Correlation of visual axis and coronal axis measurements of the optic nerve sheath diameter.

OBJECTIVE There are several approaches to sonographic imaging and measurement of the optic nerve sheath diameter (ONSD). In this study, we sought to compare visual axis measurements of the optic nerve sheath (ONS) with traditional coronal axis measurements to assess for correlation. METHODS The ONS was visualized in 2 views on both eyes of healthy volunteers using an 8.5-MHz probe. Coronal axis views were obtained with the probe placed at the lateral canthus directed nasally posterior to the globe. Subsequent imaging was made along the midline visual axis. The diameter in this view was measured at several points posterior to the sclera (2, 3, 6, 9, 12, and 15 mm). RESULTS Twenty-seven subjects were enrolled (54 scans). There was a significant difference between ONSDs measured in each axis, with a coronal axis mean diameter of 3.4 mm and visual axis mean diameters at 2, 3, 6, 9, 12, and 15 mm of 4.28, 4.32, 5.15, 5.74, 6.39, and 7.42 mm, respectively (P < .05). The Pearson coefficient showed no correlation between coronal axis and visual axis measurements, with R values ranging from 0.51 to 0.69. There was a statistically significant increase in the ONSD as the nerve coursed posteriorly when measured in the visual axis. CONCLUSIONS Visual axis measurements do not reliably correlate with coronal axis measurements. The consistently larger diameter measured in the visual axis as well as the gradually increasing diameter posteriorly suggests measurement of an artifactual shadow rather than the true ONS.