Anorectal Abscess

Introduction

An anorectal abscess is an infection of the soft tissues around the anus. For patients with Crohn's disease, an anorectal abscess will develop in approximately one third of patients.

Epidemiology

Anorectal abscesses are between 2 and 3 times more common in men than women, with most abscesses occurring in patients between 20 to 40 years of age.

Association with Fistulas

Anorectal abscesses are associated with anal fistulas in 37% of patients. If these fistulas are not recognised and treated, perirectal abscesses may recur.

Classification

Anorectal abscesses are usually classified clinically, based on the anatomy of the abscess.

Intersphincteric Abscess

Located in the space between the internal and external anal sphincter.

Perianal Abscess

Occur in the superficial soft tissues overlying the intersphincteric space.

Perirectal Abscess

Found in the ischio-rectal or post-anal spaces.

Supralevator Abscess

Occur above the anorectal ring in the supralevator space.

ANORECTAL ABSCESS CLASSIFICATION. (IMAGE BY MCORTNGHH (OWN WORK) [CC BY-SA 4.0], VIA WIKIMEDIA COMMONS)

Clinical Features

The presence of key risk factors such as a history of Crohn's disease or anal fistula should be elicited. In addition, anorectal abscesses are more common in men than women.

Symptoms

Patients usually relate a history of localised anal or perianal pain.

Pain usually begins 1 to 2 days before presentation and becomes progressively more severe.

Patients frequently complain of swelling and warmth of the perianal tissues.

Fever is common and is usually <38.6ºC.

Physical Examination

The most common finding is a tender, indurated area immediately adjacent to the anus, within the anal canal, or above the anorectal ring.

Investigations

Radiological studies are rarely helpful in the diagnosis and management of anorectal abscesses.

Ultrasonography

Occasionally used for patients with complex or atypical presentations, or those with supralevator or horseshoe abscesses. However, the severe pain frequently limits its use.

CT or MRI

May be more helpful in the evaluation of complex cases or when the diagnosis is uncertain.

Management

Surgical Drainage

The goal of treatment is to achieve adequate drainage of the abscess without damaging the anal sphincters.

Drainage should be accomplished without undue delay because of the potential for the abscess to spread into a necrotising, soft-tissue infection leading to life-threatening sepsis.

External drainage of perianal and perirectal abscess is appropriate.

Intersphincteric and supralevator abscesses should be drained internally into the anal canal and rectum, respectively, to avoid the creation of extrasphincteric or suprasphincteric fistulas.

Adequate drainage should result in prompt improvement in symptoms. If not, re-examination under anaesthesia is indicated to ensure complete drainage.

For patients with Crohn's disease, treatment of the underlying condition should be considered after the acute anorectal sepsis has been treated.

The management of a co-existing anal fistula should also be considered.

Antibiotics

Antibiotics are not an alternative to surgical drainage and should be used as an adjunctive treatment.

Usually reserved for patients with diabetes, immunocompromised status, chronic debilitation, older age, history of cardiac valvular disease, or significant associated cellulitis.

Broad-spectrum antibiotics with anaerobic and gram-negative coverage should be started preoperatively and be discontinued within 24 hours of surgery or after cellulitis has resolved.

For patients with necrotising soft-tissue infection, broad-spectrum antibiotics are mandatory.