What are the typical Nerve Conduction Velocity (NCV) findings in patients with tarsal tunnel syndrome?

NCV Findings in Tarsal Tunnel Syndrome

In tarsal tunnel syndrome, the most diagnostically valuable NCV finding is abnormal sensory nerve conduction velocity in the medial and lateral plantar nerves, which demonstrates superior sensitivity compared to motor studies, with absent nerve potentials or slowed conduction velocities present in approximately 90% of confirmed cases.

Key Diagnostic Parameters

Sensory Nerve Studies (Primary Diagnostic Tool)

• Sensory nerve conduction velocity abnormalities in the medial and lateral plantar nerves represent the superior objective diagnostic index for tarsal tunnel syndrome 1
• Sensory nerve action potential (SNAP) abnormalities occur in 19 of 21 cases (90%), manifesting as either absent nerve potentials or slow sensory nerve conduction velocity 1
• SNAPs demonstrate higher sensitivity but lower specificity compared to mixed nerve action potentials 2
• Record CMAP amplitudes to assess for axonal damage, as progressive reduction correlates with nerve injury severity 3
• Progressive reduction of sensory nerve action potential amplitude indicates axonal degeneration 3

Motor Nerve Studies (Secondary Findings)

• Prolonged terminal latency of the posterior tibial nerve occurs in approximately 52% of cases (11 of 21) 1
• Across-tarsal-tunnel motor nerve conduction shows mean + 2SD onset latency of 3.2 msec for both medial and lateral plantar nerves 4
• Peak latency measurements are 2.9 msec for both divisions 4
• Amplitude decrement across the tunnel averages 29.3% for medial plantar nerve and 27.2% for lateral plantar nerve 4
• Abnormally slow nerve conduction through the posterior tibial nerve predicts failure of conservative treatment 5

Mixed Nerve Action Potentials

• Mixed nerve action potentials after stimulation of the plantar nerves are recommended for presurgical diagnosis due to higher specificity 2
• While less sensitive than SNAPs, mixed nerve action potentials carry lower risk of false-positive results, which is critical for surgical decision-making 2
• Coexistence of both mixed nerve and sensory nerve action potential abnormalities, especially if asymmetric, is highly indicative of tarsal tunnel syndrome 2

Critical Diagnostic Considerations

When Standard NCV May Be Normal

• Standard neurophysiological techniques may be completely normal in small fiber neuropathies affecting C and Aδ fibers 3
• Early in disease, all electrodiagnostic studies may be normal, requiring repeat testing in 3-4 weeks if clinical suspicion remains high 6
• If clinical suspicion is high but all nerve conduction studies are normal, consider that tarsal tunnel syndrome can involve small fibers not detected by standard NCV 3

Complementary Testing When NCV Is Normal

• Skin biopsy examined by an experienced pathologist is the gold standard for small fiber involvement 3
• If autonomic symptoms are present, include sympathetic skin response and heart rate variability testing 3
• Electromyography may demonstrate chronic neurogenic changes due to motor axonopathy but is not routinely needed for diagnosis 3
• Consider EMG if there is clinical evidence of muscle atrophy or weakness 3

Technical Factors Affecting Interpretation

Comparison Standards

• Always compare the affected side with the contralateral healthy side in unilateral cases to establish diagnostic accuracy 2
• Normal motor NCV ranges from 40-50 m/s in lower limbs 7
• Temperature significantly affects results and must be controlled or corrected for accurate interpretation 7

Imaging Correlation

• Ultrasound of the foot may be performed when there is high clinical suspicion, as it can identify space-occupying lesions, anatomic variants, and nerve enlargement 6
• Serial neurologic examinations are recommended over repeated electromyography/nerve conduction studies for monitoring disease activity 6

Common Pitfalls to Avoid

• Do not rely solely on motor terminal latency measurements, as these are abnormal in only approximately half of cases while sensory studies detect 90% 1
• Avoid dismissing the diagnosis based on normal initial NCV studies in early disease; repeat testing in 3-4 weeks is warranted with persistent clinical suspicion 6
• Do not overlook small fiber involvement, which requires skin biopsy rather than standard NCV for diagnosis 3
• Ensure proper temperature control during testing, as this significantly impacts result accuracy 7