General MedicineTier 1Disease (DEADMAN)

Cellulitis

Description

Acute spreading infection of the dermis and subcutaneous tissue, typically unilateral, non-purulent (distinguish from abscess which needs drainage)
Erysipelas - more superficial variant with sharply demarcated, raised border, usually streptococcal

Common presentation across all ages; risk factors - obesity, lymphoedema, chronic venous insufficiency, prior cellulitis, tinea pedis (portal of entry), diabetes, immunosuppression

Streptococcus pyogenes and Staphylococcus aureus - the overwhelming majority of non-purulent cellulitis
Entry via skin breach - tinea pedis, eczema, wounds, IV drug use sites, insect bites
Purulent/abscess-forming disease more likely S. aureus (consider MRSA in relevant risk groups)
Special contexts: periorbital/orbital cellulitis (sinus spread, vision/CNS threatening), diabetic foot infection (polymicrobial, deep structures)

Clinical - erythema, warmth, swelling, tenderness, +/- lymphangitis/lymphadenopathy, poorly demarcated borders (vs erysipelas)
Blood cultures not routinely needed for uncomplicated cellulitis unless systemically unwell/immunosuppressed
Mark the erythema border to monitor progression/response
Distinguish from mimics: DVT (unilateral leg swelling, less erythema/warmth typically), stasis dermatitis (bilateral, chronic), gout, necrotising fasciitis (pain out of proportion, systemic toxicity, crepitus/bullae - surgical emergency)

1. Uncomplicated cellulitis

Oral flucloxacillin or cephalexin (covers strep + staph) for mild-moderate disease
Elevate affected limb, treat portal of entry (tinea pedis)

2. Systemic features/failure of oral therapy/immunosuppression

IV flucloxacillin +/- benzylpenicillin; escalate/broaden if not improving or risk factors for resistant organisms

3. Special situations

Periorbital/orbital cellulitis: urgent CT, IV antibiotics, ophthalmology/ENT involvement - orbital cellulitis is a surgical emergency if abscess/vision threatened
Diabetic foot infection: broader empirical cover (polymicrobial), assess for osteomyelitis (probe-to-bone, imaging), vascular and podiatry involvement
Necrotising fasciitis (if suspected): urgent surgical debridement + broad-spectrum antibiotics - do not delay for imaging if high clinical suspicion

4. Recurrent cellulitis

Treat predisposing factors (lymphoedema, tinea, venous disease); consider prophylactic antibiotics for frequent recurrence

Lymphoedema, chronic venous insufficiency, tinea pedis - both predispose to and result from recurrent cellulitis
Necrotising fasciitis as an important differential - pain out of proportion, systemic toxicity, crepitus/bullae, surgical emergency
Bacteraemia/sepsis in immunosuppressed or systemically unwell patients
Bilateral leg erythema is very rarely cellulitis - consider stasis dermatitis/lipodermatosclerosis instead

Most resolves within 7-10 days of appropriate antibiotics; mark the erythema border to distinguish true progression from an initial inflammatory flare after starting treatment
Recurrence common if predisposing factors (lymphoedema, tinea pedis) untreated
Delayed recognition of necrotising fasciitis carries high mortality - low threshold for surgical review if pain is disproportionate to appearance

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