Effects of Long-Term Simulated Weightlessness on Retinal Microcirculation and Visual Electrophysiology

To investigate the mechanisms of ocular injuries in astronauts due to gravity deficit by examining changes in retinal microcirculation and visual electrophysiology in macaques subjected to simulated weightlessness.

The head-down recumbency of macaques was used to simulate the movement of blood to the side of the head that occurs without microgravity. Head-down recumbency was performed with the head tilted downwards at a recommended angle of 10°. The macaques in the control group were similarly tethered to the rope but could be held in a normal position. The whole experiment lasted for 6 weeks and retinal microcirculation and visual electrophysiology information was collected at weeks 0, 3 and 6.

The retinal microcirculation of macaques was affected by 3 weeks of weightlessness. This includes morphological changes, such as dilation and tortuosity of the retinal microvasculature in macaques at day 21. OCT and OCTA results showed an increase in retinal and choroidal thickness and a significant decrease in vessel length density within 6×6 mm of the macula. Sustained simulated weightlessness (42 days) significantly exacerbated retina-related damage. This was evidenced by a significant decrease in the perfusion density of microcirculatory vessels, such as the macular 3×3 mm mesial vessels and the macular 6*6 mm central and medial vessels. The FAZ density in the macula 3×3 mm area began to increase. Retinal oxygen saturation testing showed a slight increase in arterial oxygen saturation. Simultaneous changes in visual electrophysiology occurred, including a significant decrease in a- and b-wave amplitudes on the dark-vision electroretinogram and a significant decrease in the amplitude of the bright-vision negative wave response. The peak timing of the flash visual evoked potential component P1 was significantly delayed compared to its baseline and time-matched control.

Sustained simulated weightlessness (42 days) significantly exacerbated retina-related damage, with both reduced macular blood supply and increased FAZ density suggesting the development of retinal ischemic changes, which disrupt visual electrophysiology. Retinal damage in human astronauts under long-term outer space conditions may be prevented by intervening in ischemic changes in the retina during the early stages of weightlessness.