantly improve identification of primary tracts (71% vs 81%), secondary tracts (63% vs 68%), or internal openings (86% vs 90%) 1.
MRI remains superior for complex fistulae: When compared head-to-head, MRI correctly classified 90% of primary tracts versus 81% for hydrogen peroxide-enhanced ultrasound, and MRI shows higher accuracy for secondary extensions in complex cases 1.
Technical Limitations
- Limited field of view and depth penetration 1
- Lower accuracy for high tracts: Extrasphincteric (50%) and suprasphincteric (67%) versus transsphincteric (93%) and intersphincteric (88%) 1
- Gas obscuration: Tract or abscess gas can obscure visualization 1
- Recurrent disease: Cannot reliably distinguish active inflammation from fibrosis/granulation tissue 1
- Patient discomfort: Limits use for initial evaluation 1
Clinical Decision-Making
Use hydrogen peroxide-enhanced endoanal ultrasound when:
- Simple or low fistulae are suspected (intersphincteric/transsphincteric)
- Surgeon is experienced with the technique and performs it themselves 7
- Real-time surgical planning is needed during the same session
- MRI is unavailable or contraindicated
Choose MRI instead for:
- Complex fistulae with suspected secondary extensions
- High fistulae (suprasphincteric/extrasphincteric)
- Crohn's disease patients requiring assessment of tract activity
- When comprehensive preoperative mapping is essential for surgical planning 1
The technique has a short learning curve and causes minimal discomfort when performed by experienced operators 8, making it practical for colorectal surgeons who integrate it into their clinical workflow 7, 2.