Fistula-In-Ano

 

Overview

Fistula-In-Ano, also referred to as “Anorectal fistula”, is an abnormal tunnel between the anal canal (hollow viscus) and the surface of the body. Over 90% are attributed to prior anorectal abscess. It may occur in the presence of Crohn’s Disease and has an association with obesity and diabetes. Patients experience intermittent rectal pain, pruritus and malodorous drainage.¹

Anatomy

Anal canal – channel connecting the rectum to the anus, located within the anal triangle of the perineum, between the two ischioanal fossae

Anal Sphincters

• Internal sphincter
  • Involuntary control – continuation of circular fibers of the colon so it receives autonomic supply
  • Surrounds upper 3/4 of anal canal
• External sphincter
  • Voluntary control
  • Surrounds entire length of anal canal
  • Consists of three parts – subcutaneous, superficial and deep

Intersphincteric plane

• Between external sphincter muscle laterally and the longitudinal muscle (inner sphincteric muscle) medially
• Contains anal glands
• Can be opened up surgically to provide access for operations on the sphincter muscles

Anal glands

• Apocrine glands
• Occasionally infected and act as a source of anal fistula

Dentate (pectinate) line

• Junction of the proximal and distal segments of the anal canal
• Divides the anus into proximal 2/3 and distal 1/3
• Situated at the middle of internal sphincter

	Above Dentate line	Below Dentate line
Embryology	Hindgut (endoderm)	Proctodeum (ectoderm)
Epithelium	Columnar	Stratified squamous
Arterial supply	Superior rectal a.	Middle rectal a.Inferior rectal a.
Venous drainage	Superior rectal v.	Middle rectal v.Inferior rectal v.
Innervation	Visceral – Inferior hypogastric plexus(insensitive to pain)	Somatic – Inferior rectal nerves: branches of Pudendal nerve(sensitive to pain)
Lymph drainage	Internal iliac	Superficial inguinal

Aetiology & Risk factors

• Anal abscess (most common)
• Crohn disease
• Obstetric injury
• Radiation proctitis
• Rectal foreign bodies
• Infectious disease – Lymphogranuloma venerum and Actinomycosis
• Malignancy

Pathophysiology

Obstruction and infection of anal glands

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Abscess formation

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Abscess tracks to the perianal skin

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Formation of epithelialised tract

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Ongoing inflammation prevents closure

Classification

Relationship to anal sphincter muscles:

• Intersphincteric
  • From dentate line to anal verge
  • Travels along intersphincteric plane
• Transsphincteric
  • Through external sphincter into ischiorectal fossa
  • Travels through both internal and external sphincter
• Suprasphincteric
  • From anal crypt to ischiorectal fossa
  • Encircles the entire sphincter apparatus
• Extrasphincteric
  • Proximal to dentate line
  • Encompasses the entire sphincter apparatus
  • Usually not of cryptoglandular origin
• Superficial (submucosal)
  • Does not involve any sphincter muscle

Complexity:

• Complex
  • High transsphincteric fistulas involving at least 30% of external sphincter
  • Suprasphincteric 
  • Extrasphincteric
  • Horseshoe – fistulas in which the tract encircles the anal canal
  • Recurrent fistula
  • Fistulas associated with: Crohn’s, radiation, malignancy, faecal incontinence
• Simple
  • Low transsphincteric involving <30% of external sphincter
  • Superficial
  • Intersphincteric

Clinical Manifestation

• Acute: severe perianal pain (particularly during defecation and sitting), swelling and erythema (+/- fever and tachycardia)
• Recurrent: regular purulent discharge +/- bleeding
• Chronic: discharge of seropurulent fluid with an identifiable punctum (opening)

Examination

• The perianal skin may be excoriated and inflamed
• External opening visible (may drain pus or blood on manual compression)
• Internal opening identification may need a proctoscope (or sigmoidoscope if internal opening in rectum)
• Palpable cord-like tract

Differential diagnosis:

• Anal abscess
• Anal fissure
• Anal ulcer or sores
• Crohn disease
• Hidradenitis
• Pilonidal disease

Diagnosis 

Clinical Diagnosis with history and physical exam findings (3Ps):

• Pain
• Purulent discharge
• Perirectal skin lesion

Investigations:

• Endoanal ultrasound
• Fistulography
• CT scan
• MRI (gold standard for demonstrating the anatomy of a fistula)

Treatment

Medical management:

• Sitz baths
• Irrigation
• Packing to ensure healing from inside to outside

Surgical management:

• Abscess drainage
• Fistulotomy – allows healing through secondary intention; used for simple uncomplicated fistulas
• Seton – used for complex fistulas; two main types:
  • Loose (draining) – keep tract open allowing drainage and preventing abscess recurrence
  • Cutting – gradually tightens to cut through tissue and allows healing behind it; associated with risk of incontinence
• Sphincter sparing procedure – at least 6 weeks after Seton placement:
  • Fistula plug
  • Fibrin glue
  • LIFT (ligation of intersphincteric fistula track)
  • Advancement flap
  • Proctectomy
  • Modified Hanley procedure ^{2 3}

Complications 

• Recurrence
• Faecal incontinence from surgery

References

1. Vogel JD. Anorectal fistula: Clinical manifestations and diagnosis. In: Weiser M, editor. UpToDate [Internet]. [updated 2025 Oct 30; cited 2025 Nov 7]. Available from: UpToDate
2. Champagne BJ. Operative management of anorectal fistulas. In: Weiser M, editor. UpToDate [Internet]. [updated 2024 Aug 22; cited 2025 Nov 7]. Available from: UpToDate
3. Simpson JA, Banerjea A, Scholefield JH. Management of anal fistula. In: BMJ Best Practice [Internet]. 2012 [cited 2025 Nov 7]. Available from: BMJ Best Practice