What are the differences between cellulitis, osteomyelitis, and necrotizing fasciitis?

Distinguishing Cellulitis, Osteomyelitis, and Necrotizing Fasciitis

These three infections differ fundamentally in tissue depth, severity, and urgency: cellulitis involves superficial skin and subcutaneous tissue requiring antibiotics alone; osteomyelitis is bone infection requiring prolonged treatment; and necrotizing fasciitis is a surgical emergency involving fascial planes with high mortality that demands immediate operative debridement.

Cellulitis

Clinical Characteristics

Cellulitis presents as a diffuse, superficial, spreading skin infection affecting the dermis and subcutaneous fat 1. Key features include:

• Rapidly spreading erythema, swelling, tenderness, and warmth
• May have "peau d'orange" appearance (skin dimpling around hair follicles)
• Possible vesicles, bullae, or petechiae
• Systemic symptoms usually mild (fever, tachycardia may occur)
• Well-defined borders distinguish erysipelas (upper dermis) from deeper cellulitis

Critical Distinction

Cellulitis terminology should NOT be used when pus collections are present (abscesses, septic bursitis, furuncles) 1. This distinction is clinically crucial because cellulitis requires antibiotics as primary treatment, while purulent collections need drainage with antibiotics playing a subsidiary or unnecessary role.

Diagnosis and Treatment

• Clinical diagnosis; cultures unnecessary for typical cases 1
• Oral antibiotics effective for most patients: penicillin, amoxicillin, cephalexin, clindamycin 1
• 5-day course as effective as 10-day course if clinical improvement occurs 1

Necrotizing Fasciitis

Clinical Red Flags

NF initially may resemble cellulitis but has distinctive features that signal deeper tissue involvement 1:

1. Pain disproportionate to clinical findings (hallmark feature)
2. Hard, wooden feel of subcutaneous tissue extending beyond visible skin involvement
3. Systemic toxicity with altered mental status
4. Edema/tenderness beyond cutaneous erythema
5. Crepitus (gas in tissues)
6. Bullous lesions
7. Skin necrosis or ecchymoses
8. Failure to respond to initial antibiotics

Diagnostic Approach

Clinical judgment is paramount 1. While CT/MRI may show fascial plane edema, these studies should never delay surgical intervention in unstable patients 2.

• CT has 89% sensitivity for soft tissue gas (specific for NF) 2
• Absence of fascial enhancement on contrast CT highly associated with NF 2
• Definitive diagnosis is intraoperative: swollen, dull gray fascia with stringy necrosis, brownish exudate, no true pus, easy blunt dissection of tissue planes 1

Laboratory Support

LRINEC score (incorporating WBC, hemoglobin, chemistry, CRP) aids diagnosis 2. Patients with NF have statistically higher leukocytes, platelets, and neutrophils compared to cellulitis 3.

Treatment Algorithm

Surgical debridement is the primary therapeutic modality 1:

1. Immediate surgery when NF confirmed or suspected
2. Return to OR every 24-36 hours until no further debridement needed
3. Aggressive fluid resuscitation (copious tissue fluid discharge)
4. Empiric broad-spectrum antibiotics covering MRSA and anaerobes:
   • Vancomycin/linezolid/daptomycin PLUS
   • Piperacillin-tazobactam OR carbapenem OR ceftriaxone + metronidazole 1

For Group A Streptococcal NF: Clindamycin + penicillin (clindamycin suppresses toxin production and superior to penicillin alone) 1

• Continue antibiotics until no further debridement needed, clinical improvement, and afebrile 48-72 hours 1
• Mortality ranges 29-80% 2

Osteomyelitis

Key Distinguishing Features

Osteomyelitis is bone infection that may complicate diabetic foot ulcers or occur from hematogenous spread. While the provided evidence focuses primarily on cellulitis and NF, osteomyelitis differs by:

• Involvement of bone cortex and marrow (not just soft tissue)
• Requires imaging (MRI most sensitive, radiographs show late changes)
• Prolonged antibiotic therapy (weeks to months)
• May require surgical debridement of infected bone
• Can coexist with overlying soft tissue infections

Clinical Context

NF can complicate chronic osteomyelitis 4, 5, and diabetic patients may develop both osteomyelitis and overlying soft tissue infections 6. The classification model for diabetic foot complications shows distinct patterns for NF versus OM 6.

Critical Pitfalls to Avoid

1. Delaying surgery for imaging in suspected NF: Clinical suspicion trumps imaging; unstable patients go directly to OR 2
2. Calling purulent collections "cellulitis": This leads to inappropriate antibiotic-only treatment when drainage is needed 1
3. Missing early NF: If "cellulitis" fails antibiotics after reasonable trial, consider NF 1
4. Underestimating polymicrobial nature: NF averages 5 pathogens per wound 1

When to Escalate from Cellulitis Diagnosis

Immediately suspect NF and obtain surgical consultation if:

• Severe pain out of proportion to exam
• Rapid progression despite antibiotics
• Systemic toxicity (hypotension, altered mental status)
• Crepitus, bullae, or skin necrosis
• Hard, woody subcutaneous tissue 1