Ocular axial length and its associations in Chinese: the Beijing Eye Study

Structural parameters associated with location of peaks of peripapillary retinal nerve fiber layer thickness in young healthy eyes

The location of the peaks of the circumpapillary retinal nerve fiber layer (cpRNFL) thickness is affected by several ocular parameters. In this study, we have generated equations that can determine the peaks of the cpRNFL. This study was a prospective, observational, cross sectional study of 118 healthy right eyes. The axial length, optic disc tilt, superiortemporal (ST)- and inferiortemporal (IT)-peaks of the cpRNFL thickness, and angles of the ST and IT retinal arteries (RA) and veins (RV) were determined. The correlations between the location of the ST- and IT-peaks and ocular structural parameters and the sex, body height and weight were calculated. The best fit equations to generate the location of the ST/IT-peaks were determined using corrected-Akaike Information Criteria. The location of the ST-peak was 0.72+(0.40 x ST-RA)+(0.27 x ST-RV)+(0.14 x height)–(0.47 x papillo-macular-position)–(0.11 x disc tilt) with a coefficient of correlation of 0.61 (P<0.0001). The location of the IT-peak was 21.88+(0.53 x IT-RA)+(0.15 x IT-RV)+(0.041 x corneal thickness)-(1.00 x axial length) with a coefficient of correlation of 0.59 (P<0.0001). The location of ST/IT peaks is determined by different parameters of the ocular structure. These equations allow clinicians to obtain an accurate location of the peaks for a more accurate diagnosis of glaucoma.

Retinal pigment epithelium cell density in relationship to axial length in human eyes

PURPOSE

To assess associations between axial length and density of retinal pigment epithelium (RPE) cells in various ocular regions.

METHODS

The histomorphometric investigation included histological sections of enucleated eyes of Caucasian patients. Using a light microscope, we counted the number of RPE cells on Bruch's membrane at the ora serrata, in the pre-equatorial region, the equatorial and retro-equatorial region, at the midpoint equator/posterior pole, and at the posterior pole.

RESULTS

The study included 65 globes with a mean axial length 25.9 ± 3.5 mm (range: 21.0-34.0 mm). Retinal pigment epithelium (RPE) cell count in the equatorial to retro-equatorial region (p < 0.001; correlation coefficient r² : 0.44), in the pre-equatorial region (p < 0.001; r² : 0.39) and at the midpoint equator/posterior pole (p = 0.03; r² : 0.12) decreased with increasing axial length. Retinal pigment epithelium (RPE) cell count at the ora serrata (p = 0.49) and posterior pole (p = 0.44) was not significantly correlated with axial length. As a corollary, mean RPE cell density was higher (p < 0.001) at the posterior pole than at the midpoint equator/posterior pole or at the ora serrata region, where it was higher than in pre-equatorial region (p < 0.001) and in the equatorial to retro-equatorial region (p < 0.001).

CONCLUSIONS

The decrease in the RPE cell density mainly in the equatorial to retro-equatorial region in association with longer axial length suggests a region of enlargement of Bruch's membrane in the equatorial to retro-equatorial area in association with axial elongation. The finding may be of interest to elucidate the process of emmetropization/myopization.

Axial length, anterior chamber depth and lens thickness: Their intercorrelations in black South Africans

Purpose: To determine means and ranges for axial length, anterior chamber depth, lens thickness values and their intercorrelations in an African population.Methods: Six hundred participants (N = 600) were selected through stratified random cluster sampling from geographically contiguous areas of Durban, South Africa. All participants underwent height measurements and standard vision testing. Repeated measures of axial length, anterior chamber depth and lens thickness were taken with the Nidek US-500 Echoscan.Results: Participants’ ages ranged from 10 to 66 years with a mean age of 28.15 ± 13.09 years (95% confidence interval, 27.09–29.19). Of all the subjects, 295 (49.17%) were females and 305 (50.83%) were males. Axial length ranged from 20.42 mm to 27.28 mm with a mean of 23.05 mm ± 0.98 mm (95% confidence interval, 22.97–23.14), anterior chamber depth ranged from 2.38 mm to 4.13 mm with a mean of 3.21 mm ± 0.37 mm (95% confidence interval, 3.18–3.24) and crystalline lens thicknesses ranged from 2.24 mm to 4.66 mm with a mean of 3.69 mm ± 0.25 mm (95% confidence interval, 3.66–3.71). All three biometric indices were significantly higher in men than in women (all p-values < 0.05). A multivariate linear regression model indicated that axial length and anterior chamber depth decreased with age, while lens thickness increased with age. All biometric indices directly correlated with the male gender and height (all p-values < 0.001). Pearson correlation coefficient tests showed that axial length was significantly positively correlated with anterior chamber (r = 0.66, p < 0.001) and negatively correlated with lens thickness (r = -0.52, p < 0.001). A significant negative correlation was found between lens thickness and anterior chamber depth values (r = -0.68, p < 0.001).Conclusion: Normative values for axial length, anterior chamber depth and lens thickness are determined for the first time in a black South African sample, aged 10–66 years. Age, gender and height were associated with biometric indices. While there was a positive correlation between axial length and anterior chamber depth, there was a negative correlation between lens thickness and both axial length and anterior chamber depth. These biometric data and their intercorrelations may provide some insights into the pathophysiological mechanisms of angle-closure glaucoma in this population.

