If the rectum were a simple tube, it would be like a fecal sluice gate every time stool arrived. But itʼs not just a tube. It has an elegant sphincter system that can automatically tell solid -from gas- from liquid. What other muscle in the human body has that sort of intelligence? Understanding anal abscesses means understanding anal sphincters because they determine where the pus will spread.
The INTERNAL ANAL SPHINCTER wraps directly around the rectum like a hand holding a stick (the rectum being the stick). The ANAL GLANDS, which lubricate the anorectum, sit just inside the internal sphincter like peas in a pod. Anal abscesses are thought to begin as suppuration of the anal glands.
The EXTERNAL ANAL SPHINCTER wraps around the internal sphincter. The external sphincterʼs job is voluntary control and volitional release of feces. Next to the external sphincter is FAT and then the ISCHIUM.
When anal glands get infected, pus can spread in any direction. Most of the time it goes straight down, causing a PERIANAL ABSCESS. The perianal space is relatively small and usually, the abscess you see next to the anus is all the abscess there is. Sometimes, the infection can involve the intersphincteric (perirectal) space or may be the head of a deeper abscess that has made a serpiginous path to the perianal area. Thatʼs why, even if the perianal abscess is small and innocuous looking, a rectal exam is so important.
The ischiorectal space is an expanse of fat sitting between the external anal sphincter and the ischium. An ISCHIORECTAL ABSCESS starts deep. By the time it reaches the skin, where it can be seen on external exam, the patient is usually sick. While a perianal abscess is right up against the anus, the ischiorectal abscess generally comes to a head several centimeters away from the anal orifice. An ischiorectal abscesses are the most common of the deep infections, but it’s not the only player. There are also intersphincteric and supralevator abscesses (which may not be visible on external exam.
The Dividing Line
The essential dividing line for these abscesses is superficial (perianal) versus deep (perirectal). A superficial/perianal abscess is in our field of play. A deep/perirectal abscess is not – that is for the surgeon. How can you tell deep from superficial? Sometimes you canʼt. The abscess may be too deep for you to feel on digital rectal exam. If there’s a fever, think deep. Pain out of proportion to what you see, think deep. This is one case where your rectal exam is important. If the rectal exam is extremely painful for the patient or you feel induration, fluctuance, or fullness, consult a surgeon. The patient needs an exam under anesthesia and/or imaging study.
Who to drain in the ED
Simple perianal abscess can be incised and drained in the ED. One word of caution on the perianal abscess: they can be associated with a deep/perirectal infection.
If a patient has an ischiorectal abscess, thereʼs no reason you canʼt give it a stab to release pressure, but most will need definitive deep exam and management in the OR. The longer an anal or perirectal abscess is left to fester, the higher the chance of sphincter damage and incontinence. You may be reluctant to do an I&D out of fear of damaging the sphincters, but itʼs the delay in treatment thatʼs the real risk for sphincter damage. If you can see the point on the skin, and itʼs going to be a while before getting the patient to the OR, make a small incision to start the drainage.
Many drainage techniques have been described in the literature: small incision and drainage, incision and cut out an ellipse of skin, cruciate incision, loop drainage, pack, no pack. No single method has proven superior. Just like any other abscess, you want to get the pus out and keep the pus draining. As long as what you do maintains an open wound, thatʼs enough. The cruciate incision is loved in surgical textbooks, but it is a huge cut to have in a sensitive area and I donʼt think it has a place in ED management. Since there are so many important sphincters in the perianal area, I avoid aggressive exploration and only probe gently.
Sitz bath and stool softeners.
Every patient with a perianal abscess should be referred to a surgeon. First time infections, and even more so repeat infections, can be associated with anal fistulas.
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