Prescribing for Presbyopia
Over-Refracting Over Multifocal Contact Lenses
BY THOMAS G. QUINN, OD, MS, FAAO
Changes as small as 0.25D can have a significant impact on multifocal contact lens visual performance (Schafer, 2007; Henry, 2008). While the ultimate goal of any over-refraction (OR) is to determine the ideal multifocal power for a given patient, it is undertaken at different times for different reasons.
Use loose lenses when performing over-refraction procedures over multifocal contact lenses to avoid inducing any of the pupillary and accommodative changes that may come into play behind a phoropter.
Finding the Most Plus Point
The intent of the Most Plus Point OR is to confirm that the lenses have the most plus distance power. Any plus that can be added to the distance power will also help near vision.
Sometimes, even with the most careful spectacle refraction, more plus power can be added to the multifocal contact lens while maintaining good distance vision. There are at least four reasons why this can occur:
1. You over-minused the patient during the spectacle refraction.
2. You forgot to vertex the spectacle power back to the corneal plane.
3. Hyperopes often accept more plus power in a contact lens than what is expected (Paugh et al, 1987).
4. Design characteristics of the multifocal contact lens provide more distance minus power than you would expect from the labeled power based on the vertexed spectacle refraction. Published work utilizing Hartmann-Shack wavefront sensing confirms this in at least some of the newer-generation, front-surface, center-near aspheric soft multifocal designs (Vogt et al, 2011).
Perform the initial over-refraction under binocular conditions, introducing +0.25D or +0.50D in front of both eyes simultaneously.
Finding the Best Balance Point
The Best Balance Point OR can help solve visual complaints that persist after you have confirmed proper distance power using the Most Plus Point OR and have attempted modifications to the lens add power per the manufacturer’s visual enhancement fitting guide.
This troubleshooting OR explores how changes in distance power in each eye impact performance at all distances. Perform it under binocular conditions, but introduce ±0.25D or ±0.50D in front of one eye at a time.
If a patient has a distance vision complaint, begin by introducing minus power in 0.25D steps in front of the dominant eye. If problems persist, attempt adding minus in front of the nondominant eye. If the patient is still unhappy with distance vision, decrease add power, starting with the dominant eye.
If the patient has a near vision complaint, begin by introducing plus power in front of the nondominant eye, moving to the dominant eye if needed. If near symptoms persist, consider modifying add power.
Although visual performance under binocular conditions is what we really care about, monocular acuity can elucidate what each eye is contributing at each distance.
In some cases I find it helpful to perform a distance over-refraction over one eye while holding a +2.00D loose lens over the other eye. This allows for a more direct assessment of how power changes are impacting vision in the tested eye. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #210.
Dr. Quinn is in group practice in Athens, Ohio. He is an advisor to the GP Lens Institute and an area manager for Vision Source. He is an advisor or consultant to Alcon and B+L, has received research funding from Alcon, AMO, Allergan, and B+L, and has received lecture or authorship honoraria from Alcon, B+L, CooperVision, GPLI, SynergEyes, and STAPLE program. You can reach him at email@example.com.