Perianal and perirectal abscesses are common anorectal problems. The infection originates most often in an obstructed anal crypt gland, with the resultant pus collecting in the subcutaneous tissue, intersphincteric plane, or beyond (ischiorectal space or supralevator space) where various types of anorectal abscesses form.
A fistula is defined as an abnormal communication between two epithelium lined surfaces: the rectum or anal canal and the skin. An anorectal fistula is the chronic manifestation of an anorectal abscess.
The most common etiology of an anorectal fistula is an infected anal crypt gland. Other causes include: Crohn’s disease, Lymphogranuloma venereum, Radiation proctitis, Rectal foreign bodies, Actinomycosis, Obstetric injury.
Patients with an anorectal abscess often present with severe pain in the anal or rectal area. The pain is constant and not necessarily associated with a bowel movement. Constitutional symptoms such as fever and malaise are common. Purulent rectal drainage may be noted if the abscess has begun to drain spontaneously.
On physical examination, an area of fluctuance or a patch of erythematous, indurated skin overlying the perianal skin may be noted in patients with a superficial (eg, perianal) abscess. Patients with a deeper (eg, supralevator) abscess, however, may not have any physical signs on external examination, and the abscess can only be felt via digital rectal examination or by imaging.
Patients with an anorectal fistula usually present with a “nonhealing” anorectal abscess following drainage, or with chronic purulent drainage and a pustule-like lesion in the perianal or buttock area. Patients experience intermittent rectal pain, particularly during defecation, but also while sitting and active. Patients may also experience intermittent and malodorous perianal drainage and pruritus.
On physical examination, the perianal skin may be excoriated and inflamed. The external opening may be visualized, or palpated as an induration just beneath the skin if the external opening is incomplete or blind. The external opening may be inflamed, tender, and/or draining purulent fluid. A palpable cord leading from the external opening to the anal canal may be present. The internal opening in the anus can be viewed by an anoscopic examination, while a sigmoidoscope may be required to view the internal opening in the rectum. In some cases, the internal opening can be palpated on digital rectal exam.
- Ischiorectal abscesses (ischioanal abscesses): penetrate through the external anal sphincter into the ischiorectal space.
- Intersphincteric abscesses – only 2 to 5 % of all anorectal abscesses. They are located in the intersphincteric groove between the internal and external sphincters. As a result, they often do not cause perianal skin changes but can be palpated during digital rectal examination as a fluctuant mass protruding into the lumen.
- Supralevator abscesses − Can originate in two different sources: the typical cryptoglandular infection that travels superiorly within the intersphincteric plane to the supralevator space, or an inflammatory pelvic process caused by Crohn’s disease or perforated colon from diverticular disease or cancer. The potential source of pelvic infection is best determined from patient history.
- Horseshoe abscesses — complex perirectal abscesses that most often form posterior to the anal canal. The potential space where the abscess originates is bound by the pelvic floor superiorly, by the anococcygeal ligament inferiorly, and by the coccyx and anal canal. Because of these relatively rigid boundaries, abscesses in this space are forced to extend into the ischiorectal space, either unilaterally or bilaterally (horseshoe).
Parks described four types of anorectal fistulas that originate from crpytoglandular infections. Fistulas can have a complicated anatomy with one or more extensions and accessory tracts. The original Parks’ classification did not include a superficial fistula tract.
- Type 1 is an intersphincteric fistula that travels along the intersphincteric plane.
- Type 2 is a transsphincteric fistula that encompasses a portion of the internal and external sphincter.
- Type 3 is a suprasphincteric fistula that encompasses the entire sphincter apparatus.
- Type 4 is an extrasphincteric fistula that extends from a primary opening in the rectum, encompasses the entire sphincter apparatus, and opens onto the skin overlying the buttock.
Anorectal abscess should be suspected in patients who present with severe pain in the anal or rectal area. A superficial anorectal abscess can be diagnosed on physical examination with findings of perianal erythema and a palpable, often fluctuant mass. A deeper abscess can be diagnosed by feeling a tender, often fluctuant mass internally on digital rectal exam or by imaging studies.
Imaging studies, such as computed tomography, magnetic resonance imaging, and transperitoneal or endorectal ultrasound, can confirm the diagnosis when a deep anorectal abscess is suspected but cannot be palpated by external examination or digital rectal examination
Imaging studies are not required for diagnosis of simple fistulas; however, they may be helpful for diagnostic evaluation of complex or recurrent fistulas
Imaging studies, such as endosonography, fistulography, computed tomography, or magnetic resonance imaging, show air or contrast material within the fistula
Treatment anorectal abscess
Once diagnosed, all perianal and perirectal abscesses should be drained promptly; lack of fluctuance should not be a reason to delay treatment. Any undrained anorectal abscess can continue to expand into adjacent spaces as well as progress to generalized systemic infection.
Perianal abscess — A perianal abscess should be drained through a skin incision. The procedure can be carried out in an outpatient setting such as the office/clinic, emergency department, or procedure room.
Perirectal abscess — Most perirectal abscesses are complex and should be drained in the operating room, preferably under local or general anesthesia. Smaller perirectal abscesses may be amenable to drainage under local anesthesia with intravenous sedation. The surgical approaches vary by the site of the abscess.
Treatment in anorectal fistulas
Surgical treatment is the mainstay of therapy and is required in patients with symptomatic anorectal fistulas, with the exception of some patients with Crohn’s disease.
The goal of surgical therapy is to eradicate the fistula while preserving fecal continence. The surgical approach depends upon correct classification of the fistula. One of the most commonly cited principles to assist in the surgical management of an anal fistula is Goodsall’s rule, which states
- All fistula tracks with external openings within 3 cm of the anal verge and posterior to a line drawn through the ischial spines travel in a curvilinear fashion to the posterior midline.
- All tracks with external openings anterior to this line enter the anal canal in a radial fashion.
Fistula tracks longer than 3 cm from the anal verge do not necessarily follow Goodsall’s rule; they often have an internal opening in the posterior midline. Although Goodsall’s rule is often quoted, it may not always be accurate.