Choroidal and scleral mechanisms of compensation for spectacle lenses in chicks

It is known that when hyperopic or myopic defocus is imposed on chick eyes by spectacle lenses, they rapidly compensate, becoming myopic or hyperopic respectively, by altering the depth of their vitreous chamber. Changes in two components--ocular length and choroidal thickness--underlie this rapid compensation. With monocular lens treatment, hyperopic defocus imposed by negative lenses resulted in substantially increased ocular elongation and a slight thinning of the choroid, both changes resulting in myopia; myopic defocus imposed by positive lenses resulted a dramatic increase in choroidal thickness, which pushed the retina forward toward the image plane, and a slight decrease in ocular elongation, both changes resulting in hyperopia. The refractive error after 5 days of lens wear correlated well with vitreous chamber depth, which reflected the changes in both choroidal thickness and ocular length. The degree of compensation for lenses was not affected by whether the fellow eye was covered or open. Both form-deprivation myopia and lens-induced myopia declined with age in parallel, but wearing a -15 D lens produced more myopia than did form deprivation. The spectacle lenses affected the refractive error not only of the lens-wearing eye, but also, to a much lesser degree, of the untreated fellow eye. At lens removal refractive errors were opposite in sign to the lense worn, and the subsequent changes in choroidal thickness and ocular length were also opposite to those that occurred when the lenses were in place. In this situation as well, effects of the spectacle lenses on the fellow eyes were observed. Eyes with no functional afferent connection to the brain because of either prior optic nerve section or intraocular tetrodotoxin injections showed compensatory changes to imposed defocus, but these were limited to compensation for imposed myopic defocus, at least for the eyes with optic nerve section. In addition, optic nerve section, but not tetrodotoxin treatment, moved the set-point of the visual compensatory mechanism toward hyperopia. Optic nerve section prevents myopia in response to negative lenses but not to diffusers, suggesting that compensation for hyperopia requires the central nervous system.

Relative peripheral hyperopic defocus alters central refractive development in infant monkeys

Understanding the role of peripheral defocus on central refractive development is critical because refractive errors can vary significantly with eccentricity and peripheral refractions have been implicated in the genesis of central refractive errors in humans. Two rearing strategies were used to determine whether peripheral hyperopia alters central refractive development in rhesus monkeys. In intact eyes, lens-induced relative peripheral hyperopia produced central axial myopia. Moreover, eliminating the fovea by laser photoablation did not prevent compensating myopic changes in response to optically imposed hyperopia. These results show that peripheral refractive errors can have a substantial impact on central refractive development in primates.

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.

Extreme hyperopia is the result of null mutations in MFRP, which encodes a Frizzled-related protein

Nanophthalmos is a rare disorder of eye development characterized by extreme hyperopia (farsightedness), with refractive error in the range of +8.00 to +25.00 diopters. Because the cornea and lens are normal in size and shape, hyperopia occurs because insufficient growth along the visual axis places these lensing components too close to the retina. Nanophthalmic eyes show considerable thickening of both the choroidal vascular bed and scleral coat, which provide nutritive and structural support for the retina. Thickening of these tissues is a general feature of axial hyperopia, whereas the opposite occurs in myopia. We have mapped recessive nanophthalmos to a unique locus at 11q23.3 and identified four independent mutations in MFRP, a gene that is selectively expressed in the eye and encodes a protein with homology to Tolloid proteases and the Wnt-binding domain of the Frizzled transmembrane receptors. This gene is not critical for retinal function, as patients entirely lacking MFRP can still have good refraction-corrected vision, produce clinically normal electro-retinograms, and show only modest anomalies in the dark adaptation of photoreceptors. MFRP appears primarily devoted to regulating axial length of the eye. It remains to be determined whether natural variation in its activity plays a role in common refractive errors.

