Which of the following is the most common type of anal fistula based on the classification by Parks and Melnick?

Explanation

## Parks and Melnick Classification of Anal Fistula **Key Point:** The Parks and Melnick classification (1976) is the gold standard anatomical classification of anal fistulas based on their relationship to the internal and external anal sphincters. Intersphincteric fistulas are the most common type, accounting for 40–50% of all anal fistulas. ### Parks and Melnick Classification | Type | Frequency | Anatomical Course | Internal Opening | External Opening | Clinical Features | |---|---|---|---|---|---| | **Intersphincteric** | 40–50% | Between internal and external sphincters | At dentate line | Perianal skin | Simple, low-risk for continence | | **Transsphincteric** | 25–35% | Crosses external sphincter | At dentate line | Perianal skin, may be distant | Moderate complexity, continence risk | | **Suprasphincteric** | 5–10% | Passes above external sphincter | Above dentate line | Variable, often distant | High complexity, high continence risk | | **Extrasphincteric** | 5% | Bypasses sphincter complex entirely | Above dentate line | Distant from anal verge | Rare, associated with Crohn's, TB, trauma | ### Why Intersphincteric is Most Common 1. **Cryptoglandular origin** — Most anal fistulas originate from infected anal glands in the crypts at the dentate line. The intersphincteric space is the natural path of least resistance for infection to track. 2. **Anatomy** — The internal anal sphincter is thin and easily breached; the fistula tracks between the two sphincters before reaching the perianal skin. 3. **Simplicity** — These are the simplest fistulas anatomically and clinically. ### Mnemonic: ITSE **I**ntersphincteric (40–50%), **T**ranssphincteric (25–35%), **S**uprasphincteric (5–10%), **E**xtrasphincteric (5%) **Clinical Pearl:** Intersphincteric fistulas have the lowest risk of postoperative incontinence because they do not significantly breach the external sphincter. This makes them ideal candidates for simple fistulotomy. **High-Yield:** The Parks classification is essential for NEET PG — it is tested in both written and viva formats. Know the frequency, anatomy, and continence risk of each type. **Warning:** Do not confuse the Parks classification with other fistula classifications (e.g., Goodsall's rule, which predicts the internal opening location based on the external opening). [cite:Sabiston Textbook of Surgery Ch 31]