Carbuncle

A cluster of interconnected furuncles forming a larger, deeper infected mass that drains through multiple openings. It is almost always caused by S. aureus, including MRSA.

• Typical sites are the nape of the neck, upper back, buttocks and thighs, where the skin is thick and fibrous septa limit drainage.
• It is much more common in diabetes, obesity, older age, malnutrition, alcohol excess, immunosuppression and corticosteroid use. Men are affected more than women.
• Systemic illness is more frequent than with a single furuncle, and bacteraemia with metastatic spread occurs in a significant minority.

• A tender, firm, red, swollen plaque 3 to 10 cm or more, which becomes fluctuant and develops several pustular openings discharging pus. The surrounding skin is indurated and may show necrosis.
• Fever, rigors, malaise and leucocytosis are common. Healing takes several weeks and leaves a scar.
• Danger signs: expanding necrosis, spinal tenderness or neurological signs (epidural abscess or vertebral osteomyelitis), new murmur (endocarditis), shock, or rapidly worsening pain (necrotising infection).
• Differential diagnosis: multiple furuncles, hidradenitis suppurativa, cutaneous anthrax, deep fungal infection, cellulitis with abscess, infected epidermoid cyst, actinomycosis and tuberculosis.
• Complications: bacteraemia, septic shock, endocarditis, osteomyelitis, septic arthritis, deep abscess, epidural abscess and disseminated infection.

• Clinical, supported by tests because of the higher risk of systemic spread:
  • Pus culture and sensitivities (identify MRSA).
  • Blood cultures, two sets, in febrile patients.
  • Full blood count, CRP, urea, creatinine, glucose, HbA1c and lactate.
  • HIV test and further immune evaluation if there is no obvious cause.
• Ultrasound defines the extent of the collection. MRI is used if spinal or deep extension is suspected.
• Echocardiography if bacteraemia persists or a murmur appears.

• Surgical drainage is essential. Use a cruciate or wide incision under local anaesthesia (plain lidocaine 1% without adrenaline, up to 4.5 mg/kg, maximum about 300 mg), break down all loculations, remove necrotic tissue, irrigate, and leave the wound open or loosely packed. Extensive disease needs formal debridement in theatre.
• Antibiotics are given in nearly all cases:
  • Mild to moderate, outpatient: flucloxacillin 500 mg to 1 g orally 6 hourly, cefalexin 500 mg orally 6 hourly, doxycycline 100 mg orally 12 hourly, co-trimoxazole 160/800 mg 1 to 2 tablets orally 12 hourly, or clindamycin 300 to 450 mg orally 8 hourly, for 7 to 14 days.
  • Systemic illness or extensive disease: flucloxacillin 2 g IV 6 hourly or cefazolin 2 g IV 8 hourly for MSSA.
  • MRSA or high risk: vancomycin 15 to 20 mg/kg IV 8 to 12 hourly (with a 25 to 30 mg/kg loading dose in severe illness) adjusted to levels, or linezolid 600 mg IV or orally 12 hourly.
  • Convert to oral therapy when afebrile and improving. Bacteraemia needs at least 14 days of therapy and echocardiography. Complicated bacteraemia or endocarditis needs 4 to 6 weeks.
• Control diabetes (target glucose 6 to 10 mmol/L during infection, using an insulin infusion or basal bolus insulin if needed), correct malnutrition and review immunosuppressive therapy.
• Paracetamol 1 g orally 6 hourly for pain, with opioids for severe pain, and daily wound dressing or packing changes until granulation. Review tetanus immunisation status.
• Decolonisation for recurrent disease: mupirocin 2% intranasal twice daily for 5 days plus chlorhexidine 4% body wash daily for 5 to 14 days, with household and linen hygiene.
• Special groups:
  • Pregnancy: use flucloxacillin, cefalexin or clindamycin, and avoid doxycycline.
  • Renal impairment: adjust vancomycin and co-trimoxazole doses.
  • Immunosuppressed and elderly: admit early, and use IV therapy.
• Admit for fever, a large or facial or spinal carbuncle, diabetes with poor control, immunosuppression, or failure of outpatient therapy.
• Failure to respond: re-image for undrained collections, culture again, escalate cover for MRSA, and exclude necrotising infection, osteomyelitis and endocarditis.
• Follow up: review at 48 to 72 hours, then weekly until healed. Refer to a surgeon for large wounds needing grafting or scar revision, and to endocrinology or immunology when the cause is not found.