A fistula-in-ano is an abnormal tunnel. It connects the anal canal to the skin near the anus. This chapter explains fistula-in-ano in full. It covers presentation, examination, differential diagnosis, investigations, and management. This is the same pattern used for every disease chapter in this manual.
12.1 What Is a Fistula-in-Ano
A fistula is a track lined by tissue. It runs from an internal opening, inside the anal canal, to an external opening, on the skin near the anus. Pus, and sometimes stool, can pass through this track. This causes ongoing discharge, and often recurring swelling.
A fistula usually starts as an abscess, as explained in Chapter 1. Small pockets called anal crypts sit along the dentate line. Anal glands open into these crypts. A gland can become blocked and infected. This forms an abscess. If the abscess is drained, but the internal opening does not heal, a fistula tract remains.
Roughly one in three patients with a drained perianal abscess goes on to develop a fistula. Every patient treated for an abscess needs a clear follow-up plan. This plan should check for this possibility, as described in the chapter on perianal abscess.
12.2 Classification of Fistula Tracts
Fistula tracts are classified by their path, relative to the sphincter muscles. This classification guides treatment choice, since it predicts how much sphincter muscle the tract crosses.
Intersphincteric — the tract runs between the internal and external sphincters. This is the most common type.
Trans-sphincteric — the tract crosses both the internal and external sphincters, before reaching the skin.
Suprasphincteric — the tract rises above the puborectalis muscle, before curving down to the skin. This type is uncommon, and carries a higher risk to continence.
Extrasphincteric — the tract bypasses the sphincter complex entirely, often linked to pelvic disease rather than a simple anal gland infection.
A simple guide, called Goodsall's rule, helps predict the path of the internal opening from the position of the external opening. An external opening in front of a line drawn across the anus usually points to a straight, radial tract. An external opening behind this line usually points to a curved tract, with its internal opening in the posterior midline.
This rule is a useful guide, not an absolute law. It is more reliable for openings close to the anus, and less reliable for openings further away, or in complex, recurrent cases. Imaging, described later in this chapter, gives a more definite answer when the picture is unclear.
12.3 Presentation
Follow the history-taking method described in Chapter 2. Certain features are typical of fistula-in-ano.
Persistent or recurring discharge — pus, blood, or serous fluid, staining the underclothes.
A history of a previous perianal abscess, often drained some weeks or months earlier.
Recurring swelling and pain. This settles each time the tract drains, then builds up again.
Itching or irritation around the anus, from constant moisture and discharge.
Occasional passage of flatus or stool through the external opening, in a long-standing or complex fistula.
Ask about the number of previous episodes, and any previous surgery for abscess or fistula. This history shapes the assessment, and often predicts a more complex tract.
12.4 Examination
Follow the sequence from Chapter 3. Fistula examination needs particular care, and sometimes needs to be completed under anaesthesia.
Inspection, as described in Chapter 4, usually shows the external opening. It appears as a small, red, raised punctum, sometimes with granulation tissue.
Gentle pressure around the external opening may express a drop of pus, confirming the tract is active.
Digital rectal examination may feel the tract as a firm, cord-like structure beneath the skin, running toward the anal canal.
Proctoscopy may show the internal opening directly, though this is not always visible in the clinic setting.
Examination under anaesthesia is often needed for a complete assessment, particularly for a complex or high tract. This allows careful probing, without causing the patient pain.
Never force a probe along a tract that resists gentle pressure. Forcing a probe can create a false passage, and can turn a simple tract into a complex one.
During examination under anaesthesia, a soft probe is passed gently from the external opening. Hydrogen peroxide, or a dye such as methylene blue, injected into the external opening, can sometimes help identify the internal opening by producing bubbling or staining. This step adds useful information, without needing to force the probe itself.
12.5 Differential Diagnosis
Several conditions can mimic a fistula, or cause a fistula for a different reason than a simple anal gland infection.
Pilonidal sinus disease — has openings in the natal cleft, above the anus, rather than at the anal margin itself.
Crohn's disease — can cause multiple, complex fistula tracts, often with less pain than a simple cryptoglandular fistula.
Tuberculosis — can cause fistula formation, especially in regions where tuberculosis remains common.
Hidradenitis suppurativa — causes multiple, interconnected sinus tracts in the skin, usually without a true internal opening into the anal canal.
Anal or rectal cancer — can rarely present with a fistula-like discharge, and must be considered in any atypical or non-healing case.
A fistula that does not fit the typical cryptoglandular pattern needs further assessment, including biopsy, as described in Chapter 8.
Some warning features stand out. These include multiple external openings not explained by a simple horseshoe pattern. They also include a tract that fails to heal after correct surgery, or a history of inflammatory bowel disease. Any of these should prompt a wider search for an underlying cause.
12.6 Investigations
Simple, low fistulas are often assessed on examination alone. More complex fistulas need imaging, as described in Chapter 8.
MRI fistulogram is the investigation of choice for a complex, recurrent, or high fistula. It shows the full course of the tract, any side branches, and its relationship to the sphincter muscles.
Endoanal ultrasound is a quicker, more accessible option in some centres. It is useful for a simpler tract, and for assessing sphincter integrity.
