Background An anorectal abscess originates from an infection arising in the cryptoglandular epithelium lining the anal canal. The internal anal sphincter is believed to serve normally as a barrier to infection passing from the gut lumen to the deep perirectal tissues. This barrier can be breached through the crypts of Morgagni, which can penetrate through the internal sphincter into the intersphincteric space. Once infection gains access to the intersphincteric space, it has easy access to the adjacent perirectal spaces. Extension of the infection can involve the intersphincteric space, ischiorectal space, or even the supralevator space. In some instances, the abscess remains contained within the intersphincteric space. The severity and depth of the abscess are quite variable, and the abscess cavity is often associated with formation of a fistulous tract. For that reason, fistulas are also discussed in this article where relevant. Anatomy Normal anatomy demonstrates anywhere from four to 10 anal glands lying at the level of the dentate line, which divides the squamous epithelium distally and the columnar epithelium proximally. Obstruction of these anal glands by debris leads to stasis, bacterial overgrowth, and abscess formation that extends into the intersphincteric groove between the internal and external anal sphincters. From this space, the abscess can spread along various potential spaces. Anorectal abscesses are classified according to their anatomic location; the following are the most common locations (see the image below): • Perianal • Ischiorectal • Intersphincteric • Supralevator Perianal abscesses represent the most common type of anorectal abscesses, accounting for approximately 60% of reported cases. These superficial collections of purulent material are located beneath the skin of the anal canal and do not transverse the external sphincter. Ischiorectal abscesses are the next most common type. These abscesses form when suppuration transverses the external anal sphincter into the ischiorectal space. An ischiorectal abscess may traverse the deep postanal space into the contralateral side, forming a so-called horseshoe abscess. Intersphincteric abscesses, the third most common type, result from suppuration contained between the internal and external anal sphincters. They may lie completely within the anal canal, leading to severe pain, and may only be found by digital rectal examination or anoscopy. Supralevator abscesses, the least common of the four major types, may form from cephalad extension of the intersphincteric abscess above the levator ani or from caudal extension of a suppurative abdominal process (eg, appendicitis, diverticular disease, gynecologic sepsis) into the supralevator space. These abscesses may be diagnosed by means of computed tomography (CT), and they cause pelvic and rectal pain. Pathophysiology Perirectal abscesses and fistulas represent anorectal disorders arising predominantly (~90% of cases) from the obstruction of anal crypts, possibly involving increased sphincter tone. Infection of the now static glandular secretions results in suppuration and abscess formation within the anal gland. Typically, the abscess forms initially in the intersphincteric space and then spreads along adjacent potential spaces. Etiology Both aerobic and anaerobic bacteria have been found to be responsible for abscess formation. The anaerobes most commonly implicated are Bacteroides fragilis, Peptostreptococcus, Prevotella, Fusobacterium, Porphyromonas, and Clostridium. The aerobes most commonly implicated are Staphylococcus aureus, Streptococcus, and Escherichia coli. More recent studies have noted community-acquired methicillin-resistant S aureus (MRSA) as a pathogen leading to abscess formation. Approximately 10% of anorectal abscesses may be caused by reasons other than anal gland infection, including Crohn disease, trauma, immunodeficiency resulting from HIV infection or malignancy (both hematologic and anorectal cancer), tuberculosis, hidradenitis suppurativa, sexually transmitted diseases, radiation therapy, foreign bodies, perforated diverticular disease, inflammatory bowel disease, or appendicitis (a rare cause of supralevator abscesses). Prognosis Overall mortality from anorectal abscesses is quite low. Early data indicated that abscess formation recurred in approximately 10% of patients, with chronic fistula-in-ano occurring in as many 50% of patients. A later study found that 37% of patients developed chronic anal fistula or recurrent sepsis. In this study, risk factors were age younger than 40 years and nondiabetic status; no difference in these complications was noted with regard to HIV status, sex, antibiotic usage, or smoking status. Approximately two thirds of patients with rectal abscesses who are treated by incision and drainage or by spontaneous drainage will develop a chronic anal fistula. After fistula formation, multiple complications may develop after surgery. As many as 43% of patients may experience fecal incontinence after surgical repair for complex fistula-in-ano. Other postoperative complications include temporary postejaculation urethral irritation and postoperative urinary retention. Constipation may also occur as a result of pain on defecation. The recurrence rate of anorectal fistulas after fistulotomy, fistulectomy, or the use of a seton is about 1.5%. The success rate of primary surgical treatment with fistulotomy appears to be fairly good. Clinical presentation