The Relationship between Crystalline Lens Power and Refractive Error in Older Chinese Adults: The Shanghai Eye Study

Purpose

To report calculated crystalline lens power and describe the distribution of ocular biometry and its association with refractive error in older Chinese adults.

Methods

Random clustering sampling was used to identify adults aged 50 years and above in Xuhui and Baoshan districts of Shanghai. Refraction was determined by subjective refraction that achieved the best corrected vision based on monocular measurement. Ocular biometry was measured by IOL Master. The crystalline lens power of right eyes was calculated using modified Bennett-Rabbetts formula.

Results

We analyzed 6099 normal phakic right eyes. The mean crystalline lens power was 20.34 ± 2.24D (range: 13.40–36.08). Lens power, spherical equivalent, and anterior chamber depth changed linearly with age; however, axial length, corneal power and AL/CR ratio did not vary with age. The overall prevalence of hyperopia, myopia, and high myopia was 48.48% (95% CI: 47.23%–49.74%), 22.82% (95% CI: 21.77%–23.88%), and 4.57% (95% CI: 4.05–5.10), respectively. The prevalence of hyperopia increased linearly with age while lens power decreased with age. In multivariate models, refractive error was strongly correlated with axial length, lens power, corneal power, and anterior chamber depth; refractive error was slightly correlated with best corrected visual acuity, age and sex.

Conclusion

Lens power, hyperopia, and spherical equivalent changed linearly with age; Moreover, the continuous loss of lens power produced hyperopic shifts in refraction in subjects aged more than 50 years.

Optic nerve sheath diameter threshold by ocular ultrasonography for detection of increased intracranial pressure in Korean adult patients with brain lesions

Optic nerve sheath diameter (ONSD) seen on ocular US has been associated with increased intracranial pressure (IICP). However, most studies have analyzed normal range of ONSD and its optimal cut-off point for IICP in Caucasian populations. Considering ONSD differences according to ethnicity, previous results may not accurately reflect the association between IICP and ONSD in Koreans. Therefore, we conducted this study to investigate normal range of ONSD and its optimal threshold for detecting IICP in Korean patients.This prospective multicenter study was performed for patients with suspected IICP. ONSD was measured 3 mm behind the globe using a 13-MHz US probe. IICP was defined as significant brain edema, midline shift, compression of ventricle or basal cistern, effacement of sulci, insufficient gray/white differentiation, and transfalcine herniation by radiologic tests. The results of the ONSD are described as the median (25th-75th percentile). The differences of ONSD according to disease entity were analyzed. A receiver operator characteristic (ROC) curve was generated to determine the optimal cut-off point for identifying IICP.A total of 134 patients were enrolled. The patients were divided into 3 groups as follows: patients with IICP, n = 81 (60.5%); patients without IICP, n = 27 (20.1%); and control group, n = 26 (19.4%). ONSD in patients with IICP (5.9 mm [5.8-6.2]) is significantly higher than those without IICP (5.2 mm [4.8-5.4]) (P < 0.01) and normal control group (4.9 mm [4.6-5.2]) (P < 0.001). Between patients without IICP and normal control group, the difference of ONSD did not reach statistical significance (P = 0.31). ONSD >5.5 mm yielded a sensitivity of 98.77% (95% CI: 93.3%-100%) and a specificity of 85.19% (95% CI: 66.3%-95.8%).In conclusion, the optimal cut-off point of ONSD for identifying IICP was 5.5 mm. ONSD seen on ocular US can be a feasible method for detection and serial monitoring of ICP in Korean adult patients.

