Effects of chronic optical defocus on the kitten's refractive status

Emmetropization and nonmyopic eye growth

Most eyes start with a hypermetropic refractive error at birth, but the growth rates of the ocular components, guided by visual cues, will slow in such a way that this refractive error decreases during the first 2 years of life. Once reaching its target, the eye enters a period of stable refractive error as it continues to grow by balancing the loss in corneal and lens power with the axial elongation. Although these basic ideas were first proposed over a century ago by Straub, the exact details on the controlling mechanism and the growth process remained elusive. Thanks to the observations collected in the last 40 years in both animals and humans, we are now beginning to get an understanding how environmental and behavioral factors stabilize or disrupt ocular growth. We survey these efforts to present what is currently known regarding the regulation of ocular growth rates.

Genetic network regulating visual acuity makes limited contribution to visually guided eye emmetropization

During postnatal development, the eye undergoes a refinement process whereby optical defocus guides eye growth towards sharp vision in a process of emmetropization. Optical defocus activates a signaling cascade originating in the retina and propagating across the back of the eye to the sclera. Several observations suggest that visual acuity might be important for optical defocus detection and processing in the retina; however, direct experimental evidence supporting or refuting the role of visual acuity in refractive eye development is lacking. Here, we used genome-wide transcriptomics to determine the relative contribution of the retinal genetic network regulating visual acuity to the signaling cascade underlying visually guided eye emmetropization. Our results provide evidence that visual acuity is regulated at the level of molecular signaling in the retina by an extensive genetic network. The genetic network regulating visual acuity makes relatively small contribution to the signaling cascade underlying refractive eye development. This genetic network primarily affects baseline refractive eye development and this influence is primarily facilitated by the biological processes related to melatonin signaling, nitric oxide signaling, phototransduction, synaptic transmission, and dopamine signaling. We also observed that the visual-acuity-related genes associated with the development of human myopia are chiefly involved in light perception and phototransduction. Our results suggest that the visual-acuity-related genetic network primarily contributes to the signaling underlying baseline refractive eye development, whereas its impact on visually guided eye emmetropization is modest.

Genome-wide analysis of retinal transcriptome reveals common genetic network underlying perception of contrast and optical defocus detection

BACKGROUND

Refractive eye development is regulated by optical defocus in a process of emmetropization. Excessive exposure to negative optical defocus often leads to the development of myopia. However, it is still largely unknown how optical defocus is detected by the retina.

METHODS

Here, we used genome-wide RNA-sequencing to conduct analysis of the retinal gene expression network underlying contrast perception and refractive eye development.

RESULTS

We report that the genetic network subserving contrast perception plays an important role in optical defocus detection and emmetropization. Our results demonstrate an interaction between contrast perception, the retinal circadian clock pathway and the signaling pathway underlying optical defocus detection. We also observe that the relative majority of genes causing human myopia are involved in the processing of optical defocus.

CONCLUSIONS

Together, our results support the hypothesis that optical defocus is perceived by the retina using contrast as a proxy and provide new insights into molecular signaling underlying refractive eye development.

Effects of direct intravitreal dopamine injections on the development of lid-suture induced myopia in rabbits

BACKGROUND

Dopamine (DA) storage and release are reduced in form deprivation myopia (FDM) in a wide range of species, from chicks to primates. FDM can be prevented by treatment with DA agonists such as apomorphine, and paradoxically by the dopamine neurotoxin 6-hydroxydopamine. In this study, we increased the DA levels by direct intravitreal DA injections to learn if FDM can be suppressed in a rabbit model.

METHODS

Seven-day-old rabbits were deprived of pattern vision by the suturing the right eyelids after natural eye opening. In the first group (FD, n=20), the right eye received form deprivation (FD) alone. In the second group (DA-FD, n=16), the deprived eye of 7-day-old rabbits received four intravitreal injections of 20 microg dopamine every 5 days. In the third group (saline-FD, n=16), the deprived eye received saline injections with the same schedule. The contralateral eye remained untreated as a control. At the end of the 8-week deprivation period, the effects of DA on refractive error, corneal curvature and ocular dimensions were assessed by streak retinoscopy, keratometry and A-scan ultrasonography, respectively.

