What Are Anal Abscess and Fistula?

An anal abscess is a pus-filled cavity that develops near the anus or rectum. About 90% of cases occur due to infection of the glands inside the anal canal. Bacteria, stool particles, or undigested materials can block one of these gland ducts, causing inflammation and the accumulation of pus through a tunnel in the tissues around the anus.

In about half of the patients, after the abscess is surgically drained, the tunnel remains open and forms a fistula. An anal fistula is an abnormal tunnel connecting the skin around the anus to an infected abscess cavity or directly to the anal canal. Sometimes a fistula develops without an abscess, leading to discharge of pus from a small opening near the anus.

Classification

Anal abscesses are classified according to their location and relationship with surrounding structures:

  • Perianal

  • Ischioanal

  • Intersphincteric

  • Supralevator

The most common type is perianal, and the least common is supralevator. If any abscess spreads in a horseshoe-shaped pattern around the anus or rectum, it is called a horseshoe abscess.

Anal fistulas usually result from a current or previous abscess and are classified according to their relationship with the anal sphincter muscles:

  • Intersphincteric

  • Transsphincteric

  • Suprasphincteric

  • Extrasphincteric

The most common type is intersphincteric, and the rarest is extrasphincteric.
These classifications are crucial for planning surgical treatment and achieving successful outcomes.

Symptoms

Common symptoms of anal abscess include:

  • Pain around the anus

  • Swelling

  • Redness of the perianal skin

  • Fever

In some cases, rectal bleeding or urinary problems (painful urination, difficulty starting to urinate) may also occur.

In patients with an anal fistula, a prior abscess history is common. Typical symptoms include anal pain, discharge from around the anus, irritation of the surrounding skin, and sometimes rectal bleeding.

Examination

A detailed medical history is taken, focusing on previous anorectal symptoms and treatments. Physical examination often reveals redness, warmth, swelling, and tenderness around the anus. Although most abscesses are visible externally, some may not show visible signs, and patients only report pain (especially during defecation). A digital rectal examination can be painful but is often helpful for diagnosis.

In anal fistulas, a small external opening that discharges pus, blood, or stool is often observed. Raised edges around this opening indicate a long-standing fistula. Some fistulas may close temporarily, making diagnosis difficult — in such cases, additional tests may be required.

Imaging Methods

In most cases, diagnosis and treatment planning are made based on clinical examination. However, to better understand the structure of the fistula and its relationship to the sphincter muscles and surrounding tissues, additional imaging may be required:

  • Endoanal Ultrasound (EUS): Useful for deep abscesses, horseshoe-shaped abscesses, and mapping the fistula tract.

  • CT Scan: Helps detect abscesses related to diseases of abdominal organs (e.g., Crohn’s disease).

  • Pelvic MRI: Especially valuable for complex fistulas, achieving up to 90% accuracy in surgical planning.

Treatment of Anal Abscess

Treatment involves surgical drainage of the infected area. An incision is made near the anus to release the pus.

  • Superficial abscesses can be drained under local anesthesia,

  • Deep abscesses usually require general anesthesia in an operating room.

In patients with diabetes or weakened immune systems, hospitalization and antibiotic therapy may be necessary.

About 50% of patients develop an anal fistula after drainage. If the outer skin opening heals while the inner connection remains active, recurrent abscesses can form. Until the fistula is surgically treated, patients may experience alternating periods of relief and recurrence with pain, swelling, and discharge.

Antibiotics are not an alternative to surgery. Their use is recommended only for:

  • Immunocompromised patients

  • Those with widespread cellulitis

  • Patients with prosthetic heart valves, prior endocarditis, congenital heart disease, or heart transplant-related valve disease

Treatment of Anal Fistula

Currently, there is no effective medical (drug) treatment for anal fistulas — surgery is almost always required.

If the fistula is simple (involving a small portion of the sphincter), fistulotomy can be performed. The fistula tract is laid open, connecting the internal and external openings. The wound then heals from the inside out with granulation tissue.

  • Fistulotomy has a 92–97% success rate, but since part of the sphincter muscle is cut, there is a small risk of gas or stool incontinence.

  • The decision depends on the type of fistula, anatomy, and bowel control of the patient.

Alternative Surgical Methods (Sphincter-Preserving):

  • Fibrin Glue: A special adhesive is injected into the tract to seal it. It causes no muscle damage but has a modest success rate (~50%).

  • Fistula Plug: A bioprosthetic implant fills the fistula tract; it’s safe for sphincter muscles but also has a success rate around 50%.

  • Endoanal Advancement Flap: Used for complex fistulas or when fistulotomy risks continence loss. The internal opening is closed with a flap of healthy tissue. Success rates are 50–70%, but lower in Crohn’s disease, cancer, prior radiation, or previous fistula surgery.

  • LIFT (Ligation of Intersphincteric Fistula Tract): The tract between internal and external sphincters is tied off. It preserves sphincter function, though long-term results are still being evaluated.

What Is a Seton?

If the fistula tract passes through a large portion of the sphincter muscles—making fistulotomy risky for continence—a drainage seton may be used.

A thin elastic thread is passed through the fistula tract and tied to form a loop around the anus. The seton is kept in place for 8–12 weeks to allow drainage of infection and promote tract maturation. It is painless and does not interfere with bowel movements.
Once inflammation decreases and the tract matures, one of the above surgical methods can be applied for definitive treatment.

Fistula Treatment in Crohn’s Disease

Anal fistulas are common in Crohn’s disease, a chronic inflammatory bowel condition that can affect any part of the digestive tract. Recurrent abscesses and surgeries can impair anal sphincter function.
In Crohn’s-related fistulas, medical therapy is prioritized, while surgery is used to control infection or complement medical treatment. Each case should be individually tailored.

Recovery Process

After surgery, painkillers, stool softeners, and a high-fiber diet help manage discomfort.
Patients are advised to rest for 1–2 weeks, take warm baths several times daily, and avoid constipation or straining.

Recurrence of Anal Abscess or Fistula

Up to 50% of abscesses may recur as another abscess or fistula. Even after complete healing, recurrence rates vary depending on the surgical method used.
Management of recurrent fistulas can be complex, often requiring multiple surgeries and combined techniques for successful resolution.