Transmission of light to the aging human retina: possible implications for age related macular degeneration

The purpose of this study is to determine the transmission properties of the anterior segment of the human eye as a function of age and relate those changes to possible consequences for retinal disorders. For this a new method has been developed. This consists of a probe which is inserted into the posterior sclera and detects light passing through the anterior segment. The probe is connected to a CCD spectrophotometer via a fibre optic bundle. Using this, the transmission properties of human cadaver eyes were determined. A young primate anterior segment has a maximum absorption of 365 nm due to the O-beta-glucoside of 3-hydroxykynurenine (3-HKG) in the lens. There is a steep increase in transmission of the human anterior segment at wavelengths longer than 400 nm. With aging there is an increase in absorption throughout the visible such that by the sixth decade only 20% of blue light is transmitted to the retina compared to the young primate eye. The rate of decrease of blue light was similar to the age related change of the ratio of absorbance at 365/320 nm of the lens. (IOVS 41:1454;1999). The age related rate of decrease in the transmission of blue light to the retina was similar to the rate of increase of lipofuscin formation in the retina, and the amount of light absorbed by A2E in the RPE is constant from the second to seventh decade. Although this yellowing is thought to be detrimental to the lens, it would appear to be beneficial to the retina. It was determined that the implantation of a standard IOL after cataract surgery increased the amount of light absorbed by A2-E by approximately a factor of five.