What are the different types of soft tissue infections?

Types of Soft Tissue Infections

Soft tissue infections should be classified based on four independent characteristics: necrotizing versus non-necrotizing nature, anatomical depth, presence of purulence, and clinical severity—with this classification directly determining management urgency and mortality risk. 1

Primary Classification Framework

Necrotizing vs. Non-Necrotizing (Most Critical Distinction)

This is the most important classification because it determines whether immediate surgical debridement is required, directly impacting mortality. 1

Necrotizing Soft Tissue Infections (NSTIs):

• Life-threatening infections involving any layer from superficial dermis to deep fascia and muscle, characterized by tissue necrosis requiring surgical debridement 1
• Include necrotizing cellulitis (dermal/subcutaneous), necrotizing fasciitis (fascial), and necrotizing myositis (muscular) 1
• Can be further classified by microbiology: Type 1 (polymicrobial), Type 2 (monomicrobial β-hemolytic streptococci or CA-MRSA), Type 3 (monomicrobial gram-negative bacilli), though this distinction is less clinically useful than recognizing the need for immediate aggressive management 1
• Delay in diagnosis or treatment directly increases mortality risk through progression to multiorgan failure 1, 2

Non-Necrotizing Infections:

• Include erysipelas, impetigo, folliculitis, furuncles, carbuncles, cellulitis, and abscesses 1
• Do not require surgical debridement (except simple abscess drainage) 1

Classification by Anatomical Depth

Superficial infections (epidermal and dermal layers): 1, 3

• Erysipelas
• Impetigo
• Folliculitis
• Furuncles
• Carbuncles

Deep infections (below dermis): 1, 3

• Cellulitis (dermis and subcutaneous tissue)
• Deep abscesses
• Fasciitis
• Myonecrosis

Classification by Purulence

The IDSA divides infections by presence of purulence, which guides antibiotic selection: 3, 4

Purulent infections:

• Abscesses, furuncles, carbuncles
• Often require drainage as primary treatment 4

Non-purulent infections:

• Cellulitis, erysipelas
• Managed primarily with antibiotics 4

Severity Classification Systems

Complicated vs. Uncomplicated (FDA Classification)

Uncomplicated SSTIs: 1, 3

• Superficial infections (cellulitis, simple abscesses, impetigo, furuncles)
• Low risk for life- or limb-threatening infection if properly treated
• Require only antibiotics or simple surgical drainage

Complicated SSTIs: 1, 3

• Deep soft-tissue infections (necrotizing infections, infected ulcers, infected burns, major abscesses)
• High risk for life-threatening infection
• Require significant surgical intervention with drainage and/or debridement
• Demand broad-spectrum empiric antibiotics immediately

Eron Classification (Clinical Severity)

This system guides admission and treatment intensity decisions: 1, 3

• Class 1: No systemic toxicity or comorbidities
• Class 2: Systemically unwell with stable comorbidities OR systemically well with complicating comorbidity (diabetes, obesity)
• Class 3: Toxic appearance (fever, tachycardia, tachypnea, hypotension)
• Class 4: Sepsis syndrome or life-threatening infection (e.g., necrotizing fasciitis)

IDSA Severity Classification for Purulent Infections

Mild: 4

• Localized without systemic signs
• No significant comorbidities
• Often requires only incision and drainage

Moderate: 4

• Systemic signs (fever, tachycardia, tachypnea, abnormal WBC)
• Requires incision and drainage PLUS systemic antibiotics

Severe: 4

• Failed outpatient management
• Immunocompromised status
• Signs of deep infection (disproportionate pain, violaceous bullae, rapid progression)
• Requires hospitalization and aggressive management

FDA ABSSSI Definition

Acute Bacterial Skin and Skin Structure Infection defined as bacterial infection with lesion area ≥75 cm² (measured by redness, edema, or induration), including cellulitis/erysipelas, wound infections, and major cutaneous abscesses. 1, 3

Critical Warning Signs Requiring Immediate Surgical Consultation

These signs suggest necrotizing infection and mandate emergency surgical evaluation: 4

• Disproportionate pain on physical examination
• Violaceous bullae
• Rapid progression of erythema or infection
• Rapid extension beyond initial boundaries
• Crepitus or subcutaneous gas

Common pitfall: Necrotizing infections, particularly Type 2 monomicrobial streptococcal infections, may present WITHOUT classic skin changes, subcutaneous air, or crepitus, making early diagnosis extremely challenging but no less urgent. 5 The LRINEC score (≥8 suggests 75% risk of NSTI) can aid early identification but should not delay surgical exploration when clinical suspicion is high. 1

Risk Stratification for Necrotizing Infections

Patients with NSTIs should be classified as: 1

• High risk of poor outcome
• Mild/moderate risk of poor outcome

This stratification identifies patients requiring immediate surgical treatment and perioperative intensive care management, as early prognostic evaluation is crucial for determining treatment aggressiveness and preventing progression to multiorgan failure. 1