An Overview of Anal or Rectal Abscess

An anorectal boil or abscess is a pus-filled cavity that forms within the furrows of the anal canal (called the anal sinuses). Depending on its location, it may also be called an anal abscess, rectal abscess, perianal abscess, or perirectal abscess.

Anorectal abscesses are most often caused by the accumulation of bacteria ​in vulnerable or compromised tissues. While abscesses can form spontaneously, they are commonly associated with inflammatory bowel disease or conditions that affect the immune system such as HIV. They usually need to be drained by a healthcare provider.

This article discusses anal and rectal abscesses, also called boils. It also goes over what causes them and how they are treated.

Anorectal Abscess Symptoms

Anorectal abscesses are usually first recognized because of dull, throbbing pain in the anus or rectum, often accompanied by sharp pain when defecating.

Perianal abscesses ("peri-" meaning "around") are the most common type and can usually be identified visually as they occur in the upper layers of tissue. When touched, the lump will usually be tender, red, and warm.

By contrast, perirectal abscesses tend to form in deeper tissues and are often more felt than seen. Of the two types, a perirectal infection tends to be more serious.

As the pus starts to consolidate and form a palpable mass, other symptoms of an anorectal abscess appear, including:

• Fever
• Fatigue
• Constipation
• Rectal discharge and bleeding
• Feeling like you need to go to the bathroom when you don’t
• Increasing and often constant pain, worsening with movement or while sitting

If you experience any of these symptoms, it is important to see a healthcare provider and have a thorough examination.

Complications

If left untreated, an abscess can lead to the development of an anal fistula, an abnormal tunneling connection between the skin around the anus and the rectal or anal canal through which stool and pus may drain. This may require intensive surgery and a prolonged period of recovery.

If you develop a high fever (over 100.4 degrees), shaking chills, persistent vomiting, the inability to have a bowel movement, or extreme anal or rectal pain (with or without a bowel movement), go to the emergency room without delay. These may indicate a systemic infection that has spread from the site of the abscess into the bloodstream.

Without proper treatment, systemic infection of this sort can lead to sepsis, toxic shock, and even death.

Causes

An anorectal abscess can develop in isolation, often due to the overgrowth of bacteria common in the digestive tract, such as Escherichia coli (E. coli).

However, in recent years, there have been an increasing number of cases associated with drug-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA).

While anyone, young or old, can get an anorectal abscess, there are some conditions that can increase your risk. They include:

• Inflammatory bowel diseases (IBD) like Crohn's disease and ulcerative colitis
• HIV and other forms of immune suppression
• Diabetes
• Anal sex
• Chronic or severe constipation or diarrhea
• Steroid drug use, including prednisone
• Chemotherapy
• Sexually transmitted infections of the anus or rectum, such as syphilis, herpes, HIV/AIDS, and chlamydia 
• Hidradenitis suppurativa, a rare skin condition
• Infected anal fissure or epidermoid cyst

Diagnosis

Most anorectal abscesses are diagnosed based on your medical history and a physical exam. If an abscess is internalized within the anal canal, your healthcare provider may want to perform an anoscopy using a flexible anoscope to get a better look inside.

Less commonly, imaging tests such as a computed tomography (CT) scan or a transrectal ultrasound (TRUS) may be used if the abscess is especially deep.

During the physical exam, your healthcare provider will want to ascertain whether the mass is an abscess or hemorrhoid. The two conditions can usually be differentiated by the presentation of symptoms. An abscess will also not respond to standard hemorrhoid treatment.

Other tests may be ordered if IBD, HIV, or diabetes is suspected, including blood tests and colonoscopy.

Treatment

Anorectal abscesses rarely go away on their own or resolve solely with antibiotic therapy. In most cases, the healthcare provider would need to drain the abscess, a relatively simple in-office procedure that involves a local numbing agent, a scalpel, and a pair of forceps.

If the abscess is especially deep or situated high in the rectum, the procedure needs to be performed in a hospital under general anesthesia. The surgery generally takes around 30 minutes. Some of the extracted pus may be sent to the lab to identify the causal bacterium.

Once the procedure is done, antibiotics are prescribed for around a week to help treat the infection and prevent any further spread. At-home measures that can help while you recover include:

• Taking a sitz bath, a shallow basin used to soak and clean the anal area
• Taking pain medication such as Tylenol (acetaminophen)
• Using stool softeners to reduce abrasion and allow the drained abscess to better heal
• Keeping the area clean

After a bowel movement, dab softly with toilet paper and rinse with a squirt bottle filled with warm water. Wash lightly with soap but avoid alcohol or hydrogen peroxide, which can slow healing. If needed, pad the wound with gauze or a maxi pad.

You can also ask your healthcare provider about the short-term use of over-the-counter topical creams and gels, which may help soothe anal tissues.

The drainage of an abscess will provide almost immediate relief. While there may be some pain after the procedure, it will usually be mild by comparison. However, if you experience excessive rectal bleeding, fever, chills, or vomiting after returning home from the procedure, call your healthcare provider immediately.

Can the Abscess Come Back?

An anal abscess can recur. In fact, up to 44% of anorectal abscesses will recur within a year of surgery. You can decrease the chances of this happening by:

• Making sure any conditions you have that increase your risk of developing an abscess are well-managed
• Practicing safe sex to reduce your risk of getting an STI

Recurrent abscesses around the anus could be related to an anal fistula. Your healthcare provider will need to surgically drain and correct the fistula and will prescribe antibiotics to clean up the infection and prevent recurrence.

Summary

An anal abscess is a pus-filled lump that forms on or around the anus. It is caused by infection with bacteria. Besides a painful lump, you may also have symptoms such as fever, constipation, and rectal discharge and bleeding. 

Abscesses usually don't go away on their own and need to be drained by a healthcare provider. Untreated abscesses can lead to the development of an anal fistula, which may require surgery to repair.