What could be causing numbness in my left foot, and how should it be evaluated and treated in an adult with no known medical history?

Evaluation and Management of Left Foot Numbness

Screen immediately for diabetes with hemoglobin A1c or fasting glucose, as diabetic peripheral neuropathy is the most common cause of foot numbness and affects up to 50% of diabetic patients, often beginning asymmetrically before becoming bilateral. 1

Initial Clinical Assessment

Determine critical symptom characteristics and timing:

• Document duration of numbness, whether constant or intermittent, and any progression pattern 1
• Ask specifically about burning sensations, sharp pain, tingling, or purely sensory changes to assess small fiber involvement 1
• Determine if symptoms worsen at night (typical of diabetic neuropathy) or with exertion (suggests vascular disease) 2, 3
• Inquire about trauma history, as up to 50% of diabetic neuropathy can be asymptomatic and patients may not recall minor injuries 4, 2

Screen for systemic risk factors:

• Diabetes history, alcohol use, vitamin B12 deficiency, family history of neuropathy, current medications, and autoimmune conditions 1
• Smoking history, hypertension, hyperlipidemia (risk factors for peripheral arterial disease) 3
• History of claudication, leg fatigue, or rest pain (suggests vascular compromise) 4, 3

Physical Examination

Perform comprehensive neurological foot examination:

Small fiber function assessment:

• Test pinprick sensation in the left foot, particularly the toes 1
• Assess temperature sensation using cold tuning fork or similar instrument 1

Large fiber function assessment:

• Perform vibration testing with 128-Hz tuning fork on bony prominences 1, 3
• Check ankle reflexes bilaterally for comparison 1

Protective sensation assessment:

• Perform 10-g monofilament testing at multiple plantar sites (balls of feet, toes) - this is the single most important test for ulcer risk stratification 1, 3
• Loss of monofilament sensation indicates loss of protective sensation (LOPS) and high risk for foot ulceration 4

Vascular assessment:

• Palpate and grade dorsalis pedis and posterior tibial pulses bilaterally (0=absent, 1=diminished, 2=normal, 3=bounding) 3
• Assess capillary refill time, rubor on dependency, pallor on elevation 4
• Listen for femoral bruits indicating stenosis 3

Structural examination:

• Inspect skin integrity for ulcers, calluses, or breakdown 4
• Assess for foot deformities (bunions, hammertoes, prominent metatarsals) that increase ulcer risk 4

Laboratory Workup

Order the following tests to identify treatable causes:

• Hemoglobin A1c or fasting glucose (diabetes is the most common cause) 1, 3
• Vitamin B12 level (deficiency causes sensory neuropathy) 1, 3
• Thyroid function tests (hypothyroidism causes neuropathy) 1
• Complete blood count (screen for anemia or hematologic disorders) 1
• Comprehensive metabolic panel (assess renal function and electrolytes) 1

Imaging and Advanced Testing

Imaging is NOT routinely indicated unless specific red flags are present 1

Do NOT order routine foot radiographs unless:

• History of trauma with inability to bear weight 1
• Point tenderness over bony structures per Ottawa Ankle Rules 1
• Suspicion of Charcot foot or osteomyelitis 4

Consider ankle-brachial index (ABI) if:

• Diminished or absent pedal pulses 4
• History of claudication or rest pain 3
• Symptoms worsen with exertion 3
• ABI <0.4 indicates critical limb ischemia requiring urgent vascular referral 3

Electrodiagnostic testing (EMG/NCS) is rarely needed but reserve for:

• Atypical clinical features 1
• Diagnosis remains unclear after initial workup 1
• Unilateral presentation without clear cause (consider central causes like spinal stenosis or parasagittal lesions) 5

Management Algorithm

If laboratory tests show abnormalities:

• Optimize glycemic control if diabetic (target HbA1c based on individual factors) 3
• Supplement vitamin B12 if deficient 1
• Treat hypothyroidism with thyroid replacement 1

If symptoms persist beyond 6-8 weeks despite treating underlying conditions:

• Refer to neurology for electrodiagnostic studies 1
• Refer to podiatry for custom orthotics and biomechanical assessment 1

If vascular disease identified:

• Urgent vascular surgery referral if ABI <0.4 or rest pain present 3
• Supervised exercise therapy (structured walking program 3 times weekly for ≥12 weeks) for claudication 3
• Aggressive risk factor modification (smoking cessation, lipid management, blood pressure control) 3

Risk Stratification and Follow-up Frequency

Based on International Working Group on the Diabetic Foot (IWGDF) criteria:

• IWGDF Risk 1 (LOPS or PAD alone): Screen every 6-12 months 1
• IWGDF Risk 2 (LOPS + PAD, or either with foot deformity/callus): Screen every 3-6 months 1, 3
• IWGDF Risk 3 (history of ulcer/amputation or end-stage renal disease): Screen every 1-3 months 1

Critical Pitfalls to Avoid

Do not assume symptoms are purely mechanical without screening for diabetes, even in young adults without known medical history 1

Recognize that unilateral presentation does not rule out systemic disease:

• Diabetic neuropathy often begins asymmetrically before becoming bilateral 1
• Up to 50% of diabetic neuropathy may be asymptomatic, so absence of other symptoms does not exclude diagnosis 2

Do not overlook peripheral arterial disease, which can coexist with neuropathy:

• The combination of PAD and neuropathy significantly increases amputation risk and requires aggressive management 3
• Any skin breakdown with PAD mandates immediate systemic antibiotics and urgent vascular referral 3

Beware of red flags requiring urgent evaluation:

• Bilateral symptoms with rapid progression (consider Guillain-Barré syndrome or transverse myelitis) 6
• Bowel/bladder dysfunction (suggests spinal cord pathology) 7
• Progressive weakness (consider central causes like cervical stenosis or parasagittal lesions) 5