What are the most likely differential diagnoses for unilateral foot pain, primarily affecting the dorsum and sole, in a patient with no specified past medical history or demographics?

Differential Diagnoses for Unilateral Foot Pain Affecting Dorsum and Sole

For unilateral foot pain involving both the dorsum and sole of the foot from the ankle level distally, the most likely diagnoses are Charcot neuro-osteoarthropathy (if diabetic with neuropathy), peripheral artery disease with critical limb ischemia, stress fractures, or nerve entrapment syndromes such as tarsal tunnel syndrome or Baxter's neuropathy.

Primary Diagnostic Considerations

Charcot Neuro-Osteoarthropathy (CNO)

• Always suspect CNO when a patient with diabetes and neuropathy presents with a unilateral red, warm, swollen foot with intact skin and no history of ulceration 1
• Clinical signs include hyperemia, increased foot skin temperature, and edema affecting both dorsal and plantar surfaces 1
• Pain may be absent or relatively mild due to sensory neuropathy, though some individuals present with more severe pain despite peripheral neuropathy 1
• Initiate knee-high immobilization/offloading promptly while performing diagnostic studies, as untreated CNO carries high risk of bone fractures, dislocations, deformity, ulceration, infection, and amputation 1
• Use infrared thermometry to measure temperature difference between feet; a temperature difference >2°C between the affected and unaffected foot supports the diagnosis 1

Peripheral Artery Disease (PAD) with Critical Limb Ischemia

• Tibial artery occlusive disease may produce calf pain or, more rarely, foot pain and numbness affecting both dorsal and plantar surfaces 1
• Critical limb ischemia presents with rest pain localized in the foot, often with permanent coldness in the feet 1
• Rest pain should be distinguished from muscle cramping or arthritis and is present in the supine position 1
• Examine all lower extremity pulses (femoral, popliteal, dorsalis pedis, and posterior tibial); absence of posterior tibial and dorsalis pedis pulses suggests PAD 1
• Look for elevation pallor/dependent rubor, asymmetric hair growth, and calf muscle atrophy 1

Stress Fractures

• Stress fractures of the metatarsals, navicular, cuboid, or cuneiform bones can cause diffuse foot pain affecting both dorsal and plantar aspects 2
• Initial radiographs detect only 12-56% of stress fractures early in their course 3, 2
• Point tenderness over specific bones with inability to bear weight suggests fracture 3
• High-level athletic activity or recent increase in training intensity increases risk 3

Nerve Entrapment Syndromes

• Baxter's neuropathy (compression of the inferior calcaneal nerve) causes heel pain radiating to the plantar foot due to calcaneal enthesophytes, plantar fasciitis, or varices 1
• Tarsal tunnel syndrome can cause diffuse foot pain affecting both dorsal and plantar surfaces with burning or tingling sensations 4
• These conditions require MRI to demonstrate denervation changes in affected muscles 1

Diagnostic Algorithm

Step 1: Initial Clinical Assessment

• Remove all lower extremity garments, including shoes and socks, for thorough examination 1
• Palpate all four lower extremity pulses bilaterally and rate as 0 (absent), 1 (diminished), 2 (normal), or 3 (bounding) 1
• Assess for diabetes and peripheral neuropathy, as these dramatically alter the differential diagnosis 1
• Measure skin temperature difference between feet using infrared thermometry if CNO is suspected 1
• Evaluate for signs of inflammation (warmth, erythema, swelling) versus ischemia (pallor, coolness, absent pulses) 1

Step 2: Initial Imaging

• Obtain weight-bearing plain radiographs of the foot as the first-line study, including anteroposterior, lateral, and oblique views 1, 2
• Weight-bearing views are essential for proper evaluation of alignment and pressure distribution 3
• Radiographs may show fractures, arthritis, or bone destruction in CNO 1

Step 3: Advanced Imaging When Radiographs Are Negative or Equivocal

• If clinical suspicion remains high despite negative radiographs, proceed directly to MRI of the foot without contrast 1, 2
• MRI has 87-93% sensitivity for detecting occult stress fractures and can detect CNO changes early 3, 4
• MRI demonstrates denervation changes in nerve entrapment syndromes and can exclude other pathologies 1, 4
• Ultrasound is an alternative for plantar fascia pathology (80% sensitivity, 88% specificity) and allows dynamic examination 4

Step 4: Vascular Assessment if PAD Suspected

• Measure ankle-brachial index (ABI); an ABI <0.90 defines PAD with 79% sensitivity and 96% specificity 1
• An ABI <0.50 indicates high risk of amputation and critical limb ischemia 1
• Auscultate for femoral bruits as signs of systemic atherosclerosis 1

Additional Differential Diagnoses to Consider

Plantar Fasciitis with Dorsal Extension

• Causes plantar heel pain that may radiate to the dorsal foot 1, 4
• MRI or ultrasound shows plantar fascia thickening, focal echogenicity changes, and perifascial edema 4

Complex Regional Pain Syndrome (CRPS) Type I

• Presents with persistent posttraumatic foot pain affecting both dorsal and plantar surfaces 1
• Three-phase bone scan or MRI with contrast assists in diagnosis 1

Infection/Osteomyelitis

• In diabetic or immunocompromised patients with foot pain, always consider infection requiring urgent evaluation 3
• Look for skin breakdown, erythema, warmth, and systemic signs 1

Critical Pitfalls to Avoid

• Do not rely solely on initial radiographs to exclude stress fractures, as they miss 44-88% early in the course 3, 2
• Do not delay immobilization in suspected CNO; the consequences of untreated disease include fracture, dislocation, deformity, ulceration, and limb loss 1
• Do not miss PAD in patients with risk factors (smoking, diabetes, hypertension, hyperlipidemia); always palpate pulses and consider ABI measurement 1
• Do not order ankle radiographs when foot pathology is suspected, as they will not properly visualize the metatarsal bases or midfoot 3
• In patients with diabetes, neuropathy, chronic renal failure, or infection who develop acute limb symptoms, assess immediately as these represent potential vascular emergencies 1