Longitudinal changes in objective accommodative response, pupil size and spherical aberration: A case study.

PURPOSE: Previous transverse and a handful of longitudinal studies have shown that the slope of the static accommodation response/stimulus curve declines as complete presbyopia is approached. Changes in pupillary miosis and ocular spherical aberration (SA) are also evident. This study further investigated longitudinal changes in the relationships between the monocular static accommodative response, pupil diameter and SA of a single adult. METHODS: A wavefront analysing system, the Complete Ophthalmic Analysis System, was used in conjunction with a Badal optometer to allow continuous recording of the aberration structure of the dominant eye in a low myope for a range of accommodative demands (-0.83 to 7.63 D) over a period of 17 years until the age of 50. Monocular accommodative response was calculated as the equivalent refraction minimising wavefront error. The associated longitudinal changes in pupil size and SA with accommodation were also recorded. RESULTS: A decrease in accommodation response with age was found at almost all target vergences, with the changes being greatest for higher vergences. In addition, although absolute pupil diameter decreased with age, the rate of change in pupil diameter with accommodative stimulus remained approximately constant with age. Pupil constriction occurred for near stimuli even in full presbyopia. SA changed linearly with the accommodation response at all ages. CONCLUSIONS: The objective amplitude of accommodation declined linearly with age as complete presbyopia was approached, while the slope of the response/stimulus curve also fell. It was hypothesised that the retinal image blur associated with the larger lags of accommodation at higher accommodative stimuli was reduced by pupil constriction and the resulting lower levels of SA.

Multiple segment spectacle lenses for myopia control. Part 1: Optics.

PURPOSE: To understand and compare the optics of two multiple segment (MS) spectacle lenses (Hoya MiyoSmart and Essilor Stellest) designed to inhibit myopia progression in children. METHODS: The optics of the two designs are presented, together with geometrical optics-based calculations to understand the impact of the lenses on the optics of the eye. Lenses were evaluated with three techniques: surface images, Twyman-Green interferometry and focimetry. The carrier lens powers and the spatial distribution, powers and forms of the lenslets were measured. RESULTS: MS lenses as manufactured were found to match most of the design specifications provided by their manufacturers, although some apparent small discrepancies were found. The focimeter-measured power of the lenslets was approximately +3.50 D for the MiyoSmart and +4.00 D for the highly aspheric lenslets of the Stellest design. For both lens designs, image contrast would be expected to become modestly reduced in the focal planes of the distance-correcting carrier lenses. Images become much more degraded in the combined carrier-lenslet focal plane, due to the generation of multiple laterally displaced images formed by adjacent lenslets within the effective pupil. The exact effects observed depended on the effective pupil size and its location with respect to the lenslets, as well as the power and arrangement of the lenslets. CONCLUSION: Wearing either of these lenses will produce broadly similar effects on retinal imagery.

Multiple segment spectacle lenses for myopia control. Part 2: Impact on myopia progression.

PURPOSE: Initial studies have suggested that multiple segment (MS) spectacle lenses can reduce the progression rate of childhood myopia and axial eye growth. This paper aimed to compare the effectiveness of two different available designs of MS lens and to explore the nature of their control effect. METHOD: Published data from the only two clinical trials in which changes in mean spherical equivalent refraction (SER) and axial length (AL) for matched groups of myopic children wearing either MS or single-vision (SV) spectacle corrections, recorded over a period of at least 2 years, were further analysed and compared. Both trials involved Chinese children of similar ages and visual characteristics, but the trials were located in different cities. The two MS lenses examined were MiyoSmart or DIMS (Hoya) and Stellest (Essilor). RESULTS: Absolute changes in SER and AL differed over time during the two trials. However, if the results were expressed in terms of efficacy over successive 6-month periods, then the two MS lenses produced broadly similar results (initial efficacy for the control of myopia progression of about 60%-80%, declining over 2 years to about 35%-55%). Control appears to be absolute rather than proportional. DISCUSSION: Myopia control may be due to either the additional myopic defocus induced by the MS lenses (i.e., asymmetry of the through-focus image changes about the distance focus) or to the general reduction in image contrast that the lenslets create in the peripheral field. CONCLUSION: Multiple segment spectacle lenses offer a valuable new approach to the control of myopia progression in children. Further work is required to clarify their mechanism of action and to optimise their design parameters.

Modelling changes in the prevalence of childhood myopia.