Posterior Corneal Characteristics of Cataract Patients with High Myopia

Purpose

To evaluate the characteristics of the posterior corneal surface in patients with high myopia before cataract surgery.

Methods

We performed a cross-sectional study at the Eye and ENT Hospital of Fudan University, Shanghai, China. Corneal astigmatism and axial length were measured with a rotating Scheimpflug camera (Pentacam) and partial coherence interferometry (IOLMaster) in a high-myopia study group of 167 eyes (axial length ≥ 26 mm) and a control group of 150 eyes (axial length > 20 mm and < 25 mm).

Results

Total corneal astigmatism and anterior corneal astigmatism values were higher in the high-myopia group than in the control group. There was no significant difference in posterior corneal astigmatism between the high-myopia study group and the control group. In the study group, the mean posterior corneal astigmatism (range 0 – −0.9 diopters) was –0.29 diopters (D) ± 0.17 standard deviations (SD). The steep corneal meridian was aligned vertically (60°–120°) in 87.43% of eyes for the posterior corneal surface, and did not change with increasing age. There was a significant correlation (r = 0.235, p = 0.002) between posterior corneal astigmatism and anterior corneal astigmatism, especially when the anterior corneal surface showed with-the-rule (WTR) astigmatism (r = 0.452, p = 0.000). There was a weak negative correlation between posterior corneal astigmatism and age (r = –0.15, p = 0.053) in the high-myopia group. Compared with total corneal astigmatism values, the anterior corneal measurements alone overestimated WTR astigmatism by a mean of 0.27 ± 0.18 D in 68.75% of eyes, underestimated against-the-rule (ATR) astigmatism by a mean of 0.41 ± 0.28 D in 88.89% of eyes, and underestimated oblique astigmatism by a mean of 0.24 ± 0.13 D in 63.64% of eyes.

Conclusions

Posterior corneal astigmatism decreased with age and remained as ATR astigmatism in most cases of high myopia. There was a significant correlation between posterior corneal astigmatism and anterior corneal astigmatism when anterior corneal astigmatism was WTR. If posterior corneal astigmatism is not accounted for when selecting toric intraocular lenses for high-myopia patients, the use of anterior corneal astigmatism measurements alone will lead to overestimation of WTR astigmatism and underestimation of ATR and oblique astigmatism.

Vascular Density in Retina and Choriocapillaris as Measured by Optical Coherence Tomography Angiography

PURPOSE

To measure the density of the superficial retinal small vessel network (SRSVN), superficial retinal capillary network (SRCN), deep retinal capillary network (DRCN), and choriocapillaris and the size of the foveal avascular zone (FAZ) in the superficial retinal layer in normal eyes.

DESIGN

Prospective observational cross-sectional study.

METHODS

In healthy Chinese volunteers, the retinal and choroidal vasculature was visualized by split-spectrum amplitude decorrelation angiography-associated optical coherence tomography (RTVueXR Avanti device; Optovue Inc, Fremont, California, USA).

RESULTS

Among 105 healthy participants (age: 35.9 ± 13.8 years) mean FAZ measured 0.35 ± 0.12 mm(2), and mean density of SRSVN, SRCN, DRCN, and choriocapillaris was 8.54% ± 0.92%, 31.8% ± 2.6%, 45.8% ± 3.3%, 44.4% ± 3.3%, and 44.5% ± 2.7%, respectively. In multivariate analysis, higher SRSVN density was associated with younger age (P = .001; standardized regression coefficient β: -0.28), male sex (P = .008; β: -0.23), lower SRCN density (P < .001; β: -0.40), and larger mean choriocapillaris vessel diameter (P = .001; β: 0.30). Higher SRCN density was correlated with male sex (P = .007; β: -0.19), lower SRSVN density (P < .001; β: -0.44), and higher density of the radial peripapillary capillary density (P = .004; β: 0.20). Higher DRCN density was correlated with younger age (P < .001; β: -0.31), female sex (P = .002; β: 0.22), higher SRCN density (P < .001; β: 0.38), and higher choriocapillaris density (P < .001; β: 0.39). Higher choriocapillaris network density in the central region was associated with higher DRCN density (P < .001; β: 0.43) and lower radial peripapillary capillary density (P = .005; β: -0.26). All retinal vascular parameters were not significantly correlated with axial length or subfoveal choroidal thickness.

