PRESCRIBING FOR PRESBYOPIA
USING PUPIL SIZE IN MULTIFOCAL CONTACT LENS PRESCRIBING
BROOKE MESSER, OD
The assessment of pupil size during a multifocal contact lens fitting has historically been a key piece of data used in selecting a presbyopic lens design. It’s now become possible to successfully fit many patients without considering their pupil size as multifocal technologies continue to improve. Today, we may evaluate pupil size for troubleshooting purposes. What do we know about factors that can affect pupil sizes, and how can we use this information to more efficiently prescribe today’s presbyopic lens designs?
Key Players: Age and Luminance
The fact that pupil sizes get smaller with age and increased light levels is well documented (Cardona and Lopez, 2016; Guillon et al, 2016). The most marked difference in pupil size between presbyopes and non-presbyopes occurs in scotopic conditions (Guillon et al, 2016). We need to consider this in our multifocal lens prescribing, especially with mature presbyopes, given that there is less dilation of the pupil in our presbyopic population in any given lighting condition. A smaller pupil size limits the area of the optical zone that a patient can utilize for near and distance optics. But fear not—an increased depth of focus with smaller pupils can help maintain visual quality and tolerance to blur (Cardona and Lopez, 2016).
Consider using a near-centered lens design for patients who have small pupils; they will fully utilize and appreciate the lens’ add power when working at near, and they will experience improved distance vision when their pupil dilates slightly when viewing distant objects (Davis, 2016). The change from near to distance power will occur rapidly over the lens surface, allowing smaller pupils to utilize the full range of lens optics through a smaller aperture. The rate at which power changes across an optical zone is determined by eccentricity; higher eccentricities have greater demands for proper lens centration over the visual axis (Davis and Eiden, 2010). If lens centration is poor, it will be difficult to make reliable changes to the lens power. A change to the base curve or diameter is needed to improve centration and the potential for a successful outcome. A successful multifocal fit places the various zones within reachable position of the pupil (Davis and Eiden, 2010).
Lenses that have lower add powers are designed with a slower, more gradual change from distance to near power. The distribution of power across the lens surface is more proportional to pupil size so that vision is balanced and not overtaken by distance or near optics. For presbyopes requiring a lower add power, a few quick questions can assist you in lens selection. Assessing their daily activities and typical lighting conditions are important additional factors. For those more focused on their distance activities, such as driving or golf, consider a distance-center multifocal, and vice-versa for patients focused on near tasks, such as sewing and reading.
Supporting Cast: Refractive Error and Working Distance
In addition to lighting levels and age, refractive error can also contribute to pupil size. In general, myopes have larger pupils compared to hyperopes and emmetropes of similar age (Guillon et al, 2016). Therefore, think about using higher-add multifocal lenses for hyperopes and emmetropes to ensure that they have access to the full range of lens powers within their pupil diameter.
Working distance can also affect pupil size. Cardona and Lopez (2016) suggest measuring working distance of specific tasks, as well as the illumination in which those tasks are performed, rather than prescribing based on standard near measurements performed in-office.
Finally, keep in mind that testing done in the exam room is often done in better lighting compared to where our patients perform their day-to-day tasks. Ask patients about their workspace environment, then simulate those conditions in the exam room for accurate measurements and lens performance. CLS
For references, please visit www.clspectrum.com/references and click on document #252.
Dr. Messer practices in Minneapolis in a private optometry office focused on specialty contact lenses. She is a Fellow of the American Academy of Optometry and of the Scleral Lens Education Society. She is also a consultant to Precilens and CooperVision, has received research funding from B+L, and has received honoraria from Alden Optical.