01OverviewDefinition, clinical context and the essential points that orientate the chapter.
Perianal Crohn disease includes inflammatory fissures, ulcers, tags, strictures, abscesses and fistulas. A fistula is an abnormal tract connecting the anorectal lumen to perianal skin or another structure, often with branches crossing the sphincter complex. The clinical problem is therefore more than a persistent skin opening: active inflammation, infection, structural damage and sphincter function interact. Some patients have perianal disease before an established intestinal diagnosis. Multiple openings, atypical fissures, recurrent abscesses or a complex tract should prompt consideration of Crohn disease, while cryptoglandular fistula and hidradenitis remain important alternatives.
Treatment works best when colorectal surgery and the IBD team share a plan with radiology, specialist nursing and nutritional support. Drainage tackles a collection; a loose seton maintains a safe outlet; medical therapy treats the inflammatory driver; selected operations attempt closure when the tissues and anatomy permit. None of these actions automatically achieves all four goals. The 2025 BSG guidance recommends prompt medical treatment after adequate drainage and suggests infliximab as the first biological therapy for perianal disease. Clinical response, deep healing, continence and quality of life should each be measured rather than condensed into an unsupported promise of cure.
Key points
- Map both sepsis and inflammation: MRI complements experienced examination under anaesthesia, while rectal assessment identifies proctitis that worsens fistula-healing prospects.
- Drain an abscess and secure drainage before biological therapy; active sepsis or an undrained abscess contraindicates infliximab.
- In eligible adults, selected Remicade induction is 5 mg/kg intravenously at weeks 0, 2 and 6; assess response after all three doses before continuing eight-weekly maintenance.
- A loose seton keeps a tract draining and protects against recurrent sepsis; it is not proof of fistula closure or a substitute for inflammatory control.
- Infliximab is BSG’s suggested first biological treatment after adequate drainage; apply NICE eligibility and product-specific screening alongside the surgical plan.
- Reduced external discharge is a clinical response, whereas MRI can show residual active disease; do not equate a closed skin opening with deep healing.
- Seton removal, definitive closure and diversion are individual MDT decisions, especially with proctitis, complex anatomy or poor quality of life.
02AetiologyUnderlying causes, associations and risk factors, with why each one matters.
Transmural inflammation
Crohn disease can produce penetrating inflammation around the anorectum, creating abnormal tracts and collections in tissue already vulnerable to ulceration and poor healing.
Mixed local mechanisms
Cryptoglandular infection can coexist with Crohn inflammation. Fistula anatomy, luminal findings and surrounding tissue therefore vary, even among people with the same intestinal diagnosis.
03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.
- 1Branching tract formation
Penetrating inflammation extends between tissue planes and may cross the sphincters, creating branches that connect bowel, skin or adjacent structures and permit recurrent contamination.
- 2Abscess accumulation
If a tract or branch loses its outlet, infected material collects under pressure. External drainage from another opening can continue while a separate deep pocket remains undrained.
- 3Proctitis and healing
Inflamed rectal mucosa and persistently active tissue create an unfavourable environment for fistula closure. Local inflammation can persist despite improvement in general bowel symptoms.
04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.
Ask about pain, swelling, discharge, recurrent filling and spontaneous drainage, continence and previous abscess procedures. Record the pattern of stool and systemic symptoms, weight change and treatment exposure. An external opening that has stopped discharging may reflect improvement, but new pain at the same time suggests obstruction of drainage and another collection.
Inspect gently with consent and a chaperone, noting oedematous tags, broad or multiple fissures, ulcers, induration and the number of openings. Avoid forceful probing or painful office examination. Severe pain can limit assessment and justify examination under anaesthesia; tissue quality and rectal inflammation affect whether a tract can later be closed safely.
Document existing liquid or solid leakage, urgency, prior obstetric injury and previous sphincter division. Ask about luminal disease activity and previous immunomodulator or biological failure, intolerance and adherence. A proposed fistulotomy must account for both tract anatomy and the greater risk of poor healing or functional harm in inflamed or damaged tissue.
