What is the differential diagnosis, appropriate work‑up, and management for a swollen lower extremity with a small wound?

Swollen Lower Extremity with Small Wound: Differential Diagnosis, Work-Up, and Management

For a swollen lower extremity with a small wound, immediately obtain plain radiographs in two views to exclude osteomyelitis and foreign bodies, then perform duplex ultrasound to rule out deep vein thrombosis, as these represent the most urgent life-threatening diagnoses that require immediate intervention. 1, 2, 3

Immediate Life-Threatening Diagnoses to Exclude First

Deep Vein Thrombosis

• Unilateral swelling always indicates an obstructive process requiring urgent evaluation 2, 3
• DVT presents with ipsilateral extremity edema (80% of cases), pain (75%), erythema (26%), and unexplained persistent calf cramping 2
• Proceed directly to duplex Doppler ultrasound without D-dimer testing if clinical suspicion is high, as ultrasound has sensitivity and specificity above 80% for detecting thrombus through visualization of echogenic material and lack of vein compression 2, 3
• If DVT is confirmed, initiate therapeutic anticoagulation immediately while completing the evaluation 2, 3

Soft Tissue Infection and Osteomyelitis

• Plain radiographs in two planes are mandatory as the first investigation to detect gas in soft tissues, foreign bodies, bone destruction, or periosteal reaction 1, 3, 4
• Infection requires at least two of: local swelling/induration, erythema, local tenderness/pain, local warmth, or purulent discharge 1, 4
• Critical pitfall: Fever and elevated white blood cell count may be absent in up to half of patients with significant infections, so do not rely on systemic signs alone 1, 4
• Probe the wound with a sterile blunt metal probe—bone touched with a probe has a characteristic stony feel and strongly suggests osteomyelitis 1, 4

Structured Diagnostic Algorithm

Step 1: Initial Imaging (Perform Immediately)

• Obtain plain radiographs in two views (AP and lateral minimum) to evaluate for fractures, bone destruction, gas in soft tissues, or foreign bodies 1, 3, 4
• Perform duplex Doppler ultrasound of the entire affected limb to exclude DVT, especially if swelling extends proximally 2, 3

Step 2: Wound Assessment and Culture

• Debride the wound to remove hyperkeratosis, necrotic tissue, and slough—this reduces bacterial colonization, facilitates proper culture collection, and permits examination for deep tissue involvement 1
• Obtain tissue specimens (not swabs) for culture before initiating antibiotics whenever possible, as tissue samples provide more accurate results than superficial swabs 1
• Measure and document wound size, depth, surrounding cellulitis extent, and drainage characteristics 1
• Use a sterile blunt metal probe to assess for foreign bodies, abscesses, joint cavity communication, or palpable bone 1, 4

Step 3: Advanced Imaging (If Initial Studies Are Negative or Equivocal)

• MRI with contrast is the preferred modality for suspected osteomyelitis, with high sensitivity and specificity—importantly, a negative MRI excludes osteomyelitis 1, 3
• MRI is superior for delineating anatomic extent of soft tissue infections, detecting fasciitis, myositis, and areas of necrosis 1
• Ultrasound is valuable for detecting soft tissue fluid collections, abscesses, and foreign bodies (especially radiolucent materials like wood or plastic) 1, 5
• CT is appropriate when MRI is contraindicated or for detecting radiodense foreign bodies and visualizing bone destruction 1, 3

Differential Diagnosis by Clinical Pattern

Infectious Causes

• Cellulitis: Erythema, warmth, tenderness extending beyond wound margins; treat with antibiotics active against streptococci (mild cases) or add MRSA coverage if penetrating trauma, injection drug use, or systemic signs present 1
• Osteomyelitis: Deep or chronic ulcer (>6 weeks), bone visible or palpable with probe, or radiographic bone destruction 1, 4
• Necrotizing soft tissue infection: Gas on imaging, compartment syndrome, or systemic sepsis—requires emergent surgical intervention 1

