ORAL EXAM PREP: ANAL ABSCESS

Schwartz's Principles of Surgery, 11th Ed.

PART 1 - ANATOMY (Brief)

Anal Canal Structure

• The anatomic anal canal extends from the dentate (pectinate) line to the anal verge
• The dentate line marks the transition between columnar rectal mucosa (above) and squamous anoderm (below)
• The dentate line is surrounded by longitudinal mucosal folds called anal columns (columns of Morgagni) - between these are the anal crypts
• Anal glands open into these crypts at the dentate line level - their ducts traverse the internal sphincter into the intersphincteric plane (this is KEY for abscess pathogenesis)

Sphincter Complex

• Internal anal sphincter (IAS): smooth muscle, involuntary, distal extension of the inner circular muscle of the rectum - innervated by sympathetic + parasympathetic fibers
• External anal sphincter (EAS): skeletal muscle, voluntary - innervated by the inferior rectal branch of the pudendal nerve
• Puborectalis: part of the levator ani, forms the anorectal ring, innervated by pudendal nerve and direct branches of S3-S5
• The intersphincteric space separates the IAS and EAS, continuous with the perianal space distally, extends cephalad into the rectal wall

Perianal/Perirectal Spaces (Memorize These - They Determine Abscess Type)

Vascular Supply (Brief)

• Superior rectal artery - from inferior mesenteric artery - upper rectum
• Middle rectal artery - from internal iliac
• Inferior rectal artery - from internal pudendal artery (branch of internal iliac)

Lymphatic Drainage

• Above dentate line: drains to inferior mesenteric AND internal iliac nodes
• Below dentate line: primarily to inguinal lymph nodes (also to inferior mesenteric and internal iliac)

This is clinically important - anal canal cancer below dentate line can spread to inguinal nodes

Nerve Supply

• Sympathetic: T6-T12, L1-L3 (inhibitory to colon motility)
• Parasympathetic: nervi erigentes from S2-S4 (stimulatory)
• Rectum is relatively insensate; anal canal below dentate line has somatic innervation (painful!)

PART 2 - PHYSIOLOGY (Brief)

Fluid & Electrolyte Exchange

• Colon absorbs ~1000-2000 mL/day of water (up to 5000 mL capacity)
• Na+ absorbed actively via Na+/K+ ATPase (up to 400 mEq/day)
• Water follows Na+ passively along osmotic gradient
• K+ is actively secreted and also passively absorbed
• Cl- absorbed via chloride-bicarbonate exchange

Motility, Defecation & Continence

• Continence depends on the anorectal angle (maintained by puborectalis) and the sphincter complex
• The rectum is a reservoir; when distended, the rectoanal inhibitory reflex relaxes the IAS
• Voluntary contraction of EAS maintains continence until defecation is appropriate

Colonic Flora

• Anaerobes predominate - Bacteroides species most common (10^11-10^12/mL)
• E. coli most numerous aerobe
• Flora important for vitamin K production and colonization resistance

PART 3 - ANAL ABSCESS (Anorectal Sepsis & Cryptoglandular Abscess)

Pathogenesis (Cryptoglandular Theory)

1. Anal glands sit in the intersphincteric plane; their ducts traverse the IAS and open into anal crypts at the dentate line
2. Infection of an anal gland → abscess in the intersphincteric plane
3. The abscess enlarges and spreads along perianal/perirectal spaces → determines the TYPE of abscess

Types of Abscess (Based on Spread)

Fig 29-36 - Pathways of anorectal infection (Schwartz's, p.1343)

Diagnosis

• Severe anal pain - most common presenting complaint
• Palpable mass on inspection or DRE
• Fever
• Urinary retention
• Life-threatening sepsis (in severe/delayed cases)

• Perianal and ischiorectal: usually diagnosed by physical exam alone
• Intersphincteric: requires Examination Under Anesthesia (EUA) - high index of suspicion
• Complex/atypical: CT or MRI to delineate anatomy

PART 4 - MANAGEMENT

General Principle

Anorectal abscesses should be drained as soon as the diagnosis is established. - Schwartz's, p.1341

• Extensive overlying cellulitis
• Immunocompromised patient
• Diabetes mellitus
• Valvular heart disease

Antibiotics alone are INEFFECTIVE at treating perianal/perirectal infection. Drainage is the definitive treatment.

