The inflammatory process that leads to abscess may begin with an abrasion or tear in the lining of the anal canal, rectum, or perianal skin, and subsequent infection by Escherichia coli, staphylococci, or streptococci. Such trauma may result from injections for treatment of internal hemorrhoids, enema-tip abrasions, puncture wounds from ingested eggshells or fishbones, or insertion of foreign objects.

Other preexisting lesions include infected anal fissure, infections from the anal crypt through the anal gland, ruptured anal hematoma, prolapsed thrombotic internal hemorrhoids, and septic lesions in the pelvis, such as acute appendicitis, acute salpingitis, and diverticulitis. Systemic illnesses that may cause abscesses include ulcerative colitis and Crohn’s disease. However, many abscesses develop without preexisting lesions. Other causes include trauma, malignancy, radiation, infectious dermatitis, and an immunocompromised state.

As the abscess produces more pus, a fistula may form in the soft tissue beneath the muscle fibers of the sphincters (especially the external sphincter), usually extending into the perianal skin. The internal (primary) opening of the abscess or fistula is usually near the anal glands and crypts; the external (secondary) opening, in the perianal skin.

Characteristics of perirectal abscess are throbbing pain and tenderness at the site of the abscess and painful swelling that’s exacerbated by defecation. A hard, painful lump develops on one side, preventing comfortable sitting.