Constant light produces severe corneal flattening and hyperopia in chickens

In this study we report on the effects of constant light (CL) on the refractive development and ocular morphology of White Leghorn chicks (Cornell K-strain). Refractive state and corneal curvature were measured by IR photoretinoscopy and IR keratometry respectively. The axial lengths of the ocular components were measured by A-scan ultrasonography. We find that constant light produces significant hyperopia compared to controls in as few as 10 days (7.4 vs 4.0 D). This is apparently the result of flatter than normal corneal curvature (radius of curvature: 3.22 vs 3.08 mm) as vitreous chamber depth is significantly deeper in CL eyes than controls at that age (5.6 vs 5.1 mm). In contrast to other reports, if CL rearing is continued for longer periods the hyperopia progresses, even though vitreous chamber depth continues to increase. After 11 weeks of CL severe hyperopia was observed (18.2 vs 2.8 D). Long term CL is also found to produce shallow anterior chambers, corneal thickening, lenticular thinning and cataracts, and damage to the retina, pigment epithelium, and choroid.

Optic disc size in a population based study in northern China: the Beijing Eye Study

AIM

To determine the optic disc size in the adult Chinese population in an urban and a rural region of Beijing.

METHODS

The population based, cross sectional cohort study included 4439 subjects out of 5324 subjects invited to participate (response rate 83.4%). It was divided into a rural part (1973 (44.4%) subjects) and an urban part (2466 (55.6%) subjects). Mean age was 56.2 (SD 10.6) years (range 40-101 years). Colour optic disc photographs were morphometrically examined. Main outcome measure was optic disc area.

RESULTS

Optic disc photographs were available for 4027 (90.7%) subjects. Mean optic disc area measured 2.65 (0.57) mm2 (range 1.03 mm2-7.75 mm2). Optic disc area was significantly (p<0.001) correlated with myopic refractive error, with a steep decrease in optic disc area from high myopia to the mid-range of refractive error, a slightly horizontal course in the refractive error range between -8 dioptres and +4 dioptres, and a further decrease in optic disc area towards higher hyperopia. Optic disc area was not related to age (p = 0.14) or sex (p = 0.93) (optic disc area, males: 2.65 (0.56) mm2 versus females: 2.65 (0.57) mm2). "Microdiscs" may be defined as smaller than 1.51 mm2, and "macrodiscs" as larger than 3.79 mm2.

CONCLUSIONS

Compared with data of preceding studies, mean optic disc size is larger in Chinese people than in white people. In Chinese people highly hyperopic eyes have significantly smaller optic discs, and highly myopic eyes have significantly larger optic discs than emmetropic eyes.

Prevalence of hyperopia and associations with eye findings in 6- and 12-year-olds

PURPOSE

To describe the prevalence of hyperopia and associated factors in a representative sample of Australian schoolchildren 6 and 12 years old.

DESIGN

Population-based cross-sectional study.

PARTICIPANTS

Schoolchildren ages 6 (n = 1765) and 12 (n = 2353) from 55 randomly selected schools across Sydney.

METHODS

Detailed eye examinations included cycloplegic autorefraction, ocular biometry, cover testing, and dilated fundus examination. Information on birth and medical history were obtained from a parent questionnaire.

MAIN OUTCOME MEASURES

Moderate hyperopia defined as spherical equivalent (SE) refraction of > or =+2.00 diopters (D), and eye conditions including amblyopia, strabismus, astigmatism, and anisometropia.

RESULTS

Prevalences of moderate hyperopia among children ages 6 and 12 were 13.2% and 5.0%, respectively; it was more frequent in children of Caucasian ethnicity (15.7% and 6.8%, respectively) than in children of other ethnic groups. Compared with children without significant ametropia (-0.49 < or = SE refraction < or = +1.99 D), the prevalence of eye conditions including amblyopia, strabismus, abnormal convergence, and reduced stereoacuity was significantly greater in children with moderate hyperopia (all Ps < 0.01). Maternal smoking was significantly associated with moderate hyperopia among 6-year-olds (P = 0.03), but this association was borderline among 12-year-olds (P = 0.055). Early gestational age (<37 weeks) and low birth weight (<2500 g) were not statistically significant predictors of moderate hyperopia in childhood.

CONCLUSIONS

Moderate hyperopia was strongly associated with many common eye conditions, particularly amblyopia and strabismus, in older children. Birth parameters did not predict moderate hyperopia.

Shape of the myopic eye as seen with high-resolution magnetic resonance imaging

We have obtained multislice magnetic resonance (MR) images of the eye and calculated ocular dimensions along the three cardinal axes: antero-posterior (A-P), equatorial, and vertical. We found no difference in the shape of hyperopic (average refractive error: +3.72 D) and emmetropic eyes, both of which had an equatorial diameter longer than the A-P and vertical diameters. Myopic eyes (average refractive error: -6.54 D) were larger than hyperopic eyes, and most had the same spheroelliptical shape as that of the emmetropic and hyperopic eyes. The results suggest that during myopic progression an overall enlargement or a radial volume expansion has occurred.

