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Oral Exam Prep: Anal Abscess - Surgery Module

Format note: This is structured exactly like your professor's case-based style - anatomy first, then pathology, then a mock case with workup.

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PART 1: ANATOMY & PHYSIOLOGY (Brief)

Anal Canal Anatomy

• Dentate line (pectinate line): The junction between columnar mucosa (above) and squamous epithelium (below). This is where the anal crypts and anal glands open - the origin of most anal abscesses.
• Anal verge: The distal margin of the anal canal at the skin.

• Internal anal sphincter (IAS): Smooth muscle, continuation of the circular muscle of the rectum. Involuntary, high resting tone - responsible for ~80% of resting anal pressure.
• External anal sphincter (EAS): Striated skeletal muscle. Voluntary control. Innervated by the pudendal nerve (S3-S4).
• Intersphincteric space: The potential space between IAS and EAS where the anal glands sit. This is ground zero for anal abscess formation.

• Above dentate line: Autonomic innervation (visceral pain - poorly localized)
• Below dentate line: Somatic innervation via pudendal nerve (sharp, well-localized pain)

• Above dentate line: Drains upward to pararectal and iliac nodes
• Below dentate line: Drains to inguinal nodes

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PART 2: ANAL ABSCESS

Definition & Etiology

• Intestinal origin (cryptoglandular): E. coli, Bacteroides spp., mixed gram-negative + anaerobes
• Skin origin (furuncle/boil): Staphylococcus aureus - these are NOT associated with fistula formation

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Classification by Location (Frequency)

1. Anal crypt infection (cryptitis) at dentate line
2. Spreads to anal gland in intersphincteric space
3. Intersphincteric abscess forms (primary)
4. Spreads:
   • Downward in intersphincteric plane → Perianal abscess
   • Laterally through EAS → Ischiorectal abscess
   • Upward in intersphincteric plane → Supralevator abscess
   • Inward through IAS → Submucous abscess

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Clinical Features

• Severe, constant anal pain (hallmark - worse with sitting, walking, defecation)
• Swelling and erythema around anus (visible in perianal type)
• Fever and constitutional symptoms (especially ischiorectal/supralevator)
• History of previous similar episode that spontaneously drained (30%)
• Secondary causes to ask about: Crohn's disease, HIV/AIDS, malignancy, previous anal surgery, diabetes (immunocompromised)

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Diagnosis

• Usually clinical - painful fluctuant swelling near anus
• Digital Rectal Examination (DRE): Essential - identifies intersphincteric/supralevator abscess not visible externally
• Needle aspiration: Confirms pus if not obvious
• Examination Under Anesthesia (EUA): For obscure/complex cases
• MRI: Investigation of choice for complex anorectal sepsis, especially to delineate supralevator/horseshoe extensions
• Anal ultrasound: Can also identify occult abscesses

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Treatment

• Must be done PROMPTLY - do NOT delay waiting for fluctuance
• Antibiotics alone are NOT sufficient and NOT the primary treatment
• Antibiotics are adjuncts ONLY when:
  • Extensive surrounding cellulitis
  • Immunocompromised (diabetes, HIV, steroids, hematologic disease)
  • Valvular heart disease (endocarditis prophylaxis)

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PART 3: COMPLICATIONS

Immediate / Short-term

Long-term

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PART 4: FISTULA-IN-ANO (the chronic complication you must know)

• External opening anterior to transverse anal line (and within 2 cm): short, straight tract to nearby crypt
• External opening posterior to transverse anal line (or anterior but >2 cm from verge): curved tract to posterior midline crypt
• Exception: Horseshoe fistula - external opening anterior >2 cm, internal opening in posterior midline; requires drainage of postanal space + secondary drainage of anterior extensions

1. Intersphincteric - most common; tract between sphincters
2. Transsphincteric - crosses EAS; low (safe to divide) vs high (risk incontinence)
3. Suprasphincteric - above puborectalis; complex
4. Extrasphincteric - outside both sphincters; very complex; rare

• Fistulotomy (lay-open): For low, simple intersphincteric/low transsphincteric fistulas
• Seton placement: For high fistulas (to preserve sphincter) - a suture/wire passed through the tract; cutting seton or loose seton
• Division of puborectalis = incontinence - NEVER do this

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PART 5: MOCK EXAM CASE (Anal Abscess Style)

Q1: What is your assessment?

• Differential: perianal abscess, pilonidal abscess, Bartholin's abscess (female), hidradenitis suppurativa

Q2: Initial workup at ER

• Blood glucose (screen for diabetes - uncontrolled DM worsens prognosis)
• Blood cultures if febrile/septic
• HIV screening (if high-risk or recurrent)

• MRI pelvis: If supralevator extension or horseshoe spread suspected
• Anal ultrasound: If available
• CT pelvis: If worried about pelvic/intra-abdominal source (especially supralevator abscess)

• WBC 18 = leukocytosis → supports acute infection/abscess
• Blood glucose elevated → suspect DM, affects healing and risk for necrotizing fasciitis
• Blood cultures → if positive, bacteremia complicating abscess

Q3: Initial management

1. IV access, IV fluids if hypotensive/septic
2. IV antibiotics as adjunct (if extensive cellulitis, DM, immunocompromised) - e.g., ampicillin-sulbactam or piperacillin-tazobactam + metronidazole
3. Analgesia (IV morphine or ketorolac)
4. Consent for I&D under anesthesia
5. Check bleeding parameters (PT/PTT, platelet count)

Q4: Definitive management

• Regional or general anesthesia
• Large cruciate incision over the abscess, as close to anal canal as possible
• Break down all loculations digitally
• Culture the pus
• Pack wound / place mushroom catheter
• Wound care and sitz baths post-op
• Follow-up at 4-6 weeks to evaluate for fistula formation

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Quick Exam Recall Summary

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