What are the possible causes and recommended work‑up for sudden swelling of the left foot?

Sudden Swelling of Left Foot: Differential Diagnosis and Work-Up

Sudden unilateral foot swelling requires urgent evaluation to exclude deep vein thrombosis (DVT), infection (especially in diabetics), acute arterial ischemia, or compartment syndrome—all of which can be limb- or life-threatening.

Immediate Clinical Assessment

Critical Red Flags to Identify Immediately

• Neurological deficits (sensory loss, motor weakness, paralysis): Suggests acute limb ischemia requiring emergency revascularization within 4-6 hours 1
• Systemic signs of infection (fever >38°C, tachycardia >90 bpm, altered mental status): Indicates severe infection requiring urgent surgical consultation 1
• Signs of necrotizing infection (hemorrhagic bullae, skin necrosis, crepitus/gas in tissues): Requires immediate surgical debridement 2, 3
• Non-compressible veins on point-of-care ultrasound: Diagnostic of DVT requiring anticoagulation 1

Focused Physical Examination Elements

For DVT evaluation 1:

• Palpate for tenderness along deep venous system (common femoral and popliteal veins)
• Assess for asymmetric calf/ankle swelling
• Check for warmth, erythema extending from a focal area

For infection (especially in diabetics) 1:

• Look for at least 2 signs of inflammation: erythema >0.5 cm around any wound, local warmth, tenderness/pain, induration, or purulent discharge
• Perform probe-to-bone test if open wound present (positive test in high-risk patient is diagnostic of osteomyelitis)
• Measure erythema extent: <2 cm = mild infection; ≥2 cm = moderate infection

For arterial ischemia 1:

• Palpate pedal pulses (dorsalis pedis, posterior tibial)
• Assess capillary refill, skin temperature, color
• Test sensation (light touch, pinprick) and motor function (dorsiflexion, plantarflexion)

For gout 1:

• Examine first metatarsophalangeal joint for acute monoarticular involvement
• Note if pain reached maximum severity within 24 hours
• Check for overlying erythema

Initial Diagnostic Work-Up Algorithm

First-Line Imaging (Obtain Immediately)

1. Plain radiographs of the foot (3 views: AP, lateral, oblique) 1:

• Identifies fractures, gas in soft tissues (suggests necrotizing infection), foreign bodies
• Shows bone destruction (osteomyelitis), joint effusion (septic arthritis)
• Mandatory before any advanced imaging

2. Point-of-care ultrasound for DVT 1:

• Perform limited compression ultrasound of common femoral and popliteal veins
• Non-compressible vein = DVT present (99% sensitivity when performed properly)
• If negative but high clinical suspicion, arrange repeat scan in 5-7 days for isolated calf DVT

Second-Line Imaging (Based on Initial Findings)

If infection suspected and radiographs show soft tissue swelling 1:

• MRI with contrast (preferred): Distinguishes cellulitis from abscess, myositis, necrotizing fasciitis; diagnoses osteomyelitis with high sensitivity
• CT with IV contrast (if MRI unavailable/contraindicated): Evaluates soft tissue compartments, guides aspiration/surgical planning

If arterial ischemia suspected 1:

• CT angiography or MR angiography: Reveals level and extent of arterial occlusion
• Doppler ultrasound: Assesses arterial flow if limb not immediately threatened
• Obtain urgent vascular surgery consultation if neurological deficits present

If DVT confirmed or PE suspected 1:

• Note: Negative lower extremity ultrasound does NOT rule out pulmonary embolism

Laboratory Studies

For all patients with swelling 1:

• Complete blood count (WBC >12,000 or <4,000 suggests severe infection)
• Erythrocyte sedimentation rate (markedly elevated suggests osteomyelitis)
• Blood glucose (hyperglycemia in diabetics worsens infection)
• Creatinine (renal impairment affects antibiotic dosing and wound healing)

If infection present 1:

• Blood cultures (before antibiotics if systemically ill)
• Wound culture (after debridement, not from superficial swab)
• Bone biopsy culture if osteomyelitis suspected (gold standard, but not always required)

If gout suspected 1:

• Synovial fluid aspiration and crystal analysis (monosodium urate crystals = definitive diagnosis)
• Serum uric acid (elevated supports diagnosis but NOT diagnostic alone)

Common Pitfalls to Avoid

• Do not rely on hyperuricemia alone to diagnose gout—crystal identification is required 1
• Do not mistake superficial veins for deep veins on ultrasound (especially in obese patients) 1
• Do not delay vascular imaging if pulses absent or neurological deficits present—tissue viability window is only 4-6 hours 1
• Do not use soft tissue cultures to guide osteomyelitis treatment—they don't reflect bone pathogens accurately 1
• Do not perform debridement of ischemic ulcers without infection unless revascularization planned 1
• In diabetics, do not assume absence of fever/leukocytosis excludes severe infection—systemic signs often blunted by neuropathy 1

Urgent Consultation Triggers

Obtain immediate surgical consultation if 1, 3:

• Severe infection with systemic inflammatory response syndrome
• Necrotizing infection, extensive gangrene, or gas in tissues
• Deep (subfascial) abscess or compartment syndrome
• Peripheral arterial disease with infected foot ulcer/gangrene

Obtain immediate vascular surgery consultation if 1:

• Neurological deficits (sensory loss or motor weakness)
• Absent pulses with acute symptom onset
• Suspected acute limb ischemia