Study of gene-environment effects on development of hyperopia: a study of 191 adult twin pairs from the Finnish Twin Cohort Study

A Review of Current Concepts of the Etiology and Treatment of Myopia

Myopia occurs in more than 50% of the population in many industrialized countries and is expected to increase; complications associated with axial elongation from myopia are the sixth leading cause of blindness. Thus, understanding its etiology, epidemiology, and the results of various treatment regiments may modify current care and result in a reduction in morbidity from progressive myopia. This rapid increase cannot be explained by genetics alone. Current animal and human research demonstrates that myopia development is a result of the interplay between genetic and the environmental factors. The prevalence of myopia is higher in individuals whose both parents are myopic, suggesting that genetic factors are clearly involved in myopia development. At the same time, population studies suggest that development of myopia is associated with education and the amount time spent doing near work; hence, activities increase the exposure to optical blur. Recently, there has been an increase in efforts to slow the progression of myopia because of its relationship to the development of serious pathological conditions such as macular degeneration, retinal detachments, glaucoma, and cataracts. We reviewed meta-analysis and other of current treatments that include: atropine, progressive addition spectacle lenses, orthokeratology, and multifocal contact lenses.

Moderate hyperopia prevalence and associated factors among elementary school students

Hyperopia is the most common refractive condition in childhood. There are few studies on moderate hyperopia and associated factors. This study aims to investigate the prevalence of moderate hyperopia and associated factors among school children. A cross-sectional study comprising 1,032 students attending 1st to 8th grades at two public schools was conducted in a Southern Brazilian urban area in 2012. Cycloplegia was used to examine both eyes and refractive error was measured through auto-refraction. A socioeconomic and cultural questionnaire was administered. Multivariable analysis was performed through Poisson regression. Moderate hyperopia prevalence was 13.4% (95% CI, 11.2-15.4) and 85% of these did not wear glasses. Age was inversely associated with moderate hyperopia, while female gender RP = 1.39 (95%CI, 1.02 - 1.90) and white skin RP = 1.66 (95%CI, 1.04 - 2.66) were risk factors for this outcome. This study makes progress in estimating mild and moderate hyperopia prevalence both by age range and specific age. It emphasizes how the lack of this condition being corrected in southern Brazil is a serious problem. It highlights the importance of detailing and characterizing the amount of time spent on close-range, long-range and outdoor activities.

The long-term outcome of the refractive error in children with hypermetropia

PURPOSE

The purpose of this study was to investigate the long-term outcome of high hypermetropic refractive errors in childhood.

METHODS

We retrospectively reviewed data from the clinical records of 164 children with spherical equivalent (SE) hypermetropic refractive errors in three medical centers collected over 29 years. Refractive errors between +1.00 and +3.00 diopter (D) on initial examination were classified as mild hypermetropia and those +5.00 D or greater were classified as high hypermetropia. The four variables studied were, age, refractive error, strabismus, and gender. The rate of reduction in hypermetropic refractive error was calculated over time in years. We identified subgroups according to age, gender, and initial refractive error.

RESULTS

Seventy-eight children with high hypermetropia and 86 children with mild hypermetropia were studied. High hypermetropia was detected at a mean age of 3.3 years, while mild hypermetropia was detected at a mean 4 years of age. The mean follow-up was 6.6 years for high hypermetropia and 6.4 years for mild hypermetropia. Over the follow-up period, children in all subgroups tended to reduce their refractive errors. The reduction in refraction power was small for both mild and high hypermetropic refractive errors. Amblyopia in the high hypermetropia group was more common and more refractory to treatment.

CONCLUSIONS

Most children with hypermetropia of less than +3.00 D experience a reduction in hyperopic refractive error over time and will outgrow any need for corrective lenses. Children with hyperopia greater than +5.00 D will not experience a significant reduction in the power of the refractive error.

