In normal vision (20/20), the cornea and the crystalline lens work together to focus the image on the fovea of the retina. This image is transformed into nerve impulses which are then transmitted by the optic nerve into the brain. Blurry vision results when refractive errors cause the light rays to not be focused on the retina. For example, the anomalous curvature of the cornea can cause myopia (nearsightedness), hyperopia (farsightedness),or astigmatism, whereas the loss of lens elasticity causes presbyopia (age-associated farsightedness).
The subjective findings reflect ocular problems experienced inflight by crewmembers, while the objective findings are ocular conditions encountered by the optometrist at the R+3 examination. The most frequently reported subjective finding is decreased near vision acuity (NVA). About 15% of the Shuttle astronauts in this data set perceived this symptom during flight. Objectively, the condition most often recorded during the postflight examination is change in refraction, although not to the extent of subjective reportings.
These post-landing data obviously cannot capture microgravity conditions. However, the perceived decreased vision acuity may be caused by a change in the shape of the eye, so that light rays refracted by the cornea and lens do not focus on the retina. This shape change may stem from either or both of the following factors: (1)lack of gravitational pull along the z-axis (head-to-toe line) and (2) engorgement of blood vessels behind the retina as a result of bodily fluid redistribution.
Changes in the shape of the eye do not exclusively account for changes in near vision. However, any change in NVA is more noticeable than changes in distant vision acuity, as most astronaut tasks involve near distances. Currently, about half of the astronauts with decreased near vision acuity report adjustment back to 1-G NVA while inflight, while the other half report no such adjustment.
These preliminary results suggest that changes in visual acuity may be the main ocular issue facing future missions, especially those involving long-term weightlessness. Further research should examine the whole Shuttle flight experience, and the association between mission length and changes in visual acuity.
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