Anal fistula

An anal fistula is a narrow tunnel joining the inside of the anus to the skin beside it. Infection lives inside it, so pus leaks from the outer opening from time to time and a painful lump keeps swelling under the skin. The tunnel almost never closes on its own: its walls acquire a lining and it turns into a permanent tube. It is not a tumour and it is not contagious, but neither is it something that clears up with ointments and rest.

Where a tunnel between bowel and skin comes from

Small glands open around the anus, on the border between the lining and the skin. When the duct of one of them blocks, pus builds up inside within days. The abscess is drained or bursts by itself, the pain eases — but the path the pus carved to the outside remains. In roughly a third to a half of people that path becomes a fistula.

Hence the typical story: a few days of severe pain and a lump beside the anus, then relief, and weeks or months later a permanently damp spot on the skin that dries up and opens again. The tunnel may be short and straight, or long, with side branches and pockets, and it may run through the muscles that close the anus. Everything depends on that: how the fistula is found, how it is treated and what the person risks in surgery.

What it feels like

• constant staining of underwear — pus, blood-stained fluid, sometimes traces of stool, with an unpleasant smell;
• itching, burning and sore, damp skin around the anus;
• a dull or throbbing pain that gets worse on sitting, walking, coughing and opening the bowels;
• quiet spells alternating with flare-ups, when everything swells and hurts again until it drains;
• a small opening or bump on the skin near the anus, which you cannot always see yourself;
• less often, trouble holding wind and stool — usually with a long-standing or complex fistula.

Fever, chills and spreading redness are signs not of the fistula itself but of a fresh abscess that has formed inside it.

When waiting is not an option

Seek urgent care if you develop:

• rapidly increasing pain in the perineum that feels far worse than the area looks;
• a temperature above 38 with chills, weakness, confusion, a racing pulse;
• redness and swelling creeping onto the buttocks, the scrotum, the vulva or the lower abdomen;
• patches of skin that darken, turn blue or lose sensation, and a crackling feel under the fingers when pressed;
• being unable to pass urine or open the bowels because of pain.

This can be the start of Fournier's gangrene, a rare but fast-moving infection of the tissues of the perineum that destroys them within hours. It is commoner with diabetes, obesity and medicines that suppress immunity, but it also strikes people who were previously well. Here the clock runs in hours, not days: emergency surgery is needed. It is the one situation around anal fistula in which delay is life-threatening, which is why it is worth knowing about in advance.

Not every fistula starts with an abscess

Nine fistulas out of ten follow an ordinary infection of an anal gland. The rest have other causes, and missing them matters: in those cases the treatment has to be aimed at the underlying disease, not at the tunnel.

• Crohn's disease. The commonest of the difficult causes. Its fistulas are often multiple and stubborn, heal badly after standard surgery, and sometimes appear before any bowel symptoms — as the first sign of the illness. Diarrhoea, abdominal pain, weight loss, blood in the stool and inflamed eyes or joints all strengthen the suspicion.
• Hidradenitis suppurativa. Chronic skin inflammation with lumps and tracts in the groin, the armpits and between the buttocks; its tracts are easily mistaken for an anal fistula.
• Tuberculosis, and less often HIV or other causes of weakened immunity. These come to mind when a fistula has been there a long time and does not respond to treatment.
• After surgery and childbirth — operations on the anus, perineal tears. A separate variety is a tunnel between the rectum and the vagina, letting wind and bowel contents through.
• Radiotherapy to the pelvis, sometimes years after treatment.
• Cancer. Rarely, the tunnel is a manifestation of cancer of the rectum or anal canal; in addition, the lining of a very long-standing fistula can itself turn malignant. That is why tissue removed at operation is sent for examination.

An inflamed tract in the cleft between the buttocks, nearer the tailbone, is a different condition, although it looks similar and is often confused with an anal fistula.

How the tunnel is traced

It starts with an examination: the doctor looks at the outer opening, feels the perineum, performs an internal examination of the rectum and a look inside with a short lit instrument. Often that is enough for the diagnosis.

The harder question is where the internal opening is and how the tunnel runs in relation to the closing muscles. MRI of the pelvis helps most here — it shows side branches, collections and hidden cavities better than anything else. Ultrasound through the anus is also used. The definitive picture often comes from an examination under anaesthetic: the surgeon traces the tunnel with a probe and decides on the spot what to do. If there are several fistulas, if they keep coming back or sit in an unusual place, the bowel is investigated so that Crohn's disease is not missed.

Probing the tunnel yourself, squeezing the contents out or putting anything into the opening is a reliable way of driving the infection deeper.

Why a surgeon is unavoidable

Antibiotics settle acute inflammation but do not close the tunnel; ointments, baths and suppositories bring relief and nothing more. Surgery has a double aim: remove the tunnel and preserve the muscles that hold stool in.

• Laying the tunnel open. It is opened along its whole length and heals as an open groove. The most reliable method, but only suitable when a small part of the muscle is involved.
• A draining thread. It is passed through the tunnel and left for weeks or months. It does not cure, but it keeps the tunnel open, stops pus collecting and puts an end to flare-ups — a way of buying time before the next step, or of living comfortably when operating is too risky.
• Tying the tunnel off between the muscles, and flap repair. The tunnel is closed without cutting the muscle: either in the space between its layers, or by covering the internal opening with a flap of the person's own lining. Healing succeeds less often than with laying open, but the muscles stay intact.
• Filling and sealing the tunnel — glues, biological plugs, laser or radiofrequency treatment from within. The muscle is untouched and recovery is quick, but the fistula returns more often.

Treatment frequently runs in two or three stages, and that is not a sign of failure but the normal course of things. With fistulas caused by Crohn's disease surgery is only part of the plan: medicines that damp down bowel inflammation are adjusted at the same time, otherwise the tunnel opens again.

Worth asking the surgeon: whether the muscle is involved and how much, what the risk of losing bowel control is with the chosen technique, and how likely the fistula is to come back.

Living with it, before and after surgery

• soft, regular stools — water, fibre and, if needed, a gentle laxative; there is no point straining or sitting on the toilet for long;
• washing with warm water after each bowel movement and patting dry; perfumed soap and alcohol wipes only irritate the skin;
• a pad or folded gauze so that underwear stays dry;
• sitting in a bowl of warm water: it does not heal anything but noticeably eases the pain;
• stopping smoking — wounds in this area heal worse in smokers, especially in Crohn's disease;
• keeping blood sugar in check if you have diabetes: healing depends on it directly.

After surgery the area heals slowly, over weeks, and that is normal. Go back to the doctor if a fever appears, if the pain grows instead of settling, if bleeding starts or if the sensation of needing to go is lost.

Online consultation

In an online consultation the doctor will go through your history: whether there was an abscess, how many times it has all repeated, what has been done already. They will tell you whether the situation needs urgent care or whether you can calmly prepare for a planned visit, explain which tests are worth doing before you see a colorectal specialist, and what to watch for if there is reason to suspect bowel disease.

This material is for information only and does not replace medical advice.