What are the possible causes of a burning sensation on the sole of the foot with unexplained shoe slipping?

Burning Sensation of Sole with Shoe Slipping: Differential Diagnosis

The combination of burning foot pain and unknowing shoe slippage strongly suggests diabetic peripheral neuropathy or small fiber sensory neuropathy, with the shoe slipping indicating loss of protective sensation (LOPS) that prevents awareness of footwear displacement. 1

Primary Diagnostic Considerations

Diabetic Peripheral Neuropathy (Most Likely)

• Burning pain affecting the soles is the hallmark symptom of small fiber involvement in diabetic neuropathy, often described as "walking barefoot on hot sand" or "walking barefoot on marbles" 1
• The unconscious shoe slipping indicates loss of protective sensation (LOPS) from large fiber dysfunction, which is a critical risk factor for diabetic foot ulceration 1
• Up to 50% of patients with type 2 diabetes develop neuropathy, and up to 50% of those with neuropathy experience painful symptoms 1
• Symptoms characteristically worsen at night, often causing sleep disturbance 1

Small Fiber Sensory Neuropathy

• Presents as burning pain in the feet with minimal objective findings on examination, despite distressing subjective symptoms 2, 3
• Can occur with normal nerve conduction studies, requiring skin biopsy for diagnosis 3, 4
• Associated with diabetes, impaired glucose tolerance, metabolic syndrome components, or may be idiopathic 4

Clinical Examination Findings to Assess

The combination of symptoms affecting both small and large fibers requires systematic evaluation:

Small Fiber Assessment

• Test pinprick and temperature sensation to evaluate small fiber function 1
• Look for allodynia (pain from normally non-painful stimuli like bedclothes or socks) 1
• Assess for dysesthesias (unpleasant burning and tingling sensations) 1

Large Fiber and Protective Sensation Assessment

• Perform 10-g monofilament testing annually to identify feet at risk for ulceration 1
• Test vibration perception using a 128-Hz tuning fork for large fiber function 1
• Assess lower extremity reflexes, particularly ankle reflexes 1
• The calcaneal compression test (squeezing calcaneus medially to laterally) can help identify stress fractures if present 5

Autonomic Involvement

• Examine for abnormalities of sweating in the feet, which indicates autonomic fiber involvement 1
• Assess for circulatory instability in the feet 1

Diagnostic Algorithm

For patients with type 2 diabetes or those with type 1 diabetes >5 years duration:

1. Annual screening for diabetic peripheral neuropathy is mandatory 1
2. If nerve conduction studies are normal but symptoms persist, validated measures of small fiber neuropathy should be used 1
3. Skin biopsy with intraepidermal nerve fiber density (IENF) quantification has sensitivity of 77-88% and specificity of 80-89% for small fiber neuropathy 1
4. Consider autonomic testing (heart rate variability, sudomotor function tests) as autonomic dysfunction occurs concurrently with small fiber involvement 1

For non-diabetic patients:

1. Screen for impaired glucose tolerance and metabolic syndrome components 4
2. Evaluate for other causes: chronic infections (HIV), toxicity from drugs, autoimmune diseases, nutritional deficiencies (B12, folate) 2, 6
3. If standard electrophysiologic tests are normal, proceed with skin biopsy as it is more sensitive than quantitative sudomotor or sensory tests 3

Additional Causes to Consider

Metabolic and Nutritional

• Alcohol-nutritional neuropathy can present with burning feet syndrome 7
• Vitamin B12 deficiency and other nutritional deficiencies 6

Structural/Mechanical (Less Likely Given Presentation)

• Haglund's deformity with retrocalcaneal bursitis causes posterior and lateral heel pain, with pain relieved when walking barefoot but worsened by shoes 5
• However, this typically does not cause burning sensations or loss of protective sensation 5

Neurologic

• Sympathetically maintained pain (rare) can cause bilateral burning foot pain with autonomic disturbances 7
• Central causes (e.g., multiple sclerosis) are rare but should be considered if other neurologic signs present 3

Critical Pitfalls to Avoid

• Do not assume normal nerve conduction studies exclude neuropathy - over one-third of patients with painful feet require skin biopsy to diagnose small fiber sensory neuropathy 3
• The absence of symptoms does not mean feet are healthy - patients may have neuropathy, peripheral vascular disease, or even ulcers without complaints due to sensory loss 1
• Electrophysiological testing shows minimal correlation with small fiber and autonomic function in diabetic patients 1
• Loss of protective sensation is a critical risk factor for foot ulceration - the unconscious shoe slipping in this patient indicates urgent need for preventive foot care education 1

Management Priorities

Optimize glucose control immediately to prevent or delay neuropathy progression 1

Initiate preventive foot care:

• Patients with demonstrated risk factors require examination every 1-6 months 1
• Educate about daily foot inspection and proper footwear 1
• Never walk barefoot due to loss of protective sensation 1

For painful symptoms:

• First-line pharmacologic treatment includes tricyclic antidepressants, SNRIs (duloxetine), or anticonvulsants (pregabalin, gabapentin) 1
• Second-line approaches include opioids (tramadol, morphine, oxycodone) 1