An anal fistula, also known as a fistula-in-ano, is an abnormal tract or cavity which runs outwards from an internal opening in the anorectal lumen to an external opening on the skin of the perineum or buttock. The fistula usually originates in the infected crypt of a anorectal abscess and tracks to the external opening .
An anal fistula is an abnormal communication between the anorectal lumen (the internal opening) and the perianal area (the external opening). Anorectal fistulae are commonly classified based on the relationship of the primary track to the external anal sphincter. Most fistulas arise from the incomplete resolution, treatment or drainage of an anorectal abscess, which leads to the chronic process of fistula formation. An anal fistula may be associated with an underlying disease such as tuberculosis or Crohn disease.
Symptoms range from the minor discomfort of perianal irritation and pus discharge to more serious sepsis. The treatment of choice is the surgical opening of the tract to drain the infection and eradicate the fistulous tract. Complete drainage and good postoperative care of an anorectal abscess are essential to prevent the development of anal fistula.
Most anal fistulas result from a previous anorectal abscess. Infection of an anal crypt glands leads to involvement of the intramuscular spaces and anorectal abscess. Sometimes the abscess bursts spontaneously draining in the skin leading to the development of a chronic fistula. Following surgical drainage of an abscess, occasionally a granulation tissue-lined tract is left behind that can lead to an anal fistula .
Other causes of anal fistulas are:
Non-specific anal fistulae are more common in men than women (1.8:1). The overall incidence is about 9 cases per 100,000 population per year. The most common age group is third to fifth decades of life, mean age of patients being 38.3 years. Various studies have shown that 7 to 40 % cases of anorectal abscess result in the formation of a fistula tract .
The majority of primary anal fistulas originate in anal crypts which become infected, leading to abscess formation. The surgical opening or rupture of the abscess leads to the formation of an anal fistula. The fistula tract is lined with some amount of granulation tissue. Although majority of anal fistula are non-specific, idiopathic or cryptoglandular, they can also be found in patients with inflammatory bowel disease, particularly Crohn disease, some other diseases of the intestines, and HIV infection. Anorectal abscesses and fistulas occur in approximately 30% of HIV patients .
Anal fistulae can be treated satisfactorily with surgery. Fistulas persist when the source of infection in anorectal abscess, has not been identified or adequately drained, when the diagnosis is incorrect, when postoperative care is insufficient, or in the presence of Crohn disease, malignancy, radiation proctitis, or unusual infection. Malignancy can rarely arise in a long-standing fistula tract.
An anal fistula causes chronic purulent, fecal or serosanguineous discharge with skin irritation. Intermittent swelling, pain and even fever occur due to fecal stasis in the tract; spontaneous rupture and discharge results in temporary improvement. A "water can" appearance is seen when there is prolonged disease course with secondary track formation .
Anal fistulas have been classified into the four general types by Parks et al, based on correlation with anatomical structures to facilitate surgical strategy: Intersphincteric, transsphincteric, suprasphincteric and extrasphincteric fistulas .
The aim is to determine the site of the internal and external openings, the course of the primary track, the presence of secondary extensions, and the presence of other conditions complicating the fistula .
Concurrent presence of an inflammatory bowel disease warrants consultation with a gastroenterologist. Asymptomatic anal fistula in presence of Crohn disease is managed conservatively because of post-surgical recurrence and poor healing. Antibiotics are given only in cases with sepsis or cellulitis. Analgesics can be given for the pain.
New experimental treatments
Long-term postoperative complications
Recurrence and continence rates
0-18% recurrence rate has been reported in standard fistulotomy, with 3-7% rate of incontinence. With surgery using seton, a recurrence rate of 0-17% has been reported, with 0-17% rate of incontinence. Surgical technique using mucosal advancement flap results in 1-17% recurrence and 6-8% incontinence.