Venous Causes

• Chronic venous insufficiency/stasis dermatitis: Aching, heaviness, cramping that worsens at day's end and improves with leg elevation; varicose veins present; hemosiderin deposition and lipodermatosclerosis on examination 1, 2
• Post-thrombotic syndrome: History of prior DVT with persistent swelling, skin changes, and potential ulceration 1

Other Causes

• Lymphedema: Excessive accumulation of protein-rich fluid; typically non-pitting initially, progresses to fibrotic changes 2
• Retained foreign body: History of puncture wound; may trigger granulomatous reaction and subsequent infection 1

Management Algorithm

For Confirmed or Suspected Infection

Mild Infection (No Systemic Signs)

• Antimicrobial agent active against streptococci (e.g., cephalexin, dicloxacillin) for typical cellulitis 1
• Duration: 5 days minimum, extend if not improved 1
• Elevation of affected extremity 1
• Examine interdigital toe spaces carefully—treating fissuring, scaling, or maceration reduces colonization and recurrent infection 1

Moderate Infection (Systemic Signs Present)

• Include coverage against MSSA in addition to streptococci 1
• Consider hospitalization if concern for deeper infection, poor adherence, severe immunocompromise, or outpatient treatment failure 1

Severe Infection (SIRS, Penetrating Trauma, MRSA Risk Factors)

• Vancomycin or another agent effective against both MRSA and streptococci 1
• For severely compromised patients: vancomycin plus piperacillin-tazobactam or imipenem/meropenem as empiric broad-spectrum coverage 1
• Urgent surgical consultation for deep abscesses, compartment syndrome, or necrotizing infection 1

Osteomyelitis Management

• Obtain bone specimen at time of surgery for culture and histopathology 1
• Prolonged antibiotic therapy (typically 4-6 weeks minimum) guided by culture results 1
• Surgical debridement often necessary, especially with destroyed soft tissue envelope or progressive bone destruction 1

For Venous Insufficiency/Stasis Dermatitis

• Compression therapy is the cornerstone: minimum 20-30 mm Hg pressure, 30-40 mm Hg for more severe disease 1
• Caution: Check ankle-brachial index before compression—if ABI <0.6, arterial revascularization is needed first; if 0.6-0.9, use reduced compression (20-30 mm Hg) 1
• Topical corticosteroids for associated dermatitis 1
• Treat predisposing factors: obesity, edema, toe web abnormalities 1
• For venous ulcers: compression therapy heals ulcers more quickly than primary dressings alone 1

For Confirmed DVT

• Initiate therapeutic anticoagulation immediately following standard DVT protocols 2, 3
• Investigate for underlying causes (malignancy, hypercoagulable state, indwelling devices) 2

Critical Pitfalls to Avoid

• Never assume absence of pain rules out serious pathology in diabetic patients—sensory neuropathy may mask pain from osteomyelitis or infection 4
• Do not rely on superficial wound swabs for culture—they yield more organisms than deep tissue but may miss true pathogens 1
• Avoid debriding dry eschar in ischemic feet—these often resolve with autoamputation 1
• Do not use topical antimicrobial dressings routinely—they show no benefit for venous ulcers 1
• Temperature asymmetry >2°C between limbs suggests active inflammatory process (DVT, infection, or Charcot arthropathy) requiring urgent evaluation 2

When to Refer Urgently

• Evidence of deep-space infection, abscess, or limb-threatening infection requires immediate surgical consultation 1, 4
• Suspected necrotizing fasciitis or compartment syndrome requires emergent surgical intervention 1
• Confirmed osteomyelitis requiring bone biopsy or extensive debridement 1, 4
• Critical limb ischemia (ABI <0.5) requires vascular surgery referral 1
• Suspected active Charcot neuro-osteoarthropathy requires immediate subspecialist referral to prevent progression 4