Perianal Abscess and Fistula in Ano

Introduction and anatomy

Perianal abscess and fistula in ano are two phases of the same disease. The abscess is the acute presentation, the fistula the chronic sequel, and between a third and a half of patients who have an abscess drained will develop a fistula.

The cryptoglandular hypothesis explains almost all of it. There are between four and ten anal glands lying in the intersphincteric space - the potential space between the internal and external anal sphincters - each draining through a duct that opens into an anal crypt at the dentate line. If a duct becomes obstructed, the gland becomes infected, and pus accumulates in the intersphincteric space. From there it takes the path of least resistance.

• Downwards between the sphincters to the anal margin, producing a perianal abscess
• Laterally through the external sphincter into the fat of the ischioanal fossa, producing an ischioanal (ischiorectal) abscess
• Upwards in the intersphincteric plane, producing a supralevator abscess
• Remaining in the intersphincteric space, producing an intersphincteric abscess, which causes severe pain with almost nothing visible externally
• Circumferentially around the anus through the deep postanal space, producing a horseshoe abscess

Once the abscess has drained, either spontaneously or surgically, a track may persist between the original infected crypt and the skin. That track epithelialises and becomes a fistula. Knowing this sequence explains why the internal opening of a fistula is almost always at the dentate line, and why a fistula that is not treated at its internal opening simply recurs.

Aetiology

Around 90% of anorectal abscesses are cryptoglandular. The remainder are secondary, and identifying them changes the management substantially.

• Crohn disease - the most important secondary cause. Perianal disease may precede intestinal symptoms by years, and fistulae are often complex, multiple and recurrent.
• Diabetes mellitus and immunosuppression - both increase incidence and the risk of extensive or necrotising infection
• Trauma and iatrogenic injury - obstetric injury, haemorrhoid banding, sclerotherapy, foreign bodies, and prostate biopsy
• Malignancy - anal and rectal carcinoma can present as perianal sepsis, and a chronic fistula can itself undergo malignant change
• Infection - tuberculosis, actinomycosis, lymphogranuloma venereum and HIV-associated infections
• Hidradenitis suppurativa and pilonidal disease, which produce perianal sinuses that mimic fistulae but do not communicate with the anal canal
• Radiotherapy to the pelvis, and previous anorectal surgery

Perianal abscess: presentation and management

Clinical features

• Constant, severe, throbbing perianal pain, worse on sitting and on defaecation, and unrelieved by opening the bowels - in contrast to a fissure, where the pain is triggered by defaecation
• A tender, erythematous, fluctuant swelling at the anal margin in a perianal abscess
• Systemic upset - fever, malaise, rigors - which is more prominent the deeper the abscess
• Purulent or blood-stained discharge if the abscess has already begun to discharge, often with sudden relief of pain
• Difficulty sitting, walking or passing urine
• In an intersphincteric or supralevator abscess there may be no external signs at all, only severe pain, tenderness on rectal examination and a fever. This is the abscess that gets missed.

Management

Incision and drainage is the treatment. Antibiotics do not penetrate an abscess cavity and cannot cure one, so prescribing them without drainage merely delays definitive treatment while the sepsis progresses.

1. Analgesia, intravenous fluids and blood cultures if febrile, with the sepsis pathway followed if criteria are met
2. Examination under anaesthetic with incision and drainage, on an emergency list. A cruciate or elliptical incision is made over the point of maximal fluctuance, as close to the anal margin as possible so that any resulting fistula is short.
3. Break down loculi digitally, irrigate, and either pack the cavity or deroof it to allow healing by secondary intention from the base upwards
4. Send pus for culture. Growth of skin organisms such as Staphylococcus aureus suggests the abscess did not arise from an anal gland and a fistula is unlikely. Growth of gut organisms such as Escherichia coli and Bacteroides suggests a cryptoglandular origin and predicts a fistula.
5. Antibiotics in addition to drainage where there is surrounding cellulitis, systemic sepsis, immunosuppression, diabetes, prosthetic heart valves, or a risk of endocarditis
6. Examine for a fistula at the same time, but do not probe vigorously in the acute setting - this risks creating a false track. Most surgeons defer definitive fistula surgery to a later elective procedure.
7. Arrange follow-up, since a persistent discharge after a few weeks means a fistula has formed

Fistula in ano: assessment

A fistula in ano is an abnormal epithelialised track connecting the anal canal (the internal opening, almost always at the dentate line) with the perianal skin (the external opening). Patients present with persistent or intermittent discharge of pus, blood or faeculent material, perianal irritation and recurrent abscesses.

