What type of anorectal abscess causes a horse shoe abscess and fistula?

Horseshoe Abscess: Intersphincteric and Ischiorectal Origins

Horseshoe abscesses can arise from both intersphincteric abscesses (Answer A) and ischiorectal/ischioanal abscesses (Answer C), with posterior extension in either anatomical plane leading to this characteristic configuration. 1

Anatomical Pathways to Horseshoe Formation

Intersphincteric Plane Extension

• Intersphincteric abscesses are located between the internal and external anal sphincters and can extend posteriorly to form a horseshoe abscess in the intersphincteric plane. 1
• The infection spreads through the deep postanal space, tracking bilaterally to create the horseshoe configuration. 2, 3
• This represents the most common pathway for posterior horseshoe abscess formation. 3

Ischiorectal Fossa Extension

• Ischiorectal (ischioanal) abscesses penetrate through the external anal sphincter into the ischioanal space, and posterior extension may result in a horseshoe abscess in the ischiorectal fossa. 1
• When suppurative inflammation spreads through the deep anal space to the bilateral ischiorectal fossae, a horseshoe fistulous abscess develops. 2

Why Other Options Are Incorrect

Transsphincteric Abscess (Answer B)

• Transsphincteric fistulas traverse through the sphincter complex but are not specifically associated with horseshoe abscess formation. 4
• These represent a different anatomical pathway that does not typically result in the bilateral posterior extension characteristic of horseshoe abscesses.

Supralevator Abscess (Answer D)

• Supralevator abscesses are located superior to the intersphincteric plane in the supralevator space. 1
• While intersphincteric infections can extend cephalad to form supralevator abscesses, this is not the mechanism of horseshoe formation. 2

Clinical Implications

The presence of a horseshoe abscess should be specifically reported with its horizontal plane spread, as this influences surgical planning and drainage strategy. 1

• Adequate drainage of the deep postanal space through a posterior midline incision is essential to prevent recurrence. 3
• Failure to maintain prolonged drainage in the midline after primary fistulotomy is associated with an 18% recurrence rate. 5
• Both anatomical planes (intersphincteric and ischiorectal) require recognition for complete eradication of the septic process. 2, 3