01OverviewDefinition, clinical context and the essential points that orientate the chapter.
A fistula-in-ano connects an internal opening in the anal canal to an external opening on perianal skin through a persistent tract. The common cryptoglandular pathway begins with infection of an anal gland and an abscess that drains spontaneously or surgically. Persistent infection and tract formation can then produce cycles of swelling, discharge and temporary improvement. The amount of visible discharge is not a reliable measure of depth: a small skin opening may connect to a substantial tract or a secondary cavity.
The Parks classification describes the relationship to the sphincter complex. An intersphincteric tract traverses the internal sphincter and reaches skin through the intersphincteric plane without crossing the external sphincter. A transsphincteric tract crosses both muscles and passes through the ischioanal region. A suprasphincteric tract rises above the puborectalis before descending, while an extrasphincteric tract connects the rectum to skin outside the usual sphincter route. An unusual high tract should prompt consideration of its cause rather than an assumption that all fistulas arose from the same gland.
Simple and complex are management descriptions as well as anatomical labels. ASCRS generally describes a simple tract as intersphincteric or low transsphincteric with less than thirty percent external sphincter involvement and without other complex features. Greater involvement, branching, horseshoe extension, recurrence, inflammatory bowel disease, radiation, malignancy or pre-existing incontinence makes the decision more difficult. Anterior fistulas in women deserve particular care because of the shorter anterior sphincter and possible obstetric injury. A low-looking tract is not permission to disregard impaired function.
Definitive treatment tries to eliminate the persistent communication without causing unacceptable continence loss. Lay-open fistulotomy can be effective for a suitable simple tract, but the tissue divided includes any sphincter crossed by that tract. Sphincter-preserving procedures such as ligation of the intersphincteric fistula tract, or LIFT, and an advancement flap trade a different healing and recurrence profile against preservation of muscle. A loose draining seton keeps infection controlled and can be an intentional long-term solution for selected patients; it is not automatically a curative operation.
Key points
- A fistula-in-ano is a persistent communication between the anal canal and perianal skin, often recognised after recurrent swelling and discharge.
- Map the relationship to both sphincters and document baseline continence before choosing an operation; skin-opening position alone is insufficient.
- Drain an associated abscess promptly, then distinguish a temporary draining seton from a later attempt at fistula closure.
- A simple low tract with normal function may suit lay-open fistulotomy; significant sphincter involvement or existing injury requires another strategy.
- MRI and EUA are complementary in complex, recurrent or Crohn-associated disease; routine MRI is not required for every obvious simple fistula.
- Record wound healing, drainage, abscess recurrence and continence separately because apparent external closure does not prove all deeper disease has resolved.
02AetiologyUnderlying causes, associations and risk factors, with why each one matters.
Persistent cryptoglandular communication
An infected anal gland and abscess can leave a tract between canal and skin, producing intermittent drainage after the initial acute infection.
Secondary fistula mechanisms
Crohn inflammation, radiation, trauma or malignancy can produce tracts whose biology and tissue quality differ from uncomplicated cryptoglandular disease.
03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.
- 1Drainage and reaccumulation
A partially open tract may decompress infection temporarily; closure at the skin while deeper disease persists can allow another painful abscess.
- 2Sphincter-crossing anatomy
The course through or around the sphincters determines how much muscle a lay-open operation would divide and therefore contributes to functional risk.
- 3Branches and horseshoe spaces
Secondary extensions can maintain infection when the most obvious tract is treated, explaining recurrence despite apparent initial surface healing.
04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.
Ask about the first abscess, every subsequent drainage or operation, periods of apparent closure and the pattern of recurrent swelling. Obtain previous operative and MRI reports if available; an unrecognised branch or internal opening may explain a failed treatment better than a presumed new disease.
Inspect scars, external openings, induration and drainage, and assess the anal canal when tolerable. Avoid forceful probing through tender inflamed tissue. Goodsall's heuristic suggests a radial course from an anterior external opening and a curved course towards a posterior midline internal opening from a posterior external opening. The rule is unreliable and does not replace operative assessment or imaging to confirm a complex tract.