CONCLUSIONS

The density of the macular vascular networks decreases with older age and is independent of axial length and subfoveal choroidal thickness in healthy individuals.

Accuracy of Intraocular Lens Power Calculation Formulas for Highly Myopic Eyes

Purpose. To evaluate and compare the accuracy of different intraocular lens (IOL) power calculation formulas for eyes with an axial length (AL) greater than 26.00 mm. Methods. This study reviewed 407 eyes of 219 patients with AL longer than 26.0 mm. The refractive prediction errors of IOL power calculation formulas (SRK/T, Haigis, Holladay, Hoffer Q, and Barrett Universal II) using User Group for Laser Interference Biometry (ULIB) constants were evaluated and compared. Results. One hundred seventy-one eyes were enrolled. The Barrett Universal II formula had the lowest mean absolute error (MAE) and SRK/T and Haigis had similar MAE, and the statistical highest MAE were seen with the Holladay and Hoffer Q formulas. The interquartile range of the Barrett Universal II formula was also the lowest among all the formulas. The Barrett Universal II formulas yielded the highest percentage of eyes within ±1.0 D and ±0.5 D of the target refraction in this study (97.24% and 79.56%, resp.). Conclusions. Barrett Universal II formula produced the lowest predictive error and the least variable predictive error compared with the SRK/T, Haigis, Holladay, and Hoffer Q formulas. For high myopic eyes, the Barrett Universal II formula may be a more suitable choice.

Axial Length and Associated Factors in Children: The Shandong Children Eye Study

PURPOSE

To investigate ocular axial length (AL) in Chinese children.

METHODS

The Shandong Children Eye Study included 6,364 children aged 4-18 years.

RESULTS

Longer AL (mean 23.45 ± 1.20 mm, range 18.80-28.59 mm) was associated (multivariate analysis; correlation coefficient r2: 0.61) with older age (p < 0.001, standardized correlation coefficient β = 0.35, unstandardized regression coefficient B = 0.13, 95% confidence interval [CI] 0.12, 0.15), male gender (p < 0.001, β = -0.24, B = -0.10, 95% CI -0.29, -0.19), urban region (p < 0.001, β = 0.10, B = 0.25, 95% CI 0.20, 0.31), body height (p < 0.001, β = 0.22, B = 0.02, 95% CI 0.01, 0.02), maternal education (p < 0.001, β = 0.07, B = 0.07, 95% CI 0.05, 0.10), paternal myopia (p < 0.001, β = 0.09, B = 0.26, 95% CI 0.20, 0.33), maternal myopia (p < 0.001, β = 0.08, B = 0.23, 95% CI 0.17, 0.30), more time spent indoors reading/writing (p < 0.001, β = 0.05, B = 0.03, 95% CI 0.02, 0.04), less time spent outdoors (p = 0.005, β = -0.03, B = -0.01, 95% CI -0.02, -0.003), longer corneal curvature radius (p < 0.001, β = 0.36, B = 1.63, 95% CI 1.53, 1.74) and higher intraocular pressure (p = 0.008, β = 0.03, B = 0.01, 95% CI 0.004, 0.02). High axial myopia (AL ≥26.0 mm) present in 202 children (3.4 ± 0.2%, 95% CI 2.92, 3.84) was associated with less time spent outdoors (p = 0.002, odds ratio 0.92, 95% CI 0.87, 0.97) in multivariate analysis.

CONCLUSIONS

In children in the less developed Eastern Chinese province of Shandong, the prevalence of high axial myopia was >10% among 16-year-olds. A modifiable factor associated with higher prevalence of high axial myopia was less time spent outdoors.

Relationship between Cornea and Lid Margin Position on the Appearance of Strabismus

PURPOSE

Previous strabismus appearance studies using images from real face photographs could not preclude the effect of other facial landmarks, such as position and contour of brow, nose, and facial outline, from pure effect of ocular deviation itself. With conceptualized drawings of eyes, we simulated strabismus deviation and collected the responses of observers.