Genome-wide meta-analysis of myopia and hyperopia provides evidence for replication of 11 loci

Refractive error (RE) is a complex, multifactorial disorder characterized by a mismatch between the optical power of the eye and its axial length that causes object images to be focused off the retina. The two major subtypes of RE are myopia (nearsightedness) and hyperopia (farsightedness), which represent opposite ends of the distribution of the quantitative measure of spherical refraction. We performed a fixed effects meta-analysis of genome-wide association results of myopia and hyperopia from 9 studies of European-derived populations: AREDS, KORA, FES, OGP-Talana, MESA, RSI, RSII, RSIII and ERF. One genome-wide significant region was observed for myopia, corresponding to a previously identified myopia locus on 8q12 (p = 1.25×10(-8)), which has been reported by Kiefer et al. as significantly associated with myopia age at onset and Verhoeven et al. as significantly associated to mean spherical-equivalent (MSE) refractive error. We observed two genome-wide significant associations with hyperopia. These regions overlapped with loci on 15q14 (minimum p value = 9.11×10(-11)) and 8q12 (minimum p value 1.82×10(-11)) previously reported for MSE and myopia age at onset. We also used an intermarker linkage- disequilibrium-based method for calculating the effective number of tests in targeted regional replication analyses. We analyzed myopia (which represents the closest phenotype in our data to the one used by Kiefer et al.) and showed replication of 10 additional loci associated with myopia previously reported by Kiefer et al. This is the first replication of these loci using myopia as the trait under analysis. "Replication-level" association was also seen between hyperopia and 12 of Kiefer et al.'s published loci. For the loci that show evidence of association to both myopia and hyperopia, the estimated effect of the risk alleles were in opposite directions for the two traits. This suggests that these loci are important contributors to variation of refractive error across the distribution.

A survey on computer aided diagnosis for ocular diseases

Background

Computer Aided Diagnosis (CAD), which can automate the detection process for ocular diseases, has attracted extensive attention from clinicians and researchers alike. It not only alleviates the burden on the clinicians by providing objective opinion with valuable insights, but also offers early detection and easy access for patients.

Method

We review ocular CAD methodologies for various data types. For each data type, we investigate the databases and the algorithms to detect different ocular diseases. Their advantages and shortcomings are analyzed and discussed.

Result

We have studied three types of data (i.e., clinical, genetic and imaging) that have been commonly used in existing methods for CAD. The recent developments in methods used in CAD of ocular diseases (such as Diabetic Retinopathy, Glaucoma, Age-related Macular Degeneration and Pathological Myopia) are investigated and summarized comprehensively.

Conclusion

While CAD for ocular diseases has shown considerable progress over the past years, the clinical importance of fully automatic CAD systems which are able to embed clinical knowledge and integrate heterogeneous data sources still show great potential for future breakthrough.

Refractive Errors in Twin Studies

It is estimated that 1.6 billion people worldwide have myopia, a refractive error, and this number is expected to increase to approximately 2.5 billion by the year 2020. It is now well established that both the environment and genetics play a role in the development of myopia. However, the exact contribution of each of these components to myopia development has yet to be completely determined. Twin studies (classical twin model) are commonly used to determine the weighting of genetic and environmental components in disease. Over the last century, twin studies have investigated the heritability of refractive errors in different sample populations and have collectively supported a genetic basis to refractive errors. However, different sample populations and methods of data collection have produced a wide range of heritability estimates ranging from .5 to .9. This article will review those twin studies that have investigated refractive error, particularly myopia, as well as biometric measures linked to refractive error, to compare heritability estimates and methodology designs.

The heritability of ocular traits

Heritability is the proportion of phenotypic variation in a population that is attributable to genetic variation among individuals. Many ophthalmic disorders and biometric traits are known to have a genetic basis and consequently much work has been published in the literature estimating the heritability of various ocular parameters. We collated and summarized the findings of heritability studies conducted in the field of ophthalmology. We grouped the various studies broadly by phenotype as follows: refraction, primary open-angle glaucoma, age-related macular degeneration (AMD), cataract, diabetic retinopathy, and others. A total of 82 articles were retrieved from the literature relating to estimation of heritability for an ocular disease or biometric trait; of these, 37 papers were concerned with glaucoma, 28 with refraction, 4 with AMD, 5 with diabetic retinopathy, and 4 with cataract. The highest reported heritability for an ophthalmic trait is 0.99 for the phenotype ≥ 20 small hard drusen, indicating that observed variation in this parameter is largely governed by genetic factors. Over 60% of the studies employed a twin study design and a similar percentage utilized variance components methods and structural equation modeling (SEM) to derive their heritability values. Using modern SEM techniques, heritability estimates derived from twin subjects were generally higher than those from family data. Many of the estimates are in the moderate to high range, but to date the majority of genetic variants accounting for these findings have not been uncovered, hence much work remains to be undertaken to elucidate fully their molecular etiology.

Heritability of spherical equivalent: a population-based twin study among 63- to 76-year-old female twins

PURPOSE

To examine the heritability of spherical equivalent (SE) in older women.

