What is cellulitis?

Cellulitis: Definition, Clinical Features, and Management

Cellulitis is a diffuse, superficial, spreading skin infection involving the deeper dermis and subcutaneous fat that presents with rapidly spreading areas of erythema, swelling, tenderness, and warmth. 1

Definition and Distinction

Cellulitis refers to an acute bacterial infection of the deep dermis and subcutaneous tissue that occurs when microorganisms enter through breaches in the skin. It's important to distinguish cellulitis from:

• Erysipelas: Affects only the upper dermis and superficial lymphatics, with more clearly delineated borders of inflammation 1
• Purulent collections: The term "cellulitis" is not appropriate for cutaneous inflammation associated with collections of pus (e.g., abscesses, septic bursitis) 1

This distinction is clinically crucial because:

• Primary treatment for cellulitis is antimicrobial therapy
• Primary treatment for purulent collections is drainage of pus 1

Clinical Features

Cellulitis typically presents with:

• Rapidly spreading areas of erythema, swelling, tenderness, and warmth
• Orange peel appearance (peau d'orange) due to superficial cutaneous edema around hair follicles
• Possible lymphangitis and regional lymph node inflammation
• Possible development of vesicles, bullae, and cutaneous hemorrhage (petechiae or ecchymoses)
• Usually mild systemic manifestations, though fever, tachycardia, confusion, hypotension, and leukocytosis may occur 1

Common Locations and Predisposing Factors

Cellulitis can occur anywhere but most commonly affects the lower legs 1. Predisposing factors include:

• Obesity
• Previous cutaneous damage
• Edema from venous insufficiency or lymphatic obstruction
• Trauma
• Preexisting skin infections (impetigo, ecthyma)
• Ulceration
• Fissured toe webs from maceration or fungal infection
• Inflammatory dermatoses (eczema)
• Surgical procedures that disrupt lymphatic drainage (saphenous venectomy, axillary node dissection) 1

Microbiology

Most cases of cellulitis are caused by:

• β-hemolytic streptococci (most common, particularly group A, but also groups B, C, or G)
• Staphylococcus aureus (less frequent, often associated with previous penetrating trauma)

Other organisms may cause cellulitis in special circumstances:

• Pasteurella species (cat or dog bites)
• Aeromonas hydrophila (freshwater exposure)
• Vibrio species (saltwater exposure) 1

Diagnosis

Cellulitis is primarily a clinical diagnosis. For typical cases:

• Cultures of blood, tissue aspirates, or skin biopsies are unnecessary
• Blood cultures should be obtained for patients with:
  • Malignancy
  • Severe systemic features (high fever, hypotension)
  • Unusual predisposing factors (immersion injury, animal bites, neutropenia, severe immunodeficiency) 1

Blood culture results are positive in only about 5% of cases 1.

Treatment

For typical cases of cellulitis:

• Antibiotic therapy should target streptococci, the most common causative organism 1
• Suitable oral antibiotics include:
  • Penicillin
  • Amoxicillin
  • Amoxicillin-clavulanate
  • Dicloxacillin
  • Cephalexin
  • Clindamycin 1
• A 5-day course is as effective as a 10-day course if clinical improvement occurs by day 5 1

For patients with severe infection requiring hospitalization, consider:

• Vancomycin plus piperacillin-tazobactam
• Ceftriaxone plus metronidazole
• Ampicillin-sulbactam 2

Prevention of Recurrence

For patients with recurrent cellulitis:

• Address predisposing factors (treat tinea pedis, manage edema)
• For patients with 3-4 episodes per year despite addressing predisposing factors, consider prophylactic antibiotics:
  • Oral penicillin V 1 g twice daily
  • Erythromycin 250 mg twice daily
  • Intramuscular benzathine penicillin 1.2 million units every 2-4 weeks 2

Common Pitfalls

1. Misdiagnosis: Many conditions mimic cellulitis, including venous stasis dermatitis, contact dermatitis, deep vein thrombosis, and panniculitis 3, 4

2. Failure to address predisposing factors: Essential for preventing recurrence 2

3. Inadequate treatment duration: Therapy should be extended if clinical response is inadequate 2

4. Failure to drain purulent collections: Antibiotics alone are often insufficient for abscesses 2

5. Overuse of broad-spectrum antibiotics: Vancomycin and newer agents should be reserved for severe infections or confirmed MRSA 2