Perianal abscess and fistula

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

Cryptoglandular infection begins when an anal gland obstructs, then spreads through anatomical spaces. The clinical priority is drainage before infection tracks further or causes systemic illness. Surface appearance underestimates some deep collections, particularly in diabetes or immunosuppression.

A fistula is a chronic epithelialised or granulation-lined communication between an internal anorectal opening and external skin. Its relation to internal and external sphincters predicts whether laying it open is safe. Cure and continence are competing outcomes in complex disease.

The central decisions are whether emergency source control is required and, after sepsis settles, whether the tract is simple enough for fistulotomy or requires imaging, seton drainage and a specialist sphincter-sparing strategy.

Key points

Most anal abscesses arise from infected anal glands, but Crohn disease, hidradenitis, trauma, malignancy, tuberculosis and sexually transmitted infection can produce similar or recurrent disease.
A superficial abscess causes constant throbbing pain, focal tenderness and swelling; an ischiorectal or supralevator collection may have few external signs despite severe deep pain.
Antibiotics alone do not replace drainage of a drainable anorectal abscess, even when they transiently reduce fever.
Persistent discharge, a recurrent abscess or an external opening after drainage suggests an anal fistula connecting epithelium to perianal skin.
Do not probe aggressively through an acute abscess cavity in an attempt to find a tract; creating a false passage can injure sphincter and complicate later repair.
MRI pelvis is the preferred map for recurrent, complex, horseshoe, supralevator or Crohn-associated fistula; simple disease may be defined at examination under anaesthesia.
Fistulotomy can cure a simple low tract, whereas sphincter-involving disease often needs a draining seton or another staged continence-preserving approach.
Baseline continence, obstetric injury, previous fistula surgery and Crohn activity must be documented before choosing any operation that divides sphincter.

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

Cryptoglandular infection

Obstruction and infection of an anal gland spreads from a crypt into intersphincteric, ischiorectal or supralevator spaces, causing most abscesses.

Crohn disease

Transmural anorectal inflammation creates ulcers, abscesses and complex fistulae, often with multiple openings or associated proctitis.

Other secondary disease

Hidradenitis, trauma, malignancy and selected infections should be considered in recurrent, atypical or non-healing perianal sepsis.

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

1
Gland obstruction and suppuration
Blocked gland drainage permits bacterial proliferation and pus formation within a potential space around the anal sphincters.
2
Anatomical spread
Pressure tracks infection along intersphincteric and fascial planes, so deep abscesses may cause severe pain with little external swelling.
3
Persistent epithelial tract
After drainage, the internal crypt opening may remain connected to skin by a chronic fistula that intermittently discharges or re-abscesses.

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

Superficial perianal abscessRed flag

Constant escalating anal pain, focal erythema, warmth, swelling and exquisite tenderness, sometimes with fever or spontaneous pus, usually permit a clinical diagnosis and urgent drainage referral.

Deep occult collectionRed flag

Severe rectal, pelvic or buttock pain with fever, urinary retention or systemic illness but little surface change suggests ischiorectal, intersphincteric or supralevator sepsis and needs urgent imaging or examination under anaesthesia.

Necrotising perineal infectionRed flag

Pain out of proportion, rapidly advancing discoloration, bullae, crepitus, shock or confusion indicates possible Fournier gangrene; normal-looking early skin does not exclude fascial spread.

Anal fistula

Intermittent purulent or blood-stained discharge from a small external opening, recurrent swelling in the same site and relief after drainage are characteristic. Palpable cord or granulation may be present.

Crohn-associated disease

Multiple or branching openings, broad ulcers, skin tags, proctitis, diarrhoea, weight loss or known Crohn disease suggest a complex inflammatory phenotype requiring joint colorectal and gastroenterology care.