PURPOSE: To re-examine the changes with time in the underlying patterns of individual refraction at different ages, which have led to an increased prevalence of myopia in a population of Asian children. METHODS: Using published cross-sectional longitudinal data, the frequency distributions of spherical equivalent refractive error (SE) in yearly cohorts of 6- and 12-year-old Japanese children during the period 1984-1996 were modelled in terms of ex- and bi-Gaussian distributions. RESULTS: Both models suggested that over the period of the study, little change occurred in the SE frequency distributions for 6-year-olds, with most children having SEs near emmetropia. In contrast, in each annual cohort of 12-year-olds, although the SE of some children remained near-emmetropic, a sub-set failed to maintain emmetropia. Most of this group became more myopic between 6 and 12 years of age. The proportion of children showing myopic progression increased over the period of study. CONCLUSIONS: The observed increase in mean levels of myopia in older Japanese children in the late 20th century is due to a greater proportion of children failing to maintain emmetropisation between the ages of 6 and 12, rather than to myopic shifts in all children. Some children, with small SE changes between 6 and 12 years of age, would not have benefitted from any treatment intended to slow myopia progression.

Factors affecting image magnification in indirect ophthalmoscopy with Volk or similar lenses and a biomicroscope.

PURPOSE: To explore the factors affecting the linear magnification of the intermediate fundus image during indirect ophthalmoscopy with a slit-lamp biomicroscope. METHODS: A simple paraxial model, based on a 'reduced' eye and a 'thin' ophthalmoscopy lens, is used to develop equations showing the effects of the power and ametropia of the eye, and the equivalent power and position of the ophthalmoscopy lens on fundus magnification. Predicted magnifications are compared with practical results found in earlier published experimental studies, which used Volk ophthalmoscopy lenses in conjunction with physical model eyes with adjustable levels of axial ametropia. RESULTS: The model's magnification predictions, as a function of the eye's ametropia, are in good agreement with previous experimental measurements, provided that the equivalent powers of the Volk lenses are used rather than their labelled nominal powers. Magnification values typically change by approximately ±10% over the practical range of each parameter if other parameters are held constant. In particular, normal variations in the equivalent power of the adult emmetropic eye result in magnifications which range from about 90-120% of the nominal value given for an eye power of +60.00 D. It is demonstrated that the recommended working distances for different powers of Volk ophthalmoscopy lenses approximate optimal matching between the various pupils of the eye-Volk lens-slit-lamp biomicroscope system. CONCLUSIONS: All the parameters considered have marked effects on magnification. The magnification values quoted by manufacturers can be regarded as only approximations of those which may be found in practice. Better estimates of magnification can be obtained by inserting the appropriate parameter values into the equations derived in this paper, using, where appropriate, the equivalent power of the indirect ophthalmoscopy lens, rather than the lens' labelled, 'nominal' power.

Flitcroft's model of refractive development in childhood and the possible identification of children at risk of developing significant myopia.

PURPOSE: To better understand juvenile myopia in the context of overall refractive development during childhood and to suggest more informative ways of analysing relevant data, particularly in relation to early identification of those children who are likely to become markedly myopic and would therefore benefit from myopia control. METHODS: Examples of the frequency distributions of childhood mean spherical refractive errors (MSEs) at different ages, taken from previously-published longitudinal and cross-sectional studies, are analysed in terms of Flitcroft's model of a linear combination of two Gaussian distributions with different means and standard deviations. Flitcroft hypothesises that one, relatively-narrow, Gaussian (Mode 1) represents a "regulated" population which maintains normal emmetropisation and the other, broader, Gaussian (Mode 2) a "dysregulated" population. RESULTS: Analysis confirms that Flitcroft's model successfully describes the major features of the frequency distribution of MSEs in randomly-selected populations of children of the same age. The narrow "regulated" Gaussian typically changes only slightly between the ages of about 6 and 15, whereas the mean of the broader "dysregulated" Gaussian changes with age more rapidly in the myopic direction and its standard deviation increases. These effects vary with the ethnicity, environment and other characteristics of the population involved. At all ages there is considerable overlap between the two Gaussians. This limits the utility of simple refractive cut-off values to identify those children likely to show marked myopic progression. CONCLUSIONS: Analysing the frequency distributions for individual MSEs in terms of bi-Gaussian models can provide useful insights into childhood refractive change. A wider exploration of the methodology and its extension to include individual progression rates is warranted, using a range of populations of children exposed to different ethnic, environmental and other factors.