The clinical presentation correlates with the anatomic location of the abscess (though it should be kept in mind that a perianal abscess sometimes is not an isolated superficial lesion but represents the point of a more involved perirectal abscess). Almost all perirectal abscesses are associated with perirectal pain that is indolent in nature. Patients with a perianal abscess typically complain of dull perianal discomfort and pruritus. The pain often is exacerbated by movement and increased perineal pressure from sitting or defecation. Those with an ischiorectal abscess often present with systemic fevers, chills, and severe perirectal pain and fullness consistent with the more advanced nature of this process. External signs are minimal and may include erythema, induration, or fluctuance. As many as 50% of patients with perirectal abscesses may present with swelling around the rectum, and as many as one quarter may present with rectal or perirectal drainage that may be bloody, purulent, or mucoid. These patients may also present with constipation, most likely due to pain on defecation, but the absence of constipation or even diarrhea does not rule out the diagnosis. Most of them report no history of fever or chills. In many cases, these patients delay presentation to a physician, or they have already presented to a physician and have been given alternative diagnoses. Furthermore, complaints of abdominal pain are rare in these patients. Physical Examination Patients with anorectal abscesses usually have normal vital signs on initial evaluation, with only 21% reporting fevers or chills. Physical examination demonstrates a small, erythematous, well-defined, fluctuant, subcutaneous mass near the anal orifice. On digital rectal examination (DRE), a fluctuant, indurated mass may be encountered. In one study, however, clinicians were unable to identify abscesses in 10% of patients on rectal examination; 4% of patients showed no signs of perirectal abscesses on initial examination. Optimal physical assessment of an ischiorectal abscess may require anesthesia to alleviate patient discomfort that would otherwise limit the extent of the examination. Patients with an intersphincteric abscess present with rectal pain and exhibit localized tenderness on DRE. Physical examination may fail to identify an intersphincteric abscess. Although rare, supralevator abscesses present a similar diagnostic challenge. As a result, clinical suspicion of an intersphincteric or supralevator abscess may require confirmation by means of computed tomography (CT), magnetic resonance imaging (MRI), or anal ultrasonography. Digital examination with anesthesia can be helpful in certain cases, because patient discomfort can significantly limit physical assessment. For example, optimal evaluation for an ischiorectal abscess is performed in this manner. Complications Complications of anorectal abscesses may include the following: • Fistula formation • Bacteremia and sepsis, including seeding of the infection to other areas by hematogenous spread • Fecal incontinence • Malignancy Approach Considerations No specific laboratory studies are indicated in the evaluation of a patient with a perianal or anorectal abscess. Certain patients, such as individuals with diabetes and those who are immunocompromised, are at high risk for developing bacteremia and possibly sepsis, as a result of an anorectal abscess. In such cases, complete laboratory evaluation is important. When an anorectal abscess is not obviously apparent but a high degree of clinical suspicion exists, imaging (eg, computed tomography [CT], ultrasonography, or magnetic resonance imaging [MRI]) may be necessary for the diagnosis. Plain films are of little utility for this purpose and should not be obtained unless other diagnoses are being considered. Although imaging studies usually are not necessary in the evaluation of patients with an anorectal abscess (which will be perianal in the majority of cases), clinical suspicion of an intersphincteric or supralevator abscess may require confirmation by means of CT, anal ultrasonography, or MRI. As a rule, use of anal ultrasonography is limited to confirming the presence of an intersphincteric abscess, though this modality can also be used intraoperatively to help identify a difficult abscess or fistula. CT is readily available in most EDs and is commonly used in the diagnosis of perirectal abscesses. In one retrospective study, CT for perirectal abscesses confirmed by surgical drainage yielded a sensitivity of 77%, with the false-negative scans being significantly more likely to come from immunocompromised patients. Although not readily used for this purpose in the ED, transperineal ultrasonography has shown good results for the detection of fistulous tracts and fluid collections in preoperative planning, with sensitivities ranging from 85% to 100% or the detection of surgically significant disease. MRI is the criterion standard for imaging of perirectal abscesses; its 91% sensitivity makes it useful in preoperative planning. In the ED, however, its use is restricted. Approach Considerations As a rule, the presence of an abscess is an indication for incision and drainage. Watchful waiting while administering antibiotics is inadequate. Clinical suspicion of anorectal abscess warrants aggressive identification and surgical incision and drainage. Delaying surgical intervention results in chronic tissue destruction, fibrosis, and stricture formation and