RESULTS

Eight weeks of FDM induced a myopic shift of -2.70+/-0.87 D (n=20) in treated eyes compared with contralateral eyes. The major structural correlate of the myopia appeared to be elongation of the vitreous chamber (0.7+/-0.3 mm, n=20) and axial elongation (0.9+/-0.3 mm, n=20), respectively. Repeated intravitreal injections of DA fully prevented the myopic shift (-0.06+/-0.37 D), elongation of the vitreous chamber (0.1+/-0.3 mm, n=16) and axial elongation (0.3+/-0.2 mm, n=16) due to lid suture, whereas saline injections had slight effect.

CONCLUSIONS

FD by suturing eyelids is an effective technique to induce a significant myopic shift, vitreous chamber and axial elongation in rabbits as a model of myopia development. These changes associated with FD were retarded by intravitreal injections of DA.

Peripheral vision can influence eye growth and refractive development in infant monkeys

PURPOSE

Given the prominence of central vision in humans, it has been assumed that visual signals from the fovea dominate emmetropization. The purpose of this study was to examine the impact of peripheral vision on emmetropization.

METHODS

Bilateral, peripheral form deprivation was produced in 12 infant monkeys by rearing them with diffusers that had either 4- or 8-mm apertures centered on the pupils of each eye, to allow 24 degrees or 37 degrees of unrestricted central vision, respectively. At the end of the lens-rearing period, an argon laser was used to ablate the fovea in one eye of each of seven monkeys. Subsequently, all the animals were allowed unrestricted vision. Refractive error and axial dimensions were measured along the pupillary axis by retinoscopy and A-scan ultrasonography, respectively. Control data were obtained from 21 normal monkeys and 3 infants reared with binocular plano lenses.

RESULTS

Nine of the 12 treated monkeys had refractive errors that fell outside the 10th- and 90th-percentile limits for the age-matched control subjects, and the average refractive error for the treated animals was more variable and significantly less hyperopic/more myopic (+0.03 +/- 2.39 D vs. +2.39 +/- 0.92 D). The refractive changes were symmetric in the two eyes of a given animal and axial in nature. After lens removal, all the treated monkeys recovered from the induced refractive errors. No interocular differences in the recovery process were observed in the animals with monocular foveal lesions.

CONCLUSIONS

On the one hand, the peripheral retina can contribute to emmetropizing responses and to ametropias produced by an abnormal visual experience. On the other hand, unrestricted central vision is not sufficient to ensure normal refractive development, and the fovea is not essential for emmetropizing responses.

The role of photoreceptors in the control of refractive state

The current state of research into experimentally induced refractive errors is reviewed. The area is analysed in three components-the transduction of defocus or deprivation, the vector for transmitting the error message from the retina to the outer tunics of the eye, and the identity of the effector for causing growth modulation in the sclera. Anatomical, pharmacological, electrophysiological and optical factors are considered in terms of which elements of the retina are necessary to support a refractive response to deprivation or defocus. Two of the current models are discussed-one emphasizing the role of the choroid in effecting ocular and refractive change, while the second model approaches the problem from the aspect of scleral changes that are associated with growth adaptation without emphasis on the error detection mechanism. A third model is proposed in which the error signal for deprivation or defocus is detected in the outer retina and where error is translated through separate signals for stimulus brightening and darkening into a net signal for fluid flow across and under the active control of the retinal pigment epithelium with the fluid communication between the vitreous chamber and the choroidal lymphatics. The directions of research both fundamental and clinical which are needed to create pharmaceutical or environmental solutions to refractive control are discussed.

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

Early in life, the two eyes of infant primates normally grow in a coordinated manner toward the ideal refractive state. We investigated the extent to which lens-induced changes in the effective focus of the eye affected refractive development in infant rhesus monkeys. The main finding was that spectacle lenses could predictably alter the growth of one or both eyes resulting in appropriate compensating refractive changes in both the hyperopic and myopic directions. Although the effective operating range of the emmetropization process in young monkeys is somewhat limited, the results demonstrate that emmetropization in this higher primate, as in a number of other species, is an active process that is regulated by optical defocus associated with the eye's effective refractive state.