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
ABCDE assessment and perineal inspectionFirst step
Why
Identify sepsis, necrosis and a drainable superficial focus rapidly.
Interpretation and limitations
Record observations, early-warning score, extent of erythema, fluctuance, openings and skin viability. Mark spreading cellulitis; do not delay theatre referral for a scoring system when necrotising infection is suspected.
02
Digital rectal examination when safe
Why
Detect tenderness, induration, proctitis or a deep mass.
Interpretation and limitations
Severe pain may make awake examination unhelpful; stop rather than traumatise the patient and arrange examination under anaesthesia. A normal superficial inspection does not exclude deep sepsis.
03
Blood count, CRP, renal profile, glucose and lactate
Why
Assess inflammation, diabetes, organ dysfunction and sepsis severity.
Interpretation and limitations
Normal inflammatory markers cannot exclude a local abscess, while raised lactate or acute kidney injury supports immediate resuscitation and critical-care review. Check blood cultures before antibiotics when feasible in systemic infection.
04
MRI pelvis
Why
Map fistula tracts, internal openings, secondary extensions and occult collections.
Interpretation and limitations
Use for complex, recurrent, Crohn-associated or deep disease and correlate with examination under anaesthesia. Imaging anatomy should guide, not replace, an experienced surgeon's operative assessment.
05
CT pelvis with contrast
Why
Identify deep pelvic sepsis when MRI is unavailable or urgent systemic illness dominates.
Interpretation and limitations
CT is useful for gas, large collections and supralevator extension but may define small fistula tracts less precisely. Urgent source control should not wait for ideal imaging in an unstable patient.
06
Examination under anaesthesia
Why
Drain sepsis and define anatomy without intolerable pain.
Interpretation and limitations
The surgeon can identify cavities and openings, place a seton or treat a clearly simple tract. Acute inflammation can obscure planes, so definitive fistula division may be deferred.
07
Ileocolonoscopy and inflammatory assessment
Why
Investigate suspected Crohn disease or proctitis underlying complex sepsis.
Interpretation and limitations
Endoscopic and histological findings influence biologic and surgical planning. Control active abscess before starting immunosuppression; faecal calprotectin alone cannot map perianal anatomy.

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

Anal fissure or thrombosed haemorrhoid

Defaecation-linked cutting pain suggests fissure, while a tender blue lump indicates thrombosis; neither usually causes deep fluctuance or systemic sepsis.

Hidradenitis or pilonidal disease

Multiple superficial sinuses in hair-bearing skin or a midline natal cleft pit lie outside the typical cryptoglandular fistula pathway.

Anal cancer or inflammatory ulcer

Induration, bleeding, atypical ulceration or non-healing tissue requires biopsy and assessment for malignancy, Crohn disease or specific infection.

Additional chapter-specific clues

Alternative suppurative disorder

Multiple recurrent nodules in groins or axillae suggest hidradenitis; pilonidal disease sits in the natal cleft; infected cyst, STI, tuberculosis and cancer require targeted assessment when the pattern is atypical.

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

01DrainageAcute anorectal abscessFirst stepFocal fluctuant swelling or deep clinical suspicion of a collection.

1Assess physiology, give analgesia, establish intravenous access when unwell and involve the emergency general or colorectal surgical team promptly; keep the patient nil by mouth if theatre is likely.
2Drain the abscess adequately under appropriate anaesthesia, opening loculations while protecting sphincter; send pus for culture in recurrent, unusual, immunocompromised or treatment-resistant infection.
3DefinitiveUse systemic antibiotics in sepsis, spreading cellulitis, immune compromise or another locally defined indication, but never regard them as definitive treatment for an undrained collection.
4Provide wound, hygiene and bowel advice plus explicit review for persistent discharge or recurrent swelling, which may reveal a fistula after the acute cavity heals.

02FistulaDefine before dividing sphincterPersistent external opening, recurrent abscess or chronic perianal discharge.

1Take a bowel, Crohn, obstetric and operative history and document baseline continence; inspect openings and obtain specialist colorectal examination without forceful office probing.
2Use MRI pelvis and examination under anaesthesia for recurrent, high, branching, horseshoe or Crohn disease; a straightforward low tract may be fully characterised at surgery.
3Lay open only a simple low fistula when the expected sphincter division and continence risk are acceptable; place a loose draining seton when ongoing drainage is needed through sphincter-involving disease.
4Discuss staged sphincter-sparing options such as advancement flap or LIFT in the specialist MDT, acknowledging variable healing, recurrence and the need for repeat procedures.

03Necrotising sepsisFournier gangrene responseDisproportionate perineal pain, rapidly spreading necrosis, crepitus or septic shock.

1Start immediate ABCDE resuscitation, blood cultures, broad-spectrum antimicrobial therapy including anaerobic cover and early critical-care support; correct glucose and organ dysfunction.
2Call senior colorectal or general surgery, urology and anaesthesia immediately and obtain CT only if it will define extent without delaying life-saving debridement.
3Undertake urgent radical debridement to viable tissue with planned re-look operations; manage urinary or faecal diversion only when the operative team judges it necessary.
4Continue physiological support, culture-directed antimicrobial review, nutrition, wound reconstruction planning and rehabilitation through a multidisciplinary service.