DESIGN

Population-based twin study.

PARTICIPANTS

Ninety monozygotic (MZ) and 86 dizygotic (DZ) female twin pairs aged 63 to 76 years who were born from 1924 through 1937.

METHODS

Ocular refraction was measured using an autorefractor and controlled by the subjective method. The contributions of genetic and environmental factors to individual differences in SE were estimated by applying an independent pathway model to twin data.

MAIN OUTCOME MEASURES

Contribution of genetic and environmental effects to the variation in SE.

RESULTS

Mean SE of the study population was 1.68 (standard deviation, ± 1.82) with no differences observed either between the MZ and the DZ individuals or between the left and the right eyes. The pairwise correlations were higher in the MZ sisters (intraclass correlation coefficient [ICC], 0.803 right eye and 0.807 left eye) than DZ sisters (ICC, 0.406 right eye and 0.435 left eye). Quantitative genetic modelling showed that 83% (95% confidence interval, 77%-87%) of the variance in SE could be explained by heritable factors.

CONCLUSIONS

Additive genetic influences explained most of the individual differences in SE among older Finnish women.

The GEnes in Myopia (GEM) study in understanding the aetiology of refractive errors

Refractive errors represent the leading cause of correctable vision impairment and blindness in the world with an estimated 2 billion people affected. Refractive error refers to a group of refractive conditions including hypermetropia, myopia, astigmatism and presbyopia but relatively little is known about their aetiology. In order to explore the potential role of genetic determinants in refractive error the "GEnes in Myopia (GEM) study" was established in 2004. The findings that have resulted from this study have not only provided greater insight into the role of genes and other factors involved in myopia but have also gone some way to uncovering the aetiology of other refractive errors. This review will describe some of the major findings of the GEM study and their relative contribution to the literature, illuminate where the deficiencies are in our understanding of the development of refractive errors and how we will advance this field in the future.

Genetic isolates in ophthalmic diseases

In recent years, noteworthy gains have been made in unravelling the genetic contribution to some complex ocular diseases, principally age-related macular degeneration. Yet, a relatively poor understanding of the genetic aetiology for many other heritable blinding diseases, such as glaucoma, keratoconus and myopia, remains. Genetic isolates, populations with varying degrees of geographical or cultural seclusion, provide an effective means for investigating the molecular mechanisms involved in human diseases. This is particularly true for rare diseases in which founded alleles can be rapidly driven to a high frequency due to restriction of gene flow in the population. Recent success in complex gene mapping has resulted from the widened linkage disequilibrium (LD) in the genome of genetically isolated populations. An improved understanding of the predisposing genetic risk factors allows for enhanced screening modalities and paves the foundations for the translation of genomic technology into the clinic. This review focuses on the role population isolates have had in the investigation of genes underlying complex eye diseases and discusses their likely usefulness given the expansion of large-scale case-control association studies.

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 0.4D 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.

The association between hypermetropia and essential hypertension

PURPOSE

To explore the relationship between the refractive state of the eye and high blood pressure in a representative population.

DESIGN

Case-control study.

METHODS

Three hundred twenty-one patients with essential hypertension (mean age 53.9 +/- 15.5 years) and 188 age-matched and sex-matched healthy control subjects (mean age 50.9 +/- 7.3 years) from the same regional Health Maintenance Organization were consecutively included for the study (P > .05 for age and sex). The refractive state of the eyes was identified objectively by an autorefractometer and retinoscopic examination, recording the autorefractometer values. Spherical equivalents between -0.50 (included) and +0.50 (included) diopters were regarded as emmetropia. Values below or above this interval were regarded as either myopia or hypermetropia. Mean spherical equivalents of the groups were compared using independent samples t test; distributions of refraction were compared with chi(2) test.

RESULTS

The mean spherical equivalent of the patients with essential hypertension was +0.88 +/- 1.34 diopters (range -3.75 to +6.38 diopters), whereas the mean spherical equivalent of the control subjects was -0.26 +/- 1.12 diopters (range -5.00 to +3.38 diopters) (P < .0001). Whereas 61.4% of hypertensive patients were hypermetropic, 18.1% of normotensive patients were hypermetropic (P < .0001).

CONCLUSIONS

There is a strong association of essential arterial hypertension with hypermetropia, which has not been previously reported. Given the findings of this study, we recommend that patients who have hypermetropia and have had no recent systemic examination should at least have their blood pressure checked.

