Anal Fistula

Anal fistulas are abnormal communications between the anorectal lumen and another body structure, often to the skin. Anal fistulas often occur due to extension of anal abscesses but are also associated with specific diseases such as Crohn’s disease. Symptoms include pain or irritation around the anus; abnormal discharge or purulent drainage; and swelling, redness, or fever if an abscess is present. Management is primarily surgical, with fistulotomy, but can include antibiotics if infection is present. Treatment is surgical. Complications after surgery include recurrence and incontinence.

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Definition and Epidemiology

Definition

  • Abnormal connection between the epithelium of the anal canal and another body structure (often to the skin) 
  • Also called fistula-in-ano

Epidemiology

  • Incidence: 9 per 100,000 adults per year
  • Sex: men > women
  • More common between the 3rd and 5th decades of life
  • 50% of patients with anorectal abscess will eventually develop a fistula.

Etiology and Pathophysiology

Etiology

  • Anorectal fistulas most often occur from:
    • An acute anal abscess → ruptures or is drained → epithelialized track forms that connects the abscess in the anus to the skin (chronic process)
    • Anal abscesses (infected anal crypt glands often with reduced blood flow)
  • Other causes:
    • Rectal foreign bodies
    • Inflammatory bowel disease (Crohn’s disease)
    • Trauma (obstetric injury)
    • Anal fissures
    • Malignancy
    • Radiation proctitis

Pathophysiology

  • Obstruction of anal glands in the wall of the anal canal → stasis and overgrowth of the bacteria → anal abscess formation → extension of the abscess into adjacent perirectal spaces → fistula formation
  • May also occur secondary to disease states (e.g., Crohn’s disease)
  • Fistulas may remain open due to: 
    • Foreign body
    • Radiation
    • Infection
    • Epithelialization
    • Neoplasm
    • Distal obstruction
    • Other: 
      • Increased flow
      • Steroids

Clinical Presentation

History

  • Non-healing anal abscess, often associated with purulent drainage or malodorous discharge
  • Rectal pain

Physical exam

  • Inflamed perianal skin
  • External fistula tract opening may be observed or palpated as an area of induration.
  • Associated with a palpable cord-like tract
    • Internal fistula tract opening may be palpated on digital rectal exam (DRE); however, anoscope or further imaging may be needed to identify the fistula tract.

Diagnosis and Management

Diagnosis

  • Based on history, physical exam, and radiology findings
  • Identification:
    • Internal opening on exam of the rectal area
    • Goodsall’s rule
      • Fistulas originating anterior to a transverse line through the anus will have a straight course and exit anteriorly.
      • Fistulas originating posterior to the transverse line will begin in the midline and have a curved tract.
    • Fistulous tract: can be detected by probing or fistulography under anesthesia

Management

  • Surgical management is 1st-line treatment
    • Goal: remove fistula without causing fecal incontinence
    • Fistulotomy: unroofs the fistula tract from the external to the internal opening
      • Allows drainage and healing by secondary intention
      • Low-lying fistula (does not involve external sphincter) → primary fistulotomy
      • High-lying fistula (involves external sphincter) → staged fistulotomy with Seton suture, a nonabsorbable suture placed through the tract to compress the fistula and encourage surrounding fibrosis
  • Post-operative measures:
    • Sitz baths and irrigation
    • Packing to ensure healing
  • Complications:
    • Recurrence
    • Worsening infection
    • Fecal incontinence (rare)

Mnemonic

Why fistulas stay open: “FRIENDS”

  • Foreign body
  • Radiation
  • Infection
  • Epithelialization
  • Neoplasm
  • Distal obstruction
  • Steroids

Differential Diagnosis

  • Anal carcinoma: neoplastic disease in which cancer cells form and grow in the anus. Symptoms include bleeding from the anus, anal pain, anal mass. or itching. Risk factors include older age, infections including human papillomavirus (HPV), multiple sexual partners, and anal sex. Anal carcinoma is diagnosed by biopsy. Treatment can include surgery, radiation, or chemotherapy. In a chronic or atypical anal fissure, anal carcinoma needs to be ruled out.
  • Hemorrhoids: enlargement of anal cushions (submucosal vessels) at the distal rectum. Depending on the location of the veins, hemorrhoids can be internal or external. External hemorrhoids are painful, but internal hemorrhoids are painless; both can bleed and appear as a soft rectal mass on exam. Commonly caused by constipation, and diagnosed on exam. Treatment includes stool softeners, topical hydrocortisone, and sitz baths. For recurrent cases, rubber band ligation or surgical removal is indicated.
  • Perianal ulcerations: erosions in the perianal mucosa. Occur secondary to inflammatory bowel disease, most notably Crohn’s disease, infections, and cancer. Symptoms include pain, bleeding, and erosions in the lining of the mucosa. Treatment is aimed at the causative disease. Differentiated from anal fissures on physical exam by deeper erosions in mucosa and evidence of other causative disease.
  • Anal fissure: a painful superficial tear of the epithelial lining (anoderm) of the anal canal. Anal fissures most often occur secondary to local trauma or irritation from constipation, diarrhea, anal intercourse, or perineal lacerations during childbirth. Treatment is generally conservative, including stool softeners, bulking agents, sitz baths, and/or topical vasodilators.

References

  1. Vogel, John D. Anorectal fistula: Clinical manifestations, diagnosis, and management principles. UpToDate. Retrieved Oct 12, 2020, from https://www.uptodate.com/contents/anorectal-fistula-clinical-manifestations-diagnosis-and-management-principles

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