Measurement of the peripheral aberrations of human eyes: A comprehensive review

Eye Size and Shape in Relation to Refractive Error in Children: A Magnetic Resonance Imaging Study

Purpose

The purpose of this study was to determine the association between eye shape and volume measured with magnetic resonance imaging (MRI) and optical biometry and with spherical equivalent (SE) in children.

Methods

For this study, there were 3637 10-year-old children from a population-based birth-cohort study that underwent optical biometry (IOL-master 500) and T2-weighted MRI scanning (height, width, and volume). Cycloplegic refractive error was determined by automated refraction. The MRI images of the eyes were segmented using an automated algorithm combining atlas registration with voxel classification. Associations among optical biometry, anthropometry, MRI measurements, and RE were tested using Pearson correlation. Differences between refractive error groups were tested using ANOVA.

Results

The mean volume of the posterior segment was 6350 (±680) mm3. Myopic eyes (SE ≤ -0.5 diopters [D]) had 470 mm3 (P < 0.001) and 970 mm3 (P < 0.001) larger posterior segment volume than emmetropic and hyperopic eyes (SE ≥ +2.0D), respectively. The majority of eyes (77.1%) had an oblate shape, but 47.4% of myopic eyes had a prolate shape versus 3.9% of hyperopic eyes. The correlation between SE and MRI-derived posterior segment length (r -0.51, P < 0.001) was stronger than the correlation with height (r -0.30, P < 0.001) or width of the eye (r -0.10, P < 0.001).

Conclusions

In this study, eye shape at 10 years of age was predominantly oblate, even in eyes with myopia. Of all MRI measurements, posterior segment length was most prominently associated with SE. Whether eye shape predicts future myopia development or progression should be investigated in longitudinal studies.

Dynamic opto-mechanical eye model with peripheral refractions

Many myopia control methods based on the peripheral defocus theory have emerged towards applications in recent years. However, peripheral aberration is a critical issue, which is still not well-addressed. To validate the aberrometer for peripheral aberration measurement, a dynamic opto-mechanical eye model with a wide visual field is developed in this study. This model consists of a plano-convex lens representing cornea (f' = 30 mm), a double-convex lens representing crystalline lens (f' = 100 mm), and a spherical retinal screen with a radius of 12 mm. To optimize the quality of spot-field images from the Hartman-Shack sensor, the materials and surface topography for the retina are studied. The model has an adjustable retina to achieve Zernike 4th item (Z4 focus) ranging from -6.28 µm to +6.84 µm. As for mean sphere equivalent, it can achieve -10.52 D to +9.16 D at 0° visual field and -6.97 D to +5.88 D at 30° visual field with a pupil size of 3 mm. To realize a changing pupil size, a slot at the back of the cornea mount and a series of thin metal sheets with 2, 3, 4, and 6 mm holes are generated. Both on-axis aberrations and peripheral aberrations of the eye model are verified by a well-used aberrometer and the eye model to mimic a human eye in a peripheral aberration measurement system is illustrated.

Comparison of an open view autorefractor with an open view aberrometer in determining peripheral refraction in children

PURPOSE

The aim of this study was to compare central and peripheral refraction using an open view Shin-Nippon NVision-K 5001 autorefractor and an open view COAS-HD VR aberrometer in young children.

METHODS

Cycloplegic central and peripheral autorefraction was measured in the right eye of 123 children aged 8 to 16 years. Three measurements each were obtained with both Shin-Nippon NVision-K 5001 autorefractor and COAS-HD VR aberrometer along the horizontal visual field up to 30° (nasal and temporal) in 10° steps. The refraction from the autorefractor was compared with aberrometer refraction for pupil analysis diameters of 2.5-mm and 5.0-mm.

RESULTS

The Shin-Nippon was 0.30 D more hyperopic than COAS-HD VR at 2.5-mm pupil and 0.50 D more hyperopic than COAS-HD VR at 5-mm pupil for central refraction. For both pupil sizes, the 95% limits of agreement were approximately 0.50 D for central refraction, and limits were wider in the nasal visual field compared to the temporal visual field. The mean difference for both J₀ and J₄₅ were within 0.15 D and the 95% limits of agreement within 0.90 D across the horizontal visual field.

CONCLUSION

Defocus components were similar between the Shin-Nippon autorefractor and the COAS-HD VR aberrometer with a 2.5-mm pupil for most visual field angles. However, there was a significant difference in defocus component between the Shin-Nippon autorefractor and the COAS-HD VR aberrometer with a 5.0-mm pupil, wherein the autorefractor measured more hyperopia. The astigmatic components J₀ and J₄₅ were similar between instruments for both central and peripheral refraction.

Effect of Accommodation on Peripheral Higher Order Aberrations

Knowledge of the effect of accommodation on image quality of peripheral retina is crucial for better understanding of the visual system, but only a few studies have been carried out in this area. This study was designed to evaluate the effect of accommodation on higher order aberrations from third to sixth Zernike polynomials in central and peripheral retina up to 23° off-axis. We used a Hartmann–Shack aberrometer to measure Zernike coefficients with both accommodated and non-accommodated eyes of 15 healthy subjects. Each Zernike coefficient, total higher order aberrations, spherical aberrations and astigmatism were compared between accommodated and non-accommodated status. Additionally, aberrations in the central retina were compared with the peripheral retina. Accommodation induced significant changes in the Zernike coefficients of vertical pentafoil C5−5 and secondary vertical tetrafoil C6−4 in central retina, secondary vertical astigmatism C4−2 on 23° of temporal retina, secondary vertical tetrafoil C6−4 and tertiary vertical astigmatism C6−2 on 10° of nasal retina, secondary vertical trefoil C5−3 and secondary vertical tetrafoil C6−4 on 23° of nasal retina, and horizontal tetrafoil C44, and secondary horizontal tetrafoil C64 on 23° of inferior retina (p < 0.05). Total higher order aberration was lower in each retinal area examined with accommodation, but it was statistically significant only on 23° temporal retina and 11.5° and 23° of superior retina (p < 0.05). Spherical aberration decreased with accommodation on 23° temporal retina (p = 0.036). Astigmatism was similar in non-accommodated and accommodated eyes. Overall, accommodation affected higher order aberration (HOA) asymmetrically in different peripheral retinal areas.