Skin Abscess

Skin abscess is a localised collection of pus within the dermis or subcutaneous tissue, most often caused by Staphylococcus aureus (including community acquired MRSA). It is among the commonest reasons for emergency surgical drainage.

• Sites: buttocks, axillae, groin, limbs, face, scalp.
• Risk factors: diabetes, injection drug use, HIV, obesity, skin disease (eczema, scabies), close contact in crowded settings, immunosuppression, previous abscess.

Painful, tender, warm, red, fluctuant swelling with or without a central pustule, with surrounding cellulitis, lymphadenopathy and sometimes fever.

• Differential diagnosis: cellulitis without abscess, epidermoid or sebaceous cyst (inflamed), hidradenitis suppurativa, boil, carbuncle, pilonidal abscess, lymphadenitis, necrotising fasciitis, pyomyositis, tumour, aneurysm, tuberculous cold abscess, cutaneous anthrax, mycetoma.
• Complications: spreading cellulitis, bacteraemia and sepsis, endocarditis, osteomyelitis, septic arthritis, necrotising fasciitis, recurrence, scarring.

Clinical; ultrasound when fluctuance is uncertain.

• Point of care ultrasound shows a hypoechoic collection with posterior enhancement.
• Pus for Gram stain and culture with sensitivity (recurrent, MRSA risk, immunosuppression, failure of treatment).
• Full blood count, CRP, glucose and HIV test in extensive, recurrent or systemic disease; blood cultures if febrile.

Incision and drainage is the essential treatment.

• Supportive: warm compresses before drainage, analgesia (paracetamol 1 g, ibuprofen 400 mg), elevation, hygiene, covering the wound, hand washing, not sharing towels, glycaemic control.
• Pharmacological: lidocaine 1 percent (maximum 4.5 mg/kg, 7 mg/kg with adrenaline) infiltration around the abscess; antibiotics are added only if: spreading cellulitis over 5 cm, systemic signs, immunosuppression, diabetes, extremes of age, abscess on the face or hand, multiple abscesses, or poor drainage. Give cotrimoxazole 960 mg twice daily or cefalexin 500 mg four times daily or flucloxacillin 500 mg four times daily for 5 to 7 days (clindamycin 300 to 450 mg three times daily if allergic or MRSA; doxycycline 100 mg twice daily). Severe: vancomycin 15 to 20 mg/kg IV every 12 hours or ceftriaxone 2 g IV daily with metronidazole for perineal abscess. Recurrent abscess: decolonisation with chlorhexidine 4 percent wash daily for 5 to 7 days and mupirocin 2 percent ointment to nostrils twice daily for 5 days, treating household contacts.
• Procedures: incision over the point of maximal fluctuance along skin lines, breaking loculations with a probe or forceps, irrigation, packing loosely (wick removal at 24 to 48 hours) or loop drainage; needle aspiration in selected children; operating theatre drainage for large, deep or perianal abscess.
• Special groups: pregnancy, flucloxacillin or cefalexin, avoid cotrimoxazole near term and doxycycline; children, weight based doses and sedation as needed; diabetes, admit when extensive; HIV, culture and treat for MRSA.
• Red flags: fever, rigors, hypotension, crepitus, severe pain out of proportion, rapidly spreading erythema, face (danger triangle), perianal abscess, sepsis.
• Follow-up: review at 24 to 48 hours for packing removal; reassess at 1 week; investigate recurrent abscess (glucose, HIV, carriage, hidradenitis).