Parks classification

The practical shorthand is low versus high, meaning whether the track crosses less or more than roughly the lower third of the external sphincter. Low fistulae can be laid open with little risk to continence; high ones cannot.

Goodsall rule

Draw an imaginary transverse line across the anus. An external opening anterior to that line usually connects to the internal opening by a short, straight, radial track. An external opening posterior to it usually curves and opens into the posterior midline. The rule reflects the anatomy of the deep postanal space, through which posterior tracks run.

Investigations

• Examination under anaesthetic remains the reference standard, allowing the internal opening to be identified, the track probed gently and the relationship to the sphincters assessed
• MRI of the pelvis - the imaging investigation of choice, particularly for recurrent, complex or Crohn-related fistulae. It maps the track, identifies secondary extensions and undrained collections, and defines the relation to the sphincter complex before surgery.
• Endoanal ultrasound - an alternative, good for the primary track and sphincter integrity, less good for supralevator extensions. Hydrogen peroxide can be instilled to enhance the track.
• Assessment of sphincter function - a careful continence history, and anorectal manometry where there is doubt, particularly in women with previous obstetric injury
• Colonoscopy and faecal calprotectin where Crohn disease is suspected
• Biopsy of any chronic, indurated or atypical fistula, since long-standing tracks can undergo malignant change

Fistula in ano: management

The aim is to eradicate the track without damaging continence, and every operation is a compromise between those two goals. There is no single best procedure; the choice depends on the height of the track, the patient's baseline continence, and whether Crohn disease is present.¹

Fistulising perianal Crohn disease

This is managed jointly by gastroenterology and colorectal surgery and follows different rules. Fistulotomy is generally avoided because wounds heal poorly and continence is often already compromised.

• MRI of the pelvis to map the disease before anything else
• Examination under anaesthetic with drainage of sepsis and placement of a loose seton, which controls symptoms without dividing muscle
• Anti-TNF therapy with infliximab or adalimumab, which is the mainstay of medical treatment and produces fistula closure in a substantial proportion. Antibiotics such as metronidazole and ciprofloxacin are used adjunctively.²
• Optimisation of luminal Crohn disease, since active proctitis makes any local procedure fail
• Definitive surgery only once sepsis is controlled and inflammation is quiescent
• Proctectomy with a permanent stoma in severe refractory disease, which some patients choose for quality of life

Complications

• Fistula formation after abscess drainage, in a third to a half of patients
• Recurrent abscess, usually indicating an undrained collection or an untreated internal opening
• Faecal and flatus incontinence, the major iatrogenic complication and the reason sphincter-preserving procedures exist
• Anal stenosis after extensive or repeated surgery
• Fournier gangrene and systemic sepsis, particularly in diabetic and immunosuppressed patients
• Chronic non-healing wounds, common in Crohn disease and after radiotherapy
• Malignant transformation of a long-standing fistula to squamous cell or mucinous adenocarcinoma - rare, but a reason to biopsy chronic indurated tracks
• Psychological and social impact of chronic perianal discharge, which is substantial and consistently underestimated

Red flags

Prognosis

A simple perianal abscess drained promptly resolves quickly, and around half to two thirds of patients have no further trouble. The remainder develop a fistula, usually declaring itself within a few weeks as persistent discharge from the drainage site.

For simple low fistulae, fistulotomy is close to curative, with recurrence rates in single figures and minimal effect on continence. For high and complex fistulae the picture is much less satisfactory: sphincter-preserving procedures cure perhaps 60 to 75%, several attempts are often needed, and each carries a small additional continence cost.

Perianal Crohn disease is the hardest group. Complete and durable fistula closure is achieved in a minority even with modern biologic therapy and careful surgical staging, and treatment is often about controlling sepsis and symptoms rather than achieving cure. Framing the goal honestly at the outset - drainage, comfort and continence, rather than a promise of closure - is part of managing the condition well.