Doctoral dissertation
The effect of spaceflight associated factors on the eye
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Abstract
The unexplained aetiology of Spaceflight Associated Neuro-ocular Syndrome (SANS) jeopardises all planned future missions to the Moon and Mars. Various intrinsic and extrinsic potential risk factors have been proposed to contribute to its manifestation. Specifically, the present thesis compared the effect of static exercise (simulating the exercise performed by astronauts on the International Space Station; ISS) and hypercapnia (simulating the ambient conditions on the ISS) on intraocular pressure (IOP) in younger and astronaut-aged individuals. Additionally, the effect of hypoxia (the anticipated ambient in future space habitats) on retinal vessels was also investigated.
These objectives were achieved in a series of four studies:
Study 1: The effect of posture and hypercapnia on IOP during handgrip exercise in young males. To compare the effect of posture (i.e., seated, supine 6° head-down tilt (HDT), and prone 6°HDT), exercise, and hypercapnia on IOP responses, 10 younger (age = 22.5 ± 3.1 yrs) males participated in six interventions consisting of rest and submaximal isometric handgrip exercise while inspiring either room air or a hypercapnic (FiCO2 = 0.01) gas mixture.
Study 2: The effect of hypercapnia on IOP during handgrip exercise in older males. To assess the effect of age, exercise, and hypercapnia on IOP responses in an astronaut-aged population, 10 older (age: 57.8 ± 6 yrs) male participants underwent trials consisting of bouts of rest and isometric handgrip exercise in a prone 6°HDT position while breathing room air or a hypercapnic (FiCO2 = 0.01) gas mixture.
Study 3: The effect of sex on IOP during static handgrip exercise in astronaut-aged individuals. To investigate the effect of sex and exercise on IOP responses in an astronaut-aged population, 12 older males (age: 57.7 ± 5.2 yrs) and 12 older females (age: 53.4 ± 5.1 yrs) performed isometric handgrip exercise in a prone 6°HDT position.
Study 4: The effect of age on hypoxia-induced changes in retinal blood vessel diameter. To assess the effect of age and hypoxia on retinal vessel diameters, 11 adults (age: 40.1 ± 4.1 yrs) and 8 children (age: 9.3 ± 1.3 yrs) spent one day in normobaric normoxia and one day in normobaric hypoxia at an equivalent altitude of 3,000 m elevation.
The principal findings of these studies are that IOP was significantly higher in a prone 6°HDT position, compared to a seated or supine 6°HDT position, and that neither exercise nor hypercapnia had any significant effect on IOP responses in younger males in either of the three positions (Study 1). Isometric exercise significantly elevated IOP in older males in the prone 6°HDT position, with the exercise-induced elevations being further augmented by hypercapnia (Study 2). The prone 6°HDT exercise-induced IOP responses were similar in males and females (Study 3). Normobaric hypoxia significantly increased the diameter of the retinal vessels in both adults and children, with no difference between the two groups (Study 4).
We conclude that IOP is significantly affected by posture, but not by sex. Additionally, IOP responses observed in the prone 6°HDT position were similar to those reported for astronauts shortly after entering microgravity. It would therefore appear that the prone, rather than supine 6°HDT position is more suitable for the simulation of ocular changes that occur in acute microgravity. Resistive exercise and hypercapnia have a significant effect on IOP in older but not in younger males, indicating the potential presence of age-related effects on SANS manifestation. Lastly, adults and children appear to be similarly sensitive to changes in ambient partial pressure of oxygen.