Ocular Refractive Error Research Articles

Ocular Growth and Refractive Error Development in Premature Infants with or without Retinopathy of Prematurity

To study factors involved in the development of refractive error in premature infants with or without retinopathy of prematurity (ROP). Premature infants in the national ROP screening program were recruited and examined longitudinally between 32 and 52 weeks' postmenstrual age. Axial length (AL), anterior chamber depth (ACD), and lens thickness (LT) were measured on the A-scan biometer. Corneal curvature was recorded with video-ophthalmophakometry and refractive state was determined with routine cycloplegic retinoscopy. Multilevel modeling techniques were used to study relationships between all the variables and stage of ROP throughout the study period, as well as individual growth rates. One hundred thirty-six infants were included. AL and ACD showed linear patterns of growth, whereas LT changed little over the study. Corneal curvature showed quadratic growth patterns in infants unaffected by ROP, but showed linear growth if ROP developed. Corneal curvature correlated well with refractive state. Most infants were myopic at the start of the study, became emmetropic around term, and were hypermetropic toward the end of the study. However, the eyes that were treated for ROP showed little change in refractive error; with significantly less hypermetropia by the end of the study. Eyes of premature infants have shorter axial lengths, shallower anterior chambers, and more highly curved corneas than eyes of full-term infants. These differences become more significant as the severity of ROP increases. Premature eyes develop less of the expected hypermetropia in full-term eyes, mainly due to differences in ACD and corneal curvature. These differences are most significant in eyes that receive laser treatment for ROP.

Refractive errors and ocular findings in children with intellectual disability: A controlled study

To evaluate the ocular findings and refractive errors in children with intellectual disability and in controls of average intellectual development of similar socioeconomic backgrounds. The study was conducted at Diskapi Children's Hospital in Ankara, Turkey: 724 subjects with intellectual disability and 151 control subjects were evaluated. The subjects with intellectual disability were subdivided into mild (IQ 50-69, n = 490), moderate (IQ 35-49, n = 164), and severe (IQ <34, n = 70) groups, and syndromic (n = 138) versus nonsyndromic (n = 586) disability. All children underwent cycloplegic autorefraction or retinoscopy, slit-lamp biomicroscopy, and dilated fundus examination. Ocular alignment was assessed by Hirschberg, Krimsky, or prism cover test. The main outcome measure was the prevalence of refractive errors and ocular findings. Seventy-seven percent of subjects with intellectual disability, and 42.4% of controls, had ocular findings. The children with intellectual disability had significantly more nystagmus, strabismus, astigmatism, and hypermetropia than controls. Children with syndromic intellectual disability had significantly more nystagmus, strabismus, astigmatism, and hypermetropia than subjects with nonsyndromic intellectual disability. Increasing severity of intellectual disability was related to higher prevalence of nystagmus, strabismus, astigmatism, hypermetropia, and anisometropia. From a public health perspective, evaluation and treatment of ocular and refractive findings in children with moderate, severe, and syndromic intellectual disability categories is urgently needed and likely to be highly effective in alleviating future health and social care costs, as well as improving the productive lives of individuals with intellectual disability.

The significance of retinal image contrast and spatial frequency composition for eye growth modulation in young chicks

Purpose: This study sought further insight into the stimulus dependence of form deprivation myopia, a common response to retinal image degradation in young animals. Methods: Each of 4 Bangerter diffusing filters (0.6, 0.1, <0.1, and LP (light perception only)) combined with clear plano lenses, as well as plano lenses alone, were fitted monocularly to 4-day-old chicks. Axial ocular dimensions and refractive errors were monitored over a 14-day treatment period, using high frequency A-scan ultrasonography and an autorefractor, respectively. Results: Only the <0.1 and LP filters induced significant form deprivation myopia; these filters induced similarly large myopic shifts in refractive error (mean interocular differences ± SEM: −9.92 ± 1.99, −7.26 ± 1.60 D, respectively), coupled to significant increases in both vitreous chamber depths and optical axial lengths ( p < 0.001). The other 3 groups showed comparable, small changes in their ocular dimensions ( p > 0.05), and only small myopic shifts in refraction (<3.00 D). The myopia-inducing filters eliminated mid-and-high spatial frequency information. Conclusions: Our results are consistent with emmetropization being tuned to mid-spatial frequencies. They also imply that form deprivation is not a graded phenomenon.