METHODS

The experiments consisted of three sessions: (1) one-eye, (2) two-eye cornea, and (3) two-eyelid deviation simulations. On a monitor, eight different ocular deviations in increments of 6 prism diopters of horizontal or vertical deviations were simulated. Nine ophthalmology residents were asked to choose between esotropia and exotropia for horizontal simulations and hypotropia or hypertropia for vertical simulations. Psychometric function fitting was performed. Detection sensitivity, point of subjective equality, and 95% thresholds were calculated.

RESULTS

Between cornea and lid deviation simulations, there was no difference in 95% thresholds, points of subjective equality, and sensitivity (all p values > 0.10), reflecting that the relative location of cornea and lid was the determinant factor in recognizing ocular deviation. The sensitivities of vertical deviation simulations were higher than that of horizontal in two-eye cornea and lid comparisons (p values < 0.04) but not in one-eye (p value = 0.37).

CONCLUSIONS

Our results suggest that the appearance of strabismus depends not on the absolute ocular alignment but rather on the reciprocal relationship of cornea and lid margin. Vertical misalignment was more apparent than horizontal misalignment.

Ocular biometry in the adult population in rural central China: a population-based, cross-sectional study

AIM

To describe the distribution and determinants of ocular biometric parameters and to ascertain the relative importance of these determinants in a large population of adults in rural central China.

METHODS

A population-based, cross-sectional study performed in rural central China included 1721 participants aged 40 or more years. Ocular biometrical parameters including axial length (AL), anterior chamber depth (ACD), radius of corneal curvature (K) and horizontal corneal diameter [white-to-white (WTW) distance] were measured using non-contact partial coherence interferometry [intraocular lens (IOL)-Master].

RESULTS

Ocular biometric data on 1721 participants with a average age of 57.0±8.7y were analyzed at last. The general mean AL, ACD, mean corneal curvature radius (MCR), WTW were 22.80±1.12, 2.96±0.36, 7.56±0.26 and 11.75±0.40 mm, respectively. The mean values of each parameter in 40 to 49, 50 to 59, 60 to 69, and 70 to 91 years age groups were as follows: AL, 22.77±0.87, 22.76±1.06, 22.89±1.41, 22.92±0.80 mm; ACD, 3.10±0.32, 2.98±0.34, 2.86±0.36, 2.77±0.35 mm; MCR, 7.58±0.25, 7.54±0.26, 7.55±0.26, 7.49±0.28 mm; WTW, 11.79±0.38, 11.75±0.40, 11.72±0.41, 11.67±0.41 mm. The AL, ACD, MCR and WTW were correlated with age and the AL was correlated with height and weight.

CONCLUSION

Our findings can serve as an important normative reference for multiple purposes and may help to improve the quality of rural eye care.

Evaluation of Anterior Segment Parameters in Obesity

PURPOSE

To investigate anterior segment parameters in obese patients in comparison to healthy individuals.

METHODS

Thirty-four obese subjects and 34 age-sex-matched healthy subjects were enrolled in this prospective cross-sectional study. Ophthalmological examinations including intraocular pressure (IOP), central corneal thickness (CCT), anterior chamber depth (ACD), anterior chamber volume (ACV), anterior chamber angle (ACA), and axial length (AL) measurements were performed on each subject. Height and weight of all subjects were recorded and body mass index (BMI) was calculated.

RESULTS

IOP was significantly higher in the obese group (p = 0.003). The mean ACD in obese subjects was significantly lower than that in control subjects (p = 0.036). AL, ACV, ACA and CCT were not significantly different between the groups. There was a positive correlation between BMI and IOP (r = 0.404, p < 0.001). ACD and ACA were negatively correlated with BMI.

CONCLUSIONS

IOP was significantly higher and ACD was significantly lower in obese subjects. AL, ACV, ACA and CCT were not significantly different between the groups. The impact of obesity on anterior chamber parameters should be further investigated.