Ametropia and ocular biometry in a U.K. university student population

PURPOSE

The prevalence of myopia is known to vary with age, ethnicity, level of education, and socioeconomic status, with a high prevalence reported in university students and in people from East Asian countries. This study determines the prevalence of ametropia in a mixed ethnicity U.K. university student population and compares associated ocular biometric measures.

METHODS

Refractive error and related ocular component data were collected on 373 first-year U.K. undergraduate students (mean age = 19.55 years +/- 2.99, range = 17-30 years) at the start of the academic year at Aston University, Birmingham, and the University of Bradford, West Yorkshire. The ethnic variation of the students was as follows: white 38.9%, British Asian 58.2%, Chinese 2.1%, and black 0.8%. Noncycloplegic refractive error was measured with an infrared open-field autorefractor, the Shin-Nippon NVision-K 5001 (Shin Nippon, Ryusyo Industrial Co. Ltd, Osaka, Japan). Myopia was defined as a mean spherical equivalent (MSE) less than or equal to -0.50 D. Hyperopia was defined as an MSE greater than or equal to +0.50 D. Axial length, corneal curvature, and anterior chamber depth were measured using the Zeiss IOLMaster (Carl Zeiss, Jena, GmBH).

RESULTS

The analysis was carried out only for white and British Asian groups. The overall distribution of refractive error exhibited leptokurtosis, and prevalence levels were similar for white and British Asian (the predominant ethnic group) students across each ametropic group: myopia (50% vs. 53.4%), hyperopia (18.8% vs. 17.3%), and emmetropia (31.2% vs. 29.3%). There were no significant differences in the distribution of ametropia and biometric components between white and British Asian samples.

CONCLUSION

The absence of a significant difference in refractive error and ocular components between white and British Asian students exposed to the same educational system is of interest. However, it is clear that a further study incorporating formal epidemiologic methods of analysis is required to address adequately the recent proposal that juvenile myopia develops principally from "myopiagenic" environments and is relatively independent of ethnicity.

Intraocular lens power calculation in short eyes

PURPOSE

To compare the accuracy of the Hoffer Q and SRK-T formulae in eyes below 22 mm in axial length, using biometry measured with partial coherence inferometry (PCI), without a customised ACD constant.

METHODS

Data were retrospectively and prospectively collected by identifying eyes of axial length below 22 mm in the records of the intraocular lens (IOL) master and in preoperative notes. Biometry was performed using PCI and IOL power was calculated using both SRK-T and Hoffer Q formulae. Refractive outcome was measured and the accuracy of the two formulae compared.

RESULTS

Forty-one eyes of 41 patients were identified with an axial length <22 mm. Axial lengths ranged from 21.96 to 20.29 with a mean of 21.51 mm, and IOL power ranging from 23 dioptres (D) to 29 D. The Hoffer Q formula showed a mean prediction error of 0.61 D (SD 0.80) compared with the SRK-T, which showed a mean prediction error of 0.87 D (SD 0.829). A paired t-test found that the Hoffer Q was significantly more accurate than the SRK-T formula (P<0.001).

CONCLUSIONS

Hoffer Q was found to be more accurate than the SRK-T formula in this series of eyes <22 mm axial length when customised ACD constants are not used. Royal College of Ophthalmologists guidelines may need to be adjusted in accordance with these findings. This study underlines the importance of monitoring outcomes, and suggests different customisations are needed for different formulae, with a higher correction if the SRK-T formula is used for short eyes.

Hereditary posterior microphthalmos with papillomacular fold and high hyperopia

Five patients had a bilateral hereditary ocular syndrome composed of posterior microphthalmos with a papillomacular fold and high hyperopia. Anterior segment dimensions were near normal; the vitreous compartment was markedly fore-shortened. A papillomacular retinal fold extending from the center of the fovea toward the optic nerve head was present. Visual acuity ranged from 0.05 (20/400) to 0.6 (20/33); refractive errors ranged from + 11.25 to + 17.50 diopters. An autosomal recessive pattern of inheritance is postulated.