Animal Models of Myopia: Learning How Vision Controls the Size of the Eye

As they grow up, approximately 25% of children in the United States become myopic (nearsighted). A much smaller fraction become significantly hyperopic (farsighted), while the majority develop little or no refractive error and are emmetropic. The causes of refractive error, especially myopia, have been the subject of debate for more than a century. Some have held that myopia is primarily an inherited disorder, and others, that myopia is caused by protracted near work and, especially, by accommodation during protracted near work. It has not been possible, based solely on clinical observations, to resolve the relative roles of heredity versus environment in the development of refractive error. In the mid-1970s, several animal models were developed to study the mechanisms underlying refractive error. Using animal models, it was found that the visual environment exerts a powerful influence on refractive state by controlling the axial length of the eye during the postnatal developmental period. Although several species have been examined, three have emerged as primary models and have played complementary roles: tree shrews (mammals closely related to primates), chicks, and monkeys. Each has advantages and disadvantages. Collectively, research on animal models has provided evidence on three issues, namely that (1) the visual environment can produce refractive error; (2) an emmetropization mechanism normally guides eyes to low refractive error; and (3) under-accommodation, rather than excessive accommodation, may cause myopia. Two decades of research on animal models have provided criteria that may be used to evaluate the usefulness of additional species as models of emmetropization.

Natural and imposed astigmatism and their relation to emmetropization in the chick

This study investigated the ocular response of young chicks to astigmatic errors imposed by spectacle lenses and as a related issue, we examined the nature and prevalence of astigmatism in young chicks, and its relation to corneal development and natural emmetropization. Normal hatchling chicks exhibited significant against-the-rule refractive astigmatism (approx. 8 D) of which 60-90% was corneal. Both types of astigmatism decreased in magnitude with normal corneal development as part of emmetropization. The apparent association with corneal growth is consistent with two further observations: (1) that smaller corneas, induced by constant light rearing, had higher than normal astigmatism (1.5 D greater at 15 days), (ii) that enlarged corneas, due to form deprivation, had reduced astigmatism (2.4 D less). When astigmatism was artificially imposed with (+/-10 DC spectacle lenses), altered ocular growth patterns were observed, although the changes were not consistent with the chicks having emmetropized to the imposed astigmatism. Irrespective of the axis setting used in positioning the lenses (45 degrees, 90 degrees, 180 degrees), eyes became hyperopic with +10 DC lenses (+8.8 +/- 1.3 D), and became slightly myopic with 10 DC lenses (-1.8 +/- 1.9 D). These refractive changes are consistent with the chicks having emmetropized to the more myopic meridian rather than the equivalent mean spherical error imposed (responses of control groups to +5 D and -5 D spherical lenses were +5.2 +/- 1.0 D and -5.1 +/- 0.8 D resp.). The same was true for chicks first prevented from accommodating by prior ciliary nerve section, except for one group wearing the 10 DC lens at 45 degrees axis where astigmatic changes consistent with partial compensation were seen, although this may represent an artefact of the surgery. These results argue against 'astigmatic emmetropization' as a normal phenomenon. Also consistent with this finding was the lack of significant astigmatic changes with accommodation-stimulating and inhibiting drugs (nicotine and vercuronium resp.), for normal chicks. These results imply that accommodation, while the most likely mechanism for astigmatic emmetropization, has little capacity to compensate for imposed astigmatic focussing errors.

Effects on the compensatory responses to positive and negative lenses of intermittent lens wear and ciliary nerve section in chicks

This study examined the ocular compensation to lens-induced defocus in chick and the effect of interrupting lens wear on a daily basis. Eyes fitted with +10 D lenses at hatching compensated rapidly, with almost complete compensation after 4 days of lens wear; they had decreased vitreous chamber depth compared to normal eyes and were thus hyperopic when the lenses were removed. In contrast, adaptation to the -10 D lenses was much slower, was still incomplete after 9 days of lens wear, and in this case, eyes had increased vitreous chamber depth and were myopic without the lenses. Adaptation improved when lens wear was delayed until 7 days after hatching. The effect of interrupting lens wear by periods of normal vision varied with the sign of the lenses worn. Hyperopia was always seen in response to +10 D lenses, although the magnitude of the response decreased as the duration of lens wear was decreased. In contrast, even brief periods of normal vision, i.e., 3 hr, prevented the development of myopia in response to the -10 D lenses; this apparent sensitivity to normal vision is similar to that reported for form-deprivation myopia. Ciliary nerve section used here to eliminate accommodation did not alter these response patterns.

