What is the protocol for nerve conduction velocity (NCV) testing in diagnosing tarsal tunnel syndrome?

Nerve Conduction Velocity Protocol for Tarsal Tunnel Syndrome

For diagnosing tarsal tunnel syndrome, perform motor nerve conduction studies of the posterior tibial nerve and its branches (medial and lateral plantar nerves), measuring distal motor latencies, compound muscle action potentials (CMAP), and nerve conduction velocities, with mixed nerve action potentials being the most specific parameter for presurgical confirmation.

Core NCV Testing Protocol

Motor Nerve Conduction Studies (Primary Test)

• Measure motor distal latencies of the posterior tibial nerve to the abductor hallucis and abductor digiti minimi muscles 1, 2
• Record CMAP amplitudes to assess for axonal damage, as progressive reduction correlates with nerve injury severity 3
• Calculate nerve conduction velocities through the tarsal tunnel segment 1, 2
• Motor nerve conduction studies demonstrate 80% sensitivity in symptomatic limbs of idiopathic tarsal tunnel syndrome 2

Mixed Nerve Action Potentials (Most Specific)

• Stimulate the medial and lateral plantar nerves and record mixed nerve action potentials 1
• Mixed nerve action potentials are less sensitive but more specific than sensory nerve action potentials, making them the recommended parameter for presurgical diagnosis to avoid false-positive results 1
• The coexistence of abnormal mixed nerve and sensory nerve action potentials, especially if asymmetric, is highly indicative of tarsal tunnel syndrome 1

Sensory Nerve Action Potentials (Complementary)

• Record sensory nerve action potentials from the medial and lateral plantar nerves after stimulation 1
• Sensory nerve action potentials are more sensitive but less specific than mixed nerve action potentials 1
• Progressive reduction of sensory nerve action potential amplitude indicates axonal degeneration 3

Bilateral Comparison Approach

• Always test both limbs for comparison, as diagnostic accuracy is calculated by comparing the affected side with the healthy side 1
• This is critical because symptoms are bilateral in 70% of idiopathic tarsal tunnel syndrome cases 2
• Asymmetric abnormalities between sides strengthen the diagnosis 1

Enhanced Testing for Negative Basic Studies

Electrodiagnostic Triple Compression Stress Test

• Perform this provocative maneuver when basic nerve conduction studies are negative but clinical suspicion remains high 4
• The ankle is placed in full plantar flexion and the foot in inversion, with constant digital pressure applied over the posterior tibial nerve during nerve conduction testing 4
• This stress test increases sensitivity, detecting 11 additional cases that had false-negative clinical tests in one study 4
• The electrodiagnostic stress test was positive in 78 feet where basic studies may have been equivocal 4

Expected Findings and Interpretation

Demyelinating Pattern (Most Common)

• "Probable demyelination" occurs in 58.2% of electrophysiologically abnormal nerves in idiopathic tarsal tunnel syndrome 2
• Manifests as slowed nerve conduction velocity with relatively preserved CMAP amplitudes initially 3, 2

Axonal Pattern

• Axonopathy is less common than demyelination but still significant 2
• Shows progressive reduction of CMAP amplitude with less pronounced slowing of conduction velocity 3

Prognostic Value

• Abnormally slow nerve conduction through the posterior tibial nerve predicts failure of conservative treatment and may indicate need for surgical intervention 5

Critical Caveats

Small Fiber Neuropathy Limitation

• Standard neurophysiological techniques may be completely normal in small fiber neuropathies affecting C and Aδ fibers 3
• 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
• In such cases, skin biopsy examined by an experienced pathologist is the gold standard 3

Autonomic Involvement

• If autonomic symptoms are present (such as abnormal sweating patterns in the foot), include sympathetic skin response and heart rate variability testing 3
• This is uncommon in tarsal tunnel syndrome but should not be missed 3

Complementary EMG

• 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

When to Order NCV Studies

• Order nerve conduction studies when clinical examination (positive Tinel's sign, dorsiflexion-eversion test, or triple compression stress test) suggests tarsal tunnel syndrome 4, 6, 5
• NCV is essential for presurgical confirmation to exclude other diseases and document objective nerve dysfunction 1, 5
• One or more neurophysiologic parameters should be abnormal to support the diagnosis, as was found in all cases in validated studies 1
• A positive Tinel's sign before surgery combined with abnormal NCV is a strong predictor of surgical relief after decompression 5