By Type:

• Most can be drained under local anesthesia in office/clinic/ER
• Larger/complicated → OR under general/regional anesthesia
• Skin incision + disk of skin excised to prevent premature closure
• No packing needed; sitz baths start next day

• Incision in overlying skin
• Horseshoe abscess: drainage of deep postanal space (incising anococcygeal ligament) + counter-incisions over one or both ischiorectal spaces

• Drained through a limited, usually posterior, internal sphincterotomy
• If diagnosis in doubt: EUA is both diagnostic and therapeutic

Immunocompromised Patient:

• Leukopenia = may have no cardinal signs of inflammation despite serious infection
• Broad-spectrum antibiotics may help, but EUA should not be delayed due to neutropenia
• Any indurated area: incise + drain + biopsy (to exclude leukemic infiltrate) + culture
• Increased pain/fever/deterioration = mandatory EUA

PART 5 - COMPLICATIONS

1. Fistula in Ano (Most Common Complication)

• Drainage of anorectal abscess results in cure in only ~50% of patients
• The other 50% develop a persistent fistula in ano
• Fistula usually originates from the infected crypt (internal opening) and tracks to the external opening (usually site of prior drainage)
• Goodsell's Rule predicts internal opening location:
  • External opening anterior to anus → short, radial tract to internal opening
  • External opening posterior to anus → curvilinear tract to posterior midline
  • Exception: anterior opening >3 cm from anal margin usually tracks to posterior midline

• Intersphincteric: tracks through distal IAS and intersphincteric space to near anal verge
• Transsphincteric: often from ischiorectal abscess; extends through both IAS and EAS
• Suprasphincteric: originates intersphincteric plane, loops over puborectalis
• Extrasphincteric: entirely outside sphincter complex

2. Necrotizing Soft Tissue Infection (Fournier's Gangrene)

• Rare but lethal complication
• Usually polymicrobial and synergistic
• Source: undrained/inadequately drained cryptoglandular abscess or urogenital infection
• Also seen post-hemorrhoidectomy
• Risk factors: immunocompromised, diabetes
• Signs: necrotic skin, bullae, crepitus; systemic toxicity, hemodynamic instability
• Treatment: urgent surgical debridement of all nonviable tissue (multiple operations may be needed) + broad-spectrum antibiotics + possible colostomy
• Mortality: ~50% despite early treatment

3. Recurrence

• Occurs when source (infected crypt/gland) is not addressed at time of drainage

4. Septicemia

• Delayed or inadequate treatment → extensive life-threatening suppuration, massive tissue necrosis, septicemia

5. Complex Fistula Formation (from Incorrect Drainage of Supralevator Abscess)

• Wrong drainage route → suprasphincteric or extrasphincteric fistula (very difficult to manage)

6. Incontinence

• Risk with extensive sphincter involvement in treatment

PART 6 - MOCK ORAL EXAM CASE (Anal Abscess)

Perianal abscess - most likely cryptoglandular in origin

CBC (WBC for infection), blood culture if systemic sepsis suspected, glucose (rule out DM), physical exam is usually sufficient - CT/MRI only if complex presentation

• WBC 18, Hemoglobin 130, Plt 220
• Blood glucose: normal
• No valvular disease, not immunocompromised

Incision and drainage under local anesthesia; excise a disk of skin to prevent premature closure; no packing; sitz baths next day. Antibiotics NOT indicated (no extensive cellulitis, not immunocompromised, no DM, no valvular disease).

50% chance of developing a fistula in ano; if persistent drainage from the wound after healing, he must return for evaluation.