Lateral Internal Sphincterotomy
What the Examiner Expects
Division of the internal anal sphincter muscle in the lateral position to treat a chronic anal fissure that has failed conservative medical therapy. The pathophysiology of chronic fissures involves internal sphincter hypertonia causing ischemia to the posterior midline (watershed zone of blood supply). Sphincterotomy reduces resting anal pressure, improving blood flow and allowing healing. The examiner expects you to first attempt 6–8 weeks of medical management (topical calcium channel blockers or nitroglycerin, dietary fiber, sitz baths, +/- botulinum toxin injection) before offering surgery. The sphincterotomy extends from the anal verge to the dentate line.
Key Examiner Focus Points
- Indicated for chronic anal fissure refractory to medical management
- Divide the internal sphincter to the level of the dentate line in the lateral position
- Medical therapy first: fiber, sitz baths, topical nitroglycerin or diltiazem, botulinum toxin
- Risk of incontinence (particularly in women and elderly) — discuss preoperatively
- Most fissures are posterior midline; lateral or multiple fissures suggest Crohn's, HIV, or malignancy
Common Curveballs
The fissure is in the lateral position, not posterior midline
Atypical fissure location raises suspicion for secondary causes: Crohn's disease, HIV/AIDS, tuberculosis, syphilis, or anal cancer. Do NOT perform a sphincterotomy without first evaluating for these conditions. Biopsy the fissure, obtain labs, and evaluate for IBD.
Female patient with prior vaginal delivery and baseline mild incontinence
Sphincterotomy carries higher incontinence risk in this patient. Consider botulinum toxin injection as an alternative — it provides temporary sphincter relaxation (2–3 months) without permanent sphincter division, allowing the fissure to heal. If surgery is chosen, perform a more conservative (tailored) sphincterotomy.
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