Impact of gaze deviation on the optic nerve sheath diameter metric: A prospective interventional study

BACKGROUND: Optic nerve sheath diameter (ONSD) is a widely used non-invasive surrogate marker of intracranial pressure. Current recommendations state that measurements should be obtained with a neutral gaze. However, the effect of gaze deviation on ONSD has never been investigated. We aimed to evaluate the reliability of ONSD in case of gaze deviation and quantify the associated measurement error. A secondary aim was to assess whether bed inclination has a significant effect on ONSD values. METHODS: We conducted a prospective interventional study on 44 healthy volunteers during routine ocular ultrasound training. ONSD was measured bilaterally along the horizontal and vertical axes at 45° head-of-bed elevation in neutral and deviated gaze. After repositioning the bed to 0° and a 2-minute stabilization period, horizontal measurements were repeated. Measurements were performed according to current consensus recommendations and ALARA principles. Nonparametric analyses were used to analyse the results. RESULTS: Gaze deviation significantly reduced ONSD values across all axes and both eyes, with a consistent median difference of 0.8-1.0 mm (all p < 0.001) and large effect sizes. In contrast, no significant differences were observed between measurements obtained at 45° and 0° (p > 0.5). CONCLUSIONS: Gaze deviation induces a predictable "stretching" effect on the optic nerve sheath, resulting in clinically relevant ONSD reduction. If confirmed in neurocritical populations, this systematic difference may allow estimation of neutral-gaze ONSD in non compliant patients and could form the basis for a dynamic test that estimates optic nerve sheath compliance.