Objective: The cause of carpal tunnel syndrome (CTS) is frequently idiopathic. Recent research concluded that thickening of the subsynovial connective tissue (SSCT) may lead to compression of the median nerve in patients with idiopathic CTS. It is possible to measure this thickness with ultrasound (US). However, the reproducibility of this technique has not been investigated before. The primary purpose of this study was therefore to determine the intra-rater reproducibility and the inter-rater reliability of SSCT thickness measurements with US in patients with idiopathic CTS, and in healthy controls. To compensate for differences in hand size, the secondary purpose was to determine the measurement properties for the thickness ratio of the SSCT to the third flexor tendon digitorum superficialis (FDS). Methods: US images of patients with idiopathic CTS and healthy controls were collected in a previous study through convenience sampling. The US images were taken longitudinally at the wrist crease (proximal tunnel), hamate level (mid-tunnel) and the distal edge (distal tunnel) of the carpal tunnel. Three examiners calculated the thickness of the SSCT and the third FDS tendon at two different moments. The intra-rater and inter-rater reliability were calculated using the intraclass correlation coefficient (ICC) (2,1). The smallest detectable change (SDC) was determined for the agreement. Results: Three examiners evaluated US images of 29 patients with idiopathic CTS and 34 healthy controls. ICC values for the intra-rater reliability of the SSCT measurement ranged from 0.151 to 0.676 for patients and from -0.24 to 0.680 for healthy controls. ICC values for the TR measurement varied from 0.187 to 0.616 for patients and 0.011 to 0.574 for healthy controls. ICC values for the inter-rater reliability of the SSCT measurement ranged from -0.129 to 0.442. For the TR measurement the ICC scores varied from -0.24 to 0.254. The SDC varied from 50% to 156% of the mean thickness measurements. Conclusion: The reproducibility of the SSCT thickness measurements in the carpal tunnel with US is low. However, there seems to be a trend for a fair to good reliability at the proximal carpal tunnel level
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