Skin and Soft Tissue Infections: Cellulitis, Necrotizing Fasciitis, and MRSA

Skin and soft tissue infections (SSTIs) represent a spectrum of infectious processes involving the skin, subcutaneous tissue, fascia, and muscle. These infections range from superficial cellulitis to life-threatening necrotizing fasciitis.

[KEY_CONCEPT] SSTIs are classified into uncomplicated and complicated infections based on severity, extent, and patient factors.

Classification

Epidemiology

• Cellulitis: 24-25 cases per 1,000 person-years
• Necrotizing fasciitis: 0.4 cases per 100,000 person-years
• MRSA prevalence: 30-50% of S. aureus isolates in healthcare settings

[HIGH_YIELD] Risk factors include diabetes mellitus, immunocompromise, chronic venous insufficiency, lymphedema, obesity, and injection drug use.

Pathophysiology

Cellulitis results from bacterial invasion through breaks in skin integrity. Necrotizing fasciitis involves rapid spread along fascial planes with vascular thrombosis and tissue necrosis. MRSA infections are characterized by resistance to β-lactam antibiotics due to altered penicillin-binding proteins.

[CLINICAL_PEARL] The majority of uncomplicated cellulitis is caused by β-hemolytic streptococci, while S. aureus (including MRSA) predominates in purulent infections and abscesses.

Cellulitis

Classic presentation includes erythema, warmth, swelling, and tenderness of affected skin. The infection typically has ill-defined borders and may be associated with lymphangitis (red streaking) and regional lymphadenopathy.

[HIGH_YIELD] Systemic symptoms (fever, chills, malaise) suggest more severe infection requiring aggressive management.

Necrotizing Fasciitis

Early signs are often subtle and may mimic simple cellulitis:

• Pain out of proportion to physical findings
• Rapid progression of erythema and swelling
• Skin changes: Initial erythema progresses to dusky discoloration, bullae, and eventual necrosis
• Systemic toxicity: High fever, hypotension, altered mental status

[CLINICAL_PEARL] The absence of fever does not rule out necrotizing fasciitis, especially in immunocompromised patients or elderly individuals.

MRSA Infections

Purulent infections with MRSA typically present as:

• Abscesses with central necrosis and surrounding cellulitis
• Furuncles and carbuncles
• Infected wounds with purulent drainage
• Folliculitis in hair-bearing areas

Risk Stratification Signs

High-Risk Features (Complicated SSTI): ├── Systemic toxicity (fever >38°C, tachycardia, hypotension) ├── Rapid progression (>24-48 hours) ├── Pain out of proportion to findings ├── Skin necrosis or bullae formation ├── Crepitus on examination ├── Immunocompromised state └── Failed outpatient therapy

[KEY_CONCEPT] The LRINEC score (Laboratory Risk Indicator for Necrotizing Fasciitis) helps distinguish necrotizing fasciitis from cellulitis, though clinical suspicion remains paramount.

Clinical Diagnosis

Cellulitis is primarily a clinical diagnosis based on characteristic findings. Laboratory studies and imaging are not routinely required for uncomplicated cases.

[HIGH_YIELD] Blood cultures should be obtained in patients with systemic toxicity, immunocompromise, or severe infection.

Diagnostic Workup for Complicated SSTIs

Laboratory Studies

• Complete blood count: Leukocytosis suggests systemic infection
• Comprehensive metabolic panel: Assess renal function, electrolytes
• Inflammatory markers: ESR, CRP (elevated in severe infections)
• Blood cultures: Positive in 5-15% of cellulitis cases
• Wound cultures: Essential for purulent infections

LRINEC Score for Necrotizing Fasciitis

Score interpretation: ≥6 points suggests necrotizing fasciitis (sensitivity 68%, specificity 85%)

[CLINICAL_PEARL] A LRINEC score <6 does not rule out necrotizing fasciitis if clinical suspicion is high.

Imaging

• CT with contrast: First-line for suspected necrotizing fasciitis
• MRI: Superior soft tissue contrast but may delay care
• Ultrasound: Can identify fluid collections and guide drainage

Necrotizing Fasciitis Diagnostic Criteria

Clinical Suspicion + Any of: ✓ Pain out of proportion to physical findings ✓ Rapid progression of infection ✓ Systemic toxicity ✓ Skin necrosis or bullae ✓ Crepitus ✓ LRINEC score ≥6

Imaging findings: ✓ Fascial thickening ✓ Fluid along fascial planes ✓ Gas in soft tissues

[KEY_CONCEPT] Surgical exploration remains the gold standard for diagnosing necrotizing fasciitis when clinical suspicion is high, regardless of imaging or laboratory results.