Factors that influence outcomes of hyperopic laser in situ keratomileusis

PURPOSE

To analyze the influence of preoperative corneal curvature, postoperative keratometric power, and the amount of correction on the outcomes of hyperopic laser in situ keratomileusis (LASIK).

SETTING

Clínica Baviera, Instituto Oftalmológico Europeo, Madrid, Spain.

METHODS

In this retrospective study, the records of 376 eyes that had LASIK for hyperopia using the Moria LSK-One microkeratome and the Technolas-Keracor 217C excimer laser were reviewed. The results were analyzed by preoperative hyperopia (5 subgroups) and by preoperative (more than and less than 43.0 diopters [D]) and postoperative (more than and less than 48.0 D) mean keratometry.

RESULTS

A statistically significant keratometry regression was found in the +3.00 to +3.90 D range (P <.01), a significant decrease in predictability in the +4.00 to +4.90 D range (P <.05), and a significant worsening in safety in the highest range (+6.00 to +7.90 D; P <.05). Comparative analysis of the > or = +4.00 D and <+4.00 D groups showed statistically significant differences in most measurement parameters. The preoperative keratometry did not influence postoperative results with the exception of poorer predictability in the group of preoperative flat corneas in which a high degree of hyperopia was corrected; ie, spherical equivalents within +/-0.50 D were found in 40.4% and 61.0% of cases with flat and steep corneas, respectively (P <.05). The efficacy and safety in eyes that achieved a postoperative keratometry >48.00 D did not differ significantly from the efficacy and safety in eyes that had a lower final keratometric power.

CONCLUSIONS

The factor that negatively influenced the outcome of hyperopic LASIK was the degree of hyperopia corrected. Preoperative keratometry did not significantly influence the postoperative results, and postoperative keratometry >48 D did not result in significant worsening of visual results when the attempted correction was less than +4.00 D.

Developmental basis of nanophthalmos: MFRP Is required for both prenatal ocular growth and postnatal emmetropization

BACKGROUND

Nanophthalmos is a genetic disorder characterized by very small, hyperopic eyes that are without gross structural defects. Recessive nanophthalmos is caused by severe mutations in the MFRP gene, which encodes a Frizzled-related transmembrane protein that is selectively expressed in the retinal pigment epithelium (RPE) and ciliary body.

RESULTS

For two MFRP -/- adults, we have obtained records of refraction that begin in early childhood. At the age of 6 months, one patient's eyes already had a refractive error of +12.25 D, and over the next 20 years this slowly increased to +17.50 D. Adults homozygous for null mutations in MFRP have eyes with axial lengths shorter than those of normal newborns. Furthermore, the unusually high curvature of their corneas is consistent with eyes that had been smaller than normal during late fetal development. MFRP protein was first detected at 14 weeks of gestation, when it was restricted to the posterior pole RPE. By 20 weeks gestation, MFRP expression had spread laterally, and was found throughout the RPE. MFRP protein was detected in both posterior and lateral RPE of the adult eye.

CONCLUSIONS

Embryonic function of the MFRP gene appears necessary for the eye to reach its full size at birth. Its onset of expression in the RPE during mid-gestation suggests that MFRP does not participate in early formation of the optic cup, and is consistent with a role in later growth and development of the eye. Patients without MFRP gene function exhibit no correction of refractive error during childhood, which suggests that this gene is essential for emmetropization, a complex process by which vision regulates axial growth of the eye.

