A perianal or perirectal abscess is a collection of pus near the anus or rectum. It is an acute condition. It often comes on over a few days. This chapter explains the abscess in full. It covers presentation, examination, differential diagnosis, investigations, and management. This is the same pattern used for every disease chapter in this manual.
13.1 What Is a Perianal or Perirectal Abscess
An abscess starts in the anal glands. Chapter 1 explains this. These glands sit along the dentate line. A gland can become blocked. Bacteria then multiply inside it. This forms a collection of pus.
The pus can spread into different spaces around the anal canal. The space it reaches decides the type of abscess. This chapter covers each of these spaces in turn.
Most abscesses are simple infections. A minority point to a deeper problem. These include Crohn's disease, diabetes, or a rare tumour. Chapter 8 covers the further tests needed in these cases.
13.2 Classification by Anatomical Space
Abscesses are classified by the space they occupy. This classification guides both urgency and the surgical approach.
Perianal abscess — sits just under the skin, close to the anal margin. This is the most common type. It is usually the easiest to diagnose and treat.
Ischiorectal abscess — sits in the fat-filled space beside the anal canal. It can grow large before it is noticed. This is because it lies deeper than a perianal abscess.
Intersphincteric abscess — sits between the internal and external sphincters. It often causes severe pain. There are few outward signs on the skin.
Supralevator abscess — sits above the levator ani muscle, high in the pelvis. This is the rarest and most serious type. It can be linked to pelvic disease, not just a simple gland infection.
Horseshoe abscess — spreads around the back of the anal canal, linking the ischiorectal spaces on both sides. It needs careful surgical planning.
An abscess in a deeper space is harder to feel on simple inspection. It often needs a more careful examination. Sometimes imaging is needed too. This confirms the diagnosis.
13.3 Presentation
Follow the history-taking method described in Chapter 2. Certain features are typical of a perianal or perirectal abscess.
Severe, constant pain near the anus. The pain often builds steadily over one or two days.
Pain worse on sitting, walking, or passing stool.
A tender swelling near the anus, sometimes with redness of the overlying skin.
Fever and a general feeling of being unwell, particularly with a deeper abscess.
Trouble passing urine. This happens with a large or deep abscess near the urethra.
Ask about any history of diabetes. Also ask about any condition or medicine that weakens the immune system. Ask about any past abscess or fistula. This history shapes the urgency of treatment.
13.4 Examination
Follow the sequence from Chapter 3. Examination must be gentle, since the area is often very tender.
See Chapter 4 for the inspection method. Inspection may show a red, swollen area near the anus. A perianal abscess is usually visible this way. A deeper abscess may show little on the skin.
Gentle palpation may find a tender, fluctuant swelling. Fluctuance means the swelling feels soft and fluid-filled. This is a sign that pus has collected under pressure.
Digital rectal examination should be done with care. It can help find a deeper abscess. Do not force it if the patient feels too much pain.
A patient may have severe pain with a normal-looking outside. This can still mean a deep abscess. Examination under anaesthesia is often the only safe way to confirm this. Do not delay treatment while waiting for a clearer picture on the surface.
13.5 Differential Diagnosis
Several conditions can cause pain and swelling near the anus. These are not always a simple abscess.
Thrombosed external haemorrhoid — causes sudden, severe pain, with a firm, bluish lump at the anal margin. Chapter 10 covers this in full.
Pilonidal abscess — sits in the natal cleft, above the anus, rather than at the anal margin.
Hidradenitis suppurativa — causes multiple, recurring abscesses in the skin folds around the anus and groin.
Anal fissure with secondary infection — causes pain on defecation, with a visible tear rather than a true collection of pus.
A tumour with secondary infection — is rare. It should be considered in an older patient. It should also be considered if an abscess behaves unusually.
Most cases are straightforward on examination. An unusual case needs wider assessment. This also applies to a case that does not settle after drainage. See Chapter 8.
13.6 Investigations
A simple, superficial abscess is often diagnosed on examination alone. It does not need imaging before treatment.
Imaging is useful for a deeper or more complex abscess. MRI gives the clearest picture of a supralevator or horseshoe abscess. Endoanal ultrasound is a faster option in some centres.