HS produces recurrent superficial nodules, tunnels and scars in flexural skin and may coexist with IBD. A solitary cryptoglandular fistula can occur in a person with Crohn disease without proving that every tract is inflammatory. Diagnosis draws together clinical history, rectal and luminal findings, imaging and operative anatomy rather than the disease label alone.
Longstanding perianal fistulising disease carries a risk of malignancy. New pain, induration, bleeding, an ulcer or a change in drainage deserves reassessment, with examination, imaging, endoscopy and EUA-directed biopsy as appropriate. BSG notes that evidence does not establish a single standard surveillance interval, so inventing an annual biopsy schedule would be misleading.
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
Pelvic MRI with fistula mappingFirst step - Why
- Define primary and secondary tracts, sphincter involvement and occult collections.
- Interpretation and limitations
- MRI complements experienced EUA and helps show branches that a superficial examination misses. It also provides a baseline for radiological response. An urgent septic collection still needs prompt surgical action; arranging elective imaging must not become a reason to delay source control in a deteriorating patient.
- 02
Examination under anaesthesia and rectal assessment - Why
- Examine painful anatomy, drain sepsis and assess proctitis with experienced colorectal input.
- Interpretation and limitations
- Record the opening, tract, collections and seton configuration. Inspection of rectal mucosa matters because active proctitis predicts poorer healing after definitive fistula surgery. EUA is both diagnostic and therapeutic, and MRI or endoanal ultrasound may add complementary information depending on expertise.
- 03
Luminal and inflammatory assessment - Why
- Assess Crohn activity and nutritional consequences beyond the perineum.
- Interpretation and limitations
- FBC, CRP, albumin and appropriate stool or endoscopic investigation support assessment of inflammation, anaemia and nutrition. A low CRP does not exclude a locally active fistula. Ileocolonoscopy and other intestinal evaluation are selected by the IBD team when diagnosis or disease extent is uncertain.
- 04
Pre-infliximab infection and safety screening - Why
- Identify preventable biological-treatment harms before the first infusion.
- Interpretation and limitations
- Evaluate active and latent TB with history, chest imaging and appropriate testing, recognising false-negative tests during immunosuppression. Test HBV and involve relevant specialists for positive findings; review vaccination, current infection, heart failure, neurological disease, malignancy history, pregnancy and concomitant immunosuppression. Record baseline blood counts, liver and renal function to interpret later toxicity and organ-risk decisions.
- 05
Drug concentration and antibodies after loss of response - Why
- Help distinguish pharmacokinetic failure from other reasons for persistent disease.
- Interpretation and limitations
- First reconsider undrained infection, anatomy and adherence. Specialist therapeutic drug monitoring may then inform optimisation or a change of therapy. Observational associations between higher trough concentrations and fistula outcomes do not create a universal target or authorise automatic dose escalation for every patient.
06Differential diagnosisRealistic alternatives and the features that help distinguish them.
Cryptoglandular fistula
Infection arising from an anal gland can create a tract without underlying Crohn inflammation. A prior abscess and tract anatomy can overlap with inflammatory disease.
Perineal hidradenitis
Follicular inflammatory nodules, superficial tunnels and scars cluster in flexural skin. The disease can coexist with Crohn disease but does not automatically communicate with the anal lumen.
Fistula-associated malignancy
A chronic tract may develop a malignant change, presenting as new induration, altered pain, bleeding or discharge. An existing inflammatory diagnosis can obscure this change.
07ManagementImmediate care, first-line treatment, alternatives and escalation.
01Worked caseDrainage followed by measured biological responseFirst stepA 34-year-old, 70 kg man has persistent active fistulising Crohn disease after conventional therapy.+
- 1MRI shows a complex trans-sphincteric tract with an abscess. The colorectal team drains the collection and inserts a loose seton; subsequent clinical review and imaging assessment confirm adequate drainage with no residual abscess. His prior antibiotics, drainage and immunosuppressive treatment have not controlled fistulising disease, satisfying the conventional-treatment history for specialist infliximab consideration.
