Anorectal (perianal or perirectal) abscess is a relatively common condition in children. It occurs most often in male infants younger than 1 year but can occur in either sex and at any age. The exact incidence and prevalence are not well established. The treatment approach varies somewhat by age and, in most instances, differs from that used in adults (see Anorectal Abscess).
A perianal abscess is an infection characterized by a collection of pus that has formed under the skin within the soft tissue just outside the anus. The abscess often appears as a raised red lesion under the skin lateral to the anus, where it may grow and become painful. Some abscesses spontaneously drain pus and heal; others require surgical intervention. Some perianal abscesses heal incompletely, with or without surgery, leaving a tiny opening at the site of drainage (anal fistula, or fistula-in-ano), which may or may not require additional surgery.
The vast majority of anorectal abscesses develop spontaneously in completely healthy children and are self-limited; however, in older children, the condition can be associated with inflammatory bowel disease (IBD) or other conditions in which the immune system is compromised.
Controversies abound in the treatment of perianal and perirectal abscesses (see Treatment). The use of antibiotics alone (rather than surgical drainage) as a means of definitive treatment with the intention of decreasing the likelihood of eventual fistula-in-ano formation is quite controversial but is supported in the literature.  Nonoperative management of fistula-in-ano via observation alone in otherwise completely healthy male babies remains controversial but is also supported in the literature. 
The anal canal and the skin around it are the site of perianal abscesses and fistulas. Just inside the anal canal, about 1-2 cm from the anal verge in most babies, are small pits in the wall of the anal canal called anal crypts (or the crypts of Morgagni). It is believed that anorectal abscesses and fistulas originate as an infection in these anal crypts. The infection then erodes through the wall of the anal canal and extends into the fat beneath the perianal skin. From here, it can continue in one of two directions, as follows:
The infection can head toward the skin; this is the more common course
The infection can track deeper into what is referred to as the ischiorectal fossa, bounded superiorly by the levator ani muscles; infections in the ischiorectal fossa are rare in the pediatric population
Pathophysiology and Etiology
The pathophysiology and etiology of anorectal abscess and fistula-in-ano have not yet been fully defined. The prevailing theory is that an infection in an anal crypt, or crypt of Morgagni—that is, cryptitis—progresses and erodes through the wall of the anal canal into the surrounding soft tissue, where a collection of pus accumulates, forming the abscess.
When an anorectal abscess drains spontaneously by eroding through the skin or is surgically drained, a communication is formed between the abscess cavity and the skin. If the infection truly originates from an anal crypt, the abscess cavity must communicate with the lumen of the anal canal. The hole in the skin would therefore also communicate all the way into the anal lumen. When this communication persists over several weeks, it is called a fistula. It is unclear why fistulas form in some individuals but not in others. 
If the etiology of fistula is abscess drainage, either spontaneously or through surgical incision, it logically follows that efforts to cure the abscess before it drains, thereby avoiding completing the communication from the anal canal to the skin, may decrease the risk of fistulization. Various studies have followed this line of thinking (see Treatment).
Some authors have suggested that some infants have abnormal crypts, which predispose them to cryptitis and abscess formation. One study showed that the anal crypts of infants with fistulas tend to be deeper (3-10 mm) than those of healthy infants (1-2 mm).  It has been proposed that androgen excess or androgen-estrogen imbalance may predispose to the formation of these abnormal crypts. 
The overall incidence of anorectal abscesses in children is unknown. It is a relatively common condition seen in a general pediatric or pediatric surgical practice. In infants, the pediatric subgroup among whom this condition is most prevalent, the estimated incidence is between 0.5% and 4.3%, with an overwhelming male preponderance. In older children, anorectal abscesses show no sexual predilection. No racial predilection is reported in any age group.
The prognosis of anorectal abscess in children is excellent for all cases that are unrelated to Crohn disease. With or without surgery, the condition will eventually be brought to a successful resolution with no impact or implications for the future.
Children with abscesses who undergo drainage are likely to develop a fistula. Fistulas in children usually resolve without intervention, but some patients require surgery for resolution. Treatment of any concomitant fistula may enhance the results of surgical treatment of first-time perianal abscesses in children.  Recurrent fistula after fistulotomy in an otherwise healthy child is very unlikely and should prompt an evaluation for other signs of Crohn disease. The prognosis of Crohn-related perianal pathology is complex and beyond the scope of this article.
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