A Novel Ultrasonographic Anthropometric-Independent Measurement of Median Nerve Swelling in Carpal Tunnel Syndrome: The "Nerve/Tendon Ratio" (NTR)

Paolo Falsetti; Edoardo Conticini; Caterina Baldi; Emilio D'Ignazio; Suhel Gabriele Al Khayyat; Marco Bardelli; Stefano Gentileschi; Roberto D'Alessandro; Miriana D'Alessandro; Caterina Acciai; Federica Ginanneschi; Luca Cantarini; Bruno Frediani

doi:10.3390/diagnostics12112621

A Novel Ultrasonographic Anthropometric-Independent Measurement of Median Nerve Swelling in Carpal Tunnel Syndrome: The "Nerve/Tendon Ratio" (NTR)

Diagnostics (Basel). 2022 Oct 28;12(11):2621. doi: 10.3390/diagnostics12112621.

Affiliations

¹ Rheumatology Unit, Department of Medical, Surgical Sciences and Neurosciences, University Hospital of Siena, 53100 Siena, Italy.
² Unit of Respiratory Diseases and Lung Transplantation, Department of Medical, Surgical Sciences and Neurosciences, University Hospital of Siena, 53100 Siena, Italy.
³ Neurorehabilitation Unit, San Donato Hospital, 52100 Arezzo, Italy.
⁴ Neurology Unit, Department of Medical, Surgical Sciences and Neurosciences, University Hospital of Siena, 53100 Siena, Italy.

Abstract

Background: There is little consensus on ultrasound (US) normative values of cross-sectional area of median nerve (MN-CSA) in carpal tunnel syndrome (CTS) because of its dependency on anthropometric parameters. We aim to propose a novel anthropometric-independent US parameter: MN-CSA/flexor radialis carpi CSA (FCR-CSA) ratio (“Nerve Tendon Ratio”, NTR), in the diagnosis of clinically and electrodiagnostic (EDS)-defined CTS. Methods: 74 wrists of 49 patients with clinically defined CTS underwent EDS (scored by the 1−5 Padua Scale of electrophysiological severity, PS) and US of carpal tunnel with measurement of MN-CSA (at the carpal tunnel inlet), FCR-CSA (over scaphoid tubercle) and its ratio (NTR, expressed as a percentage). US normality values and intra-operator agreement were assessed in 33 healthy volunteers. Results: In controls, the mean MN-CSA was 5.81 mm2, NTR 64.2%. In 74 clinical CTS, the mean MN-CSA was 12.1 mm2, NTR 117%. In severe CTS (PS > 3), the mean MN-CSA was 15.9 mm2, NTR 148%. In CTS, both MN-CSA and NTR correlated with sensitive conduction velocity (SCV) (p < 0.001), distal motor latency (DML) (p < 0.001) and PS (p < 0.001), with a slight superiority of NTR vs. MN-CSA when controlled for height, wrist circumference and weight. In CTS filtered for anthropometric extremes, only NTR maintained a correlation with SCV (p = 0.023), DML (p = 0.016) and PS (p = 0.009). Diagnostic cut-offs were obtained with a binomial regression analysis. In those patients with a clinical diagnosis of CTS, the cut-off of MN-CSA (AUROC: 0.983) was 8 mm2 (9 mm2 with highest positive predictive value, PPV), while for NTR (AUROC: 0.987), the cut-off was 83% (100% with highest PPV). In patients with EDS findings of severe CTS (PS > 3), the MN-CSA (AUROC: 0.876) cut-off was 12.3 mm2 (15.3 mm2 with highest PPV), while for NTR (AUROC: 0.858) it was 116.2% (146.0% with highest PPV). Conclusions: NTR can be simply and quickly calculated, and it can be used in anthropometric extremes.

Keywords: carpal tunnel syndrome; electrodiagnostics; median nerve; neuropathy; ultrasound.

Grants and funding

This research received no external funding.