Murchison G. Callender

Refractive plasticity of the developing chick eye.

We have developed a lightweight plastic goggle with rigid contact lens inserts that can be applied to the eyes of newly hatched chicks to explore the range and accuracy of the developmental mechanism that responds to retinal defocus. Convex and concave lenses of 5, 10, 15, 20 and +30D were applied to one eye on the day of hatching. The chick eye responds accurately to defocus between ‐10 and + 15D. although hyperopia develops more rapidly than myopia. Beyond this range there is first a levelling off of the response and then a decrease. The resulting refractive errors are caused mainly by increases and decreases in axial length, although high levels of hyperopia are associated with corneal flattening. If ±10 D defocusing lenses are applied nine days after hatching the resulting myopia and hyperopia are equal to about 80% of the inducing power. After one week of inducing myopia and hyperopia with ±10 D lenses, the inducing lenses were reversed. In this case, the refractive error did not reach the power of the second lens after another week of wear. Instead, astigmatism in varying amounts (0–12 D) was produced, being greater when reversal was from plus to minus. Finally, astigmatism can also be produced by applying 9 D toric inducing lenses on the day of hatching. The astigmatism produced varies from 2 to 6 D. and the most myopic meridian coincides with the power meridian of the inducing lens. This astigmatism appears to be primarily due to corneal toricity. Furthermore, the greatest magnitude of astigmatism was produced when the piano meridian of the inducing lens was placed 45° from the line of the palpebral fissure.

Inducing myopia, hyperopia, and astigmatism in chicks

Myopia and hyperopia have been produced in chicks by applying specially designed convex and concave soft contact lenses to the eyes of newly hatched birds. After 2 weeks of wear, the eyes develop refractive states equivalent in sign and amount (+8 and -10 D) to the lens used. However, the lenses produce an artificial hyperopic shift during the first week of wear due to corneal flattening. We have developed a new approach involving the use of goggles with hard convex and concave contact lens inserts placed between the frontal and lateral visual fields. Myopia and hyperopia (+10 and -10 D) can be produced within days (4 days for hyperopia and 7 days for myopia) if the defocus is applied from the day of hatching. We can also produce significant amounts of astigmatism (1 to 5 D) axis at 90° and 180° by using cylindrical contact lens inserts. Although these last results are preliminary, they suggest that accommodation is not likely involved at this stage of refractive development because we do not believe that the accommodative mechanism can cope with cylindrical defocus. All spherical refractive errors produced using the goggle system appear to result from alterations in vitreous chamber depth.

Inducing ametropias in hatchling chicks by defocus-aperture effects and cylindrical lenses

Light-weight translucent plastic goggles with convex or concave rigid contact lens inserts were applied unilaterally to the eyes of young chicks. Convex and concave cylindrical lenses produced astigmatic refractive errors. The magnitude of the induced astigmatism was less than that of the inducing lens and varied with axis orientation. Decreased aperture size or interruption of the defocus resulted in a decreased response to refractive defocus. Slit apertures and spherical defocus produced variable amounts of myopia, hyperopia and astigmatism. Choroidal changes (increased thickness) were observed only in birds developing hyperopia or recovering from myopia.

CHICK EYE OPTICS : ZERO TO FOURTEEN DAYS

Ocular dimensions and refractive state data for chicks 0 to 14 days of age were obtained from 234 untreated control eyes of birds treated unilaterally in previous work involving various defocussing lenses and/or translucent goggles. Refractive state and corneal curvatures were measured in vivo by retinoscopy and ophthalmometry respectively. Intraocular dimensions were measured by A-scan ultrasonography, after which the eyes were removed, weighed and measured. In some cases (n=52) intraocular dimensions and lens curvatures were obtained from frozen sections of enucleated eyes. The hyperopia of hatchling chicks (+6.5+4.0 D) initially decreases rapidly and then more gradually to + 2.0 ± 0.5 D by 16 days. The distribution of refractive errors is very broad at Day 0, but becomes leptokurtotic, with a slight myopic skew, by Day 14. Corneal radius is constant for the first four days, possible as a result of pre-hatching lid pressure, and then increases linearly, as do all lens dimensions, axial diameter and equatorial diameter. Schematic eyes were developed for Days 0, 7, and 14.

