01OverviewDefinition, clinical context and the essential points that orientate the chapter.
A perianal abscess is a collection of pus near the anal canal. Cryptoglandular infection is a common mechanism, particularly for deeper collections: obstruction and infection of an anal gland spreads into a neighbouring tissue plane. A superficial collection is accessible below the skin, whereas intersphincteric, ischioanal and supralevator disease follows the spaces around the sphincters and pelvic floor. The anatomical label matters because an incision made through the wrong route can create a difficult fistula or fail to drain the true cavity.
Pain is often constant and progressive, aggravated by sitting or defaecation. A superficial abscess may be red, hot and fluctuant, but a deep abscess can present with anal, buttock or pelvic pain and little visible swelling. Fever is not required for diagnosis. Immunosuppression can blunt the inflammatory response; neutropenic infection needs urgent senior and haematology-informed assessment rather than reassurance because fluctuance or leukocytosis is absent. A patient repeatedly treated for a fissure without a visible tear deserves reconsideration.
Source control is the essential treatment of a drainable abscess. Antibiotics may treat surrounding cellulitis or bloodstream infection but cannot be assumed to empty a closed cavity. The first decision is therefore urgency and drainage planning, not which oral antibiotic to try for several days. Imaging adds value when the collection is occult, recurrent, anatomically complex or associated with Crohn disease. It should answer a genuine unresolved anatomical question and should not become an administrative prerequisite for drainage of an obvious septic focus.
An abscess can heal completely after drainage or leave a persistent fistula connecting the anal canal to the skin. The probability varies with underlying disease and study population, so counselling should not promise either inevitable fistula formation or guaranteed cure. A recurrent swelling in the same location, intermittent purulent drainage and a nonhealing opening suggest a tract. Recovery assessment should therefore cover immediate infection control, wound healing, continence and a clear route back to colorectal care if the course is not straightforward.
Key points
- A tender fluctuant perianal swelling is usually a clinical diagnosis; a deep collection may cause severe pain without an obvious external lump.
- Drain a confirmed acute abscess promptly; routine MRI must not postpone source control in a clear, especially systemically unwell, presentation.
- Fournier warning signs require immediate broad-spectrum antibiotics, critical-care involvement and surgical debridement; skin necrosis may be absent early.
- Use adjunctive antibiotics for significant cellulitis, systemic infection or relevant immunocompromise, rather than for every uncomplicated drained cavity.
- The UK ACPGBI approach to a first abscess is drainage without searching for or treating an internal fistula opening; recurrent disease needs a different assessment.
- Assess wound recovery and recurrence, and refer persistent discharge or repeated abscess for fistula and selected Crohn disease investigation.
02AetiologyUnderlying causes, associations and risk factors, with why each one matters.
Cryptoglandular infection
An infected anal gland may produce an intersphincteric collection that spreads into adjacent tissue planes; a superficial abscess can have a more local origin.
Secondary inflammatory or immune factors
Crohn disease, impaired immunity and recurrent fistula-associated infection change both the possible source and the probability of a complicated course.
03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.
- 1A closed infected compartment
Pus under pressure produces pain and local tissue injury. A collection can remain clinically important despite systemic antibiotics because drainage has not occurred.
- 2Extension through tissue planes
Infection can track into ischioanal, horseshoe or supralevator spaces; understanding these connections helps avoid missed pockets and inappropriate drainage routes.
- 3Persistence as a tract
A communication between the anal canal and skin can remain after infection drains, allowing intermittent discharge and later recollection of pus.
04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.
Examine the perianal skin, buttocks and perineum with consent, assessing redness, fluctuance, tenderness, scars and discharge. Look for natal-cleft pits or disease elsewhere in skin folds that suggests pilonidal disease or hidradenitis. A swelling near the anus is not automatically cryptoglandular.
Deep tenderness, urinary difficulty or pelvic pain with systemic findings can justify examination under anaesthesia and selected imaging. A tolerable digital examination may help localise tenderness, but forcing a severely painful examination is unlikely to produce a reliable map.
Record diabetes, steroid or immune therapy, neutropenia, renal failure and previous abscesses. Check observations and perfusion rather than relying only on the temperature. Establish whether Crohn symptoms, previous fistulas or a recent intervention changes the expected anatomy.
Pain beyond the visible inflammation, rapid extension, systemic toxicity, dusky tissue, crepitus or dishwater-like discharge requires emergency escalation. The absence of gas or skin necrosis does not safely exclude early Fournier gangrene, especially in a person with impaired immunity or microcirculation.
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
Clinical assessment and examination under anaesthesiaFirst step - Why
- Confirm the collection and obtain an operative assessment when bedside examination is inadequate.
