Ultrasound measurement of the optic nerve and its sheath using the CLOSED protocol: comparison with MRI in a porcine in-vivo model

BACKGROUND: Finding a reliable, non-invasive method for assessing intracranial pressure in humans and other species remains a subject of ongoing scientific and clinical interest. Ultrasonographic measurement of the optic nerve and its sheath shows considerable promise and is widely used in human medicine. However, methodological challenges must be addressed when applying this approach in animal models. The aim of this study was to evaluate whether a previously developed ultrasonographic protocol is feasible under in vivo conditions in pigs and whether the obtained measurements accurately reflect anatomical dimensions. To this end, transbulbar ultrasonographic examinations were performed in sixteen pigs under general anesthesia, and optic nerve and optic nerve sheath diameters were measured in the left eye and compared with corresponding magnetic resonance imaging measurements. All measurements were performed twice and averaged. RESULTS: Ultrasonographic visualization of the optic nerve and its sheath was feasible in all animals, and accurate measurement points corresponding to the borders of these structures were established. Measurements obtained using ultrasonography showed good agreement with magnetic resonance imaging, with no significant differences between modalities (p > 0.05) and all measurements within the 95% limits of agreement. The mean optic nerve diameter was 3.26 mm for sonographic measurement and 3.18 mm for magnetic resonance measurement, while the mean optic nerve sheath diameter was 5.12 mm and 5.36 mm, respectively. CONCLUSIONS: These findings indicate that the standardized ultrasonographic protocol can be successfully applied in a porcine in vivo model and provides anatomically reliable measurements. This methodological validation represents an important step toward future experimental studies investigating intracranial pressure-related changes, for example during laparoscopic procedures.