Which has a larger impact on the "normal" decline of visual function as we age, genetic or environmental factors? This question is explored in the February issue of Ophthalmology, the journal of the American Academy of Ophthalmology.
Vision worsens for most of us as we age, even in the absence of eye diseases such as glaucoma or AMD. Elders who have good visual acuity (20/25 vision or better) may have trouble driving at night or adjusting when they move between indoor and outdoor light. Some declines are optic, such as presbyopia, reduced flexibility of the eye's lens, which causes poorer near vision for many people after age 40. Other declines are neuronal, related to the eye's ability to send images to the brain. Since crucial functions like reading and memory depend on vision, it is important to understand how "normal" aging occurs and discover what can be done to delay or prevent reduced function. In the first investigation of heredity's impact on neuronal visual decline, Ruth E. Hogg, PhD, of the University of Melbourne, Australia, and her colleagues used a classic twin study to explore genetic and environmental factors.
Study participants were a cohort of the AMD twin study by the University of Melbourne Centre for Eye Research, comprised of eighty-four twins (42 pairs, 21 identical and 21fraternal) between the ages of 57 and 75 who met study criteria for visual acuity and absence of eye disease. In each person the eye with the best visual acuity (or the right eye if acuity was equal in both eyes) was tested for adaptation to light level changes, for color detection, and for detection of gray tone gradations between image and background, termed contrast sensitivity. Taken together, these tests assess key visual functions needed in daily life. When test results showed that a visual ability declined at about the same age in identical twins, the trait was assumed to be under genetic control, and when the decline occurred at different ages, environmental factors were considered dominant. Results for identical twins and fraternal (non-identical) were compared, and when concordance between scores was higher for identical than for fraternal pairs, genetics was assumed to be the controlling factor.
Genetic factors appear to be strong determinants of sharp visual acuity and color discrimination, functions performed by cone cells pathways in the eye's retina, the tissue at the back of the eye that converts light into electronic images for relay to the brain. Genetic factors were not strongly correlated with night vision and the ability to adapt to light level changes, which are performed by retinal rod cells, implying that environmental influences are important to those functions. Autopsies and other studies have found that substantially more rod than cone cells are lost as eyes age. The flow of nutrients across the retinal membrane appears to be more important for rod cell than for cone cell function, and it is here that environmental factors—such as smoking, deficient nutrition, excessive sunlight exposure, and inflammation---may influence visual decline. Rod cell deterioration is accepted as a component of both general visual decline and age-related diseases like AMD, the most common cause of visual disability in elders.
"Our results support clinical and research efforts now underway to slow or stop age-related vision decline by modifying lifestyle factors and/or using specific medications," says Dr. Hogg.
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