What is cellulitis?

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 affecting the lower extremities. 1

Clinical Definition and Pathophysiology

Cellulitis involves the deeper dermis and subcutaneous fat, distinguishing it from erysipelas which affects only the upper dermis and superficial lymphatics. 1 The infection arises when organisms enter through breaches in the skin, though these breaks are often clinically inapparent. 1

It is crucial to understand that "cellulitis" should NOT be used to describe cutaneous inflammation surrounding collections of pus such as abscesses, septic bursitis, or furuncles—the primary treatment for these purulent collections is drainage, not antibiotics. 1

Clinical Presentation

The hallmark features include:

• Rapidly spreading areas of erythema, edema, tenderness, and warmth 1
• Peau d'orange appearance (orange peel texture) due to superficial cutaneous edema surrounding hair follicles that remain tethered to the underlying dermis 1
• Lymphangitis and regional lymph node inflammation may accompany the skin findings 1
• Vesicles, bullae, petechiae, or ecchymoses may develop on the inflamed skin 1
• Systemic manifestations including fever, tachycardia, confusion, hypotension, and leukocytosis sometimes occur and may even precede visible skin abnormalities by hours 1

Microbiology

The majority of cellulitis cases (85%) are nonculturable, making the causative bacteria unknown. 2 When organisms are identified:

• β-hemolytic Streptococcus species (especially Group A Streptococcus) are the most common cause 1
• Staphylococcus aureus (methicillin-sensitive) is less frequent but still important 1
• Blood cultures are positive in only 5% of cases 1
• Needle aspiration cultures yield organisms in only 5-40% of cases depending on technique and patient population 1

Streptococci cause diffuse, rapidly spreading infection, while staphylococcal cellulitis is typically more localized. 1

Epidemiology and Burden

Cellulitis represents a significant global health burden:

• More than 650,000 hospital admissions per year in the United States 2
• An estimated 14.5 million cases annually in the US 2
• $3.7 billion in ambulatory care costs alone 2
• Cellulitis accounts for 59.1% of complicated skin and soft tissue infections requiring hospitalization 1

Predisposing Factors and Risk Factors

Common conditions that increase cellulitis risk include:

• Obesity, previous cutaneous damage, and edema from venous insufficiency or lymphatic obstruction 1
• Tinea pedis and fissured toe webs from maceration or fungal infection—the interdigital toe spaces often harbor the responsible streptococci 1
• Pre-existing skin infections such as impetigo or ecthyma 1
• Inflammatory dermatoses such as eczema 1
• Surgical procedures disrupting lymphatic drainage including saphenous venectomy, axillary node dissection for breast cancer, and radical pelvic surgeries 1
• Prior episodes of cellulitis, chronic edema, and cutaneous lesions 3

Diagnostic Approach

Cellulitis is a clinical diagnosis based on history and physical examination—there is no gold standard diagnostic test. 1, 4

Cultures of blood, tissue aspirates, or skin biopsies are unnecessary for typical cases of cellulitis. 1 Blood cultures and tissue cultures should be reserved for:

• Patients with malignancy 1
• Severe systemic features (high fever, hypotension) 1
• Unusual predisposing factors such as immersion injury, animal bites 1
• Neutropenia or severe cell-mediated immunodeficiency 1

Common Diagnostic Pitfalls

Many conditions mimic cellulitis, and misdiagnosis is common, leading to unnecessary hospital admissions and antibiotic overuse. 5, 4 The most common mimickers include:

• Venous stasis dermatitis 6, 4
• Contact dermatitis and eczema 6, 4
• Deep vein thrombosis 6
• Panniculitis 6
• Lymphedema 4

If a patient fails to improve with appropriate first-line antibiotics, consider resistant organisms, cellulitis mimickers, or underlying complications rather than simply escalating antibiotic coverage. 2