Educational draft · awaiting clinical reviewUse Rapid for revision, not patient-care decisions. Check current national and local guidance and the BNF or BNFC before acting.
2 min synopsisUK scopeSources checked 8 Sept 2026Clinical review pending
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Deep infection and possible Fournier gangrene
Rapid progression, systemic illness, disproportionate pain, dusky skin, crepitus or thin foul discharge can indicate necrotising infection; early skin changes may be subtle.
Action: Call senior surgery and critical care, begin sepsis treatment with immediate appropriate broad-spectrum antibiotics, and arrange emergency debridement without delaying for reassuring laboratory results or routine MRI.
Synopsis
Recognise superficial and occult anorectal infection, prioritise safe drainage, select adjunctive antibiotics when justified and verify recovery while watching for a persistent fistula.
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.
Key red flags
Severe anal pain with little visible swelling can be caused by an intersphincteric or supralevator abscess.
Diabetes, immunosuppression, renal or liver disease and systemic illness lower the threshold for urgent senior assessment and expedited drainage.
Persistent fever, worsening pain or continuing cellulitis after drainage suggests residual infection or another complication and requires reassessment.
Investigation priorities
01
Clinical assessment and examination under anaesthesiaFirst step
Confirm the collection and obtain an operative assessment when bedside examination is inadequate.
Management branches
Worked caseVerify uncomplicated source control
A 33-year-old man has three days of worsening focal anal pain and a fluctuant superficial swelling.
He 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.
At 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.
Key medicines
Co-amoxiclav Sandoz 500 mg/125 mg oral tabletFor 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.
National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 8 Sept 2026; clinical approval remains outstanding.
ASCRS anorectal abscess and fistula guideline 2022Perianal abscess: Perianal abscess scope: 2022 guidance; relevant diagnosis, drainage and sphincter-preserving treatment sections read 8 September 2026.
NHS Lothian peri-anal abscess antimicrobial guidancePerianal abscess: Perianal abscess scope: Local version 2.0 reviewed 3 June 2024, review due 1 June 2027; actual indication, duration and oral-switch restrictions read 8 September 2026.