Based on Park’s classification, identify the type of anorectal fistula shown in the image.

Correct Answer: B. Intersphincteric

Park's classification (1976) categorizes anorectal fistulas based on their anatomical relationship to the internal and external sphincters. An intersphincteric fistula is the most common type (40–50% of cases) and represents a tract that originates from the internal opening at the dentate line and tracks between the internal and external sphincters before opening to the perianal skin. The discriminating feature is that the fistula tract lies entirely within the sphincter complex—specifically in the plane between the two sphincter layers. This anatomy is crucial because it explains why intersphincteric fistulas typically present with a simple, direct course without complex branching. On imaging (MRI or fistulography), the tract appears to run parallel to the anal canal, confined between the sphincter layers. Clinically, intersphincteric fistulas are associated with lower morbidity regarding continence compared to higher fistulas, and they respond well to simple fistulotomy or lay-open procedures. The internal opening is almost always located at the dentate line (site of cryptoglandular origin), and the external opening is typically in the midline posteriorly or anterolaterally. Recognition of this anatomy on imaging or during examination is essential for surgical planning in Indian colorectal practice, where fistula-in-ano remains a common presentation in outpatient departments.

Why the other options are wrong

A. High transsphincteric — High transsphincteric fistulas cross both sphincters at a level well above the dentate line, creating a tract that ascends significantly within the anal canal before exiting. This is a more complex anatomy than intersphincteric and carries higher risk of sphincter damage and continence issues. The image would show a tract crossing the external sphincter at a higher level, not confined between the sphincters. C. Extrasphincteric — Extrasphincteric fistulas are the rarest type (1–2%) and bypass the sphincter complex entirely, running outside both sphincters. They typically originate from a high internal opening above the sphincters and are associated with inflammatory bowel disease, trauma, or malignancy. The tract would be clearly external to the sphincter layers, which is not the case in a simple intersphincteric presentation. D. Suprasphincteric — Suprasphincteric fistulas are rare and represent a variant where the tract passes above the internal sphincter, crosses the external sphincter at a high level, and may have a complex course. This is distinct from intersphincteric because the tract ascends above the internal sphincter rather than remaining between the two sphincter layers. The image would show an upward trajectory, not the simple intersphincteric course.

High-Yield Facts

Mnemonics

Park's Fistula Classification (Sphincter Anatomy) Intersphincteric (between sphincters) → Transsphincteric (through both) → Suprasphincteric (above internal) → Extrasphincteric (outside both). Use I-T-S-E in order of frequency: Intersphincteric is most common (40–50%), Transsphincteric is second (30–40%), Suprasphincteric is rare (5%), Extrasphincteric is rarest (1–2%). Intersphincteric = Simple & Safe Intersphincteric = Inside sphincters = Incision safe (fistulotomy). Remember: confined anatomy = simple surgery = low continence risk. Use this when deciding operative approach.

NBE Trap

NBE may pair high transsphincteric with complex branching patterns to distract from the simpler intersphincteric anatomy. Students who focus only on "fistula crosses sphincters" without noting the level of crossing may incorrectly choose high transsphincteric instead of recognizing the confined intersphincteric tract.

Clinical Pearl

In Indian outpatient colorectal clinics, intersphincteric fistulas present with a simple midline posterior or anterolateral opening and respond excellently to lay-open fistulotomy without fear of postoperative incontinence—making early recognition on examination or MRI critical for patient counseling and operative planning.

_Reference: Bailey & Love Ch. 72 (Anorectal Fistula); Park's original classification (1976)_