An anorectal abscess is a localized collection of pus in the perirectal spaces. Abscesses usually originate in an anal crypt. Symptoms are pain and swelling. Diagnosis is primarily by examination and CT or pelvic MRI for deeper abscesses. Treatment is surgical drainage.
(See also Evaluation of Anorectal Disorders.)
An abscess may be located in various spaces surrounding the rectum and may be superficial or deep. A perianal abscess is superficial and points to the skin. An ischiorectal abscess is deeper, extending across the sphincter into the ischiorectal space below the levator ani; it may penetrate to the contralateral side, forming a “horseshoe” abscess. An abscess above the levator ani (ie, supralevator abscess) is quite deep and may extend to the peritoneum or abdominal organs; this abscess often results from diverticulitis or pelvic inflammatory disease. Crohn disease (especially of the colon) sometimes causes anorectal abscess. A mixed infection usually occurs, with Escherichia coli, Proteus vulgaris, Bacteroides,streptococci, and staphylococci predominating.
Superficial abscesses can be very painful; perianal swelling, redness, and tenderness are characteristic. Fever is rare.
Deeper abscesses may be less painful but cause toxic symptoms (eg, fever, chills, malaise). There may be no perianal findings, but digital rectal examination may reveal a tender, fluctuant swelling of the rectal wall. High pelvirectal abscesses may cause lower abdominal pain and fever without rectal symptoms. Sometimes fever is the only symptom.
Patients who have a pointing cutaneous abscess, a normal digital rectal examination, and no signs of systemic illness do not require imaging. CT scan is useful when a deep abscess or Crohn disease are suspected. Higher (supralevator) abscesses require CT to determine the intra-abdominal source of the infection. Those with any findings suggestive of a deeper abscess or complex perianal Crohn disease should have an examination under anesthesia at the time of drainage.
Prompt incision and adequate drainage are required and should not wait until the abscess points. Many abscesses can be drained as an in-office procedure; deeper abscesses may require drainage in the operating room. Febrile, neutropenic, or diabetic patients or those with marked cellulitis should also receive antibiotics (eg, ciprofloxacin 500 mg IV q 12 h and metronidazole 500 mg IV q 8 h, ampicillin/sulbactam 1.5 g IV q 8 h). Antibiotics are not indicated for healthy patients with superficial abscesses. Anorectal fistulas may develop after drainage.
Anorectal abscesses may be superficial or deep.
Superficial abscesses may be diagnosed clinically and drained in the office or emergency department.
Deep abscesses often require imaging with CT scan and typically must be drained in the operating room.
Immunocompromised patients and those with deep abscesses should receive antibiotics.