What is Cellulitis?
Cellulitis is a bacterial infection of the deep dermis and subcutaneous tissue that presents with rapidly spreading areas of erythema, edema, warmth, and tenderness with poorly defined borders. 1
Clinical Presentation
Primary Features
- Skin changes: The affected area displays redness, swelling, heat, and tenderness that spreads rapidly and lacks well-demarcated borders (unlike erysipelas which has sharp borders) 1
- Peau d'orange appearance: The skin surface may resemble an orange peel due to superficial edema surrounding hair follicles, which remain tethered to the underlying dermis causing dimpling 2, 1
- Additional cutaneous findings: Vesicles, bullae, petechiae, or ecchymoses may develop on the inflamed skin 2
Systemic Manifestations
- Mild to moderate symptoms: Fever, tachycardia, confusion, hypotension, and leukocytosis can occur, sometimes appearing hours before visible skin changes 2, 1
- Associated findings: Lymphangitis (red streaking) and regional lymph node inflammation may accompany the infection 2, 1
Anatomic Depth
- Location in skin layers: Cellulitis involves the deeper dermis and subcutaneous fat, distinguishing it from erysipelas which affects only the upper dermis and superficial lymphatics 2
- Common sites: Most frequently occurs on the lower legs, though can develop at any location 2
Causative Organisms
- Primary pathogens: β-hemolytic Streptococcus (often group A, but also groups B, C, or G) and methicillin-sensitive Staphylococcus aureus are responsible for most cases 2, 3, 4
- Diagnostic challenge: The majority of cellulitis cases are nonculturable, with organisms identified in only 15% of cases 3
- Blood cultures: Positive in only 5% of cases 2
Predisposing Factors
Skin Barrier Disruption
- Entry points: Organisms enter through breaches in the skin, including trauma, preexisting infections (impetigo, ecthyma), ulceration, fissured toe webs from maceration or fungal infection, and inflammatory dermatoses like eczema 2
- Tinea pedis: Streptococci frequently colonize macerated or fissured interdigital toe spaces, emphasizing the importance of treating fungal infections 2
Host Factors
- Conditions weakening skin integrity: Obesity, previous cutaneous damage, edema from venous insufficiency, lymphatic obstruction 2
- Surgical procedures: Saphenous venectomy, axillary node dissection for breast cancer, and gynecologic cancer operations involving lymph node dissection increase risk due to disrupted lymphatic drainage 2
Important Caveats
- Diagnostic mimics: Many non-infectious conditions present similarly, including venous stasis dermatitis, contact dermatitis, eczema, lymphedema, and deep vein thrombosis 4, 5
- Red flag for deeper infection: Widespread petechiae and ecchymoses associated with systemic toxicity should raise concern for necrotizing fasciitis 2
- No gold standard test: Cellulitis remains a clinical diagnosis based on history and physical examination 4