Ocular biometry measures and their correlation with refractive error in Aboriginal and Torres Strait Islander children

CLINICAL RELEVANCE

The ocular biometry measures of the eye determine the refractive status, and while most refractive error develops during childhood, the ocular biometry measures of Aboriginal and Torres Strait Islander children have not previously been reported.

BACKGROUND

To investigate the ocular biometry of Aboriginal and Torres Strait Islander children, including measures important in determining refractive error and those which relate to the risk of ocular disease.

METHODS

Participants included 252 primary and secondary school children (Aboriginal and Torres Strait Islander: 101; non-Indigenous: 151), aged between 4 and 18 years. Habitual monocular distance visual acuity, cycloplegic autorefraction, and ocular optical biometry were measured in all participants and intraocular pressure measured in secondary school children using rebound tonometry.

RESULTS

The mean (±SD) spherical equivalent refractive error of Aboriginal and Torres Strait Islander children was significantly less hyperopic than non-Indigenous children (Aboriginal and Torres Strait Islander: +0.52 ± 0.80 D; non-Indigenous: +0.86 D ±0.58 D; p < 0.001). There were no differences in axial length or axial length/corneal radius ratio between the two groups, however the mean lens power of Aboriginal and Torres Strait Islander children was significantly greater than that of non-Indigenous children (Aboriginal and Torres Strait Islander: 23.62 D; non-Indigenous: 22.51 D; p < 0.001). Aboriginal and Torres Strait Islander children had a thinner central corneal thickness (Aboriginal and Torres Strait Islander: 534 ± 37 µm; non-Indigenous: 543 ± 35 µm; p = 0.04), and lower intraocular pressure compared with non-Indigenous children (Aboriginal and Torres Strait Islander: 14.7 ± 3.8 mmHg; non-Indigenous: 16.0 ± 3.7; p = 0.02).

CONCLUSION

Differences exist in the refractive error, lens power, central corneal thickness, and intraocular pressure of Aboriginal and Torres Strait Islander children compared to non-Indigenous Australian children which have potential implications for the development of refractive error and ocular disease later in life.

Refractive development I: Biometric changes during emmetropisation

PURPOSE

Although there are many reports on ocular growth, these data are often fragmented into separate parameters or for limited age ranges. This work intends to create an overview of normal eye growth (i.e., in absence of myopisation) for the period before birth until 18 years of age.

METHODS

The data for this analysis were taken from a search of six literature databases using keywords such as "[Parameter] & [age group]", with [Parameter] the ocular parameter under study and [age group] an indication of age. This yielded 34,409 references that, after screening of title, abstract and text, left 294 references with usable data. Where possible, additional parameters were calculated, such as the Bennett crystalline lens power, whole eye power and axial power.

RESULTS

There were 3422 average values for 17 parameters, calculated over a combined total of 679,398 individually measured or calculated values. The age-related change in refractive error was best fitted by a sum of four exponentials (r²  = 0.58), while all other biometric parameters could be fitted well by a sum of two exponentials and a linear term ('bi-exponential function'; r² range: 0.64-0.99). The first exponential of the bi-exponential fits typically reached 95% of its end value before 18 months, suggesting that these reached genetically pre-programmed passive growth. The second exponentials reached this point between 4 years of age for the anterior curvature and well past adulthood for most lenticular dimensions, suggesting that this part represents the active control underlying emmetropisation. The ocular components each have different growth rates, but growth rate changes occur simultaneously at first and then act independently after birth.

CONCLUSIONS

Most biometric parameters grow according to a bi-exponential pattern associated with passive and actively modulated eye growth. This may form an interesting reference to understand myopisation.

Optical mechanisms regulating emmetropisation and refractive errors: evidence from animal models

Our current understanding of emmetropisation and myopia development has evolved from decades of work in various animal models, including chicks, non-human primates, tree shrews, guinea pigs, and mice. Extensive research on optical, biochemical, and environmental mechanisms contributing to refractive error development in animal models has provided insights into eye growth in humans. Importantly, animal models have taught us that eye growth is locally controlled within the eye, and can be influenced by the visual environment. This review will focus on information gained from animal studies regarding the role of optical mechanisms in guiding eye growth, and how these investigations have inspired studies in humans. We will first discuss how researchers came to understand that emmetropisation is guided by visual feedback, and how this can be manipulated by form-deprivation and lens-induced defocus to induce refractive errors in animal models. We will then discuss various aspects of accommodation that have been implicated in refractive error development, including accommodative microfluctuations and accommodative lag. Next, the impact of higher order aberrations and peripheral defocus will be discussed. Lastly, recent evidence suggesting that the spectral and temporal properties of light influence eye growth, and how this might be leveraged to treat myopia in children, will be presented. Taken together, these findings from animal models have significantly advanced our knowledge about the optical mechanisms contributing to eye growth in humans, and will continue to contribute to the development of novel and effective treatment options for slowing myopia progression in children.