Effect of Defocus Incorporated Multiple Segments Spectacle Lens Wear on Visual Function in Myopic Chinese Children.

Purpose: To compare visual function of myopic children who had worn either defocus incorporated multiple segment (DIMS) spectacle lenses or single vision (SV) spectacle lenses over two years. Methods: We included 160 Chinese myopic (-1 diopter [D] to -5 D) children aged 8 to 13 years in a randomized clinical trial; they wore either DIMS lenses (DIMS; n = 79) or regular SV spectacles lenses (n = 81) full time for 2 years. Visual function, including high-contrast visual acuity (VA) and low-contrast VA at distance and near, binocular functions, and accommodation, before, during, and after 2 years of spectacle wear were assessed when both groups wore SV corrections. Changes of visual function between the two groups and within groups were compared. Results: There were no statistically significant differences in the 2-year visual function changes between DIMS and SV groups (repeated measures analysis of variance with group as factor; P > 0.05). Statistically significant improvement in the best-corrected distance high-contrast VA (P < 0.001) and stereoacuity score (P < 0.001) were found after DIMS lens wear over 2 years. Similar findings were observed after SV spectacle lens wear. For both the DIMS and SV groups, there were statistically significant decreases in accommodative lag, monocular and binocular amplitude of accommodation after two years (P < 0.01), but not in the changes in distance low-contrast VA, near high-contrast VA, near low-contrast VA, or phoria. Conclusions: Although changes in some visual function were shown during 2 years of DIMS lens wear, similar changes were found with SV lens wear. Wear of DIMS spectacle lenses for 2 years does not adversely affect major visual function when children return to SV corrections. Translational Relevance: DIMS spectacle lenses did not cause any adverse effects on visual function.

Pharmacological Strategies for Presbyopia Correction.

PURPOSE: To summarize the pharmacological strategies that are being explored for presbyopia correction. METHODS: The review concentrates on pharmacologically induced pupillary miosis to increase depth-of-focus and lens softening or other measures to restore active accommodation. RESULTS: Several studies suggest that near vision improves and distance vision is unaffected for many hours after either monocular or binocular instillation of any one of several drug combinations that cause miosis. Unfortunately, in most studies, measurements were limited to photopic visual acuity for near and distance vision, whereas it is anticipated that pupil constriction may have adverse effects on mesopic and scotopic vision. It is not clear whether improved near vision was due entirely to increased depth-of-focus, or whether, for example, a drug-induced myopic shift in refraction was also involved. Currently, no study has provided direct evidence for drug-induced restoration/enhancement of true accommodation involving an ocular power change. CONCLUSIONS: Although it is possible that, in the future, pharmacological drops may offer a safe and reliable solution for presbyopia correction, more evidence of their effectiveness and limitations is required. [J Refract Surg. 2019;35(12):803-814.].

Small-aperture optics for the presbyope: do comparable designs of corneal inlays and intraocular lenses provide similar transmittances to the retina?

The near-vision performance of emmetropic presbyopes can be improved by the monocular surgical implantation of small-aperture corneal inlays or intraocular lenses that contain either an annular or circular stop to increase ocular depth of focus. Ray tracing is used to show that, although different stop designs and positions may produce similar axial imagery and increases in depth of focus, off-axis, the vignetting effects associated with the distance between the stop and the iris aperture result in different field dependences for the pupil transmittance. The implications of these effects, and of implanting a stop in only one eye, are discussed.

Accommodation and age-dependent eye model based on in vivo measurements.

PURPOSE: To develop a flexible model of the average eye that incorporates changes with age and accommodation in all optical parameters, including entrance pupil diameter, under photopic, natural, environmental conditions. METHODS: We collated retrospective in vivo measurements of all optical parameters, including entrance pupil diameter. Ray-tracing was used to calculate the wavefront aberrations of the eye model as a function of age, stimulus vergence and pupil diameter. These aberrations were used to calculate objective refraction using paraxial curvature matching. This was also done for several stimulus positions to calculate the accommodation response/stimulus curve. RESULTS: The model predicts a hyperopic change in distance refraction as the eye ages (+0.22D every 10 years) between 20 and 65 years. The slope of the accommodation response/stimulus curve was 0.72 for a 25 years-old subject, with little change between 20 and 45 years. A trend to a more negative value of primary spherical aberration as the eye accommodates is predicted for all ages (20-50 years). When accommodation is relaxed, a slight increase in primary spherical aberration (0.008µm every 10 years) between 20 and 65 years is predicted, for an age-dependent entrance pupil diameter ranging between 3.58mm (20 years) and 3.05mm (65 years). Results match reasonably well with studies performed in real eyes, except that spherical aberration is systematically slightly negative as compared with the practical data. CONCLUSIONS: The proposed eye model is able to predict changes in objective refraction and accommodation response. It has the potential to be a useful design and testing tool for devices (e.g. intraocular lenses or contact lenses) designed to correct the eye's optical errors.

