A 42-year-old woman with a history of recurrent perianal abscess presents with a purulent discharge from a small opening near the anus. Examination confirms an anal fistula. What is the most common type of anal fistula based on anatomical classification?

Explanation

## Classification and Epidemiology of Anal Fistula **Key Point:** Intersphincteric fistula is the most common type of anal fistula, accounting for 45–70% of all cases. ### Goodsall's Classification of Anal Fistulas | Type | Frequency | Anatomy | Clinical Features | | --- | --- | --- | --- | | **Intersphincteric** | 45–70% | Tract lies between internal and external sphincters | Simplest anatomy; best prognosis; lowest continence risk | | **Transsphincteric** | 25–35% | Tract crosses both internal and external sphincters | More complex; higher continence risk; requires careful surgical planning | | **Suprasphincteric** | 5–10% | Tract passes above external sphincter, crosses puborectalis | Rare; high risk of continence loss; requires complex surgery | | **Extrasphincteric** | 1–5% | Tract completely bypasses sphincter complex | Rarest; often secondary to trauma, IBD, or malignancy | **High-Yield:** The **intersphincteric fistula** is the most common and has the best surgical prognosis because: 1. It does not cross the external sphincter 2. Surgical treatment (fistulotomy) can be performed safely without risk of continence loss 3. Healing is faster and complication rates are lower ### Clinical Pearl: Goodsall's Rule **Mnemonic: GOOD = Goodsall's Orientation Of Opening Determines** the likely internal opening location: - **Anterior fistulas** (anterior to transverse anal line) → internal opening is usually directly anterior - **Posterior fistulas** (posterior to transverse anal line) → internal opening is usually at the posterior midline (6 o'clock position) This rule helps predict fistula tract anatomy and guides surgical planning. ### Why Intersphincteric is Most Common Intersphincteric fistulas arise most commonly from: - Infection of anal glands (cryptoglandular origin — the most common etiology of anal fistula overall) - These glands drain into the anal crypts at the dentate line - When infected, they track between the sphincters rather than crossing them **Warning:** Fistulas that do NOT fit Goodsall's rule or are extrasphincteric should raise suspicion for secondary causes: Crohn's disease, tuberculosis, malignancy, or prior trauma.