Reply to comment by Morteza Mehdizadeh on the publication “The effect of acute hyperglycemia on retinal thickness and ocular refraction in healthy subjects” by Nanouk G.M. Wiemer et al.

Dear Editor, In our study, we measured the retinal thickness and ocular refractive error in healthy subjects during acute induced hyperglycemia. In one subject, we found a hyperopic shift of 0.4 D, which could not be explained by a change in retinal thickness. Therefore, we proposed that other refractive components, such as the lens, are involved in causing blurred vision and refractive alterations during hyperglycemia. In a comment on our article, Dr. Mehdizadeh draws attention to the fact that the hyperopic shift in refraction could also be explained by a change in the refractive index of the aqueous humor and the vitreous. Subsequently, Dr. Mehdizadeh shows by calculation that a change in the refractive index of the aqueous and vitreous of 1/1000 would induce a change in the summation of the refractive power of the posterior corneal surface, and the anterior and posterior lens surface of 0.12 D. We agree with Dr. Mehdizadeh that the possible role of a change in the refractive index of the aqueous and vitreous should be considered before it can be concluded that a change in refractive error is caused by changes in the lens. However, we think that the latter is the case. Firstly, it is probable that the value of the change in the refractive index of the aqueous and vitreous of 1/1000, which was used in the comment, is rather large. This value was based on the study of Liu et al. [6], in which it was found that a change in glucose concentration of 0.1% (0.1 mg/dl or 1.0 mmol/l) is equal to a change in the refractive index of the aqueous humor of 1.4 × 10−4 (1/10000). Nevertheless, in more recent studies of Kohl et al. [5] and Esenaliev et al. [2], a change in the refractive index of the aqueous was found that was six times smaller (approximately 2.5 × 10−5 per increase in glucose levels of 1.0 mmol/l). However, if the large value of 1/1000 is used to calculate the change in refractive error, our calculations show a smaller change in refraction than the change in refractive error that was obtained in the comment. Our results differ, because the calculations in the comment were performed with a value of the refractive index of the lens (1.387), which is based on the refractive index of the cortex of the lens [1]. Instead, the equivalent refractive index of the lens as a whole, which is 1.427 [3, 4], should be included in the calculations. Another reason why our calculations are different is that in the comment the sum of the change in the refractive power of the posterior corneal surface and of the anterior and posterior lens surfaces is used to draw conclusions about a change in the refractive error of the whole eye. However, the change in refractive power in the eye (e.g., in the lens) is not equal to the change in refractive error. For example, a change in intraocular lens power of 0.5 D only gives a change in the refractive error of the eye of 0.34 D [7]. Thus, in order to calculate the effect of a change in the refractive index of the aqueous and vitreous on ocular refraction, all geometric eye parameters (i.e., the thickness of the cornea and the lens, the radius of curvature of the anterior and posterior surfaces of the cornea and the lens, the refractive indices of the cornea, aqueous, lens and vitreous, and the axial eye length) should be taken into account. In that case, calculation makes it clear that an increase in the refractive index of the aqueous and vitreous of 1/1000 would induce a change in ocular refraction of 0.08 D in subject 01 of our study, assuming that the other geometric ocular parameters were unaltered. This is too small to explain the hyperopic shift of 0.4 D found in our article. Finally, an important reason why we stated in our study that the change in refraction was caused by the lens is that we actually measured a change in the shape and the equivalent refractive index of the lens during hyperglycemia. This finding will be published in another article [8], and as a result we concluded that it is most likely that the observed hyperopic shift is caused by changes in the lens.

