Perianal and Perirectal Abscesses 

Perianal 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 of perianal abscesses is not well established. The treatment approach varies somewhat by age and, in most instances, differs from that used in adults.

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 may spontaneously drain pus and heal, whereas others may require surgical intervention. Some perianal abscesses may heal incompletely, with or without surgery, and result in a tiny opening at the site of drainage, called an anal fistula, or fistula-in-ano. This may or may not require an additional surgery.

The vast majority of perianal 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.

Frequency

The overall incidence of perianal and perirectal abscesses in children is unknown. It is a relatively common condition seen in a general pediatric or pediatric surgical practice. In infants, in whom the condition is most prevalent among pediatric patients, the incidence is estimated at between 0.5% and 4.3%, overwhelmingly in males. In older children, perianal and perirectal abscesses have no sexual predilection. No racial predilection is reported in any age group.

The etiology of perianal abscess and fistula-in-ano remains unclear. The prevailing theory implicates small glands in the wall of the anal canal, called the crypts of Morgagni. It is believed that a small infection, or cryptitis, forms in one of these crypts, leading to perianal abscess.

Some authors have suggested that some infants have abnormal crypts, which predispose them to cryptitis and abscess formation. One study showed that the crypts of infants with fistulas tend to be deeper (3-10 mm) than those in healthy infants (1-2 mm).^[1] Some have suggested that androgen excess or androgen-estrogen imbalance might predispose to the formation of these abnormal crypts.^[2]

The prevailing theory is that an infection in an anal crypt progresses and erodes through the wall of the anal canal into the surrounding soft tissue, where a collection of pus accumulates. This is the perianal abscess.

When a perianal 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 originated in 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 some individuals form fistulas while others do not.^[3]

If the etiology of fistula is abscess drainage, either spontaneously or through surgical incision, it is a logical conclusion 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. Recent studies have followed this line of thinking and are discussed in the Treatment section.

Perianal abscesses often present within the first few months of life. A perianal abscess, which in many ways is the same as a small pimple, appears as a red swollen area located just outside the anus; this is usually first noticed during a diaper change and may be tender to the touch. Affected children may appear to be irritable but are commonly asymptomatic. Differentiating an upset baby from true tenderness due to perianal abscess can be a challenge for the clinician but is important to reassure parents.

Infants with perianal abscesses generally do not have underlying medical conditions that predispose them to abscesses. Perianal abscess and fistula-in-ano are unrelated to diaper rash.

Older children with perianal abscesses fall into two roughly equally sized categories. The first category is otherwise completely healthy children who have no significant risk factors or history of symptoms suggestive of IBD. Many children are constipated; constipation is a risk factor for anal fissure but not for perianal abscess. The second category of older children with perianal abscesses are those with IBD. Some children with a known history of Crohn disease present with a new abscess or fistula. In many cases, the first manifestation of IBD or Crohn disease in a child is perianal abscess or fistula. Elements in the history that suggest IBD include weight loss, failure to thrive, diarrhea, and chronic abdominal pain.

Abscess

In infants younger than one year who present with a small perianal abscess, it is reasonable to try to attain full resolution with an antibiotic regimen and no drainage. Such an approach may decrease the likelihood of fistula formation.^[3]

More aggressive treatment is indicated in children younger than one year who present with a large, red, bulging perianal abscess and who appear to be in significant distress. The abscess should be drained, most commonly in the office, and oral antibiotics initiated. Although exceedingly rare, signs of true systemic illness (weakness, lethargy, fever) secondary to the infection in children require admission for intravenous antibiotics.

Routine perianal abscesses in babies do not require operative intervention and do not require general anesthesia for examination or drainage.

An identical approach can be followed in older children without a history of inflammatory bowel disease (IBD), with perhaps a shorter threshold to perform drainage in any patients initially deemed suitable for antibiotics alone. Older children with complex fistulas and/or in extreme pain during the examination require general anesthesia for examination and treatment.

Fistula

Babies who present with a fistula after surgical or spontaneous drainage of an abscess should undergo a period of nonoperative observation and should be observed until age 18 months provided that they remain otherwise happy and healthy.^[4] After this time, it is reasonable to consider surgical fistulotomy for resolution.

Older children with fistulas secondary to Crohn disease should be referred to a pediatric surgeon with significant IBD experience or to an adult colorectal surgeon.

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 these 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. Here it can go in one of two directions: (1) The infection can head toward the skin (most common), or (2) it 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.