Biopsy is taken if the tract does not fit the usual pattern, or if malignancy is a concern.
Assessment for Crohn's disease is added if multiple or unusual tracts are found, or if the patient has bowel symptoms.
Choosing between MRI and endoanal ultrasound depends on what is available, and on the complexity of the case. As a simple guide, endoanal ultrasound suits a straightforward, first-presentation tract. MRI suits a complex, recurrent, or high tract, or any case where surgical planning depends on precise anatomical detail.
12.7 Principles of Management
Fistula surgery balances two competing goals. The surgeon must heal the tract completely. The surgeon must also protect the sphincter muscle, and preserve continence. Every technique described below is a different way of balancing these two goals.
The amount of sphincter muscle the tract crosses is the single most important factor in this balance. A tract crossing little or no muscle can often be laid open safely. A tract crossing a large amount of muscle needs a technique that avoids cutting it, even if this means a slightly lower cure rate.
12.8 Surgical Techniques
Fistulotomy — the tract is laid open along its full length, and left to heal from the base. This is highly effective for a simple, low, intersphincteric tract, where little or no sphincter muscle needs to be divided.
Seton placement — a thread or band is passed through the tract, and left in place. A seton can drain the tract, and allow inflammation to settle, before a definitive procedure. A cutting seton, tightened gradually, slowly divides the tract while allowing fibrosis to form, protecting some sphincter function.
Ligation of the intersphincteric fistula tract, known as the LIFT procedure. The tract is identified, and tied off in the intersphincteric space. The external sphincter is not cut.
Advancement flap — healthy tissue from above the internal opening is used to cover it, after the tract below is cleared. This avoids cutting the sphincter, and suits a high or trans-sphincteric tract.
Fistula plug or fibrin glue — biological material is used to fill or seal the tract. Results are variable, and this is usually reserved for select cases.
The choice of technique depends on the tract's classification, its relationship to the sphincter, and the patient's baseline continence. A simple, low tract usually allows fistulotomy safely. A complex or high tract usually needs a sphincter-preserving technique instead.
After any fistula surgery, good wound care matters greatly. Warm sitz baths, kept up daily, help keep the wound clean as it heals from the base upward. Advise the patient on a high-fibre diet, to keep the stool soft, and arrange regular follow-up to check that healing is progressing as expected.
12.9 Special Situations
Horseshoe fistula — a tract that curves around the anal canal, usually posteriorly, with two or more external openings. This needs careful mapping, often with MRI, before surgery.
Fistula in Crohn's disease — treatment focuses on controlling the underlying bowel disease first. Long-term seton drainage is often preferred over definitive surgery, since healing can be poor.
Recurrent fistula — needs careful reassessment, usually with MRI, to identify any missed tract or side branch before repeat surgery.
Fistula in an immunocompromised patient — needs extra caution, since healing may be delayed, and infection can spread more readily.
In every special situation, explain the likely course clearly to the patient, following the approach in Chapter 9. Some patients, particularly those with Crohn's disease, may need a long-term seton rather than a single definitive cure. Setting this expectation early avoids confusion and disappointment later.
12.10 Assessing Continence Before Surgery
Before any fistula surgery, assess the patient's baseline continence carefully. Ask about any existing difficulty controlling flatus or stool, following the approach in Chapter 2. A patient with a history of childbirth injury, previous anal surgery, or an already weak sphincter needs extra caution.
In such patients, prefer a sphincter-preserving technique, even for a tract that might otherwise allow fistulotomy. Anorectal manometry, described in Chapter 8, can help in borderline cases, by giving an objective measure of sphincter function before the operation.
Document this assessment clearly before surgery. It protects the patient, and it protects the surgeon, by showing that continence risk was considered and discussed as part of the treatment plan.
12.11 Complications of Treatment
Every fistula treatment carries some risk, and this should be explained clearly to the patient.
Incontinence, of varying degree, if too much sphincter muscle is divided.
Recurrence, if the tract or an internal opening is missed during surgery.
Delayed wound healing, particularly with a large or complex tract.
Discomfort from a seton. Most patients tolerate this well. Some find it troublesome over a long period.
A patient who understands these risks in advance is better prepared, and copes better if a complication occurs.
12.12 Common Mistakes to Avoid
Attempting fistulotomy on a high or complex tract, without first confirming its course.
Forcing a probe along a tract, risking a false passage.
Missing a side branch or secondary tract, leading to early recurrence.
Treating a fistula without considering Crohn's disease, when the presentation is atypical.
Failing to explain the risk of incontinence clearly, before taking consent for surgery.
12.13 Recording and Follow-Up
Record the position of the external opening on the clock face, the likely tract classification, and the treatment plan chosen. For example: "Fistula-in-ano, external opening at 6 o'clock, intersphincteric tract on MRI. Plan: fistulotomy." Arrange a follow-up visit to check healing, and to plan further treatment if needed.
12.14 Why This Chapter Matters
A fistula-in-ano is a small tract, but it demands careful judgement. Every decision balances complete healing against the risk to continence. Understanding the tract's classification, and choosing the right technique for that classification, gives each patient the best chance of a lasting cure, without unnecessary risk. The next chapter turns to perianal and perirectal abscess, the acute condition that so often precedes a fistula.