Key points

Introduction

Benign anorectal diseases, such as anal abscesses and fistula, are commonly seen by primary care physicians, gastroenterologists, emergency physicians, general surgeons, and colorectal surgeons. A thorough understanding of the pathophysiology of each of these conditions is necessary to help guide appropriate treatment. The goals of this article are to review the pathophysiology, presentation, diagnosis, and treatment of both anal abscesses and fistulas in the context of the current literature.

Introduction

Benign anorectal diseases, such as anal abscesses and fistula, are commonly seen by primary care physicians, gastroenterologists, emergency physicians, general surgeons, and colorectal surgeons. A thorough understanding of the pathophysiology of each of these conditions is necessary to help guide appropriate treatment. The goals of this article are to review the pathophysiology, presentation, diagnosis, and treatment of both anal abscesses and fistulas in the context of the current literature.

Anal anatomy

A comprehensive appreciation of anal anatomy establishes a framework for understanding the pathophysiology of anal abscess and fistula. The surgical anal canal is 2 to 4 cm in length, beginning at the anorectal junction and ending at the anal verge ( Fig. 1 ). The upper portion of the anal canal is composed of columnar endothelium and the lower portion of the anal canal is composed of squamous epithelium. The transition between these 2 distinct areas of the anal canal occurs at the dentate or pectinate line. The dentate line is surrounded by longitudinal mucosal folds, called columns of Morgagni. Each fold contains anal crypts and each crypt contains between 3 and 12 anal glands. The distribution of the glands is not uniform; most of the glands lie in the anterior position of the anal canal, with only a few in the posterior midline.

Anatomy of the anal canal.

( From Jorge JMN, Habr-Gama A. Anatomy and embryology. In: Beck DE, Roberts PL, Saclarides TJ, et al, editors. The ASCRS Textbook of Colon and Rectal Surgery. New York: Springer; 2011; with kind permission from Springer Science and Business Media.)

Anal abscess

Anal abscesses commonly occur in healthy individuals, but they also occur in patients with inflammatory conditions, such as Crohn disease. The incidence of anorectal abscess in the United States is estimated to be approximately 68,000 to 96,000 cases per year. Anal abscesses are seen 2 times more frequently in men than in women.

There have been multiple theories addressing the causes of anorectal abscesses, while most focus on infection within the anal glands. This “cryptoglandular theory” was introduced by Eisenhammer in 1956 and is now widely accepted. The theory was developed on the principle of obstruction, in which an individual gland becomes impacted with debris and infection and abscess formation occurs. Because the crypt glands penetrate the anal sphincter complex to varying degrees, the infection and abscess follows the path of least resistance, and the abscess accumulates wherever the gland terminates.

Anorectal abscesses are classified based on their location in relation to the anal sphincters ( Fig. 2 ). Perianal abscesses are the most common and are superficial infections that extend between the internal and external sphincter and reach the anal verge. If the abscess penetrates the external anal sphincter, it becomes an ischiorectal abscess . Intersphincteric abscesses are infections in the potential space between the internal and external sphincters. When the abscess extends cephalad along the rectal wall above the levators, it is a supralevator abscess . A deep postanal abscess extending into either or both ischiorectal fossa is termed a horseshoe abscess .

Classification of anorectal abscesses.

( From Vasilevsky CA. Fistula-in-ano and abcess. In: Beck DE, Wexner SD, editors. Fundamentals of anorectal surgery. London: WB Saunders; 1998; with permission.)

Symptoms

Most patients with an anal abscess present with pain. If the abscess is low (such as perianal or ischiorectal abscess) then the pain is often associated with other complaints, including swelling and redness. High abscesses (such as supralevator abscess) do not extend to the sensate perianal tissues and are less likely to present with swelling or redness. These abscesses are more likely to be associated with systemic symptoms, such as fever and malaise.

Diagnosis

Most patients with an anal abscess can be diagnosed clinically on physical examination evidenced by erythema, warmth, tenderness on palpation, induration, and fluctuance, which are the most common physical findings. Patients with these findings often will not tolerate a rectal examination and a digital rectal examination should be deferred in the acute setting because of the low likelihood of providing additional information. Conversely, if there are no signs of infection (such as erythema, edema, tenderness to palpation, or fluctuance) that suggest a perianal or ischiorectal abscess, a supralevator or intersphincteric abscess should be considered. Severe pain and local induration found on digital rectal examination may help make the diagnosis. The differential diagnosis for anal abscesses includes anal fissure, thrombosed external hemorrhoids, malignancy, sexually transmitted diseases, proctitis, cellulitis, and levator muscle spasm. When digital examination is impossible or the infection is extensive, examination under anesthesia (EUA) is necessary to identify the site of infection and drainage can be performed in the same setting.

Radiographic imaging, such as ultrasound, computed tomography (CT) or magnetic resonance imaging (MRI), is rarely necessary to make the diagnosis of anal abscess, and should be reserved for complex cases, as seen with Crohn disease or recurrent infections. Caliste and colleagues studied 1000 patients who were diagnosed with anal or rectal abscess, and 113 of these patients had undergone a CT scan. Twenty-six (23%) of these patients with a confirmed abscess had a negative CT scan. The investigators concluded that CT scan lacks the sensitivity in identification of perirectal abscess, as nearly one-fourth of their patient population had negative CT scan imaging despite undergoing successful drainage of an abscess by a surgeon. In complex cases in which physical examination findings are not present or equivocal, a CT scan can be helpful in confirming a diagnosis of intersphincteric or suprasphincteric abscess. Imaging may also provide the practitioner with additional information used to guide management for these rare cases, such as intra-abdominal findings consistent with inflammatory bowel disease or diverticulitis.