Circumpapillary course of retinal pigment epithelium can be fit to sine wave and amplitude of sine wave is significantly correlated with ovality ratio of optic disc

The purpose of this study was to develop a method of quantifying the degree of optic disc tilt in normal eyes. This was a prospective, observational cross sectional study of 126 right eyes of 126 healthy volunteers. The optic disc tilt was determined from the circular peripapillary optical coherence tomographic (OCT) scan images. The course of the retinal pigment epithelium (RPE) layer in the peripapillary cross sectional scan images was fit to a sine wave curve, and the amplitude of the sine curve was used to reflect the degree of the optic disc tilt in the optical axis. The repeatability of the amplitude determinations was calculated. The correlation between the amplitude and the ovality ratio of the optic disc was determined. The correlation between the amplitude and the body height was also calculated. The mean amplitudewas 36.6 ± 17.5 pixels, which was significantly and inversely correlated with the ovality ratio of the optic disc (R = -0.59, P<0.001). The intra-rater and inter-rater correlation coefficients of the amplitude were significant high (P<0.001, both). The amplitude was significantly and inversely correlated with the body height (R = -0.38, P<0.001), but not with the axial length. In conclusion, a sine wave function can be used to describe the course of the RPE in the circumpapillary OCT images. The results indicate that the amplitude of the sine wave can be used to represent the degree of optic disc tilt. Thus, the sine wave analyses can be used as a quantifiable and repeatable method to determine the optic disc tilt.

Myopic foveoschisis: a clinical review

To review the literature on epidemiology, clinical features, diagnostic imaging, natural history, management, therapeutic approaches, and prognosis of myopic foveoschisis. A systematic Pubmed search was conducted using search terms: myopia, myopic, staphyloma, foveoschisis, and myopic foveoschisis. The evidence base for each section was organised and reviewed. Where possible an authors' interpretation or conclusion is provided for each section. The term myopic foveoschisis was first coined in 1999. It is associated with posterior staphyloma in high myopia, and is often asymptomatic initially but progresses slowly, leading to loss of central vision from foveal detachment or macular hole formation. Optical coherence tomography is used to diagnose the splitting of the neural retina into a thicker inner layer and a thinner outer layer, but compound variants of the splits have been identified. Vitrectomy with an internal limiting membrane peel and gas tamponade is the preferred approach for eyes with vision decline. There has been a surge of new information on myopic foveoschisis. Advances in optical coherence tomography will continually improve our understanding of the pathogenesis of retinal splitting, and the mechanisms that lead to macular damage and visual loss. Currently, there is a good level of consensus that surgical intervention should be considered when there is progressive visual decline from myopic foveoschisis.

[Results of advanced cataract tunnel surgery: about 262 cases performed at CHR Banfora (Burkina Faso)]

La cataracte demeure la première cause de cécité dans les pays en voie de développement posant un problème de santé publique. Le but de cette étude est de faire le bilan d'une activité de chirurgie avancée de la cataracte par la technique dite de « phacoalternative » ou par tunnélisation. Il s'agissait d'une étude prospective de base réalisée au Burkina Faso portant sur la chirurgie de la cataracte par la technique de tunnélisation. L’âge moyen de nos patients atteints de cataracte était de 62,47 ± 12,77 ans avec un sex-ratio de 0,98. La majorité des patients opérés (82,82%) étaient des cultivateurs et des ménagères. L'acuité visuelle de loin était réduite à la perception lumineuse dans 68,70% des cas. La profondeur moyenne de la chambre antérieure était de 2,98 ± 0,53 mm. L’épaisseur moyenne du cristallin était de 3,55 ± 0,75 mm. La longueur axiale moyenne de l’œil de nos patients opérés de la cataracte était de 22,88 ± 0,90 mm. La puissance réfractive moyenne estimée de la cornée était de 43,21 ± 2,03 dioptries chez nos patients. La puissance moyenne estimée de l'implant intra oculaire (IOL) utilisé était de23,11 ± 2,35D. La majorité (79,6%) de nos patients avait une bonne acuité visuelle après la chirurgie de la cataracte par tunnélisation. La chirurgie de la cataracte par tunnélisation est une technique révolutionnaire adaptée aux pays pauvres d'Afrique. Elle permet une lutte efficace à moindre coût contre la cécité par cataracte dans nos régions. La tunnélisation est une technique chirurgicale adaptée pour lutter efficacement contre la cécité due à la cataracte en Afrique.