- Interpretation and limitations
- A clear superficial abscess can be managed without routine imaging. EUA permits safe examination and drainage, while the surgeon documents the space, extent and any unresolved concern. If EUA finds no cause for persistent severe symptoms, MRI can help identify occult disease.
- 02
Full blood count renal profile and sepsis-directed tests - Why
- Measure organ dysfunction and factors that change resuscitation and antimicrobial use.
- Interpretation and limitations
- Use the clinical picture to select inflammatory markers, lactate, glucose and blood cultures. Check renal function before renally cleared antibiotics. Normal white cells or a modest inflammatory marker cannot cancel a concerning examination, particularly with immunosuppression.
- 03
MRI pelvis or selected urgent cross-sectional imaging - Why
- Map an occult, recurrent or complex collection and its relationship to the sphincters and levators.
- Interpretation and limitations
- MRI is particularly useful for fistula extensions and Crohn-associated disease; urgent CT may help broader pelvic assessment when appropriate. Do not postpone repeat surgical assessment in a deteriorating patient while waiting for an elective-quality fistula map.
- 04
Operative pus and selected underlying-disease tests - Why
- Direct antimicrobial refinement and investigate a nonstandard or recurrent course.
- Interpretation and limitations
- Routine cultures have limited value in a simple uncomplicated abscess, while Lothian’s local protocol requests pus during drainage. Obtain useful material when antibiotics, recurrence, immune compromise or unusual infection makes the result consequential. Crohn assessment can include faecal calprotectin, luminal investigation and selected biopsy when risk factors exist.
06Differential diagnosisRealistic alternatives and the features that help distinguish them.
Other painful anorectal lesions
A thrombosed external haemorrhoid, fissure or ulcer can cause marked pain but requires a different intervention from an infected cavity.
Pilonidal hidradenitis and Crohn disease
Natal-cleft pits, recurrent disease in other folds or complex inflammatory tracts suggest an alternative mechanism that should be investigated after urgent infection control.
Anorectal neoplasia
An indurated ulcer or persistent atypical mass can mimic local infection; the changing examination and a nonhealing course are relevant diagnostic clues.
07ManagementImmediate care, first-line treatment, alternatives and escalation.
01Worked caseVerify uncomplicated source controlFirst stepA 33-year-old man has three days of worsening focal anal pain and a fluctuant superficial swelling.+
- 1He is afebrile and haemodynamically stable, with no spreading cellulitis, diabetes, immune suppression, prior abscess or bowel symptoms. Senior examination confirms a first superficial perianal abscess. There are no features of necrotising infection. He is prepared for prompt drainage without a routine preoperative MRI.
- 2At EUA the surgeon opens the cavity through an appropriately positioned incision, breaks down loculations and confirms free drainage while protecting sphincter muscle. No internal opening is sought and no prophylactic fistulotomy or seton is performed. As this is uncomplicated infection with adequate drainage, he receives no therapeutic antibiotic course. Temporary haemostatic material, if needed, is removed and an external dressing used.
- 3Pain falls substantially over the next two days; he is eating, passing urine and opening his bowels. He is given wound-cleansing advice, an agreed analgesic plan and instructions to return for fever, increasing pain, swelling or difficulty passing urine. The wound is allowed to heal from the cavity outward rather than being judged by early surface closure alone.
- 4At six-week review the wound is epithelialised, there is no tenderness, discharge or recurrent swelling, and continence is unchanged. He understands that later recurrent drainage requires colorectal reassessment even after this satisfactory early endpoint. The documented outcome is a healed cavity without current evidence of a fistula, not a lifetime guarantee.
02Adjunctive infection treatmentTreat cellulitis after drainageA patient has a drained abscess but requires additional antimicrobial treatment for surrounding infection.+
- 1For a clinically clear abscess with cellulitis or systemic illness, arrange prompt drainage alongside sepsis assessment and indicated antimicrobial treatment; do not delay source control for routine MRI. Review severity, culture results, allergies, renal function and immune status. Confirm adequate drainage before considering an oral switch; a mild oral prescription is not a substitute for resuscitation or an undrained focus.
- 2AlternativeFor an eligible adult already improving sufficiently for oral switch, the current NHS Lothian local pathway lists co-amoxiclav 500/125 mg orally every eight hours as an alternative when age is sixty five or below and C. difficile risk is low. If cellulitis is present, the total IV-plus-oral course is five days; count treatment already received and use the selected product’s renal adjustment.
- 3Review clinical progress and cultures, and stop or refine treatment as indicated. Lothian advises stopping adjunctive treatment twenty four hours after drainage when the longer cellulitis course is not required. These are attributed local rules, not a universal antibiotic duration for all immunocompromised patients, deep infections or Fournier gangrene.