may impair anal continence. Simple perianal abscesses may be treated in the emergency department (ED); more complex perirectal abscesses are treated by an experienced surgeon in an operating room. A patient with a perirectal abscess should be admitted to the surgical service unless other medical conditions or complications from the abscess necessitate a primary medical admission, with the surgeon acting as a consultant. Consider admitting a patient with a perirectal abscess to a medical service with the surgeon as a consultant should be considered if the patient is elderly, febrile, hypotensive, or immunocompromised or has significant comorbidities. Surgical Intervention Preparation for surgery. Because of the acute nature of anorectal abscesses, preoperative bowel preparation is not possible and typically is unnecessary. Adequate analgesia before aspiration is mandatory. Lidocaine 2% with epinephrine injected subcutaneously over and around the periphery of the abscess and intramuscular (IM) or intravenous (IV) narcotics are recommended. Management of abscess Treatment of anorectal abscesses involves early surgical drainage of the purulent collection. Primary antibiotic therapy alone is ineffective in resolving the underlying infection and simply postpones surgical intervention. Any delay in surgical intervention prolongs infection and augments tissue damage, and it may impair sphincter continence function and promote stricture or fistula formation. The ability to drain an anorectal abscess depends on patient comfort and on the location and accessibility of the abscess. When the abscess is perianal or superficial, drainage can usually be accomplished in the office or ED with local anesthesia. A small incision is made over the area of fluctuance; to shorten the length of any fistula that may form, the incision should be made as close to the anus as is compatible with safety. Pus is collected and sent for culture. Hemostasis is achieved with manual pressure, and the wound is packed with iodophor gauze. After 24 hours, the gauze is removed, and the patient is instructed to take sitz baths three times a day and after bowel movements. Postoperative analgesics and stool softeners are prescribed to relieve pain and prevent constipation. Typically, the patient follows up with the physician in 2-3 weeks for wound evaluation and inspection for possible fistula-in-ano. Treatment of ischiorectal, intersphincteric, and supralevator abscesses typically requires general or regional anesthesia. To drain an ischiorectal abscess, a cruciate incision is made at the site of maximal swelling. Pus is drained and cultured. The ischiorectal fossa is probed with a finger or hemostat to disrupt loculations and facilitate drainage. Placement of a drain is indicated only for the management of complex or bilateral abscesses. To drain an intersphincteric abscess, a transverse incision is made in the anal canal below the dentate line posteriorly. The intersphincteric space is identified, and the plane between the internal and external sphincters is exposed. The abscess is opened to allow drainage, and a small mushroom catheter is sutured in situ to assist drainage and prevent premature wound closure. The optimal drainage technique for a supralevator abscess is determined by the location and etiology of the lesion. Failure to take the primary etiology into account in the management of a supralevator abscess may lead to iatrogenic fistula formation. Evaluation with magnetic resonance imaging (MRI) or computed tomography (CT) can exclude intraabdominal or pelvic pathology as possible sources. If the supralevator abscess evolved from the extension of an ischiorectal abscess, external drainage through the ischiorectal fossa would be indicated. If the abscess resulted from an upward extension of an intersphincteric abscess, appropriate drainage would be created through the rectal mucosa. In cases of posterior supralevator abscess collections, a transverse incision is made in the posterior anal canal below the dentate line. The dissection extends from the intersphincteric plane through the puborectalis sling and into the posterior anal space. A mushroom catheter is then sutured in place to ensure adequate drainage. Anterior supralevator abscesses are superficial and are more common in women than in men. Surgical drainage may be accomplished via an anteriorly directed transanal incision or by way of a transvaginal approach entering the posterior cul-de-sac. A mushroom catheter is placed to ensure adequate drainage of the abscess collection. Patients with systemic signs of toxicity are admitted to the hospital and treated with IV antibiotics. If the patient’s clinical condition does not improve over the following 24-48 hours, reevaluation of the supralevator abscess by means of CT or reoperation may be indicated. Some patients with recurrent, severe supralevator abscesses may require a diverting colostomy for optimal management. Postoperative Care Postoperatively, analgesics are given for pain, and stool bulking agents and stool softeners are given to prevent constipation. After appropriate wound care, patients may be discharged home with instructions for sitz baths and routine follow-up. Adequate outpatient analgesia (eg, codeine with acetaminophen or an oxycodone-containing compound) should be provided. Outpatient antibiotics may be indicated and are best chosen on the basis of culture and sensitivity testing of pathogens derived from the abscess.