Blood tests are useful in a patient who is unwell, or who has diabetes. A raised white cell count supports the diagnosis. Blood glucose should be checked in every patient. Undiagnosed diabetes can present this way.
Do not wait for imaging results before draining. This applies if the diagnosis is already clear on examination. Imaging is most useful when the diagnosis or the anatomy is uncertain.
13.7 Principles of Management
An abscess needs drainage. This is the single most important step in treatment. Antibiotics alone do not cure an abscess. They cannot reach pus that has no way to drain.
Drainage should happen as soon as the diagnosis is confirmed. A delay allows the infection to spread further. It also increases the risk of a deeper, more complex abscess.
Antibiotics have a supporting role. They are useful for a patient with widespread infection, diabetes, or a weakened immune system. They are not a substitute for drainage in a straightforward case.
13.8 Surgical Technique
Incision and drainage is the standard treatment. The abscess is opened at its most fluctuant point. Pus is released. The cavity is explored gently with a finger. This breaks down any pockets inside.
The incision should be placed as close to the anal margin as the anatomy allows. This shortens any later fistula tract, if one develops.
A cruciate, or cross-shaped, incision is sometimes used for a large abscess. The corners of skin are trimmed. This keeps the wound open, and allows it to drain freely as it heals.
The cavity is usually left open. It is packed lightly with gauze at first, then allowed to heal from the base upward. A tight pack is not needed, and can cause unnecessary pain.
A deep or complex abscess, such as a supralevator or horseshoe type, often needs examination and drainage under anaesthesia. This allows a full assessment of the extent of the abscess, and safe drainage of every pocket.
13.9 The Question of a Fistula
Chapter 12 explains this. Roughly one in three patients with a drained abscess goes on to develop a fistula. An internal opening is sometimes visible at the time of drainage.
Do not search aggressively for an internal opening in an acutely infected, inflamed area. Forcing a probe at this stage can create a false passage. It is safer to drain the abscess simply. The surgeon can reassess for a fistula once the acute infection has settled.
Arrange a follow-up visit after drainage. Check for a fistula at this visit. Follow the approach in Chapter 12.
13.10 Special Situations
Diabetic patient — needs careful glucose control alongside drainage. Infection can spread faster and heal more slowly in poorly controlled diabetes.
Immunocompromised patient — needs prompt drainage and close follow-up. Signs of infection can be less obvious. Spread can be more rapid.
Horseshoe abscess — needs drainage of both ischiorectal spaces. It also needs drainage of the posterior space that connects them. This often needs a specific counter-incision technique.
Recurrent abscess — needs a careful search for an underlying fistula, or for Crohn's disease, as described in Chapter 12.
Pregnant patient — needs prompt drainage under safe anaesthesia. Delay carries more risk than a well-planned procedure.
13.11 Complications of Treatment
Every abscess drainage carries some risk, and this should be explained clearly to the patient.
Fistula formation, in roughly one in three patients, as noted above.
Recurrence, if a pocket of infection is missed at the first drainage.
Bleeding, usually minor, from the drainage site.
Incontinence, rare, if the abscess or the drainage affects the sphincter muscles.
Spreading infection, rare but serious, if drainage is delayed in a patient with diabetes or a weakened immune system.
A patient who understands these risks in advance is better prepared. This patient copes better if a complication occurs.
13.12 Common Mistakes to Avoid
Treating an abscess with antibiotics alone, without drainage.
Delaying drainage while waiting for imaging, in a straightforward case.
Forcing a probe to find a fistula tract during the acute, infected stage.
Missing a deep space, such as a supralevator or horseshoe extension, at the time of drainage.
Failing to check blood glucose in a patient with an unexplained or recurrent abscess.
13.13 Recording and Follow-Up
Record the site and type of abscess, the method of drainage, and any internal opening seen. For example: "Perianal abscess, left lateral, incision and drainage under local anaesthesia. No internal opening seen." Arrange a follow-up visit to check healing, and to look for a developing fistula.
13.14 Why This Chapter Matters
A perianal or perirectal abscess is a surgical emergency, even though it often seems minor at first. Prompt drainage relieves pain and prevents the infection from spreading. It also raises the next question. Will a fistula follow? The next chapter turns to rectal prolapse. This is a different kind of anorectal problem. It comes from the supporting tissues, not from infection.