03Persistent or recurrent diseaseFind a residual cavity or tractPain, discharge or recurrent swelling continues after the initial drainage episode.+
- 1Reassess promptly for incomplete drainage, loculations, an unrecognised horseshoe extension or progressive infection. A systemically unwell patient may need urgent repeat EUA; a clinically stable person may benefit from MRI to map the unresolved space.
- 2Recurrent abscess in the same location raises the probability of a fistula. The ACPGBI recurrent-abscess approach generally favours a draining seton when a tract is identified unless clearly superficial; MRI is recommended when no fistula is found. Do not force a probe through inflamed tissue and create an artificial connection.
- 3DefinitiveAfter immediate sepsis is controlled, document tract anatomy and continence and assess for Crohn disease when age, recurrence, complexity, family history or luminal symptoms support it. Definitive fistula surgery is selected with the patient from the mapped anatomy, rather than added blindly during the next acute drainage.
Key medicines and prescribing safety1 treatment · regimens, roles and cautions+
Co-amoxiclav Sandoz 500 mg/125 mg oral tablet
For an eligible improving adult aged sixty five or below with low C. difficile risk, NHS Lothian lists one 500/125 mg tablet orally every eight hours with a meal as an alternative oral switch. For accompanying cellulitis use five days total IV plus oral treatment, counting prior doses. This is adjunctive to drainage. With CrCl 10–30 mL/min use the selected adult 500/125 mg dose twice daily; below 10 mL/min use once daily.The standard tablet regimen applies to adults at least 40 kg with CrCl above 30 mL/min; dialysis needs its separate product schedule. Contraindicated with hypersensitivity to amoxicillin, clavulanic acid or any selected-product excipient, any penicillin, severe immediate allergy to another beta-lactam, or previous co-amoxiclav-related jaundice/hepatic impairment. Use hepatic caution with monitoring. Review methotrexate, warfarin/INR, probenecid, allopurinol and mycophenolate. Stop and assess allergy, severe skin reaction, jaundice or significant antibiotic-associated diarrhoea. Pregnancy use requires an essential-benefit decision and breastfeeding a benefit-risk assessment. Do not infer that this oral switch treats shock or Fournier gangrene, and do not add metronidazole routinely to duplicate anaerobic cover.
08ComplicationsImportant consequences, why they occur and why they matter clinically.
Systemic and necrotising infection
Sepsis and Fournier gangrene can develop, especially with immune or microvascular risk; early recognition and emergency source control are decisive.
Fistula and treatment-related injury
A persistent tract may follow abscess drainage, while an inappropriate incision or unnecessary sphincter division can add continence dysfunction to the original disease.
Recurrent cavity formation
Residual loculations, premature skin closure or an underlying tract can permit another collection, with return of pain, swelling and purulent discharge.
09Monitoring and follow-upTreatment response, safety checks and longer-term review.
- After drainage follow pain, temperature, circulation, urinary function and the local inflammatory margin; deterioration requires source reassessment.
- Review the wound for progressive healing from its base and distinguish expected drainage from persistent purulence or a recurrent tender swelling.
- When antibiotics are used, check culture results, actual total duration, renal dosing, clinical response and adverse effects rather than automatically extending a course.
- Ask about ongoing discharge and continence at follow-up, and arrange colorectal or IBD evaluation for a nonhealing or recurrent course.
10Special situationsVariants, exceptions and circumstances that change the usual approach.
Drainage follows anatomy
Experienced surgical planning should establish the origin of a supralevator collection: upward extension from the intersphincteric space generally requires internal drainage, whereas upward extension from an ischioanal or ischiorectal abscess generally requires external drainage. Choosing the route by origin helps avoid creating a complex fistula; do not cross uninvolved sphincter or levator planes merely to reach a radiological pocket.
Distinguish guideline positions
ASCRS allows concomitant fistulotomy in selected simple fistulas; ACPGBI recommends against seeking or treating an opening during primary abscess drainage. The first-abscess worked case follows the UK approach.
Packing has a limited role
Routine continued packing adds pain and is not required as a universal healing strategy; initial haemostatic packing is a different, temporary decision.
A negative early review is bounded
A healed wound at six weeks is a meaningful observed result, but later fistula presentation remains possible and should be covered by return advice.
11Common pitfallsFrequent interpretation and management errors.
- 01
A normal-looking perianal surface does not exclude a deep abscess when severe anal or pelvic pain persists.
- 02
Requiring MRI before draining an obvious septic collection can postpone the intervention that controls the infection.
- 03
Searching aggressively for a fistula opening during a first abscess can create a false tract and expose a patient to unnecessary continence risk.
- 04
A routine prolonged antibiotic course neither compensates for incomplete drainage nor reliably prevents every future fistula.