Refractive properties of the healthy human eye during acute hyperglycemia

PurposeTo measure the refractive properties of the healthy human eye during acute hyperglycemia by means of Scheimpflug imaging and Hartmann-Shack aberrometry.MethodsAcute hyperglycemia was induced in five healthy subjects (two males, three females, mean age ±SD 24.8 years ± 4.6) by means of an oral glucose tolerance test (OGTT) after subcutaneous somatostatin injection. Before and every 30 minutes after the OGTT, measurements with Scheimpflug imaging and Hartmann-Shack aberrometry were performed. The main outcome measures were the thickness and shape of the lens, and the ocular refractive error and higher order aberrations. The equivalent refractive index of the lens was calculated from these parameters. Measurements at baseline and during hyperglycemia were analyzed by means of Wilcoxon signed rank sum tests.ResultsDuring hyperglycemia (mean blood glucose level at baseline: 4.0 mmol/l; mean maximal blood glucose level: 18.4 mmol/l) no changes could be found in the refractive properties within the group. In one subject, a hyperopic shift (0.4 D) was observed, together with a more convex shape of the anterior lens surface and a decrease in the equivalent refractive index of the lens.ConclusionsThis study shows that hyperglycemia generally does not cause changes in the refractive properties of the healthy eye. Nevertheless, in one subject a hyperopic shift accompanied by a change in shape and refractive index of the lens was measured. This finding could provide an explanation for the mechanism underlying the refractive changes that are often observed during hyperglycemia.

The effect of acute hyperglycemia on retinal thickness and ocular refraction in healthy subjects

PurposeTo quantify the retinal thickness and the refractive error of the healthy human eye during hyperglycemia by means of optical coherence tomography (OCT) and Hartmann–Shack aberrometry.MethodsHyperglycemia was induced in five healthy subjects who were given a standard oral glucose tolerance test (OGTT) after a subcutaneous injection of somatostatin. Main outcome parameters were the central, pericentral and peripheral thickness of the fovea, measured by means of optical coherence tomography (OCT3). Ocular refractive error was determined with Hartmann-Shack aberrometry. Measurements at baseline and during maximal hyperglycemia were analyzed, and a change was considered clinically significant if the difference between the measurements exceeded the threshold of 50 μm for retinal thickness and 0.2 D for refractive error.ResultsDuring hyperglycemia (mean blood glucose level at baseline: 4.0 mmol/l; mean maximal blood glucose level: 18.4 mmol/l) no significant changes could be found in the central, pericentral, or peripheral foveal thickness in any of the five subjects. One of the subjects had a hyperopic shift of 0.4 D, but no significant change in refractive error was found in any of the other subjects.ConclusionsThe present study shows that in healthy subjects induced hyperglycemia does not affect retinal thickness, but it can cause a small hyperopic shift of refraction.

Association between stature, ocular biometry and refraction in an adult population in rural Myanmar: the Meiktila eye study

To study the association between adult stature and ocular biometric parameters and refraction. In a population-based cross-sectional ophthalmic survey of 2418 adults (> or = 40 years old) living in the rural villages in central Myanmar, height and weight were measured using a standardized protocol, and body mass index was calculated. Non-cycloplegic refraction and corneal curvature were determined by an autorefractor. Ultrasound pachymetry was performed and ocular biometry, including axial length, anterior chamber depth, lens thickness and vitreous chamber length were measured using A-mode ocular ultrasonography. Height and weight were significantly correlated with age, gender and all the ocular biometric parameters, except lens thickness. After adjusting for age and gender, taller and heavier persons had eyes with longer axial length, deeper anterior and vitreous chambers, and flatter and thicker corneas than shorter persons. Height was not significantly correlated with refraction, and heavier persons tended to be less myopic (P < 0.001). Multivariate linear regression models revealed consistent results with the findings for association between height, weight and ocular biometry and refractive error. Adult stature is independently associated with vitreous chamber length and corneal radius in this Burmese population. Heavier persons were slightly hyperopic.

