Can the axial cross-sectional area of the tibial nerve be used to diagnose tarsal tunnel syndrome? An ultrasonography study.

INTRODUCTION: The axial cross-sectional area (CSA) of the tibial nerve can be measured with ultrasonography. In patients who have posteromedial tarsal tunnel syndrome (TTS), there is little information on the nerve's CSA even though this information could be useful for determining whether the nerve is damaged. This led us to carry out a case-control study in which the tibial nerve's axial CSA was measured in healthy patients and in patients with TTS.

HYPOTHESIS: The tibial nerve's axial CSA can be used as a diagnostic criterion for TTS.

METHODS: Twenty-three patients (27 feet) (11 men, 12 women, mean age=54±14 years), who had clinical and electroneuromyography signs of TTS, were compared to 21 healthy adults (8 men, 13 women, mean age 39±10 years). An ultrasonography examination was carried out to look for a source of nerve compression, then the axial CSA of the tibial nerve was measured 10cm above the tarsal tunnel (l CSA) and inside the tunnel itself (tt CSA). The difference between the two measurements was then calculated: Δ CSA=tt CSA-l CSA. The data were analysed using correlation tests and non-parametric tests, a multivariate linear regression and ROC tests.

RESULTS: A compressive cause was found by ultrasonography in 13 patients. The mean values of tt CSA and Δ CSA were 20.1±8.8 mm2 [6-42] vs. 10.3±2.3 mm2 [8-14] (p=0.0001) and 9.8±6.7 mm2 [0-29] vs. -0.2±1.8 mm2 [-3-4] (p<0.0001) in the patients and the controls, respectively. The differences in Δ CSA remained significant in the multivariate analysis after adjusting for age and weight. The best threshold for tt CSA in the TTS group was 15 mm2 with 74% sensitivity and 100% specificity. The best threshold for Δ CSA was 5mm2 with 81% sensitivity and 100% specificity.

DISCUSSION: The difference in the measured axial CSA of the tibial nerve by ultrasonography between the posteromedial tarsal tunnel and 10cm above the tunnel is a key data point for the diagnosis of tarsal tunnel syndrome with and without compressive etiology.

LEVEL OF EVIDENCE: III, diagnostic case-control study.