What ultrasound findings are characteristic of a fistula in ano (fistula in anus) on endoanal ultrasound?

Endoanal Ultrasound Findings for Diagnosing Fistula in Ano

On endoanal ultrasound, fistula tracts appear as hypoechoic or mixed echogenicity bands, while associated abscesses appear as anechoic or hypoechoic fluid collections, often with posterior echo enhancement and internal echoes. 1

Primary Sonographic Features of Fistula Tracts

Fistula tracts demonstrate the following characteristic ultrasound appearances:

• Hypoechoic bands representing the fistulous tract coursing through or between the sphincter muscles 1
• Mixed echogenicity patterns when the tract contains debris, granulation tissue, or gas 2
• The tract appears as a linear or curvilinear structure connecting the internal opening (at the dentate line) to the external opening or abscess cavity 1

Abscess Characteristics on Endoanal Ultrasound

When abscesses complicate fistulas, they demonstrate specific sonographic features:

• Hypo-anechoic structures ≥10 mm containing echoic fluid 1, 2
• Gas bubbles may be visible within the collection, creating echogenic foci 1, 2
• Posterior echo enhancement is a key diagnostic feature 1, 2
• Internal echoes representing debris or inflammatory material 1, 2
• Poorly demarcated margins with a rim of inflammatory tissue surrounding the fluid collection 2

Critical Anatomical Landmarks to Identify

The internal opening is the most critical diagnostic feature:

• Appears as a defect in the internal anal sphincter at the subepithelial space 1
• Located at the dentate line in cryptoglandular fistulas 3
• Endoanal ultrasound achieves 91-95% accuracy for identifying the internal opening 1

Fistula classification depends on relationship to sphincter complex:

• Intersphincteric fistulas: Tract runs between internal and external sphincter muscles (88% accuracy) 1
• Transsphincteric fistulas: Tract crosses through the external sphincter (93% accuracy) 1, 4
• Suprasphincteric and extrasphincteric fistulas: Lower accuracy (50-67%) due to limited field of view and depth penetration 1

Technical Optimization Strategies

Hydrogen peroxide enhancement significantly improves visualization:

• Injection of hydrogen peroxide into the external opening creates brightly hyperechoic tracts on ultrasound 1
• Improves conspicuity of fistula tracts and connection to abscess cavities 1
• Helps differentiate active tracts from scar tissue 1
• However, does not statistically improve identification rates of primary tracts or internal openings 1

3-D acquisitions with coronal plane reconstruction facilitate better anatomical display of the anorectal anatomy 1

Important Diagnostic Limitations and Pitfalls

Gas obscuration is a major limitation:

• Gas within the tract or associated abscess creates acoustic shadowing that obscures visualization 1, 2
• This can prevent complete tract mapping

Field of view restrictions:

• Limited depth penetration reduces accuracy for high fistulas (suprasphincteric, extrasphincteric) 1
• Cannot adequately assess tracts extending beyond the immediate perianal region

Difficulty distinguishing active from inactive disease:

• In recurrent fistulas, especially with Crohn's disease, fibrosis and granulation tissue cannot be reliably differentiated from active inflammation 1
• This is a critical clinical question that endoanal ultrasound cannot answer definitively

Comparative Performance Metrics

Endoanal ultrasound demonstrates strong overall diagnostic accuracy:

• Sensitivity: 87-92% for fistula detection 1, 4
• Specificity: 38-100% depending on fistula complexity 1, 4
• Overall accuracy: 82-93% compared to surgical findings 1, 4

MRI comparison reveals complementary roles:

• MRI shows higher accuracy (90% vs 81%) for overall fistula classification 1
• MRI is superior for suprasphincteric fistulas and complex anatomy 4
• Endoanal ultrasound is more sensitive for intersphincteric and transsphincteric fistulas 4
• MRI better identifies secondary extensions and horseshoe configurations 1

When to Use Color Doppler

Increased vascularity on color Doppler assessment:

• Helps differentiate non-vascularized abscess from inflammatory phlegmon 1, 2
• Altered perfusion patterns on contrast-enhanced ultrasound provide additional diagnostic information 1

Malignancy Surveillance Consideration

In chronic, long-standing fistulas (>5 years), endoanal ultrasound may detect malignant transformation:

• Appears as a multiloculated complex echoic mass with isoechoic solid components communicating with the fistula tract 5
• Sonography-guided biopsy under anesthesia enables definitive diagnosis 5
• Periodic surveillance should be considered for persistent fistulas with progressive symptoms 5