Perianal abscess and fistula: Clinical sciences

Perianal abscesses and fistulas are anorectal diseases that result from obstructed glandular crypts of the rectum or anus. A perianal abscess is a collection of pus that can form in a number of spaces within the anus or rectum. On the other hand, perianal fistulas can form after an abscess is drained and typically result from chronic changes of the same infectious process. Now, the first step is to obtain a focused history and physical examination. You should also obtain basic labs, such as a CBC to look for any clues of spreading infection.

Now, a patient might report intermittent pain that’s typically associated with sitting, activity, and possibly with defecation. The patient may also report intermittent malodorous drainage with or without pruritus. On a physical exam, you’ll typically find a non-healing abscess or chronic purulent drainage. As you’re performing the exam, see if you can visualize the external opening of the fistula, which might be found with excoriation, inflammation, or induration, as well as drainage of purulent fluid. On a digital rectal exam, you may be able to palpate the internal opening or an abscess, if present. Lastly, labs may reveal leukocytosis. If the patient presents with this clinical picture, you can suspect a perianal fistula.

Now that we have a potential diagnosis in mind, let’s talk about imaging. The next step is to proceed with anoscopy or sigmoidoscopy, which can help confirm the diagnosis of a perianal fistula. If you don’t see any findings suggestive of a fistula on anoscopy or colonoscopy, consider an alternative diagnosis. However, if you find an internal opening of the fistula in the anus or rectum, this supports your suspected diagnosis of a perianal fistula. The next step is to proceed with diagnostic imaging, such as pelvic MRI or endosonography, which helps you get a better picture of the fistula and determine if it’s a simple or a complex fistula.

Some high-yield facts to keep in mind! Crohn disease has a strong association with perianal and anorectal manifestations. So, in patients with Crohn disease, be sure to look out for a perianal abscess, perianal fistula, anal fissure, or anal stricture at some point.

Now, one way to tell whether the patient has a simple or a complex fistula is by determining the level of external sphincter involvement. Most simple fistulas don’t involve the external sphincter, and if involved, it will be 30% or less of its thickness. If this is the case, you can diagnose a simple fistula.

There are three main types of simple fistulas to know: superficial, low transsphincteric, and intersphincteric fistulas. Superficial fistulas don’t involve the external sphincter at all. Low transsphincteric fistulas involve the lower third of the sphincter. Lastly, intersphincteric fistulas extend between the internal and external sphincters.

Alright, let’s talk about the management of simple fistulas. First, start with empiric antibiotics, such as ciprofloxacin or metronidazole. Next, consult the surgical team for incision and drainage and a sphincter-sparing procedure, such as fistulotomy or fistulectomy. A fistulotomy involves cutting open the entire fistula to clean the area and allow it to heal. On the flip side, fistulectomy involves complete excision of the entire tract and closing the ends. After the chosen procedure is complete, counsel the patient on keeping the area clean, as well as performing Sitz baths frequently in order to help alleviate any residual pain and to prevent infection or future recurrence.

Now that we’re done with simple fistulas, let's go back to imaging and talk about complex fistulas. Unlike simple fistulas, complex ones are, well… more complex. If you see external sphincter involvement greater than 30%; a fistula that’s proximal to the dentate line; a multiple tract fistula; a recurrent fistula; or if the patient has a history of anal incontinence, you can diagnose a complex fistula.

Now, there are four main types of complex fistulas to know. These are high transsphincteric, suprasphincteric, extrasphincteric, and horseshoe fistulas. High transsphincteric fistulas involve greater than 30% of the external sphincter and extend from the external sphincter to the ischiorectal fossa. Suprasphincteric fistulas extend from the anal crypt to the ischiorectal fossa.