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

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.

Figure 1

Transverse scan over left carpal tunnel of a tall male, asymptomatic for CTS in left hand (180 cm × 85 kg, 19 cm of wrist circumference). Linear 6–18 MHz probe. The MN-CSA (area 1) results of 11 mm² are suggestive of pathologic swelling, but EDS was normal (PS = 0, performed as control side, for CTS in the right side). The flexor carpi radialis (FCR) shows a CSA (area 2) of 14 mm². MN = median nerve, FPL = flexor pollicis longus, * = transverse ligament, FDS = flexor digitorum superficialis.

Figure 2

Transverse scan over left and right carpal tunnels in male patient (172 cm × 82 kg, 18.5 cm of wrist circumference) affected with extremely severe CTS (Padua Scale 5 bilaterally). Linear 4–15 MHz probe. Both MN-CSA show frankly pathologic values (Area 1, left: 17 mm², Area 3, right: 14 mm²). NTR is calculated as the ratio between MN-CSA and FCR-CSA (expressed as a percentage), and shows bilaterally pathologic values (left 143%, right 119%) indicative for severe CTS. MN = median nerve, FCR = flexor carpi radialis.

Figure 3

ROC curves obtained comparing each US parameter (MN-CSA and NTR) in the clinical diagnosis of CTS (as gold standard).

Figure 4

ROC curves obtained comparing each US parameter (MN-CSA and NTR) in severe EDS-defined (PS > 3) diagnosis of CTS (as gold standard).

Figure 5

Transverse scan over right carpal tunnel in a small female patient (150 cm × 50 kg, 13.7 cm of wrist circumference) affected with medium-grade CTS (Padua Scale 3, SCV 37.1 m/sec, DML 4.8 ms). Linear 8–24 MHz probe. Both areas of median nerve (MN) and flexor carpi radialis (FCR) were manually traced. A MN-CSA of 12 mm² is not always considered pathologic (cut-off for MN neuropathy until 14–15 mm² in some studies on CTS) and the severity is often underestimated because of the anthropometric characteristics of the patient. The NTR shows values unequivocally pathologic (124%), as MN-CSA is compared with an anatomic structure (FCR-CSA) that maintains small dimensions in CTS, following the anthropometric characteristics of the patient.