Treatment

Treatment of anal abscesses requires incision and drainage. A potential consequence of draining an anal abscess (either by incision and drainage or spontaneous) is the risk of fistula formation in the future, which ranges from 5% to 83%. One hypothesis for the development of a fistula following incision and drainage of an anal abscess involves insufficient drainage of the abscess resulting in persistent chronic infection and the formation of a nonhealing tract that matures as a fistula.

Most patients with perianal or ischiorectal abscesses can be treated in the outpatient setting, whereas patients with more complex (loculated or large ischiorectal, intersphincteric, supralevator, or horseshoe) abscesses may benefit from EUA and drainage in an operating room under anesthesia. Certain patients with anxiety or severe pain may not be suitable candidates for bedside procedures. Following adequate incision and drainage where there are no signs of cellulitis, antibiotics are not necessary unless the patient has other medical comorbidities, such as diabetes mellitus or immunosuppression. Sozener and colleagues performed a multicenter, double-blinded, placebo-controlled randomized trial evaluating 151 patients with anorectal abscesses to determine if antibiotic use after adequate incision and drainage helped prevent future fistula formation. At 1-year follow-up, 45 (30%) patients developed an anal fistula. The rate of fistula formation was significantly higher in the group treated with antibiotics and drainage than the placebo, leading to the conclusion that antibiotic usage did not decrease the rate of fistula formation. Multiple logistic regression demonstrated that the risk of fistula formation increased eightfold in patients with ischiorectal abscesses and threefold in patients with intersphincteric abscesses compared with perianal abscesses.

Previous studies have shown that risk factors for recurrence of anal abscesses include Crohn disease, diabetes mellitus, and ischiorectal location. A more recent study in 2010 by Yano and colleagues found that patients with diabetes mellitus and ischiorectal abscesses were not at increased risk for recurrence of anal abscess. Interestingly, the investigators also studied body mass index (BMI) and found that patients with morbid obesity are not at increased risk for recurrence of anal abscess either.

The decision of whether or not to perform a fistulotomy during the original incision and drainage of the anal abscess has been debated in the literature. In a randomized clinical trial, Oliver and colleagues compared simple drainage of anorectal abscesses with and without fistula track treatment to evaluate the effectiveness and morbidity of both operations in the management of acute anal sepsis. Two hundred patients were included in the study: 100 randomized to the group that underwent drainage and treatment of the fistula, whereas the other 100 patients had only abscess drainage performed. At 1-year follow-up, the investigators found that drainage of anal abscess with fistulotomy if a fistula track was present can be safely performed in cases of subcutaneous, intersphincteric, or low transsphincteric fistula with a minimal recurrence rate (5%), compared with 29% recurrence rate in patients treated with drainage alone.

Onaca and colleagues studied 500 consecutive patients with perirectal abscess requiring incisional drainage. Forty-eight (9.6%) patients required early reoperation and 4 patients required a second operation within 2 to 8 days; the reoperation rate was 7.6%. The most common pattern of operative failure leading to reoperation was inadequate drainage either due to inadequate incision and drainage at the initial operation or premature closure of the skin edges over the abscess cavity. Half of all horseshoe ischiorectal abscesses were associated with a failed drainage procedure due to unrecognized additional loculations. This finding highlights the importance of a thorough examination that can sometimes only be accomplished under anesthesia with deliberate probing and drainage of any existing loculations followed by fastidious local wound care.

Drainage of anal abscesses can be performed in the emergency room, outpatient setting, or in the operating room. For superficial abscesses, such as perianal abscesses, drainage in the office or emergency room is entirely feasible. Local anesthesia, such as 1% lidocaine with epinephrine (and bupivacaine for extended analgesia), can be injected subcutaneously into the area affected by the abscess to provide local anesthesia for the skin. Care is taken to adequately anesthetize the area surrounding the abscess without injecting directly into the abscess cavity. A scalpel is used to make an incision, either cruciate or elliptical, over the area of fluctuance. Once in the abscess cavity, any loculations can be broken up using a Hemostat or even a finger. To maintain hemostasis and prevent the skin from closing prematurely, the wound can be packed with either the end of a gauze sponge or a wick. The packing can be removed in 24 hours. We recommend sitz baths following incision and drainage for local cleansing and patient comfort.

If the abscess is more complicated, drainage should be performed in the operating room under general anesthesia or conscious sedation and local anesthesia. Position of the patient is dependent on the location of the abscess and surgeon preference. Examination under anesthesia can be performed to identify the site of infection. The area of fluctuance or induration can be palpated directly or an internal or external opening is visualized with purulent drainage. Needle aspiration can be used to localize the abscess cavity if more deep and difficult to palpate and an incision can be made following the trajectory of the needle.

Complications

Consequences of anal abscess include recurrence, sepsis, and fistula formation. Recurrence is thought to be secondary to insufficient drainage and may be more common when drainage is delayed. Complications related to the abscess drainage procedure are similar to those described for other anorectal procedures, most commonly postoperative bleeding and urinary retention.