04Crohn diseaseCombined inflammatory and surgical controlComplex fistula, proctitis, multiple openings or established Crohn disease.

1EscalationDrain every abscess and establish durable sepsis control, commonly with a loose seton, before initiating or escalating immunosuppression.
2Stage luminal and perianal activity with gastroenterology, colorectal surgery and radiology; assess smoking, nutrition and infection screening relevant to biologic treatment.
3Select medical therapy under the current NICE and local IBD protocol while monitoring drainage and avoiding premature removal of a protective seton.
4Review healing clinically and radiologically when indicated; refractory destructive disease may require diversion or proctectomy after careful shared decision-making.

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

Treats systemic infection, spreading cellulitis, necrotising disease and selected high-risk abscess presentations.

Broad-spectrum antimicrobial therapy

Use the current local perineal or intra-abdominal sepsis regimen with organ adjustment.

Antimicrobials are adjuncts to drainage or debridement. Check allergy, renal and hepatic function, culture results and resistance patterns, then narrow duration through microbiology advice.

Controls severe pain before and after drainage without obstructing urgent assessment.

Non-constipating analgesia

Titrate regular simple analgesia with monitored opioid rescue only when necessary.

Avoid excessive opioid-related constipation and sedation; reassess escalating pain because it may indicate an undrained extension or necrotising infection rather than inadequate analgesia.

Treats active Crohn inflammation after adequate control of perianal sepsis.

Specialist Crohn biologic therapy

Agent, induction schedule and maintenance are selected by the IBD multidisciplinary protocol.

Never start over an undrained abscess. Complete infection and vaccination screening, monitor drug-specific toxicity and coordinate seton timing with colorectal surgery.

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

Sepsis and necrotising infection

Delayed drainage permits deep pelvic spread, bacteraemia or rapidly progressive perineal necrosis, especially in diabetes or immunosuppression.

Recurrent abscess and complex fistula

An untreated internal opening repeatedly seeds infection and can branch through sphincter or adjacent organs, making cure more difficult.

Continence injury

Aggressive fistula division or uncontrolled sepsis damages sphincter muscle, causing flatus or stool leakage and major quality-of-life harm.

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

After drainage, monitor temperature, pain trajectory, erythema, drainage, early-warning score and organ function; worsening findings suggest incomplete source control or another space involved.
Review wound healing and ask specifically about recurrent swelling, cyclical discharge and passage of gas or stool through an opening, which support fistula formation.
For a seton, check that it remains loose, draining and not cutting unintentionally; new pain or loss of drainage can mean recurrent obstruction.
Document continence before and after each definitive intervention using flatus, liquid and solid stool domains, with pelvic-floor referral for deterioration.
In Crohn disease, jointly follow luminal activity, perianal drainage, imaging when needed, infection risk, nutrition and adverse effects of immunomodulatory treatment.

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

Pain may be deeper than skin

An intersphincteric or supralevator abscess can cause profound pain and fever with almost no visible swelling. A superficial visual check is insufficient.

Drain first, map later

The acute goal is safe source control. Definitive tract surgery can be planned after inflammation settles, when anatomy and continence risk are clearer.

Goodsall is only a clue

The traditional rule predicting tract direction from the external opening is imperfect, especially for distant, anterior, recurrent or Crohn fistulae; MRI and operative findings take priority.

Closure can be dangerous

An external opening that seals while the internal source persists may permit another abscess. Reduced drainage with increasing pain is therefore a warning, not necessarily healing.

A seton is purposeful

A loose seton maintains drainage and controls sepsis while preserving sphincter. It may be an intermediate or long-term strategy rather than a failed operation.

Recurrence tests aetiology

Repeated abscesses should prompt review for a missed branch, Crohn disease, hidradenitis, immunodeficiency, tuberculosis or malignancy instead of repeated isolated drainage.

11Common pitfallsFrequent interpretation and management errors.

01
Treating a fluctuant anorectal abscess with antibiotics alone.
02
Reassuring from a normal skin appearance despite severe deep pain and fever.
03
Aggressively probing an acute cavity and creating a false tract.
04
Dividing sphincter without documenting baseline continence and tract anatomy.
05
Starting Crohn immunosuppression before an abscess has been drained.
06
Missing Fournier gangrene while waiting for ideal cross-sectional imaging.