- 2The IBD team documents TB and HBV assessment, appropriate vaccination review, no active infection, no relevant hypersensitivity or moderate/severe heart failure, and reviewed baseline blood counts and organ function. It gives Remicade 5 mg/kg, calculated as 70 × 5 = 350 mg IV over two hours, at weeks 0, 2 and 6, with observation for at least one to two hours after each infusion.
- 3After the three induction doses, pain and discharge have clearly decreased, the patient has no new collection or infusion reaction and blood markers improve. Because he has responded, he receives 350 mg maintenance at week 14, eight weeks after the third dose, and then continues the planned eight-weekly schedule with ongoing review.
- 4At the subsequent MDT assessment, MRI shows no abscess but residual tract activity despite the improved external symptoms. Continence is unchanged and he has returned to work. The achieved endpoint is source control and clinical response, not confirmed deep healing; the team keeps drainage secure and individualises the timing of seton removal rather than declaring a closed fistula from the skin appearance.
02Sepsis controlNew painful swelling during established treatmentA patient with known perianal Crohn disease develops renewed pain or a blocked draining tract.+
- 1EscalationAssess observations, abdomen and perineum, involve colorectal surgery urgently and investigate the possibility of another abscess. Escalate systemic deterioration or necrotising features immediately. A visible opening or an existing seton does not ensure that every branch remains drained.
- 2Drain a collection and restore appropriate drainage, using MRI and EUA together where clinically feasible. Antibiotics may assist acute sepsis management, but an antibiotic course alone cannot replace necessary drainage or reliably close the fistula. Reassess the safety of the next biological dose in the presence of active infection.
- 3Once infection is controlled, review rectal inflammation, luminal treatment, tract anatomy and previous response in the MDT. Persistent disease may reflect residual sepsis, inadequate inflammatory control or a structurally unsuitable tract; these explanations lead to different decisions and should not all be labelled drug failure.
03Definitive strategyPersistent tract after inflammatory treatmentDefinitiveSymptoms or radiological fistula activity remain despite an adequately coordinated initial plan.+
- 1Reassess both the tract and rectal mucosa before attempting closure. A loose seton can maintain drainage while medical therapy acts, but the timing of removal is uncertain and depends on disease activity, complexity and the proposed next step. Do not adopt one fixed removal week as a rule for every fistula.
- 2DefinitiveConsider selected sphincter-preserving closure such as an advancement flap or LIFT when anatomy and tissue quality permit. Active proctitis, undrained infection or severe sphincter injury can make definitive surgery unsuitable. Avoid routine division of a complex tract, and discuss the possibility of failure and continence effects even with a preserving procedure.
- 3For inadequate response to anti-TNF therapy, the IBD MDT may consider another advanced treatment after reviewing the reason for failure. Evidence for individual fistula outcomes varies between agents; later-line drug selection should account for luminal disease and the patient’s infection and organ risks rather than assuming all biologics are interchangeable.
- 4In severe refractory disease with major quality-of-life loss, discuss diversion or proctectomy openly. A temporary stoma does not guarantee eventual reversal, and proctectomy can leave a difficult perineal wound. Decisions should include function, nutrition, psychological support and the person’s own priorities, with the expected endpoint explicitly recorded.
Key medicines and prescribing safety1 treatment · regimens, roles and cautions+
Infliximab — Remicade 100 mg powder for IV infusion
Adult fistulising active Crohn disease after the specified conventional-treatment failure or intolerance pathway: 5 mg/kg IV over two hours at weeks 0, 2 and 6, only after abscess drainage and infection exclusion. Observe at least one to two hours after each infusion. If there is no response after three doses, give no further infliximab; responders may continue 5 mg/kg every eight weeks. Specialist treatment and scheduled reassessment are required.Contraindicated with hypersensitivity to infliximab, murine proteins or relevant excipients, active TB or severe infection including abscess/sepsis, and NYHA III–IV heart failure. Screen TB/HBV, review vaccines and exclude active infection before starting. Renal/hepatic impairment has not been studied and no dose recommendation can be made; obtain specialist assessment, not an invented adjustment. Use caution in mild heart failure, demyelination, malignancy history and older adults; stop for serious infection, severe allergy or worsening heart failure. Interrupt an acute infusion reaction immediately with resuscitation capability available. Avoid concurrent live vaccines and other biologics such as anakinra or abatacept. Review thiopurine-combination lymphoma risk, especially in younger men. Consider contraception during treatment and for six months after the final dose; use in pregnancy only if clearly needed. Breastfeeding may be considered, but infant live-vaccine restrictions require a documented plan: after in-utero exposure the selected SmPC recommends 12 months after birth, with its specified specialist exceptions; during maternal treatment while breastfeeding, live vaccine is not recommended unless infant drug levels are undetectable.