Optical characteristics of Alvarez variable-power spectacles.

PURPOSE: To establish the optical performance of currently available Alvarez variable-power spectacles in relation to their possible utility for ametropes and presbyopes. METHODS: Two commercial designs of variable-power (variable-focus) spectacles were studied: FocusSpecs and Adlens Adjustables. Using focimetry, power and prism at three different power settings were measured across the aperture of the lenses of each design and local optical quality was explored in more detail using interferometry. RESULTS: In general, equivalent-sphere powers at all lens settings were adequately stable across the central aperture (about 20 mm diameter) of the lenses, with prism changes following the Prentice rule. Some astigmatism (generally ≤0.5 DC over the central area) and higher-order aberration (comparable to the normal wavefront aberration of the eye) were found, the latter increasing towards the edge of the useable lens area. CONCLUSIONS: Within their designed power ranges, optical performance of currently-available Alvarez variable-power spectacles appears to be adequate to correct the distance and near vision of spherical ametropes.

François Pourfour du Petit (1664-1741): pioneer in ocular biometry.

PURPOSE: To outline the important contributions of the French physician Petit to the development of ocular biometry. CONTENT: After a brief review of Petit's life and his studies in neurology, anatomy, and cataract surgery, the methodology and results of his work in measuring many of the biometric parameters of the human eye are discussed. Among other techniques, he made use of frozen sections of eyes to explore their dimensions and employed an immersion technique to avoid the effect of corneal refraction on the appearance of the iris. His pioneering biometric results have been largely confirmed by modern studies. Those on the changes in the crystalline lens with age are particularly striking and suggest that these ocular aging effects have changed little over the last 300 years. Although largely forgotten today, his biometric work exercised a considerable influence on his more immediate successors, including Porterfield and Thomas Young. SUMMARY: François Pourfour du Petit deserves to be remembered as an important contributor to our understanding of the structure and dimensions of the human eye.

Do small-aperture presbyopic corrections influence the visual field?

PURPOSE: To explore the effect of small-aperture optics, designed to aid presbyopes by increasing ocular depth-of-focus, on measurements of the visual field. METHODS: Simple theoretical and ray-tracing models were used to predict the impact of different designs of small-aperture contact lenses or corneal inlays on the proportion of light passing through natural pupils of various diameters as a function of the direction in the visual field. The left eyes of five healthy volunteers were tested using three afocal, hand-painted opaque soft contact lenses (www.davidthomas.com). Two were opaque over a 10 mm diameter but had central clear circular apertures of 1.5 and 3.0 mm in diameter. The third had an annular opaque zone with inner and outer diameters of 1.5 and 4.0 mm, approximately simulating the geometry of the KAMRA inlay (www.acufocus.com). A fourth, clear lens was used for comparison purposes. Visual fields along the horizontal meridian were evaluated up to 50° eccentricity with static automated perimetry (Medmont M700, stimulus Goldmann-size III; www.medmont.com). RESULTS: According to ray-tracing, the two lenses with the circular apertures were expected to reduce the relative transmittance of the pupil to zero at specific field angles (around 60° for the conditions of the experimental measurements). In contrast, the annular stop had no effect on the absolute field but relative transmittance was reduced over the central area of the field, the exact effects depending upon the natural pupil diameter. Experimental results broadly agreed with these theoretical expectations. With the 1.5 and 3.0 mm pupils, only minor losses in sensitivity (around 2 dB) in comparison with the clear-lens case occurred across the central 10° radius of field. Beyond this angle, sensitivity losses increased, to reach about 7 dB at the edge of the measured field (50°). The field results with the annular stop showed at most only a slight loss in sensitivity (≤3 dB) across the measured field. CONCLUSION: The present theoretical and experimental results support earlier clinical findings that KAMRA-type annular stops, unlike circular artificial pupils, have only minor effects on measurements of the visual field.