What is Cellulitis?
Cellulitis is an acute bacterial infection of the deep dermis and subcutaneous tissue that presents with rapidly spreading erythema, warmth, swelling, and tenderness, most commonly caused by β-hemolytic streptococci and methicillin-sensitive Staphylococcus aureus. 1, 2
Clinical Definition and Pathophysiology
Cellulitis involves infection of the deeper dermal layers and subcutaneous fat, distinguishing it from erysipelas which affects only the upper dermis and superficial lymphatics 1. The infection causes:
- Rapidly spreading areas of erythema, swelling, tenderness, and warmth
- Lymphangitis and regional lymph node inflammation may accompany the primary infection
- "Peau d'orange" appearance - the skin surface may resemble an orange peel due to superficial edema surrounding hair follicles while the follicles remain tethered to underlying dermis
- Vesicles, bullae, petechiae, or ecchymoses may develop in the affected area 1
Causative Organisms
In the 80% of nonpurulent cellulitis cases where no pathogen is isolated, β-hemolytic streptococci (groups A, B, C, and G) are the presumed primary cause, with S. aureus playing a secondary role 1. This diagnostic limitation is critical - the clinical isolation rate of a pathogen in nonpurulent cellulitis is less than 20%, leaving clinicians with an unconfirmed diagnosis in the vast majority of cases 1.
MRSA is an unusual cause of typical cellulitis - prospective studies demonstrate that β-lactam therapy (cefazolin or oxacillin) was successful in 96% of patients, indicating MRSA treatment is usually unnecessary for uncomplicated cellulitis 1.
Clinical Presentation
Cellulitis typically presents with:
- Acute onset of localized skin changes
- Erythema with poorly demarcated borders (unlike erysipelas which has well-demarcated edges)
- Pain and tenderness over the affected area
- Warmth to palpation
- Swelling/edema of the involved tissue
Systemic manifestations may include:
- Fever and tachycardia
- Confusion or altered mental status
- Hypotension in severe cases
- Leukocytosis
- These systemic features may occur hours before skin abnormalities appear 1
Anatomic Distribution
The lower extremities are most commonly affected, though cellulitis can involve any body area 3, 2. In a large European multicenter study, cellulitis accounted for 59.1% of all complicated skin and soft tissue infections requiring hospitalization 3.
Key Diagnostic Pitfall
Cellulitis is frequently misdiagnosed because numerous conditions mimic its presentation 4, 5. Common mimickers include:
There is no gold standard diagnostic test for cellulitis - diagnosis relies entirely on clinical history and physical examination 4. Blood cultures, tissue aspirates, and skin biopsies are unnecessary for typical cases and should be reserved for patients with malignancy, severe systemic features (high fever, hypotension), neutropenia, severe immunodeficiency, or unusual predisposing factors like immersion injury or animal bites 1.