Evidence for a potential role of glucagon during eye growth regulation in chicks

Eye growth and refraction are regulated by visual processing in the retina. Until now, the messengers released by the retina to induce these changes are largely unknown. Previously, it was found that glucagon amacrine cells respond to defocus in the retinal image and even to its sign. The expression of the immediate-early gene product ZENK increased in this cell population in eyes wearing plus lenses and decreased in minus lens-treated chicks. Moreover, it was shown that the amount of retinal glucagon mRNA increased during treatment with positive lenses. Therefore, it seems likely that these cells contribute to the visual regulation of ocular growth and that glucagon may act as a stop signal for eye growth. The purpose of the present study was to accumulate further evidence for a role of glucagon in the visual control of eye growth. Chicks were treated with plus and minus lenses after injection of different amounts of the glucagon antagonist des-His1-Glu1-glucagon-amide or the agonist Lys17,18,Glu21-glucagon, respectively. Refractive development and eye growth were recorded by automated infrared photorefraction and A-scan ultrasound, respectively. The glucagon antagonist inhibited hyperopia development, albeit only in a narrow concentration range, and at most by 50%, but not myopia development. In contrast, the agonist inhibited myopia development in a dose-dependent fashion. At high concentrations, it also prevented hyperopia development. The amount of glucagon peptide in the retinae and choroids of lens-treated chicks and its diurnal variation was measured by using a radio-immunoassay. Retinal glucagon content decreased after minus lens treatment and choroidal glucagon content increased after plus lens treatment. No diurnal variation in the retinal amount of glucagon was detected. In addition, using an optokinetic nystagmus paradigm, the effect of glucagon and the antagonist des-His1-Glu9-glucagon-amide on suprathreshold contrast sensitivity was studied. Glucagon reduced contrast sensitivity (which might be linked to a signal for growth inhibition) whereas the antagonist des-His1-Glu9-glucagon-amide increased contrast sensitivity. The results of the study are in line with the hypothesis that glucagon plays a role in the visual control of eye growth in the chick.

Refractive error in cataract surgery after previous refractive surgery

Bilateral cataract extraction with posterior chamber intraocular lens (IOL) implantation was performed in a patient after previous photorefractive keratectomy, radial keratotomy (RK) combined with astigmatic keratotomy, and retreatment of RK. Significant hyperopic error was observed after cataract surgery, and the IOLs were eventually exchanged in both eyes. A review of this case found that the refractive error was smaller when a refraction-derived keratometric value was selected for IOL power calculation. Nevertheless, hyperopic error still occurred.

Interpseudophakos Elschnig pearls associated with late hyperopic shift: a complication of piggyback posterior chamber intraocular lens implantation

We report 3 cases of bilateral piggyback lens implantation in which late hyperopic shift occurred associated with Elschnig pearl formation in the peripheral interface between the 2 lenses.

Laser in situ keratomileusis flap margin: wound healing and complications imaged by in vivo confocal microscopy

PURPOSE

To examine the healing response of laser in situ keratomileusis flap margin in vivo.

METHODS

Forty-three eyes of 43 patients who had undergone myopic (n = 39) or hyperopic (n = 4) laser in situ keratomileusis were examined once after surgery. The flap margin was imaged by in vivo confocal microscopy at various depths, and the wound healing response, flap alignment, and complications were evaluated. Ten eyes were examined on day 3 postoperatively, 13 eyes at 1 to 2 weeks, 10 eyes at 1 to 2 months, five eyes at 3 months, and five eyes at 6 months or later.

RESULTS

At 3 days after laser in situ keratomileusis, the surface epithelium and basal epithelium appeared normal. Keratocyte activation was strongest at 1 to 2 weeks and 1 to 2 months, and an increased amount of haze was observed correspondingly. Intrastromal epithelial cells forming a plug could occasionally be perceived in the wound gape. Wound constriction was completed in most cases by 3 to 6 months or later. Good alignment was observed in 12 of 43 flaps (27.9%) and moderate and poor alignment in 17 of 43 flaps (39.5%) and 13 of 43 flaps (30.2%), respectively. Poor alignment was not associated with lamellar epithelial ingrowth. Epithelial ingrowth was associated with dense haze at the interface. Diffuse lamellar keratitis was imaged in two corneas after hyperopic laser in situ keratomileusis.

CONCLUSIONS

The laser in situ keratomileusis incision wound at the flap margin appears to heal after the sequence observed in incisional wounds in nonhuman primates. Complications, such as lamellar epithelial in growth and diffuse lamellar keratitis, were often observed, particularly after hyperopic laser in situ keratomileusis.

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.

Normal variations of the posterior corneal surface

Measurements of the sphero-cylindrical components of the rear corneal surface were taken from 80 healthy right eyes in order to determine normal variations. Dimensions of this surface were strongly influenced by the front corneal surface with the exception that the rear surface exhibited more toricity. Both surfaces tended to be flatter in males compared to females and in myopes compared to hyperopes. The corneal surfaces were also found to be flatter in younger eyes compared to older eyes, but this finding was most likely due to the preponderance of myopes in the young and hyperopes in the older group. The influence of the ratio of anterior: posterior corneal surface radius upon the estimation of total corneal power as required for intraocular lens implant calculations was also considered.

