Perianal Crohn disease

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Perianal Crohn disease arises from penetrating inflammation and sepsis in anatomically sensitive tissue. Simple low tracts without proctitis differ from branching high fistulae, horseshoe extensions, abscesses, multiple openings, stenosis or communication with vagina. Symptoms understate anatomy: drainage can reduce pain while a secondary collection persists, and a sealed external opening can trap infection. Inspection should be gentle and include scars, openings, induration, tags and active drainage; painful digital or proctoscopic examination should not delay anaesthesia and imaging.

Successful care aligns three tasks. The colorectal surgeon controls sepsis and preserves sphincter, the gastroenterology team treats luminal and fistula inflammation, and radiology defines anatomy and response. IBD nursing, stoma care, pain, sexual health, pelvic floor and psychological support address consequences often missed by a closure-only target. MRI radiological healing may lag behind cessation of external drainage, so treatment withdrawal based solely on a dry opening can lead to recurrent abscess. Diversion or proctectomy may be the most humane option for refractory destructive disease, but requires planned discussion of fertility, body image, function and wound healing.

Key points

Perianal Crohn disease includes abscess, fistula, ulceration, fissure, skin tags, stenosis and rectovaginal or anovaginal disease; it may precede recognised luminal inflammation.
Pain, fever or swelling is an abscess until assessed; urgent examination and drainage take priority over escalating biologic treatment or waiting for elective MRI.
Pelvic MRI and examination under anaesthesia are complementary: MRI maps tracts and occult collections, while EUA confirms openings and permits drainage or seton placement.
Assess rectal and upstream luminal Crohn activity because active proctitis worsens healing and changes the chance that local procedures will succeed.
A loose seton maintains drainage and reduces recurrent abscess while medical therapy controls inflammation; it is not necessarily the final closure strategy.
Infliximab has the strongest established evidence for complex fistulising disease after sepsis control, with alternatives chosen through current specialist guidance and prior response.
Antibiotics can treat cellulitis, infection and early drainage symptoms but rarely achieve durable complex fistula closure as sole long-term treatment.
Protect continence and quality of life: complex fistulotomy can divide sphincter and should not be performed without expert anatomical and functional assessment.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Transmural Crohn inflammation

Deep anorectal ulcers penetrate through tissue planes to create fistulae and abscesses, sometimes before luminal Crohn disease is recognised.

Active rectal disease

Proctitis sustains local inflammation and impairs wound healing, reducing the success of procedures directed only at the external tract.

Smoking and penetrating phenotype

Smoking and established penetrating bowel behaviour are associated with more difficult disease, while individual anatomy remains a major determinant.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Deep ulcer penetration
Transmural inflammation breaches the anal or rectal wall and tracks through sphincter, fat and adjacent pelvic structures.
2
Abscess and fistula formation
A sealed infected tract forms an abscess; spontaneous or surgical drainage may leave a persistent epithelialised fistula.
3
Fibrosis and impaired healing
Chronic inflammation, sepsis and repeated procedures produce scarring, stenosis and sphincter damage while malnutrition further limits repair.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Perianal abscess

Constant throbbing pain, tender swelling, fever, urinary difficulty or systemic upset suggests a collection; deep ischiorectal or supralevator sepsis may have little external change.

Draining fistula

Recurrent purulent or faeculent discharge from an external opening with cycles of swelling and relief suggests a tract, but drainage does not exclude undrained branches.

Complex anatomy

High trans-sphincteric or suprasphincteric course, multiple openings, horseshoe extension, associated abscess, stenosis or communication with vagina increases recurrence and continence risk.

Active proctitis

Bleeding, urgency, ulceration and inflamed rectal mucosa predict poorer fistula healing and usually make definitive local closure inappropriate until inflammation is controlled.

Anal stenosis

Painful difficult defaecation, narrow stool, obstructed evacuation or inability to examine may reflect inflammatory or fibrotic narrowing and requires expert assessment before dilation.

Psychosexual burden

Odour, leakage, pain, dressings and fear of incontinence can impair intimacy, work and mental health even when inflammatory indices appear acceptable.