Familial aggregation of hyperopia in an elderly population of siblings in Salisbury, Maryland

PURPOSE

To determine whether hyperopia aggregates in families in an older mixed-race population.

DESIGN

Cross-sectional familial aggregation study using sibships.

METHODS

We recruited 759 subjects (mean age, 73.4 years) in 241 families through the population-based Salisbury Eye Evaluation study. Subjects underwent noncycloplegic refraction if best-corrected visual acuity (BCVA) was 20/40 with spectacles, or were considered to be plano (refraction of zero) if the BCVA was >20/40 without spectacles. Preoperative refraction from medical records was used for bilaterally pseudophakic subjects.

RESULTS

Utilizing hyperopia cutoffs from 1.00 to 2.50 diopters, age-, race-, and gender-adjusted odds ratios for hyperopia with an affected sibling ranged from 2.72 (95% confidence interval [CI], 1.84-4.01) to 4.87 (95% CI, 2.54-9.30). The odds of hyperopia increased with age until 75 years, after which they remained relatively constant. Black men were significantly less likely to be hyperopic than white men, white women, or black women.

CONCLUSIONS

Hyperopia appears to be under strong genetic control in this older population.

The importance of genes and environment for ocular refraction and its determiners: a population based study among 20-45 year old twins

AIMS

To estimate the heritability for ocular refraction and its determiners in a population based cohort of 20-45 years old twins.

METHODS

114 twin pairs (53 monozygotic and 61 dizygotic) participated. Refraction was determined in cycloplegia and eye dimensions were measured with ultrasound. Educational length was assessed. The heritability was estimated employing aetiological model fitting. Evidence of gene-environment interaction was analysed. Correlations between intrapairwise differences in educational length and in refraction were evaluated.

RESULTS

The heritability was between 0.89 and 0.94 (95% CI: 0.82, 0.96) for refraction, total refraction, axial length, and radius of corneal curvature. Phenotypic variation was mostly due to additive genetic effects. Refraction revealed evidence of gene-environment interaction (r = -0.29 to -0.32; p <0.05). The heritability for anterior chamber depth and lens thickness was between 0.88 and 0.94 (95% CI: 0.81, 0.96) and dominant genetic effects were the most likely explanation. There was no correlation between age and intrapairwise differences in refraction. The dizygotic twins had significant larger intrapairwise differences in educational length (p <0.05), but the differences were not correlated with differences in refraction.

CONCLUSIONS

The results indicate a high heritability for ocular refraction and its determiners and thus suggest that environmental impact on refraction is not significant. However, the epidemiological association between educational length (near work) and myopia, the evidence of increasing myopia prevalence within a few generations, and the theory of gene-environment interaction imply that some individuals might be genetically liable to develop myopia if exposed to certain environmental factors.

Refractive errors in Singapore and Xiamen, China--a comparative study in school children aged 6 to 7 years

PURPOSE

To compare and contrast the prevalence of myopia and other refractive errors in Xiamen city, Xiamen countryside (Southern China), and Singapore.

METHODS

One hundred thirty-two schoolchildren aged 6 to 7 years from Xiamen city, 104 from Xiamen countryside, and 146 from Singapore city were recruited to join the study. Cycloplegic autorefraction, keratometry, and biometry measurements were performed on all children.

RESULTS

The prevalence of myopia was 12.3% in Singapore city, 9.1% in Xiamen city, and 3.9% in Xiamen countryside. The prevalence of astigmatism was higher in Singapore compared with Xiamen. The rates of hyperopia and anisometropia were similar in all three locations.

CONCLUSIONS

The myopia rate in Singapore city was higher than in Xiamen city; the lowest rates were found in Xiamen countryside. As the Chinese population from all three sites is of similar genetic stock (predominantly from Southern China), it is postulated that the differences in myopia rates in these three localities may be related to environmental factors.

Comparative corneal topography and refractive variables in monozygotic and dizygotic twins

PURPOSE

To investigate the role of heredity in determining corneal shape, axial length, and overall refractive error.

METHODS

Twenty monozygotic and 19 dizygotic twin pairs, age 12 to 73 years, were enrolled in the study. Zygosity was determined by physical similarity and by responses to questions adapted from surveys. Two twin pairs were excluded because of undetermined zygosity and one pair because of keratoconus (both siblings). Refractive error was determined by an automated refractor. Manifest refraction was also recorded, as well as cycloplegic refraction in subjects under age 18 years. Corneal topography data and manual keratometer readings were also obtained. Axial lengths were determined by A-scan ultrasound. Data were analyzed by Student t tests only in the right eye. Left-eye data were comparable for all variables.

