Anal Fistula

What an anal fistula is

An anal fistula is an abnormal track that connects the inside of the anal canal to the skin around the anus. Most fistulas are cryptoglandular: they begin as an infection in one of the small glands that open into the anal canal. That infection first forms an abscess, and when the abscess drains — spontaneously or surgically — a track can remain behind.

This is why an abscess and a fistula are best thought of as two stages of the same disease. A proportion of people who have had an anorectal abscess drained go on to develop a fistula, and that possibility should be explained at the time of drainage rather than presented as a complication of surgery.

How it presents

The usual story is a recurring discharge of pus or blood-stained fluid from a small opening beside the anus, often with an itchy or sore patch of skin. Pain builds when the track blocks and the area becomes swollen and tender, then eases when it discharges again. Some people describe repeated abscesses in the same place. Fever and rapidly increasing pain suggest an abscess that needs drainage rather than a fistula being managed in clinic.

Assessment

Assessment starts with a careful history and examination. The assessment aims to establish where the internal opening lies and how much sphincter muscle the track crosses, using imaging and examination under anaesthesia when appropriate, because these features determine which operations are safe. A fistula that crosses little or none of the external sphincter is described as simple; tracks that cross a significant amount of muscle, have several branches, arise anteriorly in a woman, occur with poor pre-existing continence, or occur with Crohn's disease or previous pelvic radiotherapy are described as complex, and complex fistulas are managed differently.

Not every straightforward fistula automatically needs MRI. The need for imaging depends on the anatomy and the available expertise, and current guidance favours a low threshold for using MRI or endoanal ultrasound (EAUS) to distinguish simple from complex disease rather than reserving imaging only for obvious complexity. Preoperative MRI is particularly important when the fistula is clinically suspected to be complex, recurrent or inadequately defined, or when endoanal ultrasound is insufficient or unavailable. Endoanal ultrasound can itself be used as a primary imaging tool for many patients where the expertise to perform and interpret it exists. Examination under anaesthesia is part of assessment and treatment, but it should not be relied upon as the sole diagnostic step for a complex fistula. Where inflammatory bowel disease is suspected, assessment of the bowel itself is arranged. A recurrent, multiple, unusual or persistently non-healing fistula may need investigation for causes other than routine cryptoglandular disease — for example Crohn's disease and, depending on the clinical setting, tuberculosis, other infection, hidradenitis or malignancy.

Treating the abscess first

An anorectal abscess needs prompt drainage. Antibiotics alone are not a substitute for drainage, though antibiotics are added in selected situations such as significant surrounding cellulitis, systemic illness or immunosuppression rather than routinely to try to prevent a later fistula. Performing a definitive fistula procedure such as fistulotomy at the same time as draining an acute abscess is not routinely recommended, because the track is not always clearly defined at that stage; drainage of the sepsis is usually the priority, with fistula surgery planned separately once the anatomy is clear. Delayed drainage in a diabetic or immunocompromised patient can lead to rapidly spreading perineal infection, which is a surgical emergency.

Treating the fistula

The aim is to heal the track while protecting continence. The two goals compete, because the more muscle a track crosses, the more likely a simple lay-open operation is to affect control of stool.

For an intersphincteric fistula or a low transsphincteric fistula in a patient with normal preoperative continence, fistulotomy — laying the track open so it heals from the base — offers the highest healing rates and is a reasonable operation once the anatomy, symptoms, sphincter involvement, continence risk and the patient's own expectations have all been properly assessed. Fistulotomy is not automatically the right choice simply because a fistula is described as low; the assessment above still applies.

For fistulas that cross a significant amount of muscle, sphincter-preserving approaches are used, and setons do not all serve the same purpose. A draining (loose) seton is a soft thread left through the track to keep it draining, control sepsis and settle inflammation; it is often used to prepare for later sphincter-preserving surgery, and in selected complex or recurrent cases — or where further surgery is undesirable or high-risk — a loose seton is sometimes continued as longer-term management rather than as a bridge to a further operation, although the evidence for this use is generally low or very low certainty. A cutting seton, which is gradually tightened to divide muscle slowly, is a different technique with different aims and is not sphincter-preserving. European guidance (ESCP) advises against using a cutting seton for simple anal fistula and for high anal fistula, while American guidance (ASCRS) allows selective use in some complex cryptoglandular fistulas on the basis of low-quality evidence. Because the risk to continence matters, a cutting seton is not a routine option and requires careful patient selection.

Ligation of the intersphincteric fistula tract (LIFT) is a recognised option for a new high transsphincteric fistula, and endorectal advancement flap is a recognised option for high fistula more generally; neither technique is universally best, and the choice remains dependent on the anatomy, previous surgery and available expertise. Video-assisted anal fistula treatment (VAAFT) can be considered for complex fistula, and laser ablation of the fistula tract can be considered for high fistula; both are supported only by very-low-certainty evidence, and a repeat laser procedure after a failed attempt may sometimes be considered but should be approached with caution because the cumulative effect on the sphincter over repeated procedures is uncertain. These video-assisted and laser-based techniques are used in selected fistulas and appropriate anatomy, but they should not be presented as always closing a fistula, as superior to established sphincter-preserving surgery, or as carrying no risk of recurrence or continence change.

Fibrin glue and fistula plugs are not the same thing and should not be grouped together. Current guidance is that fibrin glue should not be used for cryptoglandular anal fistula, a recommendation based on moderate-level evidence. A fistula plug, by contrast, can be considered as a sphincter-preserving option, based on low-level evidence; healing with a plug is generally less predictable than with established sphincter-preserving surgery.

Fistulas associated with Crohn's disease are managed jointly by gastroenterology and colorectal teams, because treatment of the underlying bowel disease is part of healing the fistula, and the priority is to avoid inappropriate destruction of sphincter muscle. Where Crohn's disease is suspected, that multidisciplinary assessment — rather than the approach used for cryptoglandular fistula — guides the treatment plan.

Realistic expectations

Healing time varies substantially by fistula anatomy and procedure, and more than one operation is sometimes needed, particularly for complex tracks. Recurrence is well recognised even after appropriately chosen surgery. Some degree of altered control of wind or stool is possible after any fistula operation that involves the sphincter, and the balance between healing and continence should be discussed openly before deciding. No technique, including laser or video-assisted treatment, can be promised as a painless, scarless or guaranteed permanent cure.