Perianal Abscess: What to Do, and Why It Often Becomes a Fistula

A perianal abscess produces a very particular kind of pain. It builds over two or three days, becomes constant and throbbing, makes sitting impossible and sleep difficult, and is entirely unrelieved by painkillers. Patients often describe it as among the worst pain they have experienced.

It is also one of the conditions where the correct treatment is straightforward and the relief is dramatic — provided the right thing is done, and done promptly.

Recognising it

• A painful, tender swelling beside or near the anus
• Constant throbbing pain rather than pain only on passing stool
• Redness, warmth and firmness of the overlying skin
• Pain that is worse on sitting and often worse at night
• Fever, chills and feeling generally unwell
• Difficulty passing urine in some cases
• Later, spontaneous discharge of pus with sudden dramatic relief

Why antibiotics alone are not the answer

Pus in a confined space is walled off from the bloodstream. Antibiotics reach the tissue around a collection but do not penetrate the collection itself in useful concentrations.

The consequence is that antibiotics may slow the process, take the edge off the fever, and give an impression of improvement while the collection continues to expand under the surface. This delays definitive treatment and allows the infection to spread through the tissue planes, converting a simple abscess into a more complex one.

The principle is one of the oldest in surgery and remains true: where there is pus, let it out. Drainage relieves the pain within hours. Antibiotics are an adjunct for patients with diabetes, immunosuppression, extensive surrounding cellulitis or systemic illness — not a substitute.

What drainage involves

Incision and drainage is a short procedure, usually done under a short general or spinal anaesthetic — local anaesthesia often works poorly in acutely infected tissue and can be inadequate for a proper cavity assessment.

A cruciate or elliptical incision is made over the point of maximum fluctuation, positioned as close to the anal verge as is safe, and the cavity is opened widely enough that it will not seal prematurely. All loculations — internal partitions within the cavity — are broken down so that no pocket of pus is left behind.

The cavity is irrigated and usually left open to heal from the base up, sometimes with a small drain or a light dressing. Closing it tightly would simply recreate the problem.

Relief is typically immediate and profound.

Why so many go on to form a fistula

This is the part patients are rarely told at the time, and it explains a great deal about what happens over the following months.

Most perianal abscesses originate from an infected anal gland inside the anal canal. The abscess is the endpoint of a tunnel that started at that gland and worked its way outward to the skin.

Draining the abscess empties the collection. It does not close the tunnel, and it does not address the infected gland at its origin. In roughly a third of patients, sometimes more, that tunnel persists as an anal fistula.

This is why a discharge that continues weeks after drainage, or a swelling that returns in the same place, is not a failure of the drainage. It is the underlying fistula declaring itself.

Why the fistula is not usually treated at the same time

It seems logical to find and treat the fistula during the same operation as the drainage. In practice this is often unwise.

Acutely inflamed tissue is oedematous and friable, and the anatomy is distorted. Probing aggressively in that setting risks creating a false passage — an artificial track that did not exist before — and can lead to unnecessary division of sphincter muscle.

The safer approach in most cases is to drain thoroughly, allow six to eight weeks for the inflammation to settle, then reassess with a clear field and, where indicated, an MRI. If a fistula has formed, it can then be treated properly with the right technique chosen for its anatomy.

A loose draining seton is sometimes placed at the time of drainage if a clear internal opening is obvious, which keeps the track open and controlled in the interim.

Recovery after drainage

• Pain relief is usually immediate and dramatic
• The wound is left open and heals from the base upward over two to four weeks
• Sitz baths two to three times daily and after each bowel movement are the main aftercare
• Some discharge is expected while the cavity heals
• Stool softeners keep bowel movements comfortable
• Most patients return to desk work within three to seven days
• A follow-up review at four to six weeks is important specifically to check whether a fistula has formed

Who is at higher risk

• People with diabetes, particularly poorly controlled — abscesses are more common, more severe and spread faster
• Patients with Crohn’s disease
• Anyone immunosuppressed, including those on long-term steroids or chemotherapy
• People with a previous perianal abscess
• Smokers
• Those with chronic diarrhoea or poor local hygiene

What not to do

• Do not apply hot compresses hoping it will burst on its own — this can allow it to spread deeper
• Do not attempt to squeeze or drain it yourself
• Do not take a course of antibiotics and wait a week to see if it settles
• Do not delay assessment because of embarrassment; this is an extremely common condition and the pain will not resolve without drainage