Cellulitis: Definition and Pathophysiology
Definition
Cellulitis is an acute bacterial skin and soft tissue infection characterized by rapid onset (hours to days) of unilateral erythema, swelling, warmth, and tenderness 1. The infection involves the dermis and subcutaneous tissue without purulent drainage or abscess formation 1, 2.
Key Diagnostic Features:
- Unilateral presentation is the hallmark that distinguishes true cellulitis from non-infectious mimics 1
- Exquisite tenderness on palpation and warmth are strong supportive signs 1
- Poorly demarcated borders with spreading erythema 2
- Systemic manifestations such as fever, tachycardia, or mild leukocytosis may be present but are not required for diagnosis 1
- Vesicles, bullae, or hemorrhagic bullae may occur and do not exclude the diagnosis 1
Critical Distinction:
The presence of purulent drainage, abscess, or ulceration indicates a condition other than simple cellulitis and requires incision-and-drainage rather than antibiotics alone 1.
Pathophysiology
Mechanism of Infection
Cellulitis develops when the physical skin barrier, immune system, or circulatory system are compromised, allowing bacterial invasion 3. The infection process involves:
- Bacterial entry through breaks in the skin barrier (trauma, tinea pedis, toe-web abnormalities) 1, 3
- Initial bacterial proliferation in the dermis and subcutaneous tissue 4
- Rapid immune response with infiltration of macrophages and neutrophils that quickly reduce viable bacterial numbers 4
Inflammatory Cascade
The clinical manifestations of warmth and erythema result from both residual bacteria and bacterial remnants, amplified by lymphokines and cytokines secreted by intra-epidermal immunocompetent cells 4. This explains why:
- Bacterial cultures are often negative despite clear clinical infection 4
- The inflammatory response may persist even after bacterial clearance 4
- Anti-inflammatory mechanisms play a role in resolution 4
Predisposing Factors
Key risk factors that impair skin barrier, immune function, or circulation include 3:
- Interdigital fungal infection (tinea pedis) and toe-web skin abnormalities 1
- Chronic venous insufficiency and lymphedema 1, 3
- Diabetes mellitus (impairs all three protective systems) 3
- Obesity (associated with defects in barrier, immunity, and circulation) 3
- Advanced age (cumulative defects in protective mechanisms) 3
- Recent trauma or skin disruption 1
Microbiology
Streptococcal species are the predominant pathogens in typical non-purulent cellulitis, which is why empiric β-lactam therapy (cephalexin, penicillin, or amoxicillin) is recommended without routine MRSA coverage 1, 5.
Recurrence Mechanism
Relapses occur frequently due to persistent post-inflammatory lymphatic damage combined with ongoing risk factors 3. The infection itself causes lymphatic injury that perpetuates susceptibility to future episodes 3.
Clinical Pitfalls
Common conditions frequently mistaken for cellulitis include 3, 6:
- Venous stasis dermatitis
- Lymphedema
- Deep vein thrombosis
- Eczema
- Gout
Bilateral or chronic presentations should raise suspicion for pseudocellulitis (non-infectious mimics) rather than true bacterial infection 1.