RESULTS

Mean intrapair difference in refractive error (spherical equivalent) was less for monozygotic than for dizygotic twins (RE: 0.41 vs 1.53; P = .001). Mean intrapair difference in axial length was less for monozygotic twins (RE: 0.39 vs 0.76 mm; P = .031). Corneal topography data (power and meridian) in all zones (3, 5, and 7 mm) also showed smaller mean differences among monozygotic pairs than dizygotic, but the difference was statistically significant only for the 5-mm zone. In addition, most Holladay Diagnostic Summary variables that were studied did not show any statistically significant differences.

CONCLUSIONS

Axial length and overall refractive error have a significant genetic basis. Corneal topography data appear to have other overriding determining factors for several of the variables studied.

Long-term influence of insulin dependent diabetes mellitus on refraction and its components: a population based twin study

AIM

To study whether refraction of the eye, or some of its components is influenced by duration of insulin dependent diabetes mellitus.

METHODS

From the young cohort of the population based Danish Twin Register, containing 20,888 twin pairs born between 1953 and 1982, all twin pairs having one or both partners affected with IDDM were searched. Autorefraction, autokeratometry, and ultrasonic biometric measurements were carried out on 45 twin pairs: 16 monozygotic (MZ) twin pairs, 14 dizygotic twin pairs of same sex (DZss), and 15 dizygotic twin pairs of opposite sex (DZos). To obtain an estimate of the influence of duration of diabetes, the intrapair differences in duration of diabetes were correlated with intrapair differences in refraction and each of its components.

RESULTS

Refraction was statistically significantly negatively correlated with duration of diabetes in the DZss group, and axial length correspondingly positively correlated. Surprisingly, refraction and axial length in the MZ group, adjusted for confounding factors, were correlated with diabetes duration in the opposite direction than in the DZss group, although not reaching statistical significance. Lens thickness was statistically significantly positively correlated with duration of diabetes in both MZ and DZ twins. Anterior chamber depth was negatively correlated with duration of diabetes in all the zygosity groups.

CONCLUSIONS

Studies of relations between refraction and duration of diabetes show diverging results. In the MZ group, a tendency to reduced axial length and corresponding hyperopia with increasing duration of diabetes was found. However, in the DZ group of same sex the opposite tendency was found. Increasing lens thickness and decreasing anterior chamber depth with increasing duration of diabetes have been confirmed in this study.

Lens thickness and insulin dependent diabetes mellitus: a population based twin study

AIM

To investigate the relation between lens thickness and duration of insulin dependent diabetes mellitus (IDDM).

METHODS

From the new population based Danish twin register, containing 20,888 twin pairs born between 1953 and 1982 (inclusive), all twin pairs having one or both partners affected with IDDM were searched. Among the 45 twin pairs available for clinical eye examination there were 15 monozygotic pairs, 14 dizygotic pairs of same sex, and 16 dizygotic pairs of opposite sex. Lens thickness was measured by ultrasonography. Using a twin control design, the relation between lens thickness and duration of IDDM was assessed by estimating the correlation between the intrapair difference in lens thickness and the intrapair difference in diabetes duration.

RESULTS

In monozygotic twin pairs a statistically highly significant correlation between duration of diabetes and lens thickness was found (right eye: r = 0.88, p < 0.0001; left eye: r = 0.90, p < 0.0001). In dizygotic twin pairs of the same sex the correlations were r = 0.58 (p = 0.029) and r = 0.53 (p = 0.053) for right eye and left eye, respectively. For dizygotic twin pairs of opposite sex the correlations were r = 0.58 (p = 0.018) and r = 0.69 (p = 0.005) for right eye and left eye, respectively. The slope in regression analysis were similar for monozygotic twin pairs (0.025, common for both eyes) and dizygotic twin pairs grouped (0.024, common for both eyes).

CONCLUSIONS

There is a statistically significant positive correlation between duration of IDDM and lens thickness, as assessed by the twin control method. The higher correlation in monozygotic twins compared with dizygotic twins suggests that genetic factors play an additional role in the determination of lens thickness. The similar slopes in regression analysis indicate that the effect of diabetes duration on lens thickness is independent of zygosity.