Fistula-in-Ano

Fistulas occur when there is an abnormal connection between two epithelialized surfaces that do not usually connect. Fistula-in-ano refers to a fistula between the skin and the anus. In 30 B.C., writings by Celsus advocated using a knife for treating fistula-in-ano. In cases where there were multiple openings, Celsus advocated for surgery in combination with ligature. John of Arderne (1307 – 1390 A.D.) described an operation for fistula-in-ano similar to treatments used today. 

Etiology

• Anorectal abscess
• Cryptoglandular: anal crypts become blocked by inspissated debris/stool, infection develops at the anal glands which extends through the pathway of least resistance, abscess is formed in intersphincteric space leading to fistula development
• Postoperative or traumatic
• IBD
• Anal fissure
• Tuberculosis-related
• Risk factors: obesity, diabetes, smoking, hyperlipidemia, sedentary lifestyle

Pathogenesis

• Results from persistent communication between the anal canal (internal opening) and perianal skin (external opening) following spontaneous or surgical drainage

History

• Cyclical pattern of pain and swelling
• Drainage of the area associated with relief of symptoms
• Fecal soiling
• ± Bleeding

Physical Exam

• ≥ 1 external opening with or without granulation tissue
• Multiple external openings (“watering can perineum”) → suspicious for perianal Crohn disease
• External opening on the anal margin skin with heaped-up granulation tissue tender to palpation
• Digital rectal exam: palpation of a cord-like subcutaneous structure
• Location of the fistula can help diagnose the type of fistula. Goodsall’s rule can be used to help locate the internal opening as well.
  • Submucosal: external opening in posterior midline close to anal verge
  • Intersphincteric: external opening off the midline close to anal verge
  • Low transsphincteric: external opening in anterior location
  • Transsphincteric or suprasphincteric: external opening in ischiorectal fossa
• Draining of fistula tract

Imaging

• Anoscopy
  • Direct inspection of dentate line
  • May reveal an erythematous crypt or visible internal opening
  • Can help exclude inflammatory conditions
• Exam under anesthesia
• Routine imaging is not usually necessary. It may be considered in patients with recurrent or complex anal fistula, immunosuppression, or anorectal Crohn disease.
• MRI pelvis
  • Identify primary and secondary openings
  • Delineate anatomy of fistula tracks
• Endoanal US (EAUS)
• Transperineal US (TPUS)

Treatment

• Goals for treatment
  • Eliminate sepsis
  • Remove or ablate epithelialized tracts
  • Avoid or minimize the risk of fecal incontinence
  • Prevent recurrence 
• Lay-open technique (fistulotomy)
  • Indications: simple fistula-in-ano with normal anal sphincter function
  • Recurrence and incontinence are most significant complication
  • Risk factors for postoperative anal sphincter dysfunction: preoperative fecal incontinence, recurrent fistula, female sex, complex fistulas, previous anorectal surgery, women with anterior fistulas or who have occult sphincter damage from previous birthing trauma
• Seton
  • Indications
    • Greater than lower ¼ of external anal sphincter involved (for first procedure) followed by either LIFT or anorectal advancement flap
    • When lay open technique is not possible or not advisable
    • Complex cryptoglandular anal fistulas
  • Used to control the fistula: narrow the fistula tract and prevent recurrent cyclical symptoms, shorten fistula track
• Endorectal advancement flap (ERAF)
  • Indications
    • High transsphincteric fistulas
    • Suprasphincteric fistulas
  • Involves curettage of fistula tract, sutured closure of internal opening, and covering of the internal opening with a mobilized segment of the rectum
• Ligation of intersphincteric fistula (LIFT)
  • Indications
    • Simple fistulas
    • Complex fistulas
    • Transsphincteric fistulas
  • Identification of internal opening with suture ligation of intersphincteric portion of the fistula. Tract and gland are excised and the wound is debrided. 
• Fibrin glue → relatively ineffective
• Anal fistula plug → relatively ineffective

Relevant Information

• Goodsall’s rule: predict the course of the fistula tract and location of the internal opening;  help locate internal opening
  • Fistulas with an external opening anterior to the anus connect with anus/rectum in a straight line
  • Fistulas with an external opening posterior to the anus go toward a midline internal opening in the anus/rectum in a curvilinear fashion
• Anal canal
  • Connection between the anal verge and anorectal junction; 2 – 4 cm in length
  • Dentate line lies midpoint in the anal canal
    • Proximal: longitudinal folds of columnar epithelium (columns of Morgagni)
    • Distal: smooth squamous epithelium (anoderm)
  • Anal crypts are located between the columns of Morgagni, where anal ducts empty

Classifications

• Simple vs. complex
  • Simple fistula
    • Due to glandular obstruction resulting in anorectal abscess, and ultimately, a fistula
    • Single tract, subcutaneous tract, involve < 30% of the external sphincter
  • Complex fistula
    • Any fistula that is high transsphincteric or when a fistulotomy would result in incontinence
    • Includes suprasphincteric, extrasphincteric, all anterior transsphincteric fistulas in women, fistulas caused by Crohn disease, malignancy, surgery, and trauma

Differential Diagnoses

• Acute proctitis
• Anal carcinoma
• Anorectal abscess
• Constipation
• Diverticulitis
• Hidradenitis suppurativa
• IBD
• Pilonidal cyst

Resources   

• American Society of Colon and Rectal Surgeons. “Clinical Practice Guidelines for the Management of Anorectal Abscess, Fistula-in-Ano, and Rectovaginal Fistula,” (2022)