Prevalence of Amblyopia and Strabismus in African American and Hispanic Children Ages 6 to 72 Months: The Multi-ethnic Pediatric Eye Disease Study

To determine the age- and ethnicity-specific prevalences of strabismus in African American and Hispanic/Latino children ages 6 to 72 months and of amblyopia in African American and Hispanic/Latino children 30 to 72 months. Cross-sectional study. The Multi-ethnic Pediatric Eye Disease Study is a population-based evaluation of the prevalence of vision disorders in children ages 6 to 72 months in Los Angeles County, California. A comprehensive eye examination was completed by 77% of eligible children. This report focuses on results from 3007 African American and 3007 Hispanic/Latino children. Eligible children in all enumerated households in 44 census tracts were identified. Participants underwent an in-home interview and were scheduled for a comprehensive eye examination and in-clinic interview. The examination included evaluation of ocular alignment, refractive error, and ocular structures, as well as determination of optotype visual acuity (VA) in children 30 months and older. The proportion of 6- to 72-month-olds with strabismus on ocular examination and proportion of 30- to 72-month-olds with optotype VA deficits and amblyopia risk factors consistent with predetermined definitions of amblyopia. Strabismus was detected in 2.4% of Hispanic/Latino children and 2.5% of African American children (P = 0.81), and was more prevalent in older children than in younger children. Amblyopia was detected in 2.6% of Hispanic/Latino children and 1.5% of African American children, a statistically significant difference (P = 0.02), and 78% of cases of amblyopia were attributable to refractive error. Amblyopia prevalence did not vary with age. Among Hispanic/Latino and African American children in Los Angeles County, strabismus prevalence increases with age, but amblyopia prevalence appears stable by 3 years of age. Amblyopia is usually caused by abnormal refractive error. These findings may help to optimize the timing and modality of preschool vision screening programs.

Biometria ocular, erro refrativo e sua relação com a estatura, idade, sexo e escolaridade em adultos brasileiros

To assess ocular biometric parameters and refractive error in Brazilian adults and their relationship with height, age, gender and years of formal education. Cross-sectional study that assessed 173 subjects by keratometry, echobiometry, refraction and measurement of body height. The statistical analysis was performed using Pearson's coefficient and a regression model was constructed. The correlations found were: each 10 cm - increase in height was related to a 0.32 mm longer axial length, 0.07 mm deeper anterior chamber, 0.26 mm deeper vitreous chamber and 0.50 D flatter keratometry. Each 10-year increase in age, related to a 0.15 mm smaller axial length, 0.25 mm thicker lens, 0.21 mm shallower vitreous chamber and 0.23 D more positive spherical equivalent. Each 10-year increase in education related to a 0.74 D more negative spherical equivalent. Gender did not influence the analyzed biometric parameters. Equations referring to biometric parameters and spherical equivalent were formulated. Positive correlations were found between: axial length and height; anterior chamber and vitreous cavity depth with height; lens thickness with age; keratometry with height; and spherical equivalent with age. Negative correlations were found between: anterior chamber and vitreous cavity depth with age; spherical equivalent and years of formal education.

Componentes oculares em anisometropia

To asses the correlation between ocular components (axial length, anterior segment length, corneal power, vitreous length and equivalent power of the eye) and refractive error in eyes with higher and lower ametropia of subjects with anisometropia. An analytical survey was carried out in 68 patients with anisometropia of two or more diopters, assisted at the Ophthalmological Clinic from "Hospital das Clínicas da Faculdade de Medicina" of São Paulo University. All participants were submitted to ophthalmologic examination which included objective and subjective cycloplegic refratometry, keratometry and ultrasonic biometry. No significant difference between measures of ocular components of the eyes with higher and lower ametropias was observed. The eyes with lower ametropia presented the same significant correlations observed in emmetropic eyes: of refraction with anterior segment length and axial length, and of the axial length with corneal power and vitreous length. The eyes with higher ametropia presented significant correlation of refraction with the axial length and of the axial length with vitreous length. Furthermore, we observed in both eyes, significant correlation of the power of the crystalline lens with the anterior chamber depth. The eyes with lower ametropia presented correlations more frequently observed in emmetropia. The eyes with higher ametropia did not present the same correlations observed in emmetropic eyes.

