How do we describe an abscess arising from the external sphincter in Endoscopic Ultrasonography (EUS)?

EUS Description of Abscess Arising from the External Sphincter

An abscess arising from the external sphincter on endoscopic ultrasound (EUS) appears as a hypo- or anechoic area with often poorly demarcated lesions, representing a confined fluid collection with a rim of inflammatory tissue. 1

Key EUS Characteristics

Sonographic Appearance

• Hypo-anechoic structures (≥10 mm) containing echoic fluid and sometimes gas bubbles 1
• Posterior echo enhancement with internal echoes 1
• Poorly demarcated lesions with a rim of inflammatory tissue, distinguishing it from well-defined cystic structures 1
• Mixed echogenicity may be present, particularly when the abscess contains debris or gas 1

Anatomical Classification Based on Location

When describing an abscess arising from the external sphincter region, specify the precise anatomical location:

• Intersphincteric abscess: Located between the internal and external anal sphincters 1
• Ischioanal (ischiorectal) abscess: Penetrates through the external anal sphincter into the ischioanal space 1
• Perianal abscess: Simple anorectal abscess in the subcutaneous tissue close to the anal verge 1

Essential Reporting Elements

Your EUS report should include: 1

• Anatomical location using the classification above
• Size measurements at the largest diameter in two perpendicular planes 1
• Presence of horseshoe extension with description of horizontal plane spread 1, 2
• Relationship to fistula tracts if present 1
• Vascularity assessment using color Doppler to distinguish non-vascularized abscess from inflammatory phlegmon 1

Important Clinical Considerations

Distinguishing Features

• Increased vascularity on color Doppler and altered perfusion may help differentiate a non-vascularized abscess from inflammatory phlegmon 1
• The poorly demarcated appearance on EUS contrasts with the rim enhancement seen on post-contrast T1-weighted MRI images 1

Common Pitfalls

• Gas within the abscess or tract can obscure visualization and create acoustic shadowing 1
• Distinguishing active inflammation from fibrosis in recurrent disease may be impossible with EUS alone, particularly in Crohn's disease patients 1
• Limited field of view restricts assessment of extrasphincteric and suprasphincteric extensions (accuracy only 50-67% for these locations) 1

Complementary Imaging

• MRI remains the gold standard for comprehensive perianal abscess evaluation, with superior accuracy for complex anatomy 1, 3
• EUS has comparable accuracy to MRI for intersphincteric and transsphincteric lesions (88-93%) but lower for higher tracts 1
• Consider hydrogen peroxide enhancement during EUS to improve tract visualization, though this doesn't significantly improve internal opening identification 1

Therapeutic Implications

Timely detection and accurate description of abscesses minimizes the risk of further septic complications and guides immediate therapeutic intervention such as drainage and seton placement. 1 EUS-guided drainage has been successfully used for pelvic abscesses when percutaneous approaches are not feasible, with technical success in appropriately selected cases. 4, 5