Cellulitis

Overview

•Cellulitis is a clinical diagnosis - deeper dermis and subcutaneous tissue; ill-defined, spreading erythema, warmth, swelling, tenderness

•Causative organisms: Staphylococcus aureus or Streptococcus pyogenes (group A Streptococcus)

•Lower limb is the most commonly affected site; portal of entry identifiable in ~70% (tinea pedis is a classic underappreciated entry point)

Investigations

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For a systemically well patient managed in the community (Class I), NO investigations are required. This is a common exam trap.

•No investigations - Class I cellulitis, systemically well, managed in community

•FBC, CRP, blood cultures - only when admitting (Class II-IV) or if sepsis suspected; blood cultures positive in <5% of uncomplicated cases

•Skin swab - only if open wound, ulceration, or concern for MRSA (diabetes, immunocompromise, recent healthcare contact)

•Ultrasound - only if necrotising fasciitis, deep abscess, or DVT suspected; not routine

•X-ray - only if osteomyelitis suspected (fever, bony point tenderness, difficulty weight bearing/mobilising)

•Contrast CT orbits/sinuses ± brain - if orbital cellulitis suspected in a child, to define extent and identify subperiosteal abscess

Differential diagnosis

Key differentials vs cellulitis

Condition	Distinguishing features
Erysipelas	Superficial dermis; sharply raised, well-demarcated bright red border; same organisms as cellulitis
Necrotising fasciitis	Pain out of proportion to appearance, grey/dusky skin, systemic shock; surgical emergency
DVT	Calf pain/swelling, minimal erythema, no fever; Wells score + D-dimer/duplex USS
Osteomyelitis	Fever, bony point tenderness, difficulty weight bearing; MRI gold standard
Acute gout	Sudden monoarthritis, exquisitely tender on passive movement, often 1st MTP joint

Management

•Mark the erythema border with a skin pen at presentation - essential monitoring to assess progression or improvement

•Elevate the affected limb above heart level; prescribe adequate analgesia

•First-line (community, Class I): oral flucloxacillin

•Penicillin allergy: oral clarithromycin

•Treat portal of entry - e.g. tinea pedis with topical antifungal to reduce recurrence

Prevention

•Prophylaxis: phenoxymethylpenicillin (penicillin V) - consider for patients with ≥2 episodes per year

•Treat tinea pedis promptly; moisturise dry/cracked skin in lymphoedema or venous insufficiency; good glycaemic control in diabetes

Periorbital vs orbital cellulitis

Periorbital (preseptal) vs orbital (postseptal) cellulitis

Feature	Periorbital (preseptal)	Orbital (postseptal)
Anatomical location	Anterior to orbital septum	Posterior to orbital septum
Proptosis	Absent	Present
Eye movement	Full, painless	Restricted/painful (ophthalmoplegia)
Visual acuity	Normal	Reduced
Systemic upset	Mild or absent	Fever, unwell

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Any ONE of: proptosis, ophthalmoplegia, reduced visual acuity, chemosis, or RAPD = orbital (postseptal) cellulitis until proven otherwise. Mandates urgent ophthalmology referral and CT orbits.