Red flags requiring action

Perianal pain with fever, spreading cellulitis, urinary retention, immunosuppression or sepsis requires urgent colorectal assessment and drainage; antibiotics alone are insufficient source control.
Rapid tissue necrosis, crepitus, severe pain out of proportion or shock suggests necrotising infection and needs immediate emergency surgery and broad antimicrobial treatment.
New faecal vaginal discharge, pelvic sepsis or obstetric tract symptoms needs specialist colorectal and gynaecological assessment rather than repeated empirical antibiotics.
Do not commence anti-TNF or another potent immune therapy through a known undrained collection.
A new irregular ulcer, mass or changing chronic tract warrants biopsy consideration because longstanding fistula disease carries a small malignancy risk.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

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 MRIFirst step
Why
Map primary tract, sphincter relation, secondary branches, horseshoe extension and abscess.
Interpretation and limitations
Provide the radiologist with prior operation and seton details; persistent T2 or enhancement may indicate activity after external closure, while anatomy guides continence-preserving surgery.
02
Examination under anaesthesia
Why
Define openings, drain sepsis and place a loose seton with pain controlled.
Interpretation and limitations
Avoid aggressive probing that creates false tracts; combine findings with MRI because either test alone may miss extensions.
03
Ileocolonoscopy or flexible sigmoidoscopy
Why
Assess rectal inflammation and the wider luminal Crohn burden.
Interpretation and limitations
Active proctitis lowers the likelihood of local fistula closure and strengthens the need for medical control before definitive repair.
04
Blood count, CRP, renal and liver profile
Why
Assess systemic infection, inflammation, anaemia and treatment readiness.
Interpretation and limitations
Normal CRP does not exclude a local abscess; results support resuscitation, antibiotic safety and biologic screening rather than replacing examination.
05
Culture of drained pus
Why
Guide antibiotics in sepsis, recurrence or resistant-infection risk.
Interpretation and limitations
Obtain from a meaningful deep sample during drainage when possible; superficial swabs from a chronic opening may reflect colonisation.
06
Continence and pelvic-floor assessment
Why
Establish baseline function before sphincter-affecting or closure procedures.
Interpretation and limitations
History, obstetric injury, previous operations and selected physiology influence whether fistulotomy, advancement, diversion or non-cutting strategies are acceptable.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Cryptoglandular fistula

A single simple tract arising from an anal crypt without proctitis or luminal inflammation favours common cryptoglandular disease.

Hidradenitis suppurativa

Superficial bilateral nodules and sinuses in hair-bearing groin or buttock skin, without communication to the anal canal, support hidradenitis.

Infection or malignancy

Atypical ulcer, induration, systemic exposure or non-healing tissue requires targeted microbiology and biopsy to exclude specific infection or cancer.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01SEPSISDrain before immunosuppressionFirst stepPain, swelling, fever, systemic upset or imaging suggests a perianal abscess.

1Assess ABCDE, analgesia, antibiotics for systemic infection and urgent colorectal review, recognising that deep sepsis can lack a visible fluctuant lump.
2Proceed to timely examination under anaesthesia and incision or internal drainage; obtain MRI first only when stable and it will not postpone source control.
3DefinitivePlace a loose seton when a fistula is identified and ongoing drainage is needed, avoiding definitive sphincter division in an acutely inflamed uncertain field.
4After sepsis control, reassess luminal and rectal disease and plan anti-inflammatory therapy plus interval imaging or EUA.

02MAPDefine anatomy, inflammation and functionA fistula, recurrent abscess, rectovaginal symptoms, stenosis or persistent drainage is present after immediate danger is controlled.

1Obtain specialist pelvic MRI with previous procedure and seton information, and review images jointly with an experienced colorectal surgeon and radiologist.
2Assess rectum and upstream bowel for active Crohn disease, using endoscopy and cross-sectional imaging according to distribution.
3Document tract complexity, continence, obstetric and operative history, nutrition, smoking, sexual function and the outcomes most important to the patient.
4Agree a combined drainage, medical and closure plan at the perianal IBD MDT rather than commissioning isolated sequential procedures.

03CONTROLCombine seton and medical therapyComplex fistulising Crohn disease remains active after collections are drained.

1Optimise luminal and rectal inflammation and select infliximab or another appropriate advanced therapy after infection screening and current NICE review.
2Use a loose seton to maintain drainage during induction, with antibiotics as a short adjunct for sepsis or cellulitis rather than a substitute for disease control.
3Assess symptoms, openings, calprotectin or CRP and pelvic MRI response, remembering that skin closure can precede radiological healing.
4Time seton removal and any advancement flap, ligation, stem-cell or other closure intervention through the expert MDT after inflammation and sepsis are controlled.

