Effects of head‐down tilt on optic nerve sheath diameter in healthy subjects

Abstract Purpose Intracranial pressure increases in head‐down tilt (HDT) body posture. This study evaluated the effect of HDT on the 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 11:00 h for baseline seated scans and then maintained a seated or 6° HDT posture from 12:00 to 15:00 h. Three horizontal axial and three vertical axial scans were obtained at 11:00, 12:00 and 15:00 h 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 3 mm behind the globe. Results In the seated visit, ONSDs were similar across time ( p > 0.05), with an overall mean (standard deviation) 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 12:00 and 15:00 h ( p < 0.001 horizontal and p < 0.05 vertical). Mean (standard error) horizontal ONSD change from baseline was 0.37 (0.07) HDT versus 0.10 (0.05) seated at 12:00 h ( p = 0.002) and 0.41 (0.09) HDT versus 0.12 (0.06) seated at 15:00 h ( p = 0.002); mean vertical ONSD change was 0.14 (0.07) HDT versus −0.07 (0.04) seated at 12:00 h ( p = 0.02) and 0.19 (0.06) HDT versus −0.03 (0.04) seated at 15:00 h ( p = 0.01). ONSD change in HDT was similar between 12:00 and 15:00 h ( p ≥ 0.30). Changes at 12:00 h correlated with those at 15:00 h for horizontal ( r = 0.78, p < 0.001) and vertical ONSD ( r = 0.73, p < 0.001). Conclusion The ONSD increased when body posture transitioned from seated to HDT position without any further change at the end of the 3 h in HDT.