Abstract
purpose. To determine the effects of simultaneously presented myopic and hyperopic defocus on the refractive development of chicks.
methods. A novel form of dual-power lens was designed. Normal chicks 7 to 8 days of age were fitted with a dual-power lens over one eye and a plano lens over the fellow (control) eye. Dual-power lenses of +20/−10, +10/−10, +5/−10, and plano/−10-D were tested, along with +10/−10-D lenses having differing ratios (50:50, 33:67, and 25:75) of surface area devoted to each power. Ocular refraction and axial ocular component dimensions were assessed after 6 days of lens wear, by retinoscopy and high-frequency ultrasound, respectively. In a separate experiment designed to test the effect of dual-power lens wear on the refractive development of myopic eyes, chicks were fitted with a dual-power +10/−10-D lens for 6 days, after myopia had been induced by 6 days of −10-D lens wear.
results. For each of the dual-power lenses tested, the refractive end point of the treated eye was found to lie between the two optical powers of the lens (but with the response weighted in favor of the effect of myopic defocus). Refractive development appeared to be modulated by the sign, dioptric magnitude, and relative contribution (relative contrast) of the imposed optical defocuses through an integrative mechanism. Chicks with myopia induced by −10-D lens wear recovered when treated with a +10/−10-D dual-power lens.
conclusions. The chick retina can discern both the sign and the magnitude of optical defocus. Chick eyes were able to integrate blur cues from simultaneously presented images focused either side of the photoreceptors and to modulate their refractive development accordingly. This implies that the complex nature of defocus in the visual environment may play a critical role in the pathogenesis of myopia. The results suggest a rational method for arresting or reversing the development of myopia, which may be useful in the treatment of human myopia if the primate retina is also capable of responding to simultaneously presented opposing defocus cues.
Myopia typically occurs due to excessive enlargement of the eye, such that in the unaccommodated state an image is focused in front of, rather than onto, the photoreceptor layer of the retina. In recent years myopia has reached epidemic proportions in parts of East Asia, including China, Hong Kong, Japan, Singapore, and Taiwan, with up to 70% to 90% of 17- to 18-year-olds in the region affected.
1 2 3 4 5 Growing evidence suggests that the prevalence of myopia is increasing in the Caucasian populations in Australia and the United States as well.
6
Myopia incurs more than a minor inconvenience, since high myopia is a leading cause of retinal degeneration
7 and visual impairment.
8 In fact, myopic degeneration is the second leading cause of low vision in Hong Kong
9 and is the fifth leading cause of blindness in the United States.
10 Optical aids are usually prescribed purely to correct refractive error and, at present, there is no proven clinical treatment for retarding myopia’s progression.
Myopia is a multifactorial disorder. Twin and family studies indicate a genetic predisposition to myopia.
11 By contrast, epidemiology studies have shown that myopia is more prevalent in early adults than in older adults within a population that shares the same gene pool.
12 13 14 Therefore, environmental factors are strongly indicated in the recent epidemic of myopia.
Naturally occurring refractive errors (myopia and hyperopia) are scarce and small in magnitude among both wild and domesticated adult animals—for example, the pigeon,
15 chick,
16 tree shrew,
17 rhesus monkey,
18 fish,
19 marmoset,
20 and guinea pig.
21 This phenomenon of the scarcity of naturally occurring refractive errors is driven by the process of emmetropization: a robust homeostatic mechanism found in diverse species that guides postnatal eye growth (reviewed in Ref.
22 ). Refractive errors are common at birth; hence, images of distant objects are focused in front of the photoreceptors (myopic defocus) in some subjects and behind the photoreceptors (hyperopic defocus) in others. However, the growth of the component parts of the eye is carefully coordinated in such a way that, as the animal matures, the position of the photoreceptors becomes increasingly well matched to the combined focal length of the eye’s refractive elements. Eventually, the eye becomes relatively free of refractive error (emmetropic) for targets at infinity. (In reality, this is a somewhat simplified view: hyperopia is more common than myopia in early infancy, and, in most populations, most individuals undergo emmetropization to a refractive state of low hyperopia rather than precise emmetropia.
16 18 21 ) It is yet to be answered why children so commonly develop myopia if the dimensions of the eye are under homeostatic control to match its optical power. In fact, the exact mechanism of emmetropization remains to be unraveled, although it is known to operate largely locally within the eye and does not necessarily require innervation from the central nervous system.
23 Experimentally, the emmetropization process can be manipulated by mounting ophthalmic lenses before the eyes of young animals and then monitoring the changes in refractive status and ocular dimensions. Results have shown that school work and other intensive close work are risk factors for myopia.
10 24 Emmetropization is known to be principally visually guided,
22 and therefore, intuitively, extreme environments such as predominantly close reading distance may somehow upset the natural balance of visual input for normal eye growth.
Previous studies of reading habits,
25 accommodative stress reduction,
26 27 and lens-induced emmetropization have typically used relatively simple visual targets. However, visual scenes typically comprise multiple spatially distinct objects, forming images at a variety of levels of defocus. These rich spatial and temporal visual inputs may provide important cues to the emmetropization system. In this study, we tested the hypothesis that ocular growth is modulated by the integration of such differentially defocused images in the visual field.
In the first experiment, we simultaneously imposed both myopic and hyperopic defocus on one eye of young chicks and studied how the emmetropization system integrated these disparate growth cues. The simultaneous myopic and hyperopic defocus was achieved by using custom-designed and manufactured dual-power lenses, which took the form of concentric annuli of alternating power. These lenses simultaneously introduced overlapping optical images that in an unaccommodated eye would be focused on either side of the retina. Chicks wore a plano (zero power) lens over the fellow eye as a control. Lenses were cleaned at least once every 2 days. More frequent cleaning was unnecessary, since eyes covered with plano lenses did not develop myopia. For comparison, additional groups of chicks were raised wearing a single vision lens (+20, +10, +5, plano, or −10 D) over one eye and a plano lens over the fellow eye. The four dual-power treatment groups were processed and measured together in a series of experiment, whereas the five single-vision control groups were processed and measured together in another series.
Experiment 3: Effect of a Switch from a Negative Single-Vision Lens to a Dual-Power Lens and Vice Versa
Experiment 1: Integration of Competing Myopic and Hyperopic Defocus during Emmetropization