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

Abstract

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. 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. 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. US of the median nerve cross-sectional area can be efficiently taught and results in measurements consistent with that of an experienced operator.