Ultrasound techniques to measure the optic nerve sheath: is a specialized probe necessary?

Ultrasound measurement of the optic nerve sheath diameter (ONSD) has been shown to correlate with intracranial pressure (ICP). This study looked at intra-observer and inter-observer reliability among three emergency physicians (EPs) using three different imaging protocols.

Ocular ultrasound was performed in 20 healthy volunteers. Each physician obtained 3 measurements of the right eye ONSD with both the 10-5 MHz and 13-6 MHz probes using the axial imaging technique (experiment 1). The ONSD was then measured with the 10-5 MHz probe using the axial technique and the intracavitary 8-5 MHz probe using the coronal technique (experiment 2).

In experiment 1, larger measurements were obtained with the standard 10-5 MHz probe with an adjusted difference of 0.21 mm (p<0.001). The standard deviation between intra-observer measurements for the 10-5 MHz probe was slightly but not significantly larger with an adjusted difference of 0.039 (p=0.10). In experiment 2, larger measurements were obtained with the standard 10-5 MHz probe with an adjusted difference of 0.91 mm (p<0.001). The standard deviation between intra-observer measurements was greater for the 10-5 MHz probe with an adjusted difference of 0.06 (p=0.03). The inter-observer measurements in experiment 2 had significantly larger variance with the 10-5 MHz probe (p=0.001).

These data suggest comparable precision and inter-observer reliability can be achieved. However, larger values when using the 10-5 MHz probe suggest different cut-off thresholds for normal upper limits. The coronal imaging technique demonstrated less inter-rater variance and may be a better technique when such small measurement differences are important.