Changes in eye growth produced by drugs which affect retinal ON or OFF responses to light

The interaction between drugs which affect ON and OFF responses to light and ocular growth was investigated in 154 eyes of newly-hatched chicks raised with normal vision or monocular occlusion with regular intravitreal injections of APB (2-amino-4 phosphonobutyric acid) or PDA (cis 2,3-piperidine-dicarboxylic acid). The experimental results indicate that APB, at a dose sufficient to abolish the b-wave of the electroretinogram (but not to show extensive damage to the retinal neurons at the light microscopic level), caused a significant decrease in the axial growth of the eyes when compared with normal eyes. APB did not result in a significant difference in the growth rates of the occluded eyes compared with occluded controls. By contrast, injection of PDA into occluded eyes at a dose which reduced the response at light offset in the electroretinogram (and also affected the ON-response), caused a dramatic reduction in elongation of those eyes compared with occluded controls. Injection of PDA into eyes raised in a normal visual environment did not induce a significant difference from the growth rate of normally reared control eyes. These growth changes support the hypothesis that drugs which affect the physiological function of the retina, in particular, the strength of the ON and OFF responses, interact with the visual rearing environment to cause a consistent pattern of changes to eye growth and refractive error.

Retinal-image mediated ocular growth as a mechanism for juvenile onset myopia and for emmetropization. A literature review

The very common ocular clinical ocular condition in children juvenile onset myopia results from axial elongation of the eye. In humans, some studies have found an association of myopia with greater levels of nearpoint activity and with differences in accommodation and convergence function. This paper reviews a variety of laboratory and clinical studies which are consistent with the hypothesis that retinal image defocus is biochemically transformed into an axial elongation expressed through increased posterior segment growth, and thus myopia. This paper also reviews theories of emmetropization, and classifies them as correlational, feedback, and combination. Evidence is presented to suggest that a combination theory, which combines both correlation of the ocular dioptric components and some feedback mechanism for growth of the eye, is the most correct. Current laboratory research suggests that quality and/or focus (defocus) of retinal imagery is involved in this feedback mechanism and that experimentally induced myopia might be enhanced, reduced or eliminated by pharmaceutical application. Direction of defocus may affect the rate of posterior segment growth, and thus the rate of ocular axial elongation.

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

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.

The absence of a photopic influence on the refractive development of the embryonic eye of the clearnose skate (Raja eglanteria)

The clearnose skate (Raja eglanteria) develops in an almost opaque eggcase and lays its eggs in pairs. One sibling from each of eight pairs of skates was removed from its eggcase during embryonic development, while the other sibling developed inside the eggcase. The refractive development of the eyes at hatching was examined to see if ambient light exposure during embryonic development could influence the refractive states of hatchlings. Measurements included refractive states, ocular dimensions and lens focal properties. The differences in measurements between the two groups were not significant, which would indicate that environmental light does not influence the refractive development of the embryonic skate eye.

Nearwork-induced transient myopia: a critical review

The literature on nearwork-induced transient myopia (NITM) is reviewed, with NITM being defined as the short-term myopic far point shift immediately following a sustained near visual task. A majority of these investigations demonstrated the presence of NITM for a variety of test parameters, e.g., visual acuity, contrast sensitivity and far point. Overall, these studies reported relatively small myopic shifts, with a mean of approximately 0.40 D and a range from 0.12 to 1.30 D. The subsequent decay is characterized by an exponential function with a relatively short time course. While the precise etiology and implications of NITM remain unclear, speculations regarding its origin and relevance to clinical myopia are discussed. Studies that did not demonstrate NITM are also reviewed.

6-Hydroxy dopamine does not affect lens-induced refractive errors but suppresses deprivation myopia

Degradation of the retinal image by translucent occluders during postnatal development induces axial myopia in chickens, tree shrews and monkeys. Local visual deprivation produces myopia even in local regions of the eye and neither accommodation nor intact connection between the eye and the brain are necessary. Therefore, it is an important question whether a similar local-retinal pathway translating visual information into growth or stretch signals to the underlying sclera is acting to emmetropize the growing eye. It is not known until now whether occluder deprivation triggers similar eye growth (or scleral stretch) mechanisms that are also responsible for visual guidance of normal refractive development. We here report that, in chickens, 6-hydroxy dopamine suppresses deprivation-induced myopia but has no effect on the magnitude of changes in axial eye elongation that are induced by spectacle lenses. The result suggests that, in chickens with normal accommodation, two pharmacologically different feedback loops may be responsible for deprivation myopia and lens-induced refractive errors.

