NASCA report 2: Longitudinal study of relationship of exposure to space radiation and risk of lens opacity

The NASA Study of Cataract in Astronauts (NASCA) was designed to measure the impact of exposure to space radiation on progression rates of cortical, nuclear, and posterior subcapsular cataract in U.S. astronauts who have flown in space and comparison groups of astronauts who had not flown in space, and subjects with a history of military aviation. We present our analyses of 5 years of data with an average of 3.8 exams per subject. All subjects had digital lens images with the Nidek EAS 1000 Lens Imaging System. Because of high variability and skewness of opacity measures, nonparametric methods were used to test for association between rates of opacification and space radiation exposure. First, median regression was used to collapse longitudinal data into robust estimates of progression rates (opacity severity compare to time for each eye of each subject). To quantify and test for a radiation effect, median regression with the dependent variable being the maximum of the two slopes (OD and OS) per subject was then used, adjusting for the confounding variables of age, nutritional, and sun-exposure histories. Median regression showed evidence of an association between the rate of cortical progression in the worse eye with radiation dose and age. The estimated median progression rate from space radiation being 0.25 ± 0.13 lens area/Sv/year (P 0.062). We found no relationship between radiation exposure and progression of aggregate area of posterior subcapsular cataract or nuclear progression rates. However, longer follow-up may be needed to further understand any impact of space radiation on progression rates for posterior subcapsular cataracts and nuclear cataracts, and to characterize changes to visual acuity.