## Anatomy and Epidemiology of Anal Fissure **Key Point:** The posterior midline is the most common site of anal fissure, accounting for approximately 90% of cases in adults. ### Why Posterior Midline? The posterior anal canal is the most vulnerable site due to: 1. **Poorest blood supply** — The posterior midline has the least collateral blood supply compared to lateral and anterior regions 2. **Maximum internal sphincter pressure** — The posterior aspect experiences the highest resting pressure, predisposing to mucosal trauma 3. **Mechanical trauma** — During defecation, the posterior anal canal bears maximum stress from hard or large stools ### Anatomical Distribution | Site | Frequency | Clinical Note | | --- | --- | --- | | Posterior midline | 90% | Most common; excellent healing with conservative management | | Anterior midline | 8–10% | More common in females; associated with childbirth trauma | | Lateral walls | <2% | Rare; consider secondary causes (IBD, TB, malignancy) | **Clinical Pearl:** Fissures at sites other than the midline (anterior or lateral) should raise suspicion for secondary causes such as inflammatory bowel disease, tuberculosis, or malignancy. These atypical fissures often require further investigation. **High-Yield:** In a patient with anal fissure: - Posterior midline = primary/idiopathic fissure (most common) - Non-midline or multiple fissures = think secondary causes ### Pathophysiology The posterior midline's poor blood supply (supplied by the inferior rectal artery with minimal anastomoses) impairs healing and perpetuates the pain-spasm-ischemia cycle that characterizes chronic fissures.
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