Experimental myopia in a diurnal mammal (Sciurus carolinensis) with no accommodative ability

1. We examined the functional morphology of the intra-ocular muscles of the grey squirrel using pharmacological and histological methods. Using sympathomimetic (phenylephrine) and parasympathomimetic (carbachol) agents, administered by transcorneal iontophoresis, the response of the iris sphincter and dilator muscles and the ciliary muscle were recorded. Measurement techniques included both streak retinoscopy and coincidence optometry for measurement of ocular refraction and high resolution ultrasonography to monitor changes in the intra-ocular component dimensions. 2. The grey squirrel was found not to possess a functional accommodative system. No change in ocular refraction or intra-ocular dimensions could be induced with 40% carbachol. Marked changes in pupil diameter occurred with topical application of both phenylephrine (dilation) and carbachol (constriction). Histological findings were in agreement with pharmacological findings in showing well developed iris sphincter and dilator muscles but only a poorly developed ciliary muscle. 3. Calculation of the depth of focus of the grey squirrel eye reveals that this could be sufficient to account for the behavioural observations of near viewing habits. 4. We then determined whether we could induce axial elongation of the vitreous chamber and a consequent myopia by monocular deprivation (MD) of pattern vision. 5. Monocular deprivation of pattern vision produced a significant experimental myopia due to axial elongation of the vitreous chamber in the deprived eye. 6. The results demonstrate that a functional accommodative system is not necessary to induce experimental myopia in the grey squirrel eye.

The influence of study habits on myopia in Jewish teenagers

The prevalence and degree of myopia were measured in 870 teenagers, males and females. We found a statistically significant higher prevalence and degree of myopia in a group of 193 Orthodox Jewish male students who differed from the rest in their study habits. Orthodox schooling is characterized by sustained near vision and frequent changes in accommodation due to the swaying habit during study and the variety of print size. A possible myopic effect of this unique visual demand is postulated.

Neonatal eye growth and emmetropisation--a literature review

The refractive development of the neonatal eye has been the subject of much study and debate. In this paper the hypothetical mechanisms of emmetropisation and their relationship to the development of refractive errors will be reviewed. The evidence supporting visual feedback control of eye growth will be described, and the role of ocular accommodation will be discussed.

Properties of the feedback loops controlling eye growth and refractive state in the chicken

Recent experiments in chickens provide evidence that axial eye growth and refractive state are guided by mechanisms sensitive to refractive error. To determine whether or not the sign of refractive error is derived from longitudinal chromatic aberration we raised chicks with spectacle lenses in monochromatic light. The eyes showed an appropriate growth response to correct for the defocus imposed by the lenses no different than in previous experiments in white light. Thus, in normally accommodating chicks chromatic cues are not necessary for emmetropization to occur. We examined the linearity of feedback loops controlling axial eye growth: positive spectacle lenses were found to inhibit axial growth very efficiently making the eyes shorter than normal whereas negative lenses had little effect on axial elongation: feedback loops for regulation of axial growth are highly nonlinear and act most efficiently on the myopic side. We found that, subsequent to a period of binocular deprivation of form vision, the refractive errors acquired are highly correlated in both eyes. Since both eyes grew without visual feedback we conclude that the gains in the feedback loops that control axial growth must be similar in both eyes. We suggest that the gains are genetically determined and are typical for each individual. Chicks made near-sighted in both eyes by "deprivation of form vision" were corrected by appropriate negative lenses. Three out of five chicks recovered from myopia despite the correction. Also two chicks that were made near-sighted in one eye recovered with no regard to the correcting lens. Three chicks remained more myopic than the correcting lens required and finally started to recover while the lens was still in place. Two out of three chicks that were made far-sighted showed recovery despite appropriate correction by positive lenses. We conclude that there must be a nonvisual mechanism highly sensitive to abnormal eye shape. During expt (4) we found unexpectedly that the development of form deprivation myopia is inhibited if no part of the retina in an animal is exposed to normal visual experience. The result indicates that some communication between both eyes exists, although form deprivation myopia itself has been shown to develop independently in both eyes.