Ask about wind, liquid and solid stool leakage, urgency, pad use and lifestyle restriction. Record obstetric trauma, previous sphincterotomy, pelvic radiation and chronic diarrhoea. A structured baseline makes a postoperative change recognisable and informs the acceptable extent of muscle division.
Multiple or atypical openings, proctitis, diarrhoea, weight loss or characteristic tags may support Crohn disease. Disease in other skin folds suggests hidradenitis; natal-cleft pits suggest pilonidal disease. An unexplained anal mass or ulcer warrants the appropriate cancer assessment.
05InvestigationsWhat to request, why it matters and how to interpret it.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
MRI pelvis for selected complex diseaseFirst step - Why
- Define the primary tract, internal opening, secondary extensions and associated abscesses.
- Interpretation and limitations
- MRI is especially useful in recurrent, branching, high or Crohn-associated disease and can reveal extensions missed clinically. The report should be reconciled with the planned operative approach. It complements EUA rather than removing the need to assess active sepsis and tissue quality.
- 02
Examination under anaesthesia - Why
- Confirm anatomy and perform the agreed drainage or fistula intervention.
- Interpretation and limitations
- The surgeon assesses the internal opening and sphincter involvement without relying only on the skin opening. If inflammation prevents confident mapping, source control and a staged approach may be safer than an unplanned division of muscle.
- 03
Selected endoanal ultrasound or physiology - Why
- Investigate suspected sphincter injury and functional vulnerability before repair.
- Interpretation and limitations
- Ultrasound can identify a structural defect and, in experienced hands, assist fistula mapping. Physiology can help characterise continence dysfunction. Neither a single normal test nor an apparently low tract supersedes the clinical history of leakage or injury.
- 04
Targeted luminal and tissue assessment - Why
- Investigate inflammatory disease or atypical lesions when the presentation supports it.
- Interpretation and limitations
- Faecal calprotectin, ileocolonoscopy or small-bowel assessment may follow an IBD hypothesis. A changing indurated or ulcerated lesion may require biopsy; an unexplained anal mass or ulcer meets a direct suspected cancer referral consideration without preliminary FIT.
06Differential diagnosisRealistic alternatives and the features that help distinguish them.
Pilonidal sinus
Midline natal-cleft pits and a tract arising from the cleft suggest a different origin from an anal internal opening.
Perineal hidradenitis
Multiple superficial tunnels with lesions in other flexural areas suggest follicular inflammatory disease, although coexistence with an anal fistula is possible.
Inflammatory or malignant disease
Complex recurrent tracts with luminal symptoms or changing indurated tissue can reflect Crohn disease or neoplasia rather than an isolated benign fistula.
07ManagementImmediate care, first-line treatment, alternatives and escalation.
01Worked caseStage a complex cryptoglandular repairFirst stepA 41-year-old man has recurrent right-sided drainage after two abscess operations.+
- 1He has preserved continence and no diarrhoea or other IBD features. MRI shows a transsphincteric tract crossing a substantial portion of the external sphincter with a small associated cavity. EUA confirms the map. The surgeon drains the cavity and places a loose draining seton rather than dividing the involved external sphincter.
- 2At six-week review pain and the cavity have resolved, the seton drains freely and there has been no further abscess or new leakage. This is an achieved infection-control endpoint, not yet tract eradication. After discussion of persistence and recurrence, he chooses a planned LIFT procedure for the mapped transsphincteric fistula.
- 3The surgeon identifies, ligates and divides the tract in the intersphincteric plane and manages the residual external tract as planned. The operation is documented against the MRI anatomy; the patient receives wound instructions and a route back for fever, recurrent swelling or worsening pain.
- 4At three months the wounds are healed, drainage has stopped, examination finds no recurrent collection and his baseline continence remains unchanged. The clinician records this observed clinical healing and explains that a later recurrence remains possible; MRI is reserved for a concerning or uncertain subsequent course rather than used to promise lifetime cure.
