prescribing for presbyopia
The Importance of Pupil Size in Presbyopic Fitting Success
BY CRAIG W. NORMAN, FCLSA
Many measurements are important when