A 35-year-old woman presents with severe anal pain during defecation and visible blood on toilet paper for 3 weeks. On examination, a visible tear is seen in the anal canal at the posterior midline. In contrast, a 42-year-old man with a 2-year history of recurrent anal discharge and a palpable nodule in the perianal skin is found to have a tract with an external opening. Which single feature best distinguishes an anal fissure from an anal fistula?

Explanation

## Distinguishing Anal Fissure from Anal Fistula ### Definition and Pathology **Key Point:** An anal fissure is a linear tear in the anoderm (distal to the dentate line), whereas an anal fistula is an abnormal tract with both an internal opening (at the dentate line or above) and an external opening in the perianal skin. ### Comparative Features | Feature | Anal Fissure | Anal Fistula | |---------|--------------|-------------| | **External opening** | Absent | Present (hallmark finding) | | **Internal opening** | No tract; simple tear | Present at dentate line | | **Pain character** | Severe, sharp, during/after defecation | Mild to moderate; often painless | | **Discharge** | Minimal; blood on paper | Purulent, blood-stained, continuous | | **Location** | Posterior midline (90%), anterior (10%) | Variable; depends on fistula type | | **Palpable finding** | Linear tear visible on anoscopy | Nodule/sinus tract palpable externally | | **Bleeding** | Bright red, visible on paper | May be present but not prominent | ### Clinical Pearl **High-Yield:** The **presence of an external opening in the perianal skin** is the single most discriminating feature of an anal fistula. This opening may drain purulent material, blood, or mucus. A fissure, by contrast, is a simple epithelial tear with no tract formation and no external opening. ### Pathophysiology 1. **Anal Fissure** — results from trauma (hard stool, childbirth) or spasm of the internal sphincter, causing ischemia and ulceration. No communication with the skin. 2. **Anal Fistula** — arises from cryptitis and abscess formation in anal glands (located in the crypts at the dentate line). The abscess tracks through tissue planes, creating an internal opening and an external opening. ### Clinical Examination Clues **Tip:** On physical examination, if you can palpate a nodule or sinus tract in the perianal skin and express discharge, you are dealing with a fistula. If you see only a linear tear on anoscopy with no external findings, it is a fissure. [cite:Sabiston Textbook of Surgery Ch 28]