Effects of Head-Down Tilt on Optic Nerve Sheath Diameter in Healthy Subjects

Date

2023-05-15

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Abstract

PURPOSE Intracranial pressure increases in head-down tilt (HDT) body posture. This study evaluates the effect of HDT on optic nerve sheath diameter (ONSD) in normal subjects. METHODS Twenty-six healthy adults (age 28 (4.7) years) participated in seated and 6° HDT visits. For each visit, subjects presented at 11am for baseline seated scans and then maintained a seated or 6° HDT posture from 12pm to 3pm. In both visits, three horizontal axial and three vertical axial scans were obtained at 11am, 12pm and 3pm with a 10 MHz ultrasonography probe on the same eye, randomly chosen per subject. At each time point, horizontal and vertical ONSD (mm) were quantified by averaging three measures taken 3mm behind the globe. Repeated-measures ANOVA and Tukey HSD test compared means across time. Paired t-tests compared horizontal vs vertical ONSD and changes from baseline between visits. RESULTS In the seated visit, ONSDs were similar across time (p>0.05), with an overall mean (SD) of 4.71 (0.48) horizontally and 5.08 (0.44) vertically. ONSD was larger vertically than horizontally at each time point (p<0.001). In the HDT visit, ONSD was significantly enlarged from baseline at 12pm and 3pm (p<0.001 horizontal, p<0.05 vertical). Mean (SE) horizontal ONSD change from baseline was 0.37 (0.07) HDT vs 0.10 (0.05) seated at 12pm (p=0.002), 0.41 (0.09) HDT vs 0.12 (0.06) seated at 3pm (p=0.002); mean vertical ONSD change was 0.14 (0.07) HDT vs -0.07 (0.04) seated at 12pm (p=0.016), 0.19 (0.06) HDT vs -0.03 (0.04) seated at 3pm (p=0.013). ONSD change in HDT was similar between 12pm and 3pm (p≥0.30). Changes at 12pm correlated with those at 3pm for horizontal (r=0.78, p<0.001) and vertical ONSD (r=0.73, p<0.001). CONCLUSION The ONSD significantly increased when body posture transitioned from seated to HDT position without further change at the end of the 3 hours in HDT.

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Keywords

Optic nerve sheath diameter, Head-down tilt, Ophthalmic ultrasound, Intracranial pressure, Translaminar pressure

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