SYDNEY - Are genetic or environmental factors to blame for weakening of vision, even in the absence of diseases like glaucoma?
Elders who have good visual acuity 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' ageing 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, University of Melbourne and her colleagues used a classic twin study to explore genetic and environmental factors.
Volunteers comprised 84 twins (42 pairs, 21 identical and 21 fraternal) between the ages of 57 and 75 who met study criteria for visual acuity and absence of eye disease.
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 colour 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.
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, said a Melbourne release.
The study was published in the February issue of Ophthalmology.