A comparison of refractive development between two subspecies of infant rhesus monkeys ( Macaca mulatta)

Purpose Different subspecies of rhesus monkeys ( Macaca mulatta) that are derived from different geographical locations, primarily Indian and China, are commonly employed in vision research. Substantial morphological and behavioral differences have been reported between Chinese- and Indian-derived subspecies. The purpose of this study was to compare refractive development in Chinese- and Indian-derived rhesus monkeys. Methods The subjects were 216 Indian-derived and 78 Chinese-derived normal infant rhesus monkeys. Cross-sectional data were obtained at 3 weeks of age for all subjects. In addition, longitudinal data were obtained from 10 Indian-derived (male = 5, female = 5) and 5 Chinese-derived monkeys (male = 3, female = 2) that were reared with unrestricted vision. Ocular and refractive development was assessed by retinoscopy, keratometry, video-based ophthalmophakometry, and A-scan ultrasonography. Results Although the course of emmetropization was very similar in these two groups of rhesus monkeys, there were consistent and significant inter-group differences in ocular dimensions and refractive error. Throughout the observation period, the Chinese-derived monkeys were on average about 0.4 D less hyperopic than the Indian-derived monkeys and the Chinese-derived monkeys had longer overall axial lengths, deeper anterior and vitreous chamber depths, thicker crystalline lenses, flatter corneas and lower powered crystalline lenses. Conclusions The ocular differences observed in this study presumably reflect genetic differences between subspecies but could reflect the differences in the genetic pool between isolated colonies rather than true subspecies differences. Nonetheless, the substantial ocular differences that we observed emphasize that caution must be exercised when comparing and/or pooling data from rhesus monkeys obtained from different colonies. These inter-subspecies differences might be analogous to the ethnic differences in ocular parameters that have been observed in humans.

Refractive Error, Ocular Biometry, and Lens Opalescence in an Adult Population: The Los Angeles Latino Eye Study

To characterize age- and gender-related differences in refractive error, ocular biometry, and lens opalescence (NOP) in a population-based sample of adult Latinos. Also assessed were the determinants of age-related refractive differences. Participants in the Los Angeles Latino Eye Study (LALES), a population-based study of Latinos aged 40 years and more, underwent an ophthalmic examination, including ultrasonic measurements of axial length (AL), vitreous chamber depth (VCD), anterior chamber depth (ACD), lens thickness (LT), and noncycloplegic automated and subjective refraction. Corneal curvature/power (CP) was measured using an autorefractor. NOP was graded at the slit lamp by an ophthalmologist using the Lens Opacity Classification System II. Age- and gender-related differences were calculated. Multiple regression models were used to identify the determinants of age-related refractive differences. Of the 6357 LALES participants, 5588 phakic individuals with biometric data were included in this analysis. Older individuals had shallower ACDs, thicker lenses, more NOP, and more hyperopia compared to younger individuals (P < 0.001). There was no age-related difference in AL (P > or = 0.05). Women had significantly shorter AL, shallower ACD and VCD, than did men (P < or = 0.01). The strongest determinants of refractive error were AL (primarily VCD) and CP. NOP was a small but significant determinant of refractive error in older individuals. Age- and gender-related differences in ocular biometric, refractive error, and NOP measurements are present in adult Latinos. While the relative contribution of NOP in determining refractive error is small, it is greater in older persons compared to younger individuals.

Long-term visual pathology in children with significant perinatal complications

Eye research in infants who experience significant perinatal complications has been restricted to evaluation of structural ocular disease and spatial vision, and results show that these aspects of vision are at increased risk for abnormal or delayed development. To expand upon previous work, a battery of 17 vision tests was employed to assess, comprehensively, long-term outcome of functional vision. Seventy-six children (38 males, 38 females), between the corrected ages of 2 years 11 months and 10 years 2 months (mean 6y 6mo) with various significant complications (e.g. very preterm birth, bronchopulmonary dysplasia, intraventricular hemorrhage) were compared to normally developing, age-matched control children (n = 61; mean age 7y 1mo) on measures of visual acuity, contrast sensitivity, stereoacuity, peripheral vision, color vision, astigmatism, and binocular alignment. Results showed that at-risk children had more test results that fell within the suspect or abnormal range. At-risk children also had a slightly higher incidence of ocular disorders (e.g. strabismus) and refractive error. These data imply that children who experienced significant perinatal risk factors are at risk for long-term deficits of functional vision. However, most of these deficits appear to be relatively mild.

