Neurology India Medknow Publications on behalf of the Neurological Society of India ISSN: 0028-3886 EISSN: 1998-4022 Vol. 59, Num. 2, 2011, pp. 296-297

Neurology India, Vol. 59, No. 2, March-April, 2011, pp. 296-297

Letter to Editor

Combination of ultrasound and nerve conduction studies in the diagnosis of tarsal tunnel syndrome

AK Therimadasamy¹, RC Seet², YH Kagda³, EP Wilder-Smith²

¹ Neurology Diagnostic Laboratory, National University Hospital, Singapore
² Division of Neurology, National University Hospital, Singapore
³ Department of Orthopaedic Surgery, National University Hospital, Singapore

Correspondence Address: A K Therimadasamy, Neurology Diagnostic Laboratory, National University Hospital, Singapore mdcwse@nus.edu.sg

Date of Submission: 08-Nov-2010
Date of Decision: 28-Nov-2010
Date of Acceptance: 29-Nov-2010

Code Number: ni11080

PMID: 21483139

DOI: 10.4103/0028-3886.79152

Sir,

Tarsal tunnel syndrome is a rare compression neuropathy of the posterior tibial nerve as it travels through the fibro-osseous tunnel at the level of medial ankle. ^[1] Nerve conduction studies are useful in supporting the diagnosis. Combining ultrasound with nerve conduction studies adds a morphological component to functional aspects of nerve damage in entrapment neuropathies. ^[2] We present a case of tarsal tunnel syndrome where this approach provided comprehensive information regarding etiology and location of tibial nerve compression at the tarsal tunnel.

A 44-year-old female presented with numbness involving the right foot sole of 2 months duration. Neurological examination revealed reduced sensation to light touch over the inferior aspects of the toes and positive Tinel's sign at the tarsal tunnel. There was no intrinsic foot muscle wasting and foot architecture was normal. Nerve conduction studies were performed following standard protocol for diagnosing tarsal tunnel syndrome. ^[3] The orthodromic medial and lateral plantar sensory nerve action potentials were absent when recorded above the tarsal tunnel. The medial and lateral plantar mixed nerve action potentials were also absent following mid-foot stimulation. Compound muscle action potentials recorded from abductor hallucis (AH) and abductor digiti quinti (ADQ) muscles showed prolonged distal latency (AH 5.55 vs. 3.80 ms and ADQ 6.55 vs. 5.15 ms) and low amplitude (AH 1.0 vs. 7.3 mV and ADQ 0.8 vs. 2.0 mV) compared to the contralateral side. There was no dispersion of the compound muscle action potential. Electrodiagnostic findings were consistent with a mixed axonal and demyelinating pathology of the posterior tibial nerve at or distal to the tarsal tunnel. Ultrasonography of the posterior tibial nerve was performed with a 10 MHz linear array transducer. The posterior tibial nerve was scanned from above the ankle to its terminal branching at the distal tarsal tunnel. The tibial nerve appeared normal above the ankle, but showed marked enlargement at the tarsal tunnel (cross-sectional area of 0.17 cm ² vs. 0.10 cm ² unaffected side) [Figure - 1] and [Figure - 2]. The Intraneural fascicles were unevenly enlarged with inferior fascicles larger and more hypoechoic. A hypoechoic cystic structure (area 0.14 cm ² ) was observed immediately distal to the nerve enlargement extending from the flexor hallucis longus tendon [Figure - 3]. The cyst was in close communication with the nerve, infiltrating inferior tibial nerve aspects.

Although nerve conduction studies are standard in evaluating entrapment neuropathies, our patient demonstrates the additional value of ultrasonography in eliciting underlying etiology and site of nerve compression in tarsal tunnel syndrome. Nerve conduction studies are limited in localizing the site of tibial nerve compression which can be within the tarsal tunnel or distal to it. ^[4] Ultrasound provides direct evidence of nerve compression by demonstrating focal nerve enlargement and change in nerve echogenicity. In our patient, ultrasound localized the tibial nerve pathology at the tarsal tunnel due to a synovial cyst attached to flexor hallucis tendon. Our patient serves as an example how conventional nerve conduction testing can be complimented by adding ultrasound evaluation, providing comprehensive information for better treatment planning.

References

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