Space radiation and the potential for early cataract development

Long duration spaceflight (LDSF) exposes atronauts to radiation including solar particle events (SPE) and galactic cosmic rays (GCR) (Fig. 1). SPE are released by our Sun and this level of emission is dependent on the solar cycle. GCR produce radiation at a steady state and also pose a potentially serious threat to astronaut health during LDSF. The eye, and particularly the lens is one of the most radiosenstive regions in the human body. A cataract is defined as when the lens of the eye becomes clouded leading to visual impairment. Although the exact pathophysiology of radiation-induced cataracts is not fully eluctidated, the effects of ionizing radiation on biological tissues include: damaging DNA base-pairs, cell membranes and lysosomes, causing early sensescence and eventually cell death. Although GCR and SPE can potentially cause cataract in the anterior segment, posterior segment radiation exposure could theoretically produce the cotton wool spots seen in SANS.

Fig. 1.

Diagram of solar particle events, galactic cosmic rays and intravehicular secondary radiation entering the eye.

When lens epithielial cells are exposed to ionizing radiation, increased proliferation rates have also been seen [1]. This phenotypic change is likely due to the altered expression of cellular growth genes after radiation exposure, which include alterations to: fibroblast growth factor 2, CDKNA1A, transforming growth factor beta and matrix metalloproteases [2]. Following this process, these radiation-damaged lens epithelial cells are believed to migrate to the posterior portion of the lens leading to opacification. Further research is required to determine if reactive oxygen species (ROS) formation is also involved in the coagulation of lens proteins [3]. Visual changes during cataract development occur gradually and are difficult to detect. Three main types of cataract exist: posterior subcapsular, cortical and nuclear. Nuclear cataracts are the most common type of cataract seen in aging, and develop in the center of the lens. In contrast, posterior subcapsular cataracts are the most common type associated with exposure to ionizing radiation, and form at the back of the lens [4]. Further research is required to further understand the effects of low dose radiation on the eye, as many of the previous studies involved high dosages of ionizing radation.

Virtual reality for early cataract detection

Currently during spaceflight, astronaut visual assement includes: a self-reported survey, visual acuity testing, and an Amsler grid [5]. As the duration of space missions increases, astronauts will be exposed to the risks of LDSF for longer, including increased levels of radiation and microgravity. More frequent visual testing as well as the addition of other tests such as contrast sensitivity would be useful to help detect the subtle visual changes that occur during the initial developmental period of a cataract. The addition of dynamic visual acuity testing in spaceflight would also be particularly useful as spaceflight is a rapidly changing environment [6].

Our group is currently developing a NASA-funded, head-mounted, visual assessment system to help monitor visual changes during LDSF [7, 8]. This technology allows astronauts to accurately assess their vision, without requiring an extensive set-up process. Future terrestrial applications of this technology include being a low-cost way to screen for cataracts in developing countries. Cataracts are one of the leading cause of preventable blindness throughout the world, and further screening to further understand its prevalence can help increase regional ophthalmic services [9]. Our assessment system is developed in Steam VR, allowing this technology to be used in the majority low-cost VR headsets [10].

Author contributions

EW—Conceptualization, writing. JO—Conceptualization, Writing. AGL—Review, intellectual support.

Competing interests

The authors declare no competing interests.