The role of the retinal pigment epithelium in eye growth regulation and myopia: A review

Myopia is increasing in prevalence world-wide, nearing epidemic proportions in some populations. This has led to expanded research efforts to understand how ocular growth and refractive errors are regulated. Eye growth is sensitive to visual experience, and is altered by both form deprivation and optical defocus. In these cases, the primary targets of growth regulation are the choroidal and scleral layers of the eye that demarcate the boundary of the posterior vitreous chamber. Of significance to this review are observations of local growth modulation that imply that the neural retina itself must be the source of growth-regulating signals. Thus the retinal pigment epithelium (RPE), interposed between the retina and the choroid, is likely to play a critical role in relaying retinal growth signals to the choroid and sclera. This review describes the ion transporters and signal receptors found in the chick RPE and their possible roles in visually driven changes in eye growth. We focus on the effects of four signaling molecules, otherwise implicated in eye growth changes (dopamine, acetylcholine, vasoactive intestinal peptide (VIP), and glucagon), on RPE physiology, including fluid transport. A model for RPE-mediated growth regulation is proposed.

Support for Polygenic Influences on Ocular Refractive Error

Refractive errors, myopia, and hyperopia are common conditions requiring corrective lenses. The familial clustering of myopia has been well established. Several chromosomal regions have been linked to high myopia (12q, 17q, and 18q), to quantitative refraction among twins (3q, 4q, 8p, and 11p), and to families with moderate myopia (22q). This study examined the familial aggregation and pattern of inheritance of ocular refraction in an adult population, by using data from the Beaver Dam Eye Study. Familial correlations were examined and segregation analysis was performed on the average refractive error measurements in the right and left eyes after adjustment for age, sex, and education. Analyses were based on 2138 individuals in 620 extended pedigrees with complete data on age, sex, education, and spherical equivalent. Substantial positive correlation was found between siblings (0.33), parents and offspring (0.17), and cousins (0.10) and lower correlation among avuncular pairs (0.08) after adjustment for age, sex, and years of education. The results of this segregation analysis do not support the involvement of a single major locus throughout the entire range of refractive error. However, models allowing for familial correlation, attributable in part to polygenic effects, provided a better fit to the observed data than models without a polygenic component, suggesting that several genes of modest effect may influence refractive error, possibly in conjunction with environmental factors. These results support the involvement of genetic factors in the etiology of refractive error and are consistent with reports of linkage to multiple regions of the genome.

Repeatability of an automated Landolt C test, compared with the early treatment of diabetic retinopathy study (ETDRS) chart testing

Purpose To evaluate the repeatability of visual acuity scores from the automated test and compare them with the Early Treatment of Diabetic Retinopathy Study (ETDRS) chart. Design Instrument validation study based on a model of repeatability study in two observations. Methods Setting: a prospective, clinic-based, comparative study. A total of 206 participants without ocular diseases and refractive errors in their right eyes were randomly enrolled in the automated group in which 107 participants performed the automated test and the ETDRS group in which 99 participants read the ETDRS chart. All participants were tested with only their right eyes without corrections at 4 meters and came back to have the same tests 1 week later. The automated test used the Landolt rings as optotypes and was conducted by a low-ended personal computer with a 15-inch monitor and a wireless keyboard. The “letter” score calculated by counting every correct response to optotypes, and the “threshold curve” score interpreted from the optotype size at the midpoint of a visual acuity threshold curve. Results The 95% confidence interval of test–retest of visual acuity scores from the automated test are comparable to the ETDRS chart (.143 compared with .125 for letter scores, .145 compared with .122 for threshold curve scores). The score repeatabilities, calculated from the standard deviations of test-retest, from the automated test are also comparable to the ETDRS chart (.201 compared with .177 for letter scores, .206 compared with .172 for threshold curve scores). All comparisons demonstrated no statistical difference ( P > .05). Conclusions The automated testing system in this study enables practical measuring visual acuity by the Landolt rings. The system’s repeatability, which is comparable to the ETDRS chart, supports its role as an alternative tool for measuring outcome in new clinical research. Its ability to practically generate visual acuity threshold curves may also be useful in future clinical research studies.

Ocular growth and refractive error development in premature infants without retinopathy of prematurity.

