title: Intraocular pressure, retinal vascular, and visual acuity changes during 48 hours of 10 degrees head-down tilt.
abstract: Intraocular pressures, retinal vascular diameters, and visual acuities of nine men (ages 19-29), were repeatedly measured while the subjects were tilted 10 degrees head-down for 48 h and while they were seated before (baseline), and after the tilt. An immediate increase in intraocular pressure, measured by pneumatonometer (4.7 +/- 0.6 mm Hg, p less than 0.001) was recorded when subjects assumed the head-down position, and diurnal variations in intraocular pressures were observed for the 48 h. The initial and final head-down intraocular pressures were not significantly different (18.9 +/- 1.2 mm Hg vs. 17.9 +/- 1.4 mm Hg, respectively). However, when subjects resumed the sitting position, intraocular pressures fell below the initial sitting values (14.2 +/- 0.9 pre vs. 11.2 +/- 0.5 post, p less than 0.04). Computer image analysis of the retinal vasculature detected a 6% and 2% reduction in the caliber of arteries and veins, respectively, as compared with sitting baseline values. No changes in visual acuity were documented during the 48 h of head-down tilt. Our data suggest that the choroidal blood reservoir increases in volume over 48 h at continuous head-down position with a compensatory decrease in aqueous volume. These findings may explain intraocular pressure changes noted in astronauts during previous space missions and in studies associated with change in body position.
authors.affiliation: University of Texas Medical Branch Clinical Research Center, Galveston.
authors.forename: T H
authors.initials: TH
authors.lastname: Mader
authors.forename: G R
authors.initials: GR
authors.lastname: Taylor
authors.forename: N
authors.initials: N
authors.lastname: Hunter
authors.forename: M
authors.initials: M
authors.lastname: Caputo
authors.forename: R T
authors.initials: RT
authors.lastname: Meehan
mesh_terms: D000328:Adult; D002940:Circadian Rhythm; D006112:Gravitation; D006801:Humans; D007429:Intraocular Pressure / Q000502:physiology*; D008297:Male; D011187:Posture / Q000502:physiology*; D012171:Retinal Vessels / Q000502:physiology*; D013997:Time Factors; D014792:Visual Acuity / Q000502:physiology*