04REFRACTORYPrioritise function when closure failsPersistent fistula, severe proctitis, stenosis, incontinence or repeated sepsis continues despite optimised combined care.

1Re-map anatomy and exclude missed collection, ongoing rectal disease, treatment underexposure, smoking, malnutrition and malignancy before another repair attempt.
2AlternativeDiscuss chronic seton drainage, alternative advanced therapy, selected local closure, faecal diversion or proctectomy with realistic probabilities and burdens.
3Provide stoma education, fertility and sexual-function counselling, psychological support and nutrition optimisation before irreversible surgery whenever urgency permits.
4Define success with the patient, which may be freedom from sepsis and pain rather than radiological closure at the cost of continence.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

Promotes fistula response and luminal control after abscess drainage and appropriate seton management.

Infliximab

Weight-based induction and maintenance through a specialist fistulising Crohn protocol.

Never substitute for source control; screen serious infection, tuberculosis and hepatitis, monitor immunogenicity and ensure a durable maintenance plus surgical review plan.

Treats bacterial infection and may reduce early fistula drainage while definitive surgical and inflammatory therapy begins.

Antibiotic adjunct

Short culture- and local-protocol-guided course for cellulitis, sepsis or postoperative drainage.

Antibiotics rarely close complex fistulae alone; repeated exposure causes resistance, C. difficile and class-specific toxicity, so drainage must not be delayed.

Relieves severe perianal pain and enables examination, hygiene, dressing and mobilisation.

Analgesia

Individualised multimodal regimen reviewed after drainage and during wound care.

Escalating opioids can mask recurrent sepsis and worsen bowel function; avoid NSAIDs where they may aggravate IBD and always treat the pain source.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Pelvic sepsis

Undrained collections can spread deeply, cause systemic sepsis and make immunosuppressive escalation dangerous until source control is achieved.

Complex fistulation and incontinence

Branching tracts may involve vagina, skin or bowel, while disease and repeated surgery threaten sphincter function and continence.

Stenosis, pain and nutritional decline

Fibrosis narrows the anus or rectum, and chronic discharge or painful defaecation restricts intake, activity, intimacy and quality of life.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Track pain, fever, drainage, swelling, dressings, continence, work and sexual impact rather than recording fistula open or closed alone.
Inspect seton position and surrounding skin, teaching when loss, cutting-in, blockage or increasing pain requires urgent colorectal contact.
Use calprotectin, CRP, endoscopy and luminal imaging to follow bowel inflammation separately from pelvic tract response.
Repeat pelvic MRI or EUA when it will change seton removal, closure or escalation; external dryness without deeper healing can be misleading.
Monitor advanced therapy for infection, exposure and class-specific toxicity, with a clear plan around operations and wound healing.
Review nutrition, smoking, anaemia, mental health, continence and reproductive goals through the multidisciplinary pathway.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Drainage changes the biology

A pressurised infected cavity cannot reliably heal under immune therapy. Source control converts an unsafe septic process into a condition that medical treatment can address.

A seton is purposeful

A loose seton prevents premature skin closure over deeper infection, buys time for inflammation control and protects sphincter compared with indiscriminate cutting.

The rectum predicts healing

Severe active proctitis bathes internal openings in inflammation and makes local closure unreliable; rectal assessment is therefore part of every fistula plan.

Dry is not always healed

External drainage may stop while MRI still shows an active tract, creating a risk of recurrent collection if therapy or seton is withdrawn prematurely.

Continence outranks neat anatomy

A technically simple tract division can cause lifelong harm when sphincter, obstetric or previous-operative reserve is limited. Baseline function belongs in surgical consent.

11Common pitfallsFrequent interpretation and management errors.

01
Ordering elective pelvic MRI while a septic painful abscess waits without urgent surgical assessment.
02
Starting biologic therapy before draining a known collection.
03
Treating chronic fistula discharge with repeated antibiotic courses but never mapping tracts or controlling luminal Crohn disease.
04
Removing a seton when the skin looks dry without assessing deeper activity and the maintenance plan.
05
Performing fistulotomy in complex or high disease without documenting sphincter anatomy and continence risk.
06
Measuring success only as closure and overlooking pain, leakage, intimacy, work and the patient's preference for reliable drainage.