Shear Wave Elastography of the Abductor Pollicis Brevis in Carpal Tunnel Syndrome: Relationship with Quantitative Motor Unit Potential and Nerve Conduction Parameters

Background/Objectives: We examined the association between shear wave elastography (SWE) of the abductor pollicis brevis (APB) and quantitative motor unit potential (MUP) parameters in carpal tunnel syndrome (CTS), using the ulnar-innervated abductor digiti minimi (ADM) as a reference muscle. Methods: In this prospective cross-sectional study, 73 patients with electrodiagnostically confirmed CTS (125 hands) and 37 healthy controls (69 hands) underwent SWE of the median nerve, APB, and ADM. Quantitative MUP analysis of the APB was performed in 65 hands with moderate or severe CTS. Both hands were included and analysed using mixed-effects models with participant as a random intercept; correlations were corrected using the Benjamini–Hochberg procedure. Results: The median nerve cross-sectional area, Young’s modulus, and shear wave velocity were higher in CTS hands than in controls (all p < 0.001), as were APB Young’s modulus (difference 2.85 kPa, 95% CI 0.46 to 5.24; p = 0.020) and shear wave velocity (p = 0.027). The ADM measurements and the within-hand APB/ADM ratio did not differ significantly. APB stiffness did not correlate with any MUP or motor conduction parameter. Weak correlations with sensory distal latency and sensory conduction velocity did not survive correction for multiple comparisons. Conclusions: Increased stiffness of the median nerve and the APB in CTS was not accompanied by a difference in the ulnar-innervated ADM, but the within-hand APB/ADM ratio did not differ either, so anatomical specificity was not established. Increased APB stiffness represented a group-level mechanical change that, in moderate-to-severe CTS, did not track neurogenic motor unit remodelling as measured by needle electromyography, and its clinical role remains to be defined.