The multifunctional choroid.

Homeostasis of Eye Growth and the Question of Myopia

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1475-1313.2011.00884.x

Worldwide prevalence and risk factors for myopia.

PURPOSE. To quantify the degree of association between juvenile myopia and parental myopia, near work, and school achievement. METHODS. Refractive error, parental refractive status, current level of near activities (assumed working distance-weighted hours per week spent studying, reading for pleasure, watching television, playing video games or working on the computer), hours per week spent playing sports, and level of school achievement (scores on the Iowa Tests of Basic Skills [ITBS]) were assessed in 366 eighth grade children who participated in the Orinda Longitudinal Study of Myopia in 1991 to 1996. RESULTS. Children with myopia were more likely to have parents with myopia; to spend significantly more time studying, more time reading, and less time playing sports; and to score higher on the ITBS Reading and Total Language subtests than emmetropic children ( 2 and Wilcoxon rank-sum tests; P 0.024). Multivariate logistic regression models showed no substantial confounding effects between parental myopia, near work, sports activity, and school achievement, suggesting that each factor has an independent association with myopia. The multivariate odds ratio (95% confidence interval) for two compared with no parents with myopia was 6.40 (2.17‐18.87) and was 1.020 (1.008 ‐1.032) for each diopter-hour per week of near work. Interactions between parental myopia and near work were not significant (P 0.67), indicating no increase in the risk associated with near work with an increasing number of parents with myopia. CONCLUSIONS. Heredity was the most important factor associated with juvenile myopia, with smaller independent contributions from more near work, higher school achievement, and less time in sports activity. There was no evidence that children inherit a myopigenic environment or a susceptibility to the effects of near work from their parents. (Invest Ophthalmol Vis Sci. 2002;43:3633‐3640)

/pdf/parental-myopia-near-work-school-achievement-and-children-s-4apzz438fy.pdf

Parental Myopia, Near Work, School Achievement, and Children’s Refractive Error

PURPOSE. To describe the pattern and magnitude of diurnal variation of choroidal thickness (CT), its relation to systemic and ocular factors, and to determine the intervisit reproducibility of diurnal patterns. METHODS. A prospective study was conducted on 12 healthy volunteers who each underwent sequential ocular imaging on two separate days at five fixed, 2-hour time intervals. Spectral domain optical coherence tomography (OCT) with enhanced depth imaging and image tracking was performed using a standardized protocol. Choroidal and retinal thicknesses were independently assessed by two masked graders. CT diurnal variation was assessed using repeated-measures ANOVA. RESULTS. A significant diurnal variation in CT was observed, with mean maximum CT of 372.2 m, minimum of 340.6 m (P 0.001), and mean diurnal amplitude of 33.7 m. Retinal thickness (mean, 235.0 m) did not exhibit significant diurnal variation (P 0.621). The amplitude of CT variation was significantly greater for subjects with thicker morning baseline CT compared with those with thin choroids (43.1 vs. 10.5 m, P 0.001). There were significant correlations between amplitude of CT and age (P 0.032), axial length (P 0.001), and spherical equivalent (P 0.001). The change in CT also correlated with change in systolic blood pressure (P 0.031). Comparing CT on two different days, a similar diurnal pattern was observed, with no significant difference between corresponding measurements at the same time points (P 0.180). CONCLUSIONS. There is significant diurnal variation of CT, with good intervisit reproducibility of diurnal patterns on two different days. The amplitude of variation varies with morning baseline CT, and is correlated with age, axial length, refractive error, and change in systolic blood pressure. (Invest Ophthalmol Vis Sci. 2012;53:261‐266) DOI:10.1167/iovs.11-8782

Diurnal variation of choroidal thickness in normal, healthy subjects measured by spectral domain optical coherence tomography.

The influence of visual experience on ocular development in higher primates is not well understood. To investigate the possible role of defocus in regulating ocular growth, spectacle lenses were used to optically simulate refractive anomalies in young monkeys (for example, myopia or nearsightedness). Both positive and negative lenses produced compensating ocular growth that reduced the lens-induced refractive errors and, at least for low lens powers, minimized any refractive-error differences between the two eyes. These results indicate that the developing primate visual system can detect the presence of refractive anomalies and alter each eye's growth to eliminate these refractive errors. Moreover, these results support the hypothesis that spectacle lenses can alter eye development in young children.

Spectacle lenses alter eye growth and the refractive status of young monkeys.

The role of optical defocus in regulating refractive development in infant monkeys.

Relative peripheral hyperopic defocus alters central refractive development in infant monkeys.

Purpose
Because of the prominence of central vision in primates, it has generally been assumed that signals from the fovea dominate refractive development. To test this assumption, the authors determined whether an intact fovea was essential for either normal emmetropization or the vision-induced myopic errors produced by form deprivation.

