prescribing for presbyopia
Keeping a Lid on GP Lenses
BY CRAIG W. NORMAN, FCLSA
For those who periodically read this column, it should come as no surprise that I believe that GP lenses play an important role in managing presbyopic patients. GPs are my option of choice for previous GP wearers and for those patients who have high visual demand.
Recently, I was lecturing on GP presbyopic designs at one of the optometry schools and a student asked, "If I had to pick one measurement that was critical to success during my evaluation, what would it be?"
For me, the answer is quite simple — upper and lower lid position. Sure, distance and near prescription and tasks are important, as are corneal curvature and HVID. These measurements are a must to be able to accurately calculate the initial GP bifocal or multifocal lens parameters. The lid position, though, will be the primary indicator of whether we should use a multifocal lens design, how it should be fit, and whether a translating lens makes more sense.
I was reminded of the importance of the lid position when I saw an excellent fitting tips sheet by TruForm Optics, Inc. TruForm has graphically presented the possible lid positions (Figure 1) along with a suggestion as to what type of presbyopic lens to consider for the initial design.
Figure 1. A graphic presentation of possible lid positions.
Finding the Correct Lid Position
The rules of thumb regarding lid position are simple. Let's begin with multifocal GPs, which are usually my lens of choice for early-to-moderate presbyopes who have add powers 2.00D.
If the upper lid position drapes over the superior cornea by a millimeter or two, a traditional design multifocal (aspheric or spheric back surface) GP design should be able to tuck underneath this lid in a central to superiorcentral position. This allows the distance optics to be properly positioned and allows for easy translation to intermediate and near vision tasks.
If the upper lid is above the limbus, you can still use a multifocal, but it must be fit steeper than normal for the back surface of the lens to control the positioning properly.
For more mature presbyopes who have add powers 2.00D, a prism-ballasted translating bifocal design is more often my lens of choice. The lids are key here also, but the lower lid position plays the bigger role.
Translating bifocals inherently position downward due to the squeezing of the upper lid against the thinner superior lens area, combined with the gravitational effects of a thicker lens. Therefore, we need the lower lid to accomplish two things for us — one to hold the lens in proper position during distance gaze, the other to keep the lens stable when the eye is moving from distance to near viewing.
Ideally, this lower lid is resting at the inferior limbus. If too high the lid may cause the lens (and seg height) to be pushed upward. If too low, the lens will drop too far and not provide good distance visual acuity while often not translating properly.
Improve GP Fitting Success
So, "keep the lid on it." Look for these ideal lid positions to improve your GP bifocal and multifocal results. CLS
Craig Norman is director of the Contact Lens Section at the South Bend Clinic in South Bend, Indiana. He is a fellow of the Contact Lens Society of America and is an advisor to the GP Lens Institute. He is also a consultant to B&L.