Definition and Overview

An anal fistula is a benign anorectal disorder characterised by an abnormal, infected channel that develops between the skin near the anus or rectum and the end of the bowel. Also commonly referred to as fistula-in-ano, it is typically filled with pus and can be caused by an obstruction due to foreign or fecal matter, bacteria, and acute infection in the internal glands of the anus.

Anal fistula are classified into two: low and high anal fistula. They are categorised based on how far they go beyond the anal sphincters. Low anal fistulas are more common, superficial, and pose little threat to continence. Thus, they are easier to manage. High anal fistulas, on the other hand, pass above or through a large amount of muscle with route that is further away from the skin. They develop typically due to ulcerative colitis and Crohn’s disease, among others. The management of high anal fistulas is more complicated compared to low anal fistulas.

Examples of high anal fistulas extrasphincteric fistulas (penetrates the levator muscle, which forms the main part of the pelvic diaphragm) and suprasphincteric fistulas (travels through the internal and external sphincters over the puborectalis muscle).

Causes of Condition

While less serious cases of fistula are caused by an anal abscess, high anal fistulas typically develop due trauma to the rectal area and medical conditions, such as:

  • Abnormal growth in the rectal area (tumors)

  • Birth abnormalities

  • Cancer of the anorectal region

  • Crohn’s disease

  • Diverticulitis

  • Hidradenitis suppurativa, a long-term skin condition that develops where the skin rubs together such as between the buttocks. This leads to chronic infection and the formation of an anal fistula secondary to deep infection.

  • Intestinal inflammation

  • Sexually transmitted diseases such as AIDS, HIV, syphilis, and chlamydia

  • Surgical complications following operative procedures near the anus

  • Tuberculosis

  • Ulcers

Key Symptoms

High anal fistulas caused various obvious and worrying symptoms particularly if the condition has already reach the advanced stage. Patients typically experience:

  • Chronic constipation

  • Constant anorectal pain

  • Difficulty passing urine

  • Discharge of foul-smelling blood or pus rom an opening around the anus

  • Fever

  • General feeling of fatigue

  • Infection

  • Nausea

  • Pain with bowel movements

  • Recurrent anal abscesses

  • Skin irritation around the anus (perianal cellulitis)

Diagnosing High Anal Fistula

Unlike low anal fistulas, high anal fistulas do not typically cause external opening on the skin near the anus. As such, diagnostic tests that aim to determine the exact depth and direction of the fistula tract are performed. These include:

  • Anoscopy – Using an anal speculum, this test is performed to evaluate problems of the anal canal.

  • Colonoscopy – Using a lighted, flexible instrument, colonoscopy is performed to examine the entire large intestine to determine if high anal fistula to rule out other disorders such as Crohn’s disease and ulcerative colitis

  • Fistulography – Patients are injected with a contrast solution before they undergo x-ray procedures. This test helps determine the depth and direction of the fistula.

  • Blood tests

  • 2D and 3D endoanal ultrasound – Recognised to be highly effective in assessing deep perirectal abscess

  • Pelvic MRI (magnetic resonance imaging scan)

  • CT Scan – This is often performed in patients with complicated infections and other medical conditions such as Crohn’s disease.

Who to See and Types of Treatments Available

Patients with high anal fistula are managed by colorectal surgeons, doctors who specialises in the treatment and management of disorders of the colon, rectum, and anus.

Treatment of high anal fistula largely depends on the on how close the abnormal channel to the anal sphincter muscles is and the strength of the patient’s sphincter muscles. The goal of the treatment is to remove the fistula with as little impact as possible on the sphincter muscles.

High anal fistulas require more complex treatment when compared to low anal fistula because of their complexity. Treatment include ligation intersphincteric fistula tract in which an incision is made between the external and internal sphincters to ligate the intersphincteric tract and the fistula tract is scraped with a curette. This is often combined with endorectal advancement flap in which the opening is covered with the anorectal wall.

50% of patients who have undergone treatment for high anal fistula reported recurrence particularly those suffering from other conditions such as cancer and those who are heavy smokers. There are also reports of incontinence.


  • Simpson JA, Banerjea A, Scholefield JH. Management of anal fistula. BMJ. 2012;345:e6705
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