Choroidal thickness predicts ocular growth in normal chicks but not in eyes with experimentally altered growth

BACKGROUND

In hatchling chicks, the thickness of the choroid is quite variable. It has been postulated that thickness per se or the changes occurring during early life might play a causal role in the regulation of ocular growth. We tested this notion by measuring ocular dimensions in several experimental conditions that alter ocular growth and in the fellow eyes.

METHODS

Chicks aged 12 to 14 days wore monocular lenses or diffusers (+10 D, n = 23; -10 D, n = 16; diffusers, n = 16) for four to five days. Fellow untreated eyes served as controls. A separate group of completely untreated birds aged eight days were also tested (n = 12). We tested two drugs known to alter ocular growth. The dopaminergic agonist quinpirole was injected daily for five days into eyes wearing negative lenses (n = 47). The muscarinic agonist oxotremorine was injected one time into normal eyes (n = 27). All eyes were measured using high-frequency A-scan ultrasonography at the start and end of the experiment. Spearman's correlation coefficient was used in all analyses.

RESULTS

Choroidal thickness predicted ocular growth rates in normal eyes: eyes with thinner choroids grew faster than those with thicker choroids (p = 0.0001). Furthermore, there was a negative correlation between initial thickness and the change in thickness (p = 0.0353). By contrast, eyes wearing lenses or diffusers did not show a correlation between initial thickness and growth rate. For lens-wearing eyes injected with quinpirole, which slowed growth, initial choroidal thickness predicted subsequent growth rate (p = 0.0126), similar to normal eyes. This was not so for oxotremorine, which stimulated growth.

CONCLUSIONS

The loss of the association between choroidal thickness and subsequent growth rate in eyes with experimentally altered growth implies an uncoupling of the choroidal response from the visual regulation of ocular growth. The negative correlation between initial thickness and ocular growth in eyes injected with quinpirole suggests potential therapeutic benefits to thicker choroids.

Longitudinal study of spherical refractive error in infantile nystagmus syndrome

PURPOSE

To explore the onset and progression of spherical refractive error in a population with infantile nystagmus syndrome.

METHODS

Retrospective refractive error data were obtained from 147 medical records of children with infantile nystagmus syndrome (albinism n = 98; idiopathic infantile nystagmus n = 49), attending a low vision clinic in Northern Ireland, over a 24 year period (1986-2010). Data were categorised by age to allow for comparisons with published studies. A prospective group of participants with Infantile nystagmus syndrome (INS) [n = 22 (albinism n = 18, idiopathic infantile nystagmus n = 4)] (aged 0-4) were also recruited. Cycloplegic streak retinoscopy was performed biannually, over a 3 year period. Spherical equivalent refractive error and most ametropic meridian were analysed.

RESULTS

The mean spherical equivalent refractive errors for albinism and idiopathic infantile nystagmus groups (across all age categories) were hypermetropic, with highest levels demonstrated by the participants with albinism aged 1 ≤ 4 years (Mann-Whitney U test, p = 0.013). Mean most ametropic meridian was highest in the albinism group aged 1 ≤ 12 years (Mann-Whitney U test, p < 0.05). Individual data demonstrated relatively static spherical equivalent refractive errors over time. Prospective participants were hypermetropic at all visits and those with albinism had, on average, higher refractive errors than those with idiopathic infantile nystagmus (IIN). No significant correlations were noted between visual acuity and spherical equivalent refractive errors or most ametropic meridian.

CONCLUSIONS

Hypermetropia is the most prevalent spherical refractive error in the INS population, irrespective of level of visual acuity. Individuals with infantile nystagmus syndrome fail to demonstrate typical patterns of emmetropisation, particularly in the presence of albinism.

Is emmetropia the natural endpoint for human refractive development? An analysis of population-based data from the refractive error study in children (RESC)

PURPOSE

To determine the natural end-point for refractive development during childhood.

