Sonographic Measurement of Optic Nerve Sheath Diameter: How Steep is the Learning Curve for a Novice Operator?

Transbulbar sonography for measuring optic nerve sheath diameter (ONSD) is noninvasive bedside technique for detection of raised intracranial pressure. This study aims to evaluate the number of sonographic evaluations required for a novice operator to learn proper measurement of ONSD after a formal training and supervised scanning session.

Three novice operators and one expert operator measured ONSD of 27 healthy volunteers using linear array transducer HFL38x (frequency range of 6-13 MHz) (MicroMaxx®; SonoSite, USA). In each eye, ONSD was measured three times by each observer and mean value was determined. Correlation coefficient was calculated between the novice and the experienced operator and in between the novice operators. Number of scans, after which the significant correlation developed between novice and the experienced operator and between the novice operators, was analyzed.

Cronbach's alpha was tested to evaluate the reliability of the values obtained from intra- and interobservations. For all 27 cases, the value of Cronbach's alpha was high: 0.80-0.99 for the right eye and 0.69-0.97 for the left eye. The mean interobserver variations were plotted on a graph which fluctuated largely in the first 17 cases but oscillated around 0.5-0.30 in the last 10 cases.

Learning curve for novice operators to measure ONSD is steep and they can be taught to measure ONSD in healthy volunteers by sonography in supervised clinical session with an acceptable clinical precision and accuracy comparable to an experienced operator.