This investigation studied the factors involved in the development of refractive error (RE) in premature infants unaffected by retinopathy of prematurity (ROP). Premature infants enrolled in the national ROP screening program were recruited and examined at 32, 36, 40, 44, and 52 weeks' postmenstrual age. At each examination, axial length (AXL), anterior chamber depth (ACD), and lens thickness (LT) were measured on the A-scan biometer. Corneal curvature (CC) was recorded with a video-ophthalmophakometer, and refractive state was determined with routine cycloplegic refraction. Multilevel modeling techniques were used to determine the relationships between all the variables throughout the study period, as well as individual growth rates. Sixty-eight premature infants were included. AXL and ACD showed linear patterns of growth, whereas LT changed little over the study period. CC showed a quadratic growth pattern, and unlike the previous variables, correlated well with refractive state. Premature infants were myopes at the start of the study, with refraction becoming emmetropic as they neared full term and then hypermetropic toward the end of the study. Most of the components of refractive status showed linear patterns of growth during this early phase of ocular development. CC displayed a more complex pattern of growth, which correlated well with refractive state. Compared with full-term infants examined around term, this group has shorter AXLs, shallower anterior chambers, and more highly curved corneas. In addition, less of the expected hypermetropia developed in the premature group, which seems mainly due to the differences in ACD and corneal curvature.

Model of retinal surface area and neuron distribution in the avian eye

Changes in retinal neuron distribution may reflect normal or pathological changes in retinal function. The quantitative study of retinal neurons provides a better understanding of anomalous mechanisms, such as those controlling ocular growth and refractive error development in experimental animals. We developed a method to facilitate the quantitative analysis of amacrine neuron populations in wholemount chick retinae, since the domestic chicken is used extensively as an animal model in myopia studies. This method involved automated cell counting from confocal microscopic images and mathematical estimation of total cell numbers based on image cell density and retinal surface areas. Cell densities and cell counts were obtained from immunohistochemically-labeled amacrine neuron populations, using derived formulae to calculate retinal surface area based on vitreous chamber depth, equatorial width, ora serrata diameter and scleral thickness. Normalized total cell counts in each eye were compared, rather than cell densities, since changes in eye growth can affect cell densities. We also compared neuron distribution in central versus peripheral portions of the retina. This is an alternative technique for retinal analysis that supplements traditional anatomical cell counting methods, allowing higher numbers of specimens to be rapidly analyzed.

Statistical analysis when dealing with astigmatism: assessment of different spherocylindrical notations

Ophthalmic epidemiological studies frequently deal with ocular refractive errors, which are commonly expressed in the form sphere/cylinder x axis. However, this representation has been shown not to be the most suitable one for performing statistical analysis. Although alternative analytical and graphic methods to represent this kind of data have been developed, these formalisms have often gone unnoticed by researchers, despite their usefulness and versatility. Besides, there has been no discussion of how each of them fits in with a particular type of study. In this paper, several mathematical representations of dioptric power are revisited in a comprehensive way. The aim is to encourage researchers in ophthalmology and optometry to use these formalisms in their epidemiological studies, thus profiting from their exactitude and simplicity. Consequently, the emphasis is not on complicated mathematical derivations but on how to use these representations. Their potential and suitability in different applications is analyzed in detail. In addition, some examples are presented to illustrate the mathematical methods considered.

Detection system for ocular refractive error measurement

An automatic and objective system for measuring ocular refractive errors (myopia, hyperopia and astigmatism) was developed. The system consists of projecting a light target (a ring), using a diode laser , at the fundus of the patient's eye. The light beams scattered from the retina are submitted to an optical system and are analysed with regard to their vergence by a CCD detector (matrix). This system uses the same basic principle for the projection of beams into the tested eye as some commercial refractors, but it is innovative regarding the ring-shaped measuring target for the projection system and the detection system where a matrix detector provides a wider range of measurement and a less complex system for the optical alignment. Also a dedicated electronic circuit was not necessary for treating the electronic signals from the detector (as the usual refractors do); instead a commercial frame grabber was used and software based on the heuristic search technique was developed. All the guiding equations that describe the system as well as the image processing procedure are presented in detail. Measurements in model eyes and in human eyes are in good agreement with retinoscopic measurements and they are also as precise as these kinds of measurements require (0.125D and ).