A 48-year-old man with a 2-year history of recurrent perianal abscess and drainage presents with a persistent opening at the anal verge discharging purulent material. Examination reveals a firm cord palpable in the perianal tissue. Anoscopy shows an internal opening at the 6 o'clock position. MRI pelvis confirms a simple linear tract from the internal opening to the external opening at the anal verge. There is no branching or complex anatomy. What is the most appropriate definitive treatment?

Explanation

## Diagnosis: Simple Anal Fistula **Key Point:** A simple anal fistula (low, linear, single tract with no branching) is best treated by fistulotomy (laying open the tract), which provides the highest cure rate (>95%) with acceptable continence outcomes. **High-Yield:** Classification of anal fistulas guides treatment: | Feature | Simple Fistula | Complex Fistula | |---------|---|---| | **Anatomy** | Low, linear, single tract | High, branching, multiple tracts | | **Internal opening location** | Below puborectalis | Above puborectalis | | **Sphincter involvement** | Minimal or <30% of sphincter | >30% of sphincter | | **First-line treatment** | Fistulotomy | Seton or LIFT | | **Cure rate (fistulotomy)** | >95% | 50–70% (high recurrence) | | **Continence risk** | Low (<5%) | Moderate–high (10–20%) | **Clinical Pearl:** The internal opening at 6 o'clock (posterior midline) and linear tract on MRI indicate a **low fistula**. Fistulotomy is safe and curative because it divides minimal sphincter muscle. **Mnemonic: FISTULA GRADE** — **F**ibrin glue (complex), **I**nternal opening (location), **S**eton (high/complex), **T**ract (simple = fistulotomy), **U**ltra-low (fistulotomy safe), **L**ow (fistulotomy), **A**natomy (defines treatment). ## Why Other Options Are Suboptimal **Seton placement:** Reserved for complex fistulas with high sphincter involvement. A simple fistula does not require staged treatment; fistulotomy in one stage is curative. **Fibrin glue:** Used for complex fistulas or patients at high continence risk (e.g., prior anal surgery, female with obstetric trauma). Cure rate is only 40–60% and recurrence is common. Not first-line for simple fistula. **Antibiotics alone:** Fistulas do not close spontaneously because of the epithelialized tract. Antibiotics may control infection temporarily but will not eliminate the fistula.