Uncomplicated Cellulitis

First-Line Oral Therapy

Outpatient Management Algorithm: ├── Non-purulent cellulitis │ ├── Cephalexin 500mg PO q6h × 5-10 days │ └── PCN allergy: Clindamycin 300-450mg PO q6h ├── Purulent/MRSA-risk infection │ ├── Clindamycin 300-450mg PO q6h × 5-10 days │ ├── Doxycycline 100mg PO BID × 5-10 days │ └── TMP-SMX DS 1-2 tablets PO BID × 5-10 days └── Follow-up in 24-48 hours for response assessment

[HIGH_YIELD] Duration: 5-10 days is typically adequate for uncomplicated cellulitis. The ACORN trial demonstrated non-inferiority of 5-day vs 10-day courses.

Complicated SSTIs - Inpatient Management

Empiric IV Antibiotic Therapy

[CLINICAL_PEARL] Clindamycin should be added to β-lactam therapy for necrotizing fasciitis due to its anti-toxin effects and superior tissue penetration.

MRSA-Active Oral Options (Step-Down Therapy)

Based on the MERINO trial findings:

• Linezolid 600mg PO BID: Non-inferior to vancomycin
• Doxycycline 100mg PO BID: Effective for skin infections
• Clindamycin 300-450mg PO q6h: Good tissue penetration

Necrotizing Fasciitis Management

Emergent Management Protocol:

1. Immediate surgical consultation
2. Aggressive IV fluid resuscitation
3. Broad-spectrum antibiotics: • Vancomycin 15-20mg/kg IV q8-12h • Piperacillin-tazobactam 4.5g IV q6h • Clindamycin 600-900mg IV q8h
4. Surgical debridement within 6-12 hours
5. ICU monitoring for septic shock
6. Consider IVIG for streptococcal TSS

[KEY_CONCEPT] Time to surgical intervention is the most important prognostic factor in necrotizing fasciitis.

Source Control

• Abscesses >5cm: Require incision and drainage
• Necrotizing infections: Emergent surgical debridement
• Foreign bodies: Must be removed

Adjunctive Therapies

• Hyperbaric oxygen: May be considered for necrotizing fasciitis
• IVIG: For streptococcal toxic shock syndrome
• Supportive care: Pain management, wound care, physical therapy

Cellulitis Complications

Local Complications

• Abscess formation: Requires drainage
• Chronic lymphedema: From recurrent infections
• Post-infectious glomerulonephritis: Rare with streptococcal infections
• Bacteremia: 5-15% of severe cases

[HIGH_YIELD] Recurrent cellulitis occurs in 15-20% of patients, often at the same anatomic site due to predisposing factors.

Systemic Complications

• Sepsis and septic shock: Following Surviving Sepsis Campaign guidelines
• Endocarditis: Rare but serious in bacteremic patients
• Osteomyelitis: Extension to underlying bone

Necrotizing Fasciitis Outcomes

Prognostic Factors

Poor prognosis indicators:

• Delayed diagnosis (>24 hours from symptom onset)
• Advanced age (>65 years)
• Diabetes mellitus
• Immunocompromise
• Shock at presentation
• Streptococcal etiology with toxic shock syndrome

[CLINICAL_PEARL] Mortality rates range from 15-30% overall, but can exceed 50% with delayed treatment or shock.

Long-term Sequelae

• Amputation: Required in 10-20% of survivors
• Chronic pain: Due to extensive tissue loss
• Functional impairment: From surgical debridement
• Psychological trauma: PTSD in severe cases

MRSA Infection Outcomes

Treatment Failure Risk Factors

• Inadequate source control
• Biofilm formation on prosthetic devices
• Immunocompromise
• Inappropriate antibiotic selection

Prevention Strategies

Primary Prevention

Risk Reduction Measures: ├── Proper wound care and hygiene ├── Treatment of predisposing conditions │ ├── Diabetes management │ ├── Compression therapy for venous insufficiency │ └── Weight management ├── Skin barrier protection └── Prompt treatment of minor injuries

Secondary Prevention

• Recurrent cellulitis: Consider prophylactic antibiotics
• High-risk patients: Penicillin V 250mg PO BID or erythromycin 250mg PO BID
• Duration: Typically 6-12 months

[KEY_CONCEPT] Prophylactic antibiotics should be reserved for patients with ≥3-4 episodes of recurrent cellulitis per year after addressing predisposing factors.

Prognosis Summary