Cellulitis
Spreading bacterial infection of the dermis and subcutaneous tissue that follows a breach in the skin barrier and tracks diffusely through tissue planes rather than forming a discrete collection.
In a nutshell
Cellulitis follows a breach in the skin barrier that lets skin flora, chiefly Streptococcus pyogenes and Staphylococcus aureus, invade the dermis and spread diffusely through tissue planes rather than forming a collection. Chronic oedema impairs lymphatic clearance and predisposes to recurrence.
Classic presentation
Unilateral spreading erythema, warmth and swelling of the lower limb with an ill-defined margin, with or without fever, often following a minor skin breach.
Key points
- Cellulitis is a diffuse spreading infection of the dermis and subcutis, not a discrete collection: that distinguishes it from an abscess.
- Empirical antibiotics must cover both Streptococcus pyogenes and Staphylococcus aureus, as either can cause the clinical picture.
- Chronic venous or lymphatic oedema impairs bacterial clearance and predisposes to recurrent episodes.
- Marking the leading edge of erythema allows objective monitoring of response to treatment.
- Disproportionate pain, rapid spread, bullae or crepitus should raise suspicion of necrotising fasciitis, a surgical emergency.
First-line investigation
Clinical assessment and severity classification; blood cultures and bloods are reserved for systemically unwell or admitted patients.
First-line management
Oral antibiotics covering Streptococcus and Staphylococcus for mild-to-moderate disease, with admission for IV antibiotics if systemically unwell or rapidly progressive.
Exam traps
- Pain out of proportion to the visible signs is necrotising fasciitis until proven otherwise: it is a surgical, not antibiotic, emergency.
- Cellulitis is usually unilateral; bilateral leg erythema is more often venous stasis or another cause.
- A high white cell count and CRP alone do not distinguish cellulitis from fasciitis: clinical assessment of pain and rate of spread is key.
Educational content pending clinical review. Not medical advice.