What is the differential diagnosis for a 26-year-old male with extensive cellulitis of the left forearm and a closed, semi-translucent, egg-sized bulbous protuberance with faintly visible vascularity and purulence?

Differential Diagnosis for Injection Drug User with Forearm Cellulitis and Bulbous Protuberance

In a 26-year-old homeless injection drug user with extensive cellulitis and a closed, semi-translucent, egg-sized bulbous protuberance with visible vascularity and purulence, the primary differential includes: (1) subcutaneous abscess with surrounding cellulitis, (2) pseudoaneurysm, (3) necrotizing fasciitis, (4) pyomyositis, and (5) septic thrombophlebitis.

Critical Immediate Assessment

This presentation demands urgent evaluation for life-threatening complications given the high-risk population and concerning features 1.

Red Flags Requiring Emergent Surgical Consultation

• Pain disproportionate to physical findings - suggests necrotizing infection 1
• Semi-translucent appearance with vascularity - raises concern for pseudoaneurysm or vascular structure 2
• Visible purulence within closed protuberance - indicates deep collection requiring drainage 1
• Extensive surrounding cellulitis - suggests aggressive infection or deeper process 1

Mandatory Initial Workup

Blood work should include: blood cultures, complete blood count with differential, creatinine, bicarbonate, creatine phosphokinase (CPK), and C-reactive protein 1. Hospitalization is indicated if any of the following are present: hypotension, elevated creatinine, low bicarbonate, CPK 2-3 times upper limit of normal, marked left shift, or CRP >13 mg/L 1.

Primary Differential Diagnoses

1. Subcutaneous Abscess with Surrounding Cellulitis (Most Likely)

This is the most common presentation in injection drug users 2.

• Polymicrobial infection occurs in >50% of cases in this population, including anaerobic organisms and aerobic gram-positive cocci 2
• The semi-translucent appearance with visible purulence strongly suggests a fluid collection 3
• Incision and drainage is mandatory - antibiotics alone are insufficient 1
• Ultrasound features supporting abscess include: cobblestoning or branching interstitial fluid, posterior acoustic enhancement, and swirling with compression 3

Common pitfall: The closed nature and semi-translucent appearance may lead to misdiagnosis as a simple cyst, delaying necessary surgical drainage 2.

2. Pseudoaneurysm (Critical to Exclude)

The semi-translucent appearance with "faintly visible vascularity" is highly concerning for a vascular structure 2.

• Injection drug users frequently develop pseudoaneurysms when attempting venous access 2
• Do NOT incise without imaging - ultrasound with Doppler or CT angiography is essential to differentiate abscess from pseudoaneurysm 2
• Incising a pseudoaneurysm can cause life-threatening hemorrhage 2

Critical action: Obtain urgent ultrasound with Doppler or CT scan before any incision to clarify whether this is a vascular structure 2.

3. Necrotizing Fasciitis

This diagnosis must be actively excluded given the high mortality (50-70%) in this population 1.

• Injection sites are a common portal of entry for necrotizing infections 1
• Clinical features to assess: wooden-hard feel of subcutaneous tissues (cannot palpate underlying fascial planes), skin anesthesia, violaceous bullae, cutaneous hemorrhage, skin sloughing, rapid progression, or gas in tissue 1
• The extensive cellulitis with a bulbous protuberance could represent early necrotizing fasciitis with fascial plane involvement 1
• If suspected, emergent surgical exploration is required - both diagnostic and therapeutic 1

4. Pyomyositis

This is a rare but well-documented abscess-forming infection of skeletal muscle in injection drug users 2.

• More than 20 cases reported in temperate climates 2
• The proximal forearm location near muscle groups and the egg-sized protuberance could represent deep muscle abscess 2
• Requires imaging (MRI or CT) for diagnosis and surgical drainage 2

5. Septic Thrombophlebitis

Thrombosed veins can be confused with abscesses in injection drug users 2.

• May present as a tender, cord-like structure with surrounding cellulitis 2
• Ultrasound with Doppler can differentiate thrombosed vein from abscess 2

Recommended Diagnostic Algorithm

Step 1: Assess for Systemic Toxicity

• Check vital signs: fever/hypothermia, tachycardia (>100 bpm), hypotension (SBP <90 mmHg) 1
• If present, obtain blood cultures and laboratory parameters immediately 1

Step 2: Urgent Imaging BEFORE Incision

• Ultrasound with Doppler is first-line to differentiate abscess from pseudoaneurysm and assess for thrombophlebitis 2, 3
• CT or MRI if concern for deeper infection (necrotizing fasciitis, pyomyositis) or if ultrasound is inconclusive 2

Step 3: Surgical Evaluation

• Emergent surgical consultation if any signs of necrotizing infection (pain out of proportion, wooden-hard tissues, skin changes) 1
• Urgent surgical consultation for incision and drainage if abscess confirmed and no vascular structure identified 1, 2

Step 4: Empiric Antibiotic Coverage

• Broad-spectrum coverage is mandatory for injection drug users with complex abscesses: Gram-positive, Gram-negative, and anaerobic bacteria 1
• Consider vancomycin PLUS piperacillin-tazobactam or a carbapenem given the polymicrobial nature and MRSA risk 1, 4
• Do not rely on antibiotics alone - source control through drainage is essential 1

Key Clinical Pearls

• Never incise a semi-translucent structure with visible vascularity without imaging - pseudoaneurysm must be excluded first 2
• Polymicrobial infections are the rule, not the exception in injection drug users 2
• Complete, often repeated, incision and drainage is prerequisite for successful outcome in subcutaneous abscesses 2
• The term "cellulitis" should not be used for cutaneous inflammation surrounding purulent collections - the correct terminology is "abscess with surrounding inflammation" 1
• Failure to improve with antibiotics alone strongly suggests an undrained collection requiring surgical intervention 1