What is the most likely diagnosis and appropriate management for a 36‑year‑old woman who developed a spreading erythematous, painful lesion on her chest after blunt trauma with a barbell?

Most Likely Diagnosis: Post-Traumatic Cellulitis or Pyoderma Gangrenosum

The most likely diagnosis is post-traumatic cellulitis developing at the site of blunt chest trauma, though pyoderma gangrenosum must be considered if the rash continues to spread despite appropriate antibiotic therapy.

Immediate Assessment Required

Critical Life-Threatening Conditions to Exclude First

Before attributing this to a simple skin infection, you must rule out internal chest trauma:

• Obtain a 12-lead ECG immediately to exclude blunt cardiac injury, which can present with arrhythmias, elevated troponin, or ECG abnormalities even without classic chest pain 1
• Perform bedside ultrasound (FAST exam) to screen for hemopericardium, pleural collection, or cardiac tamponade—hypotension and tachycardia in isolated chest trauma suggest these diagnoses 1
• Check for pneumothorax with physical exam (unilateral absent breath sounds, hyperresonance) and chest X-ray, as rib fractures from barbell impact can cause delayed pneumothorax 1
• Assess vital signs carefully: tachycardia occurs in >90% of significant chest trauma complications 1

Physical Examination of the Rash

Document these specific characteristics:

• Morphology: Is it maculopapular, petechial, diffusely erythematous, vesiculobullous-pustular, or nodular? 2
• Distribution pattern: Centrally located versus spreading peripherally 2, 3
• Borders: Well-demarcated with violaceous undermined edges suggests pyoderma gangrenosum 4
• Associated findings: Warmth, tenderness, induration, purulent drainage, or necrotic tissue 2, 5
• Systemic signs: Fever, which would indicate infectious etiology 2, 3

Differential Diagnosis Algorithm

If the Rash is Warm, Tender, and Erythematous with Fever:

Diagnosis: Post-traumatic cellulitis or abscess

• Blunt trauma creates a portal of entry for bacteria, most commonly Staphylococcus aureus or Streptococcus species 2
• The spreading nature indicates active bacterial infection requiring immediate treatment 5

Management:

• Start empiric antibiotics covering MRSA: cephalexin 500mg PO QID PLUS trimethoprim-sulfamethoxazole DS 1-2 tablets PO BID (or doxycycline 100mg PO BID if sulfa-allergic) 2
• If systemically ill or immunocompromised: vancomycin 15-20mg/kg IV Q8-12h 5
• Obtain blood cultures if febrile 5
• Mark the borders of erythema with a pen to monitor progression 2
• Re-evaluate in 48 hours—if spreading despite antibiotics, consider pyoderma gangrenosum 4

If the Rash is Spreading Despite Antibiotics with Sterile Cultures:

Diagnosis: Pyoderma gangrenosum (post-traumatic variant)

This is a critical diagnosis that mimics severe bacterial infection but is not infectious 4:

• Presents as progressively necrotizing ulcerative lesions at trauma sites 4
• Pathergy phenomenon: trauma triggers lesion development 4
• Cultures remain sterile despite septic appearance 4
• Debridement worsens the condition (pathergy) 4

Management:

• Stop antibiotics and avoid further debridement 4
• Start systemic corticosteroids: prednisone 0.5-1mg/kg/day PO 4
• Consider hyperbaric oxygen therapy as adjunct 4
• Urgent dermatology consultation required 4
• Local wound care with non-adherent dressings 4

If Associated with Systemic Symptoms Beyond Local Infection:

Consider autoimmune/rheumatologic causes 6:

• Urticarial lesions (non-pruritic): Anti-C1q vasculitis, Still's disease 6
• Annular or psoriasiform lesions: Subacute cutaneous lupus 6
• Purpuric lesions: Vasculitis in Sjögren's syndrome, Behçet's disease 6

Check: ANA, RF, anti-CCP, complement levels, ESR/CRP 6

Common Pitfalls to Avoid

• Do not assume all spreading rashes are infectious—pyoderma gangrenosum will worsen with antibiotics and debridement 4
• Do not perform aggressive debridement without confirming infection, as this triggers pathergy in pyoderma gangrenosum 4
• Do not dismiss internal chest injury based on normal initial presentation—blunt cardiac injury can be clinically silent initially 1
• Do not delay imaging if the patient develops dyspnea, as pneumothorax can present hours after trauma 1

Red Flags Requiring Immediate Hospital Transfer

• Hemodynamic instability (SBP <100, HR >100 or <50) 1
• Respiratory distress or oxygen saturation <90% 1
• Rash spreading to >10% body surface area despite treatment 5, 4
• Development of necrotic tissue or bullae 5, 4
• Fever >38.5°C with systemic toxicity 5
• Abnormal ECG or elevated troponin 1

Follow-Up Timeline

• 48 hours: Re-examine to assess antibiotic response—if spreading, consider pyoderma gangrenosum 4
• 72 hours: If no improvement with antibiotics, obtain dermatology consultation 5, 4
• 1 week: Repeat chest X-ray if any respiratory symptoms develop 1