Is there a hyperopic shift in myopic eyes during the presbyopic years?
BACKGROUND: Studies of age-related changes in myopia during the presbyopic years have produced contradictory results. Some studies have shown that myopic eyes undergo a hyperopic shift after the age of 40 or 45 years, whereas others have provided evidence for a myopic shift. In this paper, we report the results of both a cross-sectional study and a retrospective longitudinal study of age-related changes in refraction during the presbyopic years. METHODS: In the cross-sectional study, refractive error data were tabulated for the right eyes of 559 patients over the age of 45 years, who had been examined during a 17-month period in an established optometric practice. In the longitudinal study, we tabulated refractive error data on 100 myopes, 100 hyperopes and 100 emmetropes, who had been examined for periods varying from 10 to 26 years after age 40, all of whom had 6/6 visual acuity at all examinations. RESULTS: In the cross-sectional study, the prevalence of myopia during the later presbyopic years was found to be greater when eyes with all visual acuities were included than when only eyes with acuity of 6/6 or better were considered. In the longitudinal study, it was found that almost all hyperopic and emmetropic eyes showed an age-related hyperopic shift; but only a small proportion of myopic eyes shifted toward hyperopia, with others remaining relatively stable and still others increasing in myopia. DISCUSSION AND CONCLUSIONS: We suggest that when healthy myopic, hyperopic, or emmetropic eyes shift in the hyperopic direction, it is because of an age-related decrease in the gradient index of the lens; but when healthy myopic eyes shift in the myopic direction, it is because of axial elongation that more than compensates for the decrease in the gradient index of the lens. Another possible cause of a myopic shift, even for eyes having 6/6 visual acuity, is the presence of early, sub-clinical nuclear sclerosis.
Grosvenor
Theodore
T
Department of Optometry and Vision Science, The University of Auckland, New Zealand.
Skeates
Peter D
PD