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# Anorectal abscess and fistula

An anorectal abscess is a pocket of pus near the anus, usually from an infected anal gland; a tunnel that stays open to the [skin](https://durgical.com/en/anatomy/skin/) afterwards is an anal fistula.

Updated 2026-10-29

## Structures involved

Tap one to see what it is and open it in 3D.

- [Rectum](https://durgical.com/en/anatomy/rectum/)
- [External anal sphincter](https://durgical.com/en/anatomy/external-anal-sphincter/)
- [Puborectalis](https://durgical.com/en/anatomy/puborectalis/)
- [Iliococcygeus](https://durgical.com/en/anatomy/iliococcygeus-muscle/)
- [Pubococcygeus](https://durgical.com/en/anatomy/pubococcygeus-muscle/)
- [Coccyx](https://durgical.com/en/anatomy/coccyx/)
- [Internal pudendal artery](https://durgical.com/en/anatomy/internal-pudendal-artery/)

## In the clinical exam

- [Digital rectal examination](https://durgical.com/en/clinical-exam/digital-rectal-examination/)

## Why it happens

- [Outcomes of acute inflammation: resolution, abscess, scar or chronic inflammation](https://durgical.com/en/general-pathology/outcomes-of-acute-inflammation-resolution-abscess-scar-or-chronic-inflammation/)
- [Large intestine, appendix and anal canal](https://durgical.com/en/histology/large-intestine-appendix-and-anal-canal/)

## What it is

A collection of pus in the spaces around the anus and [rectum](https://durgical.com/en/anatomy/rectum/), usually from an infected anal gland. A chronic tract that remains between the [anal canal](https://durgical.com/en/anatomy/anal-canal/) and the skin after the abscess is an anal fistula (fistula-in-ano).

## Who gets it

A common surgical emergency, more frequent in young and middle-aged adults and in men. A substantial proportion of drained abscesses go on to form a fistula.

## Causes

- Cryptoglandular infection: blockage and infection of an anal gland at the dentate line (most cases)
- Crohn disease
- Trauma, foreign bodies or previous anorectal surgery
- Infections such as tuberculosis or sexually transmitted infections (less common)
- Malignancy or radiation (rare)

## Risk factors

- Diabetes mellitus
- Crohn disease or other inflammatory bowel disease
- Immunosuppression, including HIV infection and chemotherapy
- Obesity
- Smoking
- Previous anorectal abscess

## Pathophysiology

Anal glands drain into the crypts at the dentate line. When a duct blocks, the gland becomes infected in the intersphincteric space, and pus spreads along the path of least resistance: down to the perianal skin, across the external sphincter into the ischioanal fossa, or up above the [levator ani](https://durgical.com/en/anatomy/levator-ani/). If the internal opening stays open after drainage, an epithelialized tract forms: a fistula.

## Classification

- Abscess by location: perianal (most common), ischioanal (ischiorectal), intersphincteric, supralevator; horseshoe when it spreads around both sides
- Fistula by Parks classification: intersphincteric, transsphincteric, suprasphincteric, extrasphincteric
- Simple fistula (low, little sphincter involved) or complex fistula (high, branching, anterior in women, Crohn disease, recurrent)

## Symptoms and signs

- Constant, throbbing perianal or rectal pain, worse on sitting, coughing or defecation
- Tender, red, warm swelling near the anus (perianal abscess)
- Fever and malaise, especially with deep abscesses
- Deep abscesses may show few skin signs but cause rectal pain and fever
- Fistula: intermittent or persistent discharge of pus or blood from an opening near the anus, itching, and recurring abscesses

## Red flags

- Fever, tachycardia or hypotension (sepsis)
- Rapidly spreading pain, skin discolouration or crepitus (necrotizing soft-tissue infection, Fournier gangrene)
- Diabetes or immunosuppression with any anorectal abscess
- Urinary retention
- Recurrent abscesses or multiple fistulas (consider Crohn disease)

## Diagnosis

**Clinical**
Mostly clinical: inspection and digital rectal examination; examination under anaesthesia when pain prevents assessment or a deep abscess is suspected. For a fistula, the external opening is located and the internal opening sought (Goodsall's rule as a guide).

**Tests**
Blood count, glucose and inflammatory markers in unwell patients; pus culture in recurrent or atypical cases; HIV testing when risk is present.

**Imaging**
Not needed for simple perianal abscesses. Pelvic MRI or endoanal ultrasound for deep, recurrent or complex disease and to map fistulas; CT for suspected supralevator abscess or in emergencies.

## Differential diagnosis

- Thrombosed external haemorrhoid
- Pilonidal abscess
- Hidradenitis suppurativa
- Anal fissure
- Bartholin abscess (in women)
- Anorectal cancer

## Goal of treatment

Drain the pus promptly to control infection, then cure any fistula while protecting the anal sphincter and continence.

## Treatment

**Acute**
Prompt incision and drainage of the abscess, in the clinic for simple perianal abscesses or in theatre for deep ones; antibiotics alone are not enough.

**Medical**
Antibiotics in addition to drainage for cellulitis, systemic signs of infection, diabetes, immunosuppression or prosthetic devices; analgesia and stool softeners.

**Surgical**
Fistula surgery: fistulotomy (laying open) for simple low fistulas; draining seton, ligation of the intersphincteric fistula tract (LIFT) or advancement flap for complex fistulas to protect continence.

**Medical**
In Crohn disease, fistulas are managed jointly with gastroenterology using immunomodulators or biologic therapy alongside setons.

**Follow-up**
Wound care and follow-up to detect a persistent fistula or recurrence.

## Complications

- Anal fistula (the main chronic sequel)
- Recurrent abscess
- Spread into a horseshoe or supralevator abscess
- Necrotizing soft-tissue infection (Fournier gangrene)
- Sepsis
- Faecal incontinence after fistula surgery

## Prognosis

Drainage relieves symptoms quickly. A substantial minority of patients develop a fistula; simple fistulas are usually cured by surgery, while complex and Crohn-related fistulas often recur.

## Prevention

- Good control of diabetes
- Treating constipation and diarrhoea
- Early drainage of small abscesses before they spread

## Test yourself

Think of the answer, then tap the question to see it.

## What is the origin of most anorectal abscesses?

Cryptoglandular infection: a blocked, infected anal gland at the dentate line in the intersphincteric space.

## What is the treatment of choice for an anorectal abscess?

Prompt incision and drainage; antibiotics are only an adjunct in selected patients.

## Which is the most common location of an anorectal abscess?

Perianal, followed by ischioanal (ischiorectal).

## What does Goodsall's rule predict?

The likely internal opening of an anal fistula: posterior external openings curve to the posterior midline, anterior ones run radially (with exceptions).

## Why are setons used in complex anal fistulas?

They drain the tract and control sepsis without cutting much sphincter, protecting continence.

[Add to my cards (5)](https://durgical.app/estudiar/?add=/en/conditions/anorectal-abscess-and-fistula/)

## Related conditions

- [Haemorrhoids](https://durgical.com/en/conditions/haemorrhoids/)
- [Colorectal cancer](https://durgical.com/en/conditions/colorectal-cancer/)
- [Type 2 diabetes mellitus](https://durgical.com/en/conditions/type-2-diabetes-mellitus/)
- [HIV infection and AIDS](https://durgical.com/en/conditions/hiv-infection-and-aids/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Anorectal Abscess](https://www.ncbi.nlm.nih.gov/books/NBK459167/) (StatPearls (NCBI Bookshelf), 2026).

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Source: https://durgical.com/en/conditions/anorectal-abscess-and-fistula/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/conditions/anorectal-abscess-and-fistula/