METHODS

Cycloplegic (1% cyclopentolate) autorefraction was performed on 38, 811 children aged 5 and 15 in population-based samples at eight sites in the Refractive Error Study in Children (RESC). Refractions (right eye) were categorized as myopic (≤-0.5 D), emmetropic (>-0.5 to ≤+0.5 D), mildly hyperopic (>+0.5 to ≤+2.0 D and hyperopic (>+2.0 D).

RESULTS

At five sites (Jhapa - rural Nepal, New Delhi - urban India, Mahabubnagar - rural India, Durban - semi-urban South Africa and La Florida - urban Chile), there was <20% myopia by age 15. Mild hyperopia was the most prevalent category at all ages, except for Mahabubnagar where emmetropia became the marginally most prevalent category at ages 14 and 15. At the other sites (Gombak - semi-urban Malaysia, Shunyi - semi-rural China and Guangzhou - urban China), there was substantial (>35%) myopia by age 15. At these sites, mild hyperopia was the most prevalent category during early childhood, and myopia became the predominant category later. In Gombak district and Guangzhou, emmetropia was a minor category at all ages, with myopia increasing as mild hyperopia decreased. In Shunyi district, emmetropia was the most prevalent category over the ages 11-14.

CONCLUSION

Emmetropia was not the predominant outcome for refractive development in children. Instead, populations were predominantly mildly hyperopic or substantial amounts of myopia appeared in them. This suggests that mild hyperopia is the natural state of refractive development in children and that emmetropia during childhood carries the risk of subsequent progression to myopia.

Weaning children with accommodative esotropia out of spectacles: a pilot study

BACKGROUND/AIM

Many children with accommodative esotropia must continue spectacle use throughout life. This study was undertaken to determine which factors are predictive of successfully weaning children with accommodative esotropia out of spectacles.

METHODS

A retrospective review of 10 children with accommodative esotropia, who were gradually weaned from their hyperopic correction, and three age matched controls was performed. The main outcome measure was resolution or non-resolution of esotropia following weaning and eventual discontinuation of spectacles. Secondary outcome measures were final refractive error and the final esotropic or esophoric angle without correction.

RESULTS

Six patients were successfully weaned from spectacles. At the completion of the weaning period one child was orthophoric and the other five children had well controlled esophorias. The other four patients remained spectacle dependent because of persistent esotropia or decreased vision without spectacles. The baseline and final refractive errors were significantly lower in the children successfully weaned from spectacles (p = 0.014). While the children who were successfully weaned from spectacles were older when initially diagnosed with accommodative esotropia (4.6 v 2.5 years), this difference was not statistically significant (p = 0.09).

CONCLUSION

Some children with accommodative esotropia may be weaned out of spectacles during the grade school years with resolution of their esotropia. It is likely that gradual reduction of the hyperopic correction increases divergence amplitudes, but it is unclear whether this facilitates emmetropisation.

Emmetropisation following preterm birth

BACKGROUND/AIMS

Even in the absence of retinopathy of prematurity (ROP), premature birth signals increased risk for abnormal refractive development. The present study examined the relation between clinical risk factors and refractive development among preterm infants without ROP.

METHODS

Cycloplegic refraction was measured at birth, term, 6, 12, and 48 months corrected age in a cohort of 59 preterm infants. Detailed perinatal history and cranial ultrasound data were collected. 40 full term (plus or minus 2 weeks) subjects were tested at birth, 6, and 12 months old.

RESULTS

Myopia and anisometropia were associated with prematurity (p<0.05). More variation in astigmatic axis was found among preterm infants (p<0.05) and a trend for more astigmatism (p<0.1). Emmetropisation occurred in the preterm infants so that at term age they did not differ from the fullterm group in astigmatism or anisometropia. However, preterm infants remained more myopic (less hyperopic) than the fullterm group at term (p<0.05) and those infants born <1500 g remained more anisometropic than their peers until 6 months (p<0.05). Infants with abnormal cranial ultrasound were at risk for higher hyperopia (p<0.05). Other clinical risk factors were not associated with differences in refractive development. At 4 years of age 19% of the preterm group had clinically significant refractive errors.

CONCLUSION

Preterm infants without ROP had high rates of refractive error. The early emmetropisation process differed from that of the fullterm group but neither clinical risk factors nor measures of early refractive error were predictive of refractive outcome at 4 years.