Armando Hasudungan
Biology and Medicine videos



VIDEO: Fistula in Ano



Overview Ano in fistula is an abnormal tunnel between the anal canal (hollow viscus) and the surface of the body. It is very common, especially in otherwise fit young adults. The great majority result from an initial abscess forming in one of the anal glands that pass from the submucosa of the anal canal to open within its lumen. May occur in the presence of Crohn’s Disease, and has an association with obesity and diabetes. Treatment is mainly surgical, medical treatment such as antibiotics can treat associated sepsis (usually secondary to abscess).

Fistula: An abnormal connection of two epithelial surfaces
Fistula in Ano: abnormal connection of the anorectal epithelial surface to the perineal or vaginal skin.
Sinus: Granulating track leading from a source of infection to a surface
Remember Crohn Disease is associated with ano-rectal disease

Anorectal anatomy

Overview The hindgut makes up the last 1/3 of the transverse colon, the descending colon, sigmoid colon, rectum and part of the anus. The anal canal commences at the level where th rectum passes through the pelvic diaphragm and ends at the anal verge

Layers of the rectum mucosa (from inner to outer)

  • Mucosa
  • Submucosa
  • Muscularis
    • Outer longitudinal
    • Inner Circular
  • Serous


Anal Sphincters

  • Internal sphincter
    • Continuation of the circular fibers of the colon. Thus, the internal sphincter is under autonomic control (Autonomic nervous system)
    • Involuntary control
    • 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 the external sphincter muscle laterally and the longitudinal muscle (inner sphincteric muscle) medially
  • Contains anal glands
  • The plane can be opened up surgically to provide access for operations on the sphincter muscles

Anal glands

  • Apocrine glands
  • There are typically 8 to 10 anal crypt glands, arranged circumferentially within the anal canal at the level of the dentate line.
  • The glands penetrate the internal sphincter and end in the intersphincteric plane.
  • Glands are occasionally infected and acts as a source of anal fistula

anus 2

Pectinate line (dentate line)

  • Muco-cutaneous junction of anal canal
  • Divides anal canal into upper and lower lowers which differ in
    • Blood supply
    • Lymph drainage
    • Nerve supply
  • Corresponds with position of anal valves
  • Situated at the middle of internal sphincter
Distinction Above Pectinate line Below Pectinate line
Embryological origin Endoderm Ectoderm
Epithelium Colomnar epithelium Stratified squamous epithelium
Lymph drainage Internal iliac lymph node Superficial inguinal lymph nodes
Artery Superior rectal artery Middle and inferior rectal arteries
Veins Superior rectal veins Middle and inferior rectal veins
Innervation Inferior hypogastric plexus Inferior rectal nerve
Haemorrhoids Internal haemorrhoids External haemorrhoids
Hilton’s Line also called while line/anocutaneous line indicates lower end of the internal sphincter. Ischiorectal abscess when communicates with anal canal usually  opens at or below Hilton’s line.


A fistula is an abnormal connection of two epithelial surfaces and the two surfaces joined in fistula-in-ano are anorectal lining and the perineal or vaginal skin. Anal fistulae are classified according to their position and relation to the internal and external anal sphincters.

  • Perianal
  • Intersphincteric
  • Trans-sphincteric
  • Suprasphincteris
  • Extrasphincteris


Risk Factors

Signs and Symptoms

Clinical Presentation Patient usually complains of a non-healing abscess. Acute perianal abscess presents as rapid onset of severe perianal pain, swelling and erythema (+/- fever and tachycardia).

  • Acute – Severe perianal pain, swelling and erythema (abscess)
  • Recurrent – regular purulent discharge +/- bleeding
  • Chronic – chronic discharge of seropurulent fluid with an identifiable punctum (opening)


  • The perianal skin may be excoriated and inflamed
  • External opening visible
  • Internal opening identification may need a proctoscope (or simoidoscope if internal opening in rectum)
  • Palpable cord-like tract
Goodsall’s rule (law): Relates the external opening of an anal fistula to its internal opening. Fistulas can be described as anterior or posterior relating to a line drawn in the coronal plane across the anus. Anterior fistulas will have a direct track into the anal canal. Posterior fistulas will have a curved track with their internal opening lying in the posterior midline of the anal canal. An exception to the rule are anterior fistulas lying more than 3cm from the anus, which may open in the anterior midline of the anal canal.

Differential Diagnosis

  • Anal Abscess
  • Anal fissure
  • Anal ulcer or sores (secondary to another disease or infection)
  • Crohn disease


  • Endoanal ultrasound
  • CT scan
  • Magnetic resonance (MR) can demonstrate the anatomy of a fistula very clearly

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

  • Pain
  • Purulent drainage
  • Perirectal skin lesion
Remember 3Ps for anal fistula diagnosis: Pain, Purulent drainage, perirectal skin lesion


The majority of anorectal fistulas originate from an infected anal crypt gland

  • Anal abcess (most common)
  • Crohn’s Disease
  • Lymphogranuloma venereum
  • Radiation proctitis
  • Rectal foreign bodies
  • Actinomycosis


Optimal treatment depends upon correctly classifying the fistula. Surgery is gold standard.


  • Drainage of abscess
  • Fistulotomy – allows healing through secondary intention
  • Seton – slowly tightened allowing for healing
  • Anal plug
  • Flap Surgery
  • Fibrin glue


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


  • Recurrence
  • Faecal incontinence from surgery