The Inlet and Outlet Ratio: Retrospective and Prospective Study on an Improved Diagnostic Ultrasound Tool for Carpal Tunnel Syndrome

PURPOSE: This study hypothesized that ratios of sonographic cross-sectional areas (CSAs) throughout the median nerve provide a more reliable tool for diagnosing carpal tunnel syndrome (CTS) than a single CSA value. We first tested this hypothesis in a retrospective cohort and subsequently confirmed it in a prospective blinded case-control series. METHODS: Seventy patients were included in the retrospective study, and 50 patients and matched controls were included for the prospective study. We evaluated 4 CSAs, at the forearm, inlet, tunnel, outlet, and their ratios (Rforearm, Rinlet, Routlet, Routlet forearm) to evaluate compression of the median nerve. All patients underwent nerve conduction studies. For the prospective cohort, Disabilities of the Arm, Shoulder, and Hand scores and Boston Carpal Tunnel Questionnaire scores were evaluated, and ultrasound was performed by 2 examiners for each participant. RESULTS: The Boston and Disabilities of the Arm, Shoulder, and Hand scores showed worse subjective function in patients with CTS than in controls. Three ultrasonography parameters (CSAs at the inlet, Rinlet, and Routlet) correlated significantly with subjective function. Age and Rinlet were significantly correlated with severity of CTS in the nerve conduction studies. In both the retrospective and prospective patient groups, the numbers of CSAs at the inlet and outlet were significantly higher than that of CSAs at the tunnel, whereas in the control group, no such compression was found. Of the single measurements, CSAs at the inlet had the best diagnostic performance with an optimized cutoff of 11.75 mm(2). The Rinlet and Routlet ratios performed even better and showed the highest adjusted odds ratios for predicting CTS of all parameters (cutoff Rinlet, 1.25; Routlet, 1.45). Inter-observer correlation was generally high, with better values for single CSAs than for ratios. CONCLUSIONS: The 3 CSA measurements of the median nerve and the associated ratios improved diagnostic power for ultrasonography in CTS in our study. TYPE OF STUDY/LEVEL OF EVIDENCE: Diagnostic I.