What causes swelling in one foot?

Unilateral Foot Swelling: Differential Diagnosis and Diagnostic Approach

Unilateral foot swelling requires immediate evaluation to exclude life-threatening conditions including deep vein thrombosis, infection (cellulitis, abscess, osteomyelitis), and in diabetic patients with neuropathy, Charcot neuro-osteoarthropathy—all of which demand urgent intervention to prevent catastrophic outcomes including limb loss.

Immediate Life-Threatening Diagnoses to Exclude

Deep Vein Thrombosis

• Unilateral swelling indicates an obstructive process at the level of the brachiocephalic, subclavian, or axillary veins 1
• Patients typically present with ipsilateral extremity edema, pain, and paresthesia 1
• Must be excluded with appropriate clinical assessment and D-dimer/ultrasound if indicated 2, 3
• Ultrasound duplex Doppler is the initial imaging modality of choice, with diagnostic sensitivities and specificities above 80% 1

Infection (Cellulitis, Abscess, Osteomyelitis)

• Infection requires at least two of the following clinical signs: local swelling/induration, erythema >0.5 cm, local tenderness/pain, local warmth, or purulent discharge 1, 2, 3, 4
• Fever, elevated white blood cell count, or elevated C-reactive protein suggest infection, though these may be absent in up to half of patients with significant infections 4
• A swollen foot in a diabetic patient with history of foot ulceration should raise suspicion for osteomyelitis 1
• MRI is the most accurate imaging study for defining bone infection 1

Charcot Neuro-Osteoarthropathy (in Diabetic Patients)

• Suspect active Charcot in any diabetic patient with neuropathy presenting with a unilateral red, warm, swollen foot with intact skin 2, 3
• A temperature difference ≥2°C between the affected and unaffected foot at the same anatomical location strongly suggests active CNO 2, 3
• Initiate knee-high immobilization/offloading immediately, even before imaging confirmation—this is critical as delayed treatment leads to devastating consequences including bone fractures, dislocations, deformity, ulceration, and amputation 2
• Plain radiographs may be normal in early CNO; MRI is the preferred advanced imaging showing bone marrow edema 2
• Never delay immobilization while waiting for imaging in diabetic patients with neuropathy and a hot swollen foot 2, 3

Other Important Differential Diagnoses

Trauma

• Foot fractures typically present with bony point tenderness and swelling of the affected area 5
• Ottawa foot and ankle rules can help determine the need for radiography after acute ankle inversion injury 3, 5
• Plain radiographs (anteroposterior, lateral, and oblique views with weight-bearing) should be obtained when fracture is suspected 5

Gout

• Consider in patients with acute onset, severe pain, and history of hyperuricemia or prior attacks 2, 3, 4
• Joint aspiration showing urate crystals is diagnostic 4

Venous Insufficiency

• Chronic venous insufficiency can cause unilateral leg edema 6
• Isolated foot lesions with venous cause may arise from insufficient perforator veins with accompanying areas of hemosiderosis 7

Lymphedema

• May be primary or secondary to neoplasm, lymphangitis, or retroperitoneal fibrosis 6
• Lymphatic obstruction leads to swelling through impaired fluid drainage 1

Other Systemic Causes

• Heart failure, cirrhosis, nephrosis, and hypoproteinemic states typically cause bilateral edema but can present unilaterally 6
• Medication-related edema (e.g., calcium channel blockers) usually presents bilaterally 6

Diagnostic Algorithm

Step 1: Initial Clinical Assessment

• Measure skin temperature using infrared thermometry at multiple sites on both feet, comparing the highest temperature on affected foot to the same anatomic point contralaterally 2, 3
• Palpate for point tenderness at Achilles insertion, lateral malleolus, posterior superior calcaneus, lateral calcaneal wall, and calcaneofibular ligament 2, 4
• Perform calcaneal compression test (medial-to-lateral squeeze) to evaluate for stress fracture 2
• Assess for signs of infection: erythema, warmth, tenderness, purulent discharge 1, 2, 3, 4
• Check for visible or palpable bone in any ulcer (suggests osteomyelitis) 1

Step 2: Initial Imaging

• Obtain plain radiographs of the foot in all cases to evaluate for fractures, dislocations, or bone destruction 1, 2, 5
• Plain films may be normal in early stress fractures, early osteomyelitis, and early Charcot 1, 2, 4

Step 3: Advanced Imaging (When Indicated)

• For suspected DVT: Ultrasound duplex Doppler 1
• For suspected Charcot or osteomyelitis with negative X-rays: MRI is the preferred modality 1, 2
• For suspected stress fracture with negative X-rays: Technetium bone scan or MRI 2, 4
• For suspected soft tissue infection or abscess: Ultrasound to identify fluid collections 4

Critical Pitfalls to Avoid

• Never assume absence of pain rules out serious pathology in diabetic patients with neuropathy—pain may be minimal or absent due to sensory neuropathy 2, 3
• Do not assume absence of fever or elevated white blood cell count excludes infection 4
• Do not diagnose based on imaging alone—combine symptoms, clinical signs, and imaging findings 2
• Other inflammatory conditions must be excluded: trauma, gout, acute Charcot, fracture, thrombosis, and venous stasis 2
• The presence of ulceration and infection does not preclude underlying active Charcot—both can coexist 2

When to Refer Urgently

• Immediate subspecialist referral for suspected active Charcot neuro-osteoarthropathy to prevent progression to deformity and ulceration 2, 3
• Suspected osteomyelitis requiring bone biopsy for definitive diagnosis or antibiotic sensitivity testing 1, 4
• Suspected septic arthritis requiring joint aspiration 3
• No improvement after 6-8 weeks of appropriate conservative treatment 2, 4