Transscleral ultrasonographic measurements of the optic nerve sheath diameter and a regression analysis with morphometric measures of the globe in dogs

To describe transscleral ultrasonography as a novel alternative approach for visualising the optic nerve sheath and measuring its diameter and to analyse the linear regressions of the optic nerve sheath diameter value with the weights and morphometric measurements of the globe in dogs. Forty healthy dogs admitted for routine sterilisation were examined. Under general anaesthesia, a B-mode ultrasonography with a linear probe (9-18 MHz) was applied transscleral in the dorso-temporal quadrant. The optic nerve sheath diameter was measured 3 mm behind the caudal aspect of the globe. The morphometric measurements, including the axial globe, lens thickness and vitreous chamber, were estimated by two observers using the direct corneal approach. Univariate and multivariate multiple linear regression analyses were performed to explore the associations of the independent predictors with dependent variables. The optic nerve sheath diameter intraclass correlation coefficient (ICC) analyses revealed interobserver 0.91 (ICC = 0.83-0.95) and intraobserver 0.93 (ICC = 0.87-0.96) reliability. The multiple regression analysis revealed that the optic nerve sheath diameter was associated with the weight (R2 = 0.60, P < 0.0001) but not with the axial globe (P = 0.48), the lens thickness (P = 0.73) or the vitreous chamber (P = 0.99). The findings of this study suggest that transscleral ultrasonography may be a valid alternative approach for the optic nerve visualisation and optic nerve sheath diameter measurements with excellent intra- and interobserver repeatability. The optic nerve sheath diameter was associated with the body weight, but not with the morphometric measurements of the globe.