Does pupil size matter in contact lens fitting?

Is it necessary to measure pupil size when initiating myopia management? Should pupil size be a factor while selecting a strategy, and does it influence efficacy of myopia control treatments? This question was put forth to the Myopia Profile community. Here is the post asking questions about myopia controlling and multifocal (MF) contact lens designs, along with orthokeratology.

There are currently two main types of soft contact lenses are used in myopia control treatment on the market. These are myopia control specific designs (eg. concentric dual focus MiSight and extended depth of focus Mylo), and aspheric multifocal designs which are also used for presbyopia (eg. Biofinity D center and NaturalVue).

The myopia control efficacy of MiSight has been shown to be independent of pupil size.¹ The BLINK study, which investigated medium- and high-add aspheric multifocal contact lenses for myopia control, measured pupil size but data has not yet been reported on any relationship between pupil size and myopia control efficacy. For reference, the average pupil size in the BLINK study was 5.4 ± 0.7mm in mesopic and 6.5 ± 0.7mm in photopic conditions, in multi-ethnic children aged 7-11 years.²

Vision outcomes can also be influenced by pupil size. In both myopia control and multifocal designs, modelling has shown that a larger pupil size will increase the the area of the pupil covered by the varying focus powers - near and far, but smaller pupils may experience better distance vision.³  

Different contact lens designs will influence vision outcomes - two studies of the  MiSight dual focus concentric contact lens have shown better than 6/6 or 20/20 equivalent, on average,^1,4 while the Biofinity centre distance aspheric +2.50 Add has been shown to need a mean -0.62D over-minus correction for best acuity.⁵

We don't yet have the data available to see how pupil size may interact with visual acuity outcomes in children wearing these contact lenses for myopia control.

One small but influential study reported that orthokeratology lens wearers with larger pupil diameters experienced greater myopia control efficacy than those with smaller pupils. They hypothesized that a large pupil allows more myopic defocus shift for the peripheral retina, but did not measure it.⁶

This paper led to the interest in the interaction between the back optic zone diameter (BOZD) of ortho-k lenses, pupil size and myopia control efficacy. In a retrospective study, it was found that when the pupil size was larger than the BOZD, the patient experienced slower axial elongation.⁷ No methodology was provided on how the BOZD was chosen for each patient.

A randomized controlled trial⁸ fit children with either a 5mm or 6mm BOZD, and found that the 5mm BOZD group exhibited only 0.15mm axial growth over two years, compared to 0.35mm in the 6mm BOZD group. Pupil size was measured, but any interaction between pupil size and myopia control was either not present or not reported in this research abstract.

The sum of this data indicates that there is likely a relationship between pupil size, ortho-k lens design and myopia control efficacy, but we don't yet have enough guidance to make changes to clinical practice. An important note is that 5mm BOZD's can influence vision in lower lighting conditions,⁹ which may be important to some children and young adults.

There is still a lot to learn on this topic. In the case of orthokeratology, the presumption that a larger pupil size will increase the relative peripheral myopic shift has been shown incorrect.¹⁰ A larger pupil size will typically result in increased higher-order aberrations of the eye, which have shown a relationship with better myopia control outcomes in ortho-k.¹¹

While smaller treatment zone diameters may increase myopia control efficacy in ortho-k,^7,8 but a new discussion has emerged on whether treatment zone decentration may¹² or may not¹³ also have an influence. Authors in this arena caution that fitting properly centered ortho-k lenses is "recommended to ensure the safety of Ortho-K lens wear and to maintain visual quality."¹³

1. Pupil size may play a role in visual outcomes in myopia control and multifocal soft contact lenses, but it does not impact efficacy with MiSight, and the relationship in center-distance multifocal contact lenses is unknown.
2. A complex relationship between pupil size and back optic zone diameter in orthokeratology may relate to myopia control efficacy, but there is still more to learn.