What is the appropriate workup for left foot numbness?

Workup for Left Foot Numbness

Begin with a comprehensive vascular and neurological examination, including pulse palpation, monofilament testing, and ankle-brachial index (ABI) measurement, as foot numbness may indicate peripheral artery disease (PAD), diabetic neuropathy, or nerve compression—conditions that significantly impact morbidity and quality of life if left undiagnosed. 1

Initial Clinical Assessment

History Taking

Focus your history on these specific elements:

• Vascular symptoms: Ask about exertional leg symptoms (aching, burning, cramping in buttock, thigh, calf, or ankle), rest pain, and whether symptoms resolve within 10 minutes of rest 1
• Neuropathic symptoms: Inquire about pain, burning sensations, paresthesias, and duration of numbness 1
• Risk factor assessment: Document diabetes status, smoking history, hypertension, dyslipidemia, chronic kidney disease, age ≥50 years, and known atherosclerotic disease in other vascular beds 1
• Wound history: Ask about any current or previous foot ulcers, slow-healing wounds, or skin changes 1

Physical Examination

Vascular examination (perform immediately):

• Palpate all lower extremity pulses: femoral, popliteal, dorsalis pedis, and posterior tibial arteries 1
• Auscultate for vascular bruits in the groin 1
• Inspect for elevation pallor, dependent rubor, asymmetric hair growth, nail bed changes, and calf muscle atrophy 1

Neurological examination (perform immediately):

• Use 10-g monofilament testing at multiple plantar sites to detect loss of protective sensation (LOPS) 1, 2
• Test vibration sensation using a 128-Hz tuning fork 2
• Assess pinprick and temperature sensation 2
• Check ankle reflexes 2

Note: At least two normal tests (with no abnormal test) rules out LOPS, while absent monofilament sensation suggests LOPS 2. Normal foot sensation threshold is approximately 3.63 on monofilament testing; inability to feel 5.07 represents loss of 98% of sensory ability 3

Diagnostic Testing

First-Line Testing

Ankle-Brachial Index (ABI) 1, 4:

• Perform ABI immediately if any abnormal pulse findings or vascular symptoms are present 1
• ABI >1.4 suggests non-compressible vessels (common in diabetes/renal disease) and requires toe-brachial index (TBI) 4, 5
• Critical threshold: TBI <0.4 indicates critical limb-threatening ischemia requiring urgent imaging and revascularization evaluation 4

Laboratory tests:

• HbA1c to diagnose or assess diabetes control 1, 5
• Serum creatinine and eGFR to evaluate renal function 5
• Fasting glucose if diabetes not yet diagnosed 1
• Complete blood count and CRP if infection suspected 1

Advanced Imaging (When Indicated)

Plain radiographs 1:

• Obtain weight-bearing foot X-rays if structural deformity, Charcot arthropathy, or osteomyelitis suspected 1

Vascular imaging (if ABI/TBI abnormal or high clinical suspicion):

• Duplex ultrasound as first-line imaging for arterial anatomy assessment 4
• CT angiography or MR angiography when revascularization is being considered, especially if TBI <0.4 4
• Catheter angiography for below-the-knee lesions to guide intervention 4

Perfusion measurements (if wound present or TBI <0.4):

• Transcutaneous oxygen pressure (TcPO₂): values >30 mm Hg predict wound healing 4
• Skin perfusion pressure (SPP): values >40 mm Hg associated with increased healing likelihood 4

Risk Stratification and Referral

High-Risk Features Requiring Urgent Evaluation

• TBI <0.4 (requires urgent vascular surgery consultation) 4
• Non-healing ulcer with signs of infection (fever, elevated WBC, erythema >1.5 cm around wound) 1
• Absent pulses with rest pain or tissue loss 1
• Rapid deterioration of existing wound 1

Moderate-Risk Features Requiring Specialist Referral

• Loss of protective sensation on monofilament testing 1, 2
• Foot deformity with numbness 1
• ABI 0.4-0.9 or TBI 0.4-0.7 1, 4

Common Pitfalls to Avoid

• Do not rely on ABI alone in diabetic patients: ABI >1.4 due to calcified vessels can mask severe PAD; always obtain TBI in this population 4, 5
• Do not interpret monofilament testing in isolation: Combine with at least one other sensory test (vibration, pinprick, or ankle reflexes) 2
• Do not delay vascular imaging if TBI <0.4: This represents critical limb-threatening ischemia requiring urgent revascularization evaluation 4
• Do not assume numbness is "just neuropathy": Always assess vascular status, as PAD and neuropathy frequently coexist and both impact outcomes 1

Differential Diagnosis Considerations

While performing the workup, consider alternative causes if vascular and neuropathic testing is normal:

• Lumbar radiculopathy or spinal stenosis (symptoms worse with sitting, relieved when supine) 1
• Nerve root compression (sharp lancinating pain radiating down leg) 1
• Foot/ankle arthritis (aching pain after variable exercise, not quickly relieved by rest) 1