A fistula is an abnormal communication between two epithelial-lined organs. A fistula-in-ano is a fistulous connection on the body surface near the anus. It is usually secondary to perianal infections (ie, acquired); in rare cases, it is secondary to a congenital defect.  In an acquired fistula-in-ano, the communication is typically between the anal canal and the perineal skin. In a congenital fistula-in-ano, the fistulous tract may extend from the anal canal to the vestibule, vagina, or urethra.
As a clinical entity, fistula-in-ano has well been recognized from ancient times. Hippocrates (460 BC) used a seton to cure fistula-in-ano. The great Indian surgeon Sushruta (1600 BC) used a caustic ligature (Kshara sutra) to treat fistula-in-ano. In 1337, John Anderne was the first to surgically lay open a fistula-in-ano.
The anal canal is the part of the hindgut that extends from the anal ring to the anal verge. The dentate line, which is the site of the cloacal membrane, divides the anal canal into the proximal part, lined by columnar epithelium, and the distal part, lined by squamous epithelium. At the level of the dentate line, transverse folds of mucosa form a ring of valves with pockets called the crypts of Morgagni. The anal glands open in the crypts. The glands branch out and lie in the submucosal plane or, most frequently, in the intersphincteric plane.
For more information, see Anal Canal Anatomy.
In the congenital form of fistula-in-ano, the fistulous tract is lined with stratified squamous epithelium, columnar epithelium, or both. The acquired form manifests with repeated perianal abscesses and has an inflamed fibrous tract lined by granulation tissue and no epithelial lining upon microscopic examination.
In the infected (acquired) form, the crypts of Morgagni are deeper (3-10 mm) than normal (1-2 mm). This abnormality facilitates the trapping of bacteria, which cause cryptitis that leads to perianal abscess formation and fistulae. Abnormal anal glands  and hormonal imbalances have been described.
Abnormal anal glands  and hormonal imbalances have also been proposed as causes of fistula-in-ano. Androgen excess may stimulate the sebaceous glands, resulting in secondary infection.
Fistula-in-ano is not common in children compared with adults; most cases occur in adults. Fistula-in-ano is more common in boys than in girls, and 96% of cases occur in infants younger than 1 year.
At King Edward Memorial (KEM) Hospital in Pune, India, this condition is present in 0.18% of admitted patients.
Outcome is good for both acquired and congenital varieties of fistula-in-ano. Recurrence and scarring are common in patients with tuberculosis or Crohn disease.
In a retrospective study of fistula-in-ano in children, Novotny et al demonstrated that recurrence is more likely in older children and in children who had previous episodes of perianal abscess or if pus was noted at the time of surgery. 
A study by Inoue et alreported good long-term success rates with seton placement in infants with fistula-in-ano and suggested that this procedure may be a useful treatment option in this population.