Diagnosing Tarsal Tunnel Syndrome with NCS and EMG
The diagnosis of tarsal tunnel syndrome using electrodiagnostic testing relies primarily on demonstrating abnormal nerve conduction across the tarsal tunnel, with mixed nerve action potentials being the most specific test, though no single electrodiagnostic parameter diagnoses TTS with certainty. 1, 2
Key Electrodiagnostic Findings
Nerve Conduction Studies (Primary Diagnostic Tool)
Mixed nerve action potentials (MNAPs) after stimulation of the plantar nerves are the most reliable test, offering high specificity with fewer false-positives compared to sensory studies alone, making them essential for presurgical confirmation. 2
Sensory nerve action potentials (SNAPs) are more sensitive but less specific than MNAPs, and sensory responses are frequently absent in TTS (a nonlocalizing finding that limits diagnostic utility). 3, 2
Coexistence of both MNAP and SNAP abnormalities, especially if asymmetric, is highly indicative of TTS and strengthens the diagnosis. 2
Segmental conduction velocity measurements across the flexor retinaculum can be obtained using near-nerve recording techniques, which theoretically improve sensitivity by isolating the exact compression site. 3
Motor Nerve Conduction Studies
Across-tarsal-tunnel motor latencies should be measured for both medial and lateral plantar nerves, with abnormal values defined as onset latency >3.2 msec, peak latency >2.9 msec, or amplitude decrement >29.3% for medial plantar nerve and >27.2% for lateral plantar nerve. 4
Motor distal latency abnormalities are present in only 52% of TTS cases, making motor studies alone insufficient for diagnosis. 3
Abnormally slow nerve conduction through the posterior tibial nerve predicts failure of conservative treatment and may guide surgical decision-making. 1
Electromyography Findings
EMG examination of intrinsic foot muscles may show denervation changes in chronic or severe cases, though EMG is less sensitive than NCS for TTS diagnosis. 1
Critical Diagnostic Algorithm
Step 1: Perform Bilateral Comparison
- Always compare the affected side with the contralateral healthy side to calculate the diagnostic value of each neurophysiologic parameter, as this improves accuracy. 2
Step 2: Prioritize Testing Sequence
- Begin with mixed nerve action potentials of the medial and lateral plantar nerves as the primary diagnostic test. 2
- Add sensory nerve conduction studies if MNAPs are equivocal or to increase diagnostic confidence. 2
- Measure across-tarsal-tunnel motor latencies to localize compression precisely to the proximal tarsal tunnel versus distal sites. 4
Step 3: Interpret Results in Clinical Context
- One or more abnormal neurophysiologic parameters should be present to support the diagnosis, though no test alone is definitive. 2
- Correlation with clinical history, imaging, and physical examination (particularly positive Tinel's sign and dorsiflexion-eversion test) is essential, as electrodiagnostic studies lack perfect sensitivity. 1, 5
Important Limitations and Pitfalls
Normal electrodiagnostic studies do not exclude TTS, as conventional techniques only assess large myelinated fibers and may miss early or mild compression before significant axonal loss occurs. 6
Sensory responses are frequently absent in TTS, which is a nonlocalizing finding that does not confirm or exclude the diagnosis. 3
Results are operator-dependent and require specialized training and expert interpretation in the context of clinical findings. 7
Early in the disease process, electrodiagnostic results may be normal or nearly normal, requiring clinical judgment to proceed with treatment. 7
Predictive Value for Treatment Decisions
A positive Tinel's sign before surgery combined with abnormal nerve conduction studies strongly predicts successful surgical decompression. 1
Abnormally slow nerve conduction through the posterior tibial nerve usually predicts failure of conservative treatment and supports surgical intervention. 1
Clear identification of compression on electrodiagnostic testing combined with failed conservative management represents the primary indication for surgical decompression. 1