Do multi-zone myopia control lenses maintain their defocus profiles at all viewing distances?

Paper title: Modelling the refractive and imaging impact of multi-zone lenses utilised for myopia control in children's eyes

Authors: Raman Prasad Sah (1), Matt Jaskulski (1), Pete S Kollbaum (1)

1. School of Optometry, Indiana University, Bloomington, Indiana, USA

Date: May 2022

Reference: Sah RP, Jaskulski M, Kollbaum PS. Modelling the refractive and imaging impact of multi-zone lenses utilised for myopia control in children's eyes. Ophthalmic Physiol Opt. 2022 May;42(3):571-585. [Link to open access paper]

The majority of light entering an aberrated eye remains defocused, despite entering the eye via the pupil.  Myopic defocus has been found to be a 'stop' signal for axial growth in young eyes, with hyperopic defocus being a 'grow' signal.¹ Multi-zone spectacle and contact lenses fitted for children are intended to present myopic defocus in the retinal image at all distances regardless of the accommodative state.²

This study developed an optical model of the accommodating child's eye to investigate the effect of accommodation, pupil size and target distance on the quality of the retinal image when fitted with multi zone spectacle and contact lenses. The models were based on laboratory data from 16 myopes aged 8 to 13yrs.

The wavefronts of 4 multi-zone lenses (dual focus designs MiSight 1 day and DIMS spectacle lenses, Biofinity centre-distance multifocal and extended depth of focus design NaturalVue) were measured ex-vivo using an aberrometer.  These were then included in "eye+lens" wavefront modelling by adding them to the eye-only model wavefront in the pupil plane.  The optical characteristics of the models alone and with the addition of the multi zone lens wavefronts were measured, with pupil sizes, aberrations and accommodative lags adjusted for viewing a 0.0 logMAR (6/6) letter at 400cm and 16pt type letters at near distances of 100, 33 and 20cm.  These gave target viewing vergences of -0.25, -1.00, -3.00 and -5.00 respectively.

Simulated images were created with paraxial and marginal focused light and allowed for -0.50D accommodation being present for near viewing.  Spherical aberration (SA) was predicted by a mixed quadratic model for average pupil sizes ranging from 2.81 to 3.48mm.

• The modelling of spherical aberration and pupil radius showed a wide spread of defocus for near distances in the model eye, particularly at 20cm.
• MiSight 1-day (CooperVision) was shown to provide myopic defocus at all viewing distances. At the 20cm viewing distance, the effective power within the treatment zone was lessened by the negative spherical aberration (SA). Defocused light reduced image contrast, but even with accommodative lag present, an increased image size at near helped maintain letter clarity for the distance and treatment zones.
• Biofinity centre-distance multifocal contact lens (CooperVision) has a single distance zone centrally, surrounded by a near Add zone. This lens gave the maximum myopic defocus at 400cm although pupil miosis and negative SA reduced the maximum myopic defocus possible at 33 and 20cm near distances. If the Add zone was used to focus at those distances instead, the defocus became hyperopic.
• DIMS spectacle lens (MiYOSMART from Hoya) has a central distance zone surrounded by an annulus which includes multiple positive-powered lenslets. Myopic defocus was found at all viewing distances and the outcome of viewing through the central zone, regardless of accommodative lag, was comparable to the model eye alone.  The image quality at 33 and 20cm was shown to be sufficient to view the 16pt type letters at near despite low contrast arising from the lenslets, even if the peripheral optics covered half the pupil.
• NaturalVue multifocal 1 day (Visioneering Technologies) has a power profile of highly aspheric optics across the central 3mm zone. Defocus was found to be mostly hyperopic at all viewing distances and to have widest distribution at greater viewing distances (400, 100 and 33cm).  At 20cm, the positive SA generated from the lens design was counteracted by the high negative SA of the model eye and there was little myopic defocus at 20cm. However, the 16pt type letter was legible, even with 0.50D lag.

Multi-zone lenses are able to provide myopic defocus at far distances. However, for very near viewing distances, pupil miosis and increased negative SA for near viewing may reduce the full potential of myopic defocus for each design, if the eye was viewing through a distance portion.

• Children should be discouraged from looking at very close working distances, e.g., 20cm with their myopia treatment lens. This should ensure that myopic defocus is still provided by the lens, even if there is an accommodative lag.
• This also aligns with results from previous studies which show that children should avoid long periods of close work to help avoid developing myopia or higher myopia^4,5

• This study has used simulated retinal images to investigate how multi-zone lenses perform at different distances when used for myopia control. Using a modelled eye allowed control of variables when comparing the lens performance. However, because pupil miosis, accommodative lag and SA will vary between individuals, this could mean these results are not directly applicable to every child.
• Walline et al found that higher add powers in multi zone lenses had more effect on myopia progression that medium powered lenses.⁶ The extent of the defocus may be more relevant for controlling myopic progression then the proportion of defocus.

Title: Modelling the refractive and imaging impact of multi-zone lenses utilised for myopia control in children's eyes

Authors: Raman Prasad Sah, Matt Jaskulski, Pete S Kollbaum

Purpose: To develop an optical model of a child's eye to reveal the impact of target distance and accommodative behaviour on retinal image quality when fitted with multi-zone lenses.

Methods: Pupil size, aberration levels and accommodative lag were adjusted for models viewing stimuli at 400, 100, 33 and 20 cm. Distributions of defocus across the pupil and simulated retinal images were obtained. An equivalent 16-point letter was imaged at near viewing distances, while a 0.00 logMAR (6/6) letter was imaged at 400 cm. Multi-zone lenses included those clinically utilised for myopia control (e.g., dual-focus, multi-segmented and aspherical optics).

Results: Viewing distance adjustments to model spherical aberration (SA) and pupil radius resulted in a model eye with wider defocus distributions at closer viewing distances, especially at 20 cm. The increasing negative SA at near reduced the effective add power of dual-focus lenses, reducing the amount of myopic defocus introduced by the centre-distance, 2-zone design. The negative SA at near largely compensated for the high positive SA introduced by the aspheric lens, removing most myopic defocus when viewing at near. A 0.50 D accommodative lag had little impact on the legibility of typical text (16-point) at the closer viewing distances.

Conclusions: All four multi-zone lenses successfully generated myopic defocus at greater viewing distances, but two failed to introduce significant amounts of myopic defocus at the nearest viewing distance due to the combined effects of pupil miosis and negative SA. Typical 16-point type is easily legible at near even in presence of the multi-zone optics of lenses utilised for myopia control and accommodative lag.

[Link to open access paper]