Rapid, accurate, and non-invasive measurement of zebrafish axial length and other eye dimensions using SD-OCT allows longitudinal analysis of myopia and emmetropization

Refractive errors in vision can be caused by aberrant axial length of the eye, irregular corneal shape, or lens abnormalities. Causes of eye length overgrowth include multiple genetic loci, and visual parameters. We evaluate zebrafish as a potential animal model for studies of the genetic, cellular, and signaling basis of emmetropization and myopia. Axial length and other eye dimensions of zebrafish were measured using spectral domain-optical coherence tomography (SD-OCT). We used ocular lens and body metrics to normalize and compare eye size and relative refractive error (difference between observed retinal radial length and controls) in wild-type and lrp2 zebrafish. Zebrafish were dark-reared to assess effects of visual deprivation on eye size. Two relative measurements, ocular axial length to body length and axial length to lens diameter, were found to accurately normalize comparisons of eye sizes between different sized fish (R2=0.9548, R2=0.9921). Ray-traced focal lengths of wild-type zebrafish lenses were equal to their retinal radii, while lrp2 eyes had longer retinal radii than focal lengths. Both genetic mutation (lrp2) and environmental manipulation (dark-rearing) caused elongated eye axes. lrp2 mutants had relative refractive errors of -0.327 compared to wild-types, and dark-reared wild-type fish had relative refractive errors of -0.132 compared to light-reared siblings. Therefore, zebrafish eye anatomy (axial length, lens radius, retinal radius) can be rapidly and accurately measured by SD-OCT, facilitating longitudinal studies of regulated eye growth and emmetropization. Specifically, genes homologous to human myopia candidates may be modified, inactivated or overexpressed in zebrafish, and myopia-sensitizing conditions used to probe gene-environment interactions. Our studies provide foundation for such investigations into genetic contributions that control eye size and impact refractive errors.

Retinal degeneration increases susceptibility to myopia in mice

PURPOSE

Retinal diseases are often associated with refractive errors, suggesting the importance of normal retinal signaling during emmetropization. For instance, retinitis pigmentosa, a disease characterized by severe photoreceptor degeneration, is associated with myopia; however, the underlying link between these conditions is not known. This study examines the influence of photoreceptor degeneration on refractive development by testing two mouse models of retinitis pigmentosa under normal and form deprivation visual conditions. Dopamine, a potential stop signal for refractive eye growth, was assessed as a potential underlying mechanism.

METHODS

Refractive eye growth in mice that were homozygous for a mutation in Pde6b, Pde6b(rd1/rd1) (rd1), or Pde6b(rd10/rd10) (rd10) was measured weekly from 4 to 12 weeks of age and compared to age-matched wild-type (WT) mice. Refractive error was measured using an eccentric infrared photorefractor, and axial length was measured with partial coherence interferometry or spectral domain ocular coherence tomography. A cohort of mice received head-mounted diffuser goggles to induce form deprivation from 4 to 6 weeks of age. Dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) levels were measured with high-performance liquid chromatography in each strain after exposure to normal or form deprivation conditions.

RESULTS

The rd1 and rd10 mice had significantly greater hyperopia relative to the WT controls throughout normal development; however, axial length became significantly longer only in WT mice starting at 7 weeks of age. After 2 weeks of form deprivation, the rd1 and rd10 mice demonstrated a faster and larger myopic shift (-6.14±0.62 and -7.38±1.46 diopter, respectively) compared to the WT mice (-2.41±0.47 diopter). Under normal visual conditions, the DOPAC levels and DOPAC/dopamine ratios, a measure of dopamine turnover, were significantly lower in the rd1 and rd10 mice compared to the WT mice, while the dopamine levels were similar or higher than WT in the rd10 mice. Lower basal levels of DOPAC were highly correlated with increasing myopic shifts.

CONCLUSIONS

Refractive development under normal visual conditions was disrupted toward greater hyperopia from 4 to 12 weeks of age in these photoreceptor degeneration models, despite significantly lower DOPAC levels. However, the retinal degeneration models with low basal levels of DOPAC had increased susceptibility to form deprivation myopia. These results indicate that photoreceptor degeneration may alter dopamine metabolism, leading to increased susceptibility to myopia with an environmental visual challenge.

Investigating mechanisms of myopia in mice

Genetic and environmental factors have been shown to control visually-guided eye growth and influence myopia development. However, investigations into the intersection of these two factors in controlling refractive development have been limited by the lack of a genetically modifiable animal model. Technological advances have now made it possible to assess refractive state and ocular biometry in the small mouse eye and therefore to exploit the many genetic mouse mutants to investigate mechanisms of visually-guided eye growth. This review considers the benefits and challenges of studying refractive development in mice, compares the results of refractive error and ocular biometry from wild-type strains and genetic models in normal laboratory visual environments or with disrupted visual input, and discusses some of the remaining challenges in interpreting data from the mouse to validate and standardize methods between labs.