/pdf/effects-of-foveal-ablation-on-emmetropization-and-form-1hu0ribbby.pdf

Effects of Foveal Ablation on Emmetropization and Form-Deprivation Myopia

Soon after birth, the eyes of most infants normally grow in a highly coordinated manner toward the ideal optical state that is then maintained throughout childhood and into early adult life, a process called emmetropization. Evidence from many different species indicates that this process is actively regulated by visual feedback associated with the eye's refractive state—in essence, optical defocus.1–3 For example, making the eyes of young animals artificially myopic with positive lenses or hyperopic with negative lenses produces compensating ocular growth that can, within certain operational limits, eliminate the imposed refractive error.4–10 However, for reasons not currently understood, a substantial and increasing proportion of the human population develop myopia during early adolescence.11–13 It is possible that myopia onset and progression in children are triggered by visual experience, acting through the vision-dependent biochemical cascade that normally promotes the development of the optimal refractive state or possibly the failure of this focus-dependent process to stop axial growth. In this respect, most optical treatment strategies that have been designed to prevent or reduce myopia in children are based on manipulating accommodative effort and/or the effective focus of the retinal image. The fact that some recent optical treatment strategies that impose relative myopic defocus over a large portion of the retina have been shown to produce clinically meaningful reductions in myopia progression14–20 has reinforced the predominant view that visual factors other than defocus do not influence refractive development.

However, the local mechanisms that mediate the effects of optical defocus on ocular growth are complex, involving many different retinal, choroidal, and scleral components.21–23 Although optical defocus appears to be the primary stimulus, some individual components in this signal cascade (e.g., dopaminergic amacrine cells) have been shown to be influenced by other stimulus attributes (e.g., light levels).1 As a consequence, the operational efficiency of this defocus-driven feedback loop is potentially influenced by a variety of external factors, including other visual factors. Specifically, recent observations in humans and laboratory animals suggest that ambient light levels may also influence this vision-dependent loop and consequently refractive development. For example, individuals who spend more time outdoors have more hyperopic refractive errors and a lower prevalence of juvenile-onset myopia.24–27 The protective effect of time outdoors is not associated with sporting activities nor is it a substitution effect for time spent in activities that are linked to myopia (e.g., near work).26,28 Instead, it is the total amount of time outdoors that appears to be important. In this respect, the absolute differences in the amount of outdoor activities between myopic and nonmyopic children are relatively small; however, these behavioral differences are present up to 3 years before the onset of myopia,24 which suggests that lower amounts of outdoor activities may contribute to myopia onset. Although the mechanisms underlying this protective effect are not well understood, it has been proposed that these protective effects are due to the relatively flat dioptric topographies of outdoor scenes23 and/or to the high ambient lighting levels typically encountered outdoors,29 which are often 100 times higher than indoor levels. In this respect, it may be significant that most human studies that have reported the protective effects of outdoor activities have been conducted in climates with substantial amounts of sunlight (e.g., Singapore and Sydney).26–28

Research in chickens has provided direct evidence that high lighting levels can have a protective effect against myopigenic visual stimuli. For example, exposing young chicks to high illuminances, either from sunlight or intense laboratory lights, reduces the degree of axial myopia produced by form deprivation by 65% over a 4-day treatment period, accelerates the hyperopic compensation to positive lenses, and slows myopic compensation to negative lenses, although full compensation is still achieved by the end of a 6-day treatment period.29,30 In addition, the ability of a brief period of unrestricted vision to prevent form-deprivation myopia increases with increasing ambient lighting levels,29 and in chicks reared with unrestricted vision, emmetropization is slowed by high light levels, leading to more hyperopic refractive errors.31

It is not reasonable to extrapolate the results from chickens to humans, because light levels and lighting cycles can have qualitatively different effects in primates and birds. Although there are many similarities in the vision-dependent mechanisms that regulate refractive development in birds and primates,1–3 there are qualitative differences. In particular, although lighting cycles can affect refractive development in monkeys,32,33 lighting levels and light cycles can influence refractive development in chickens via mechanisms that do not appear to operate in primates (e.g., direct pineal stimulation)34 and to alter ocular growth in chickens in ways that do not occur in primates. For example, exposure to continuous light produces dramatic changes in corneal curvature in chickens,35,36 but it does not affect the anterior segment in monkeys.32,33

Therefore, the purpose of this study was to evaluate the potential protective effects of high ambient lighting on vision-induced myopia in primates by comparing refractive development in monocularly form-deprived infant rhesus monkeys reared under normal laboratory lighting with that of those exposed to high levels of artificial light.

Li-Fang Hung

Papers

Spectacle lenses alter eye growth and the refractive status of young monkeys.

The role of optical defocus in regulating refractive development in infant monkeys.

Relative peripheral hyperopic defocus alters central refractive development in infant monkeys.

Effects of Foveal Ablation on Emmetropization and Form-Deprivation Myopia

Protective effects of high ambient lighting on the development of form-deprivation myopia in rhesus monkeys.