Evolution of excimer laser corneal surgery

Interest in the potential of the excimer laser for refractive surgery was generated by a series of experiments performed in IBM's T.J. Watson laboratories. The development of photoablative decomposition was applied to corneas with immediate implication for corneal refractive control. Scientific investigations demonstrated that submicron erosion ablation of material was possible without damage to remaining tissues. After refinement of the beam and analysis of thresholds, we subsequently demonstrated that healing of large ablated areas in rabbits and then primate eyes would occur without scar formation or loss of transparency. This prompted a number of commercial efforts to develop this device for both refractive and general corneal surgical applications. The early clinical investigations show that a predictable refractive change could be induced in a cornea and this change was stable over many months. Furthermore, the transparency of the cornea was unimpaired. These favorable early results have led to a number of clinical investigations of excimer laser corneal surgery including refractive keratectomy, a direct laser keratomileusis.

Excimer laser photorefractive keratectomy for hyperopia

PURPOSE

To achieve less variation in the refractive outcome of hyperopic photorefractive keratectomy (PRK) by enlarging the treatment zone to 9.0 mm.

SETTING

Marienhospital, Amberg, Germany, and Klinika Ocni A Esteticke Chirurgie, Zlin, Czech Republic.

METHODS

This prospective clinical study was based on the results of PRK in 68 hyperopic eyes (62 patients) using an MEL 60 excimer laser. Mean attempted correction was +4.85 diopters (D) +/- 1.45 (SD) (range +2.00 to +8.25 D). Maximum follow-up was 12 months (68 eyes).

RESULTS

One year after PRK, 55 eyes (81%) were within 1.00 D and 40 eyes (59%) were within 0.50 D of the intended correction (predictability). Best corrected visual acuity was unchanged or improved in 62 eyes (92%) (safety). Four eyes (6%) lost one line, 1 eye (1%), two lines, and 1 eye (1%), three lines. Sixty-six eyes (97%) had an uncorrected visual acuity of 20/40 or better (efficacy) and 27 (40%), 20/20 or better.

CONCLUSION

Photorefractive keratectomy with a 9.0 mm treatment zone was an efficient and relatively safe procedure for correcting hyperopia of up to 8.25 D. The predictability was good. Great care must be taken to improve the centration of the optical zone.

Does experimentally-induced amblyopia cause hyperopia in monkeys?

We assessed refractive errors in 19 monkeys (Macaca nemestrina) raised with experimentally produced strabismus or unilateral defocus. These procedures resulted in hyperopic anisometropia in 10 monkeys. All 10 of the hyperopic animals were amblyopic; the amblyopic eye was always the more hyperopic eye. The degree of anisometropia was correlated with the degree of amblyopia. Hyperopic anisometropia did not develop in non-amblyopic animals. There was an association between early onset of visual abnormality and later development of hyperopic anisometropia. Since the refractive changes were correlated with changes in axial length and vitreous chamber depth, we suggest that amblyopia may cause alterations in eye growth and late-onset hyperopia.

Choroidal thickness and ocular growth in childhood

The involvement of the choroid in ocular growth regulation has been postulated in studies showing that refractive errors correlate with alterations in choroidal thickness (ChT). The advent of optical coherence tomography imaging has enabled qualitative and quantitative assessment of the choroid. In children, ChT changes correlate with a number of ocular pathologies, including myopia, retinopathy of prematurity, and amblyopia. We synthesize mechanisms and evidence regarding choroidal thickness variation during childhood. Subfoveal ChT is influenced by a number of factors including age, ethnicity, gender, axial length, and intraocular pressure. Myopic eyes have thinner choroids compared to emmetropic and hyperopic eyes. ChT may in fact serve as a marker of myopic progression, as ChT thinning occurs early during myopic development, but this association has not been established quantitatively. In addition, subfoveal ChT appears thicker in amblyopic eyes, while prematurity and retinopathy of prematurity may be associated with thinner ChT. Overall, both animal models and clinical research indicate that ChT induces or reflects physiological changes in the eye pertaining to ocular growth or maturation.