08ComplicationsImportant consequences, why they occur and why they matter clinically.
Recurrent pelvic sepsis
Branching tracts and intermittent obstruction of drainage can produce repeated abscesses, systemic illness and additional tissue damage despite periods of less external discharge.
Continence impairment
Inflammatory injury, scarring and previous sphincter procedures can combine to impair continence. Functional loss can persist even after infection and luminal inflammation improve.
Chronic burden
Pain, odour, leakage and uncertainty can impair intimacy, employment and mental health. Severe refractory disease may leave lasting anatomical and functional consequences beyond bowel activity.
09Monitoring and follow-upTreatment response, safety checks and longer-term review.
- At each clinical review ask about pain, discharge, new swelling, seton function, bowel symptoms and continence, alongside the effects on sleep, work and sexual wellbeing.
- Before each infusion review infection symptoms and recent exposures, previous reactions and the need for safety blood tests; record the administered product and batch for traceability.
- Assess response after the three fistulising-disease induction doses. Continuing eight-weekly maintenance requires a demonstrated response, rather than merely reaching the planned date.
- Compare clinical response with objective assessment, including MRI and rectal or luminal evaluation as indicated; persistent internal activity can remain despite less external discharge.
- Monitor for cytopenia symptoms, neurological changes, skin lesions and liver injury. Under the Remicade SmPC, jaundice or ALT at least five times the upper limit of normal requires discontinuation and investigation.
- Reassess long-term need by 12 months under NICE TA187 and at least annually if continued; discuss the benefits and relapse risks of withdrawal in stable remission using objective disease assessment.
- Continue infection vigilance after treatment ends because infliximab elimination can take up to six months; ensure pregnancy and infant vaccination information reaches the relevant clinicians.
10Special situationsVariants, exceptions and circumstances that change the usual approach.
Separate three forms of success
Freedom from abscess, reduced external drainage and deep radiological healing describe different outcomes. A patient may achieve the first two while retaining MRI activity; recording all three makes the next surgical or medical decision intelligible.
Antibiotics have a bounded role
BSG found no established fistula response or remission benefit from ciprofloxacin or metronidazole alone, although antibiotics can help acute sepsis or accompany advanced therapy. Avoid describing a short antibiotic course as definitive inflammatory-fistula treatment.
Seton timing is individual
A seton is a drainage device and its ongoing need changes with inflammation and the closure strategy. Evidence does not justify one universal duration; removal should follow an agreed anatomical and inflammatory assessment, not convenience alone.
Do not recycle early stem-cell certainty
Evidence for local cell therapies has evolved, including an unsuccessful later confirmatory trial noted by BSG. Discuss such options through specialist current pathways instead of presenting earlier positive findings as a guaranteed available cure.
Intestinal and perianal evidence differ
A drug can control luminal Crohn disease without equally strong evidence for fistula closure. The source of each recommendation matters, including whether results concern a primary fistula outcome or a small subgroup from a luminal trial.
11Common pitfallsFrequent interpretation and management errors.
- 01
Starting infliximab while an abscess remains undrained because the external opening is already leaking.
- 02
Calling a fistula healed solely because the skin opening closes or discharge becomes less frequent.
- 03
Removing every seton at a fixed week without reviewing tract anatomy, rectal inflammation and sepsis control.
- 04
Using the luminal-Crohn two-dose nonresponse rule in place of the selected fistulising-Crohn three-dose assessment.
- 05
Applying a numerical infliximab trough target or escalated dose universally from observational perianal data.
- 06
Ignoring a new mass or changed chronic fistula because the patient already has a Crohn diagnosis.