Effects of continuous light on experimental refractive errors in chicks

It is possible to induce ametropias in young chicks either by depriving the developing eye of clear form vision with a translucent goggle or by defocusing the retinal image with convex or concave lenses. The refractive properties of the developing chick eye are also altered by raising young birds in a continuous light environment. The effects of superimposing form deprivation or defocus treatments on chicks raised in continuous light are unclear. Newly hatched (n = 31) chicks were raised for 2 weeks under continuous light while wearing either translucent goggles or + 10 or -10 diopter (D) lenses over one eye. Refractive states, corneal curvature and intraocular dimensions were measured periodically by retinoscopy, keratometry and A-scan ultrasound. The birds were sacrificed after 2 weeks and the eyes removed and measured with calipers. Under continuous light, all eyes treated with translucent goggle and -10 D lens developed moderate myopia (-2.6 +/- 0.5 D and -1.4 +/- 0.3 D, respectively) by day 4. The eyes treated with a + 10 D lens developed moderate hyperopia (+ 4.8 +/- 0.5 D) at day 4. Corneal curvatures of all treated eyes were slightly, but significantly, larger than contralateral control eyes by day 4. After 2 weeks of goggle or lens application, all the treated eyes were hyperopic due to corneal flattening. But the eyes treated with a goggle or a -10 D lens still showed relative myopia compared to the fellow eyes (treated minus untreated = -3.8 +/- 0.4 D and -2.8 +/- 0.4 D, respectively), and the eyes treated with a + 10 D lens showed more hyperopia than fellow eyes (treated minus untreated = + 5.1 +/- 0.6 D). Compared with the control eyes, the axial length (mainly vitreous chamber depth) was slightly, but significantly, increased in the eyes treated with a goggle or a -10 D lens, and the axial length decreased slightly in the eyes treated with + 10 D lens. The results suggest that form deprivation and retinal defocus (induced by +/- 10 D lenses) could still induce experimental refractive errors (myopia and hyperopia) in chicks kept under continuous light, but the effects of form deprivation and retinal defocus were partially suppressed by continuous light.

Accuracy of Javal's rule in the determination of spectacle astigmatism.

Javals rule and Grosvenors simplification of it are commonly used formulas for predicting spectacle astigmatism from keratometric measurements. We assessed the accuracy of these two rules. Spectacle astigmatism was estimated using both rules from measurements of corneal astigmatism on 100 eyes of 100 subjects. These estimates were then compared to the subjectively determined spectacle astigmatism. Grosvenors simplification of Javals rule gave slightly more accurate assessments than the original rule. However, only 66% of results gave estimates within 0.50 D, and 7% differed by more than 1.00 D. This can be compared to previous reports on the accuracy of autorefractors, where approximately 95% of cylinder results were within 0.50 D of the spectacle astigmatism. These results indicate that using Javals rule or Grosvenors simplification of it to determine spectacle astigmatism from corneal cylinder readings is of limited clinical value.

A quantitative study of human tear proteins before and after adaption to non-flexible contact lenses.

&NA; The total protein concentration of daily tear samples from eight male subjects, before lens placement and after lenses had been worn for a specific period, were monitored. The resulting transient instability in protein concentration and the associated physiological implications are discussed.

Afocal magnification does not influence chick eye development.

In defocus-induced ametropia experiments, retinal blur circles are a likely source of information as to the magnitude but not the sign of the defocus. However, magnification (and minification) produced by the lenses may be a cue. In this study, 1-day-old broiler chicks (N = 13) were treated monocularly for 7 days with special goggles containing approximately afocal iseikonic lenses which were designed to produce 10% retinal image magnification. This is a little less than the magnification produced by +10 D defocusing lenses used to produce about 10 D of hyperopia in earlier work. Intraocular dimensions of both eyes were measured by A-scan ultrasonography on the first and last day. Refractive states of both eyes were measured daily with a retinoscope and trial lenses. After the birds were sacrificed, the eyes were enucleated, weighed, and measured with calipers. Before the treatment there was no difference in the refractive state or dimensions of the right and left eyes. After 1 week of goggle wear there was still no significant difference between the eyes in spite of the magnification produced by the goggles. These data suggest that factors other than magnification are responsible for the ability of the eye to respond to the sign of defocus.

Refractive plasticity of the developing chick eye: a summary and update

To summarize the OPO 1992 Classic Paper: Refractive plasticity of the developing chick eye (12: 448–452) and discuss recent findings in refractive development.

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.