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GABA antagonists inhibit experimental myopia in chick and GABA receptors have been localized to chick sclera and the retinal pigment epithelium (RPE). The RPE and the choroid alter scleral DNA and glycosaminoglycan (GAG) content in vitro; opposite effects have been observed for tissues from myopic and hyperopic eyes. The aim was to determine the effect of GABAergic agents on the DNA and GAG content of chick scleral fibroblasts directly and in co-culture with ocular tissues from myopic and hyperopic chick eyes.Primary cultures of fibroblastic cells expressing vimentin and α-smooth muscle actin were established. GABAergic agents were added separately (i) to the culture medium of the scleral cells and (ii) to the culture medium of the scleral cells with the addition of posterior eye cup tissue (retina, RPE, retina + RPE, choroid + RPE) to cell culture inserts. Ocular tissues were obtained from chick eyes wearing + 15D (lens-induced hyperopia, LIH) or -15D lenses (lens-induced myopia, LIM) for three days (post-hatch day 5-8) (n = 12). GAG and DNA content of scleral fibroblasts were measured.GABA agents had a small direct effect on scleral cell GAG and DNA content but a larger effect was measured when GABA agents were added to the culture medium with myopic and hyperopic RPE and choroid + RPE tissues. GABA agonists increased (p = 0.002) whereas antagonists decreased (p = 0.0004) DNA content of scleral cells; effects were opposite for scleral GAG content. GABA agents significantly altered the effect of both LIM and LIH tissues (p = 0.0005) compared to control; the effects were greater for LIM tissue versus LIH tissue co-culture (p = 0.0004).GABAergic agents affect the DNA and GAG content of scleral fibroblasts both directly and when co-cultured with ocular tissues. GABA antagonists that prevent myopia development in chick model could act via a scleral mechanism utilizing the RPE/choroid.

Purpose : Contrast adaptation, produced by prolonged viewing of high contrast gratings, has been suggested to occur at both retinal and cortical locations within the visual pathway. We sought to investigate the characteristics of retinal contrast adaptation using the multifocal electroretinogram (mfERG). Methods : Twenty subjects, with a mean age of 27.8 +/- 5.3 years, underwent mfERG testing using VERIS I. The mfERG was measured after 10 minutes of adaptation to 94% contrast, 5 cyc/deg, sinusoidal, vertical gratings and to an equi-luminance blank control. The mfERG stimulus array consisted of 61-scaled hexagons and flickered according to a pseudorandom binary m-sequence (2(13) -1). Changes in amplitude and implicit time of the first-order kernel were analyzed to determine the effect of contrast adaptation on retinal responses. Results : Adaptation to the vertical grating pattern produced a 2.5 ms increase in implicit time, and the response delay was greatest in the more peripheral parts of the retina (7.6 degrees to 30 degrees). Contrast adaptation did not produce statistically significant changes to the amplitude of the mfERG waveform. Conclusions : Contrast adaptation produced by prolonged viewing of high contrast gratings had a significant effect on retinal responses. It has been suggested that contrast adaptation may play a role in the development of nearwork induced myopia; further work investigating retinal contrast adaptation in myopic individuals may be of interest.

While intraocular pressure has been shown to have some influence on eye growth, the stress exerted on the outer wall of the eye and the rigidity of the outer coats of the eye may be of greater importance. No studies have assessed the significance of both of these variables in terms of childhood myopia and its progression.Twenty myopic and twenty non-myopic children aged 8 to 12 years participated in the study. Refractive error (including refractive error shift over the past year), ocular dimensions (anterior chamber, lens thickness, vitreous chamber, axial length), intraocular pressure, equatorial scleral rigidity, and outer wall thickness were measured for right eyes. Outer wall stress was calculated using the approximation, p = IOP*r/2t.The myopic group was 3.43 D more myopic and had 1.43 mm longer eyes on average than the non-myopic group. The myopic children had experienced a mean refractive shift of -0.30 D/yr over the past year. Equatorial wall thickness was significantly less in myopes than non-myopes (difference of 0.09 mm, p = 0.02) and in the combined sample was correlated to refractive error (r = 0.312, p = 0.05), but not refractive error shift. Ocular rigidity and eye wall stress values were similar in the two groups. The refractive shift increased as wall stress increased both in the combined sample (r = -0.386, n = 35, p = 0.022) and the myopic group (r = -0.600, n = 16, p = 0.014).We did not find large differences in wall thickness, ocular rigidity, or wall stress in myopic and non-myopic children. While reduced ocular rigidity, increased wall stress and scleral thinning may have a role in myopia progression in childhood we were not able to clearly show this in our study of 40 children.

Myopia has been found to be predominantly axial in nature, i.e. myopic eyes have longer than normal axial lengths, with corneal radius variations having only a small influence on the magnitude of the refractive error. In this study we assess whether a similar relationship exists for hyperopia.Biometric data were collected on 57 subjects with either emmetropic or hyperopic refractive errors ranging in magnitude from -0.37 D to +17.25 D. Our main analysis concentrated on subjects with less than +10 D of hyperopia (group 1, n = 53), as subjects with +10 D of hyperopia or more (group 2, n = 4) exhibited marked differences in their biometric characteristics.Analysis of group 1 data revealed a significant relationship (r2 = 0.611, p = 0.0001) between the degree of hyperopia and the measured axial lengths. A weak but statistically significant relationship (r2 = 0.128, p = 0.009) was also found between mean corneal radius measures and mean spherical refractive errors, with the mean corneal radius flattening with increasing hyperopia. In group 2, three of the four subjects exhibited much steeper corneal characteristics than predicted from the group 1 data.Our results suggest that hyperopia, like myopia, is predominantly axial in nature, although the corneal radius also plays a role in determining refractive error magnitude. These results have implications for refractive surgery and visual performance in hyperopic eyes.

Compensatory responses to focusing errors imposed by spectacle lenses in chicks, tree shrews and primates leave little doubt that active emmetropization can occur, and debate is now centered on whether this process is uni-directional or bi-directional in nature. To provide further insight into this emmetropization process, the studies reported in this paper addressed the question of whether access to sharp vision is necessary for compensation to myopic defocus in the chick.Two different experimental paradigms were used to address the above question: (A) Myopic defocus was imposed, either with +15 or +40 d lenses alone or with +15 D lenses on eyes made myopic by 7 days of form deprivation; these treatments result in a shift in the plane of focus of the eye (far point) to 6.67, 2.5 cm and approximately 3.5 cm resp., with only objects at or closer than these planes being in focus. The addition to the lenses of stand-off cones, either 2.5 or 5 cm in length, further limited access to (or precluded) sharp vision by controlling how closely the chicks could approach objects. One group that had sharp vision precluded also underwent optic nerve section. (B) A range of positive lenses (+15 to +65 D) were used on their own to impose myopic defocus; for the high power lenses, access to sharp vision was very restricted because of the close proximity of the new far point (1.54 cm for +65 D lens). Refractive errors and axial ocular dimensions were measured in all experiments.In the first study (A), preclusion of sharp vision not only prevented compensation but resulted in increased eye growth and myopia. This myopia, like form-deprivation myopia, was unaltered by optic nerve section surgery. Limiting but not precluding sharp vision resulted in partial compensation. In the second study (B), good compensation was observed with the +15 D lens but compensation progressively declined for higher powers, with the +50 D lens having no apparent effect on eye growth and refraction and the +65 D lens inducing myopia instead of hyperopia.Together these results argue that some sharp vision is fundamental to compensation to impose myopia. The significance of this new finding in relation to the processes underlying active emmetropization is discussed.