Key Facts

Cellulitis is one of the most common reasons for acute medical admission in the UK. Accurate diagnosis is important as many conditions mimic cellulitis ('pseudocellulitis'), leading to unnecessary antibiotic use.

Epidemiology

• Very common: ~20–50 per 1,000 person-years
• Accounts for ~2% of emergency hospital admissions in UK
• Lower limbs: ~70% of cases
• Face (periorbital): important in children
• Recurrence: ~30% within 3 years

Aetiology

• Beta-haemolytic streptococci (particularly Group A — S. pyogenes): most common cause
• S. aureus: second most common; predominates in abscesses, MRSA
• Portal of entry: tinea pedis (most common modifiable risk factor), leg ulcers, eczema, insect bites, trauma, surgical wounds
• Risk factors: lymphoedema (strongest risk factor for recurrence), obesity, venous insufficiency, diabetes, previous cellulitis, immunosuppression

Pathophysiology

• Bacteria enter through breached skin barrier → infection of dermis and subcutaneous tissue
• Bacterial toxins and enzymes (streptolysins, hyaluronidase) → tissue destruction and spread
• Inflammatory response → erythema, oedema, warmth, pain
• Lymphatic damage from recurrent cellulitis → lymphoedema → predisposes to further cellulitis (vicious cycle)

Typical Presentation

• Acute onset erythema, warmth, swelling, tenderness
• Poorly demarcated, spreading margins
• Usually unilateral (lower limb)
• May have associated fever, rigors, malaise
• Regional lymphadenopathy and lymphangitis (red streaking)

Erysipelas

• Variant with more superficial involvement (upper dermis)
• Well-demarcated, raised, erythematous border ('step' edge)
• More common on face (butterfly distribution)
• Often higher fever and more systemic upset than cellulitis

Periorbital vs Orbital Cellulitis

• Periorbital (preseptal): swelling of eyelid, no visual compromise, no proptosis
• Orbital (postseptal): proptosis, ophthalmoplegia, pain on eye movement, visual loss — EMERGENCY

Red Flags — Necrotising Fasciitis

• Pain DISPROPORTIONATE to clinical signs — key early feature
• Rapidly spreading erythema/necrosis
• Crepitus on palpation
• Systemic toxicity, shock
• Haemorrhagic blisters, dusky skin
• Failure to respond to antibiotics

Bedside

• Mark the edge of erythema with skin marker and date/time — monitor progression
• Observations: temperature, HR, BP, RR, SpO2 — NEWS2 score
• Assess for portal of entry: examine feet for tinea pedis, interdigital maceration

Bloods

• FBC: leucocytosis
• CRP: elevated; useful for monitoring response
• U&Es: renal function (especially if IV antibiotics)
• Blood cultures: if systemically unwell (Class III–IV) — yield low (~5%)
• Lactate: if sepsis suspected
• HbA1c/glucose: screen for diabetes

Imaging

• Not routinely required for uncomplicated cellulitis
• USS Doppler: if DVT suspected
• CT/MRI: if necrotising fasciitis, abscess, or deep tissue infection suspected
• X-ray: if crepitus (gas gangrene) or foreign body

Special Tests

• Wound swab: from portal of entry or draining discharge — guide antibiotic choice
• LRINEC score: laboratory risk indicator for necrotising fasciitis (CRP, WBC, Hb, Na, creatinine, glucose) — score ≥6 suggests necrotising fasciitis

Non-Pharmacological

• Leg elevation: above heart level — reduces oedema and pain
• Mark erythema borders: re-assess at 48 hours
• Treat portal of entry: tinea pedis (topical antifungal), eczema (emollients)
• Compression: after acute phase in patients with lymphoedema/chronic venous insufficiency

Pharmacological

Eron Class I (mild, no systemic upset):

• Flucloxacillin 500 mg QDS for 5–7 days: first-line oral
• Clarithromycin 500 mg BD: if penicillin-allergic
• Increase to 1 g QDS flucloxacillin if poor response at 48 hours

Eron Class II–III (moderate-severe/systemic upset):

• IV flucloxacillin 1–2 g QDS: plus IV benzylpenicillin 1.2 g QDS (if streptococcal suspected)
• IV co-amoxiclav 1.2 g TDS: alternative
• IV vancomycin: if MRSA suspected (dose per protocol, trough monitoring)
• Step down to oral when improving (typically 48–72 hours)
• Consider OPAT (outpatient parenteral antibiotic therapy) for IV treatment at home

Eron Class IV (necrotising fasciitis/sepsis):

• Urgent surgical exploration and debridement — DO NOT delay for imaging
• Broad-spectrum IV antibiotics: piperacillin-tazobactam 4.5 g TDS + IV clindamycin 600 mg QDS (toxin inhibition)
• ICU admission for septic shock

Prevention of recurrence:

• Phenoxymethylpenicillin 250 mg BD long-term (PATCH trial — BMJ 2017): prophylaxis for patients with ≥2 episodes in 12 months; reduces recurrence by ~45% while taking antibiotic
• Treat chronic oedema/lymphoedema (compression)
• Treat tinea pedis — key modifiable risk factor

Referral Criteria

• Hospital admission: Eron Class III–IV, failure of oral antibiotics, suspected necrotising fasciitis
• Surgical: abscess drainage, necrotising fasciitis
• Dermatology: if diagnostic uncertainty or recurrent cellulitis
• Lymphoedema service: chronic oedema management

• Uncomplicated cellulitis: excellent prognosis; responds to antibiotics within 48–72 hours
• Recurrence: ~30% within 3 years; reduced by prophylactic penicillin (PATCH trial)
• Necrotising fasciitis: mortality ~20–30% even with treatment; delays in diagnosis worsen outcomes
• Complications: abscess formation (5–10%), bacteraemia, sepsis, lymphoedema (from lymphatic damage)
• Orbital cellulitis: risk of visual loss, cavernous sinus thrombosis — requires urgent ENT/ophthalmology input

Eron Classification

PATCH Trial (BMJ 2017)

Cellulitis Mimics ('Pseudocellulitis')