Ultrasound Measurement of the Cross-Sectional Area of the Median Nerve: The Effect of Teaching on Measurement Accuracy

Abstract

Background: The purpose of this study was to determine the effect of concise instruction and guidance on the accuracy of measuring the cross-sectional area of the median nerve at the carpal tunnel inlet. Methods: Seven orthopedic residents and 5 hand fellows obtained serial measurements of the median nerve at the carpal tunnel inlet using a 15-6 MHz ultrasound (US) probe. After a 5-minute teaching session, all participants repeated measurements. A single cadaveric specimen was used. Measurements were compared with the measurement of a fellowship-trained hand surgeon with extensive experience in US diagnosis of carpal tunnel syndrome. This was considered the reference standard. Results: The rate of participants selecting the correct structure to measure on US was 36% before instruction and 97% after. Discarding the measurements of the incorrect structure, the average measurement was 4.8 mm2 before instruction and 5.2 mm2 after. The standard measurement was 6 mm2. The average deviation from the standard measurement −.2 mm2 before instruction and −0.8 mm2 after. The percent of measurements (of the correct structure) that fell within 1 mm2 of the standard measurement increased from 62% to 74%. Participant self-reported confidence in performing measurements elevated from 2.4/10 before instruction to 6.5/10 after. Conclusions: US of the median nerve cross-sectional area can be efficiently taught and results in measurements consistent with that of an experienced operator.