A cataract is a clouding of the eye's natural lens, which lies behind the iris and the pupil. Cataracts are the most common cause of vision loss in people over age 40 and is the principal cause of blindness in the world. Cataract prevalence increases with age. As the worlds population ages, cataract-induced visual dysfunction and blindness is on the increase.
There are three types of cataracts. A subcapsular cataract occurs at the back of the lens. People with diabetes or those taking high doses of steroid medications have a greater risk of developing a subcapsular cataract. A nuclear cataract forms deep in the central zone (nucleus) of the lens. Nuclear cataracts usually are associated with aging. A cortical cataract is characterized by white, wedge-like opacities that start in the periphery of the lens and work their way to the center in a spoke-like fashion. This type of cataract occurs in the lens cortex, which is the part of the lens that surrounds the central nucleus.
Age-related nuclear cataract is associated with a loss of GSH in the lens centre, and extensive modifications to the nuclear proteins. These include colouration, oxidation, insolubilisation and crosslinking. The transparency of the eye lens depends on maintaining the native tertiary structures and solubility of the lens crystalline proteins over a lifetime. Cataract is caused by protein aggregation within the protected lens environment. With age, covalent protein damage accumulates through pathways thought to include UV radiation, oxidation, deamination, and truncations. Experiments suggest that the resulting protein destabilization leads to partially unfolded, aggregation-prone intermediates and the formation of insoluble, light-scattering protein aggregates. These aggregates either include or overwhelm the protein chaperone content of the lens.