02Simple tractSelect a safe lay-open operationA symptomatic fistula is low, unbranched and confidently mapped in a patient with normal function.+
- 1Confirm that the lesion is simple in context: there is no inflammatory disease, prior injury, significant baseline leakage or other feature making division hazardous. Discuss the wound-healing period and the functional risk even when the amount of muscle involved is small.
- 2A lay-open fistulotomy converts the tract into an open wound that can heal from the base. The surgeon divides only the tissue justified by the known course; an unexpected high extension or uncertainty changes the operation rather than being ignored to complete the original plan.
- 3Review pain, wound progress, discharge and continence. Persistent drainage after the expected healing course or recurrent swelling should prompt reassessment of a missed branch, internal opening or underlying inflammatory diagnosis before repeating the same procedure.
03Complex or vulnerable functionChoose sphincter preservation deliberatelyThe tract crosses substantial muscle, has recurred or lies in a patient with impaired functional reserve.+
- 1Control abscesses and use a loose seton when needed to maintain drainage. Explain that ongoing small-volume discharge with a seton is expected and that a cutting seton is a different intervention which progressively divides tissue, not a sphincter-preserving equivalent.
- 2Discuss LIFT for a suitable transsphincteric tract or an advancement flap selected for the internal opening and tissue quality. Active proctitis, smoking, previous repair and Crohn disease can affect healing. No single technique fits every complex fistula, and published percentages should not be presented as a personal guarantee.
- 3Set expectations about repeat procedures or long-term drainage when closure would carry unacceptable functional risk. Fibrin glue and plugs have relatively poor efficacy in contemporary guideline evidence; endoscopic or laser approaches have less certain long-term results. Use these uncertainties in shared selection rather than claiming that novelty proves superiority.
08ComplicationsImportant consequences, why they occur and why they matter clinically.
Recurrent abscess and sepsis
A fistula can repeatedly obstruct and form a collection, causing renewed pain and occasionally systemic illness despite periods of mild drainage.
Continence impairment
The disease, previous injury or operative muscle division can compromise continence; the effect may be evident as wind leakage, soiling or more substantial loss.
Persistent burden and failed healing
Long-term discharge, dressings and repeated procedures can impair quality of life even in the absence of a current abscess.
09Monitoring and follow-upTreatment response, safety checks and longer-term review.
- Record recurrent abscess episodes, pain and quantity of drainage as separate measures of disease control.
- At every procedural review compare current wind and stool control with the documented baseline and ask about urgency or pad use.
- Check the loose seton’s comfort and patency and provide advice for a newly tense swelling, fever or unexpected pain.
- Investigate persistent nonhealing or changing tissue rather than allowing repeated antibiotics or dressings to obscure a missed tract, Crohn disease or malignancy.
10Special situationsVariants, exceptions and circumstances that change the usual approach.
Loose seton endpoints
A comfortable freely draining seton with no recurrent abscess is meaningful control even before closure; document that limited objective clearly.
Primary and recurrent abscess differ
The UK approach avoids searching for an opening during a first abscess, whereas recurrent infection in the same location makes a persistent tract much more consequential.
Function can outweigh speed
A longer staged treatment may be acceptable when it reduces the amount of muscle divided; discuss work, dressing burden and recurrence alongside continence.
Antibiotics have a separate indication
Antimicrobials can treat associated cellulitis or systemic infection but do not eradicate an uncomplicated chronic cryptoglandular tract by themselves.
11Common pitfallsFrequent interpretation and management errors.
- 01
Treating the external opening as a complete map can miss a branch, horseshoe extension or a high internal communication.
- 02
Calling a cutting seton sphincter-preserving conceals the fact that it progressively divides the tissue it encircles.
- 03
Offering fistulotomy solely because a tract appears low ignores baseline incontinence, anterior female anatomy and previous injury.
- 04
Equating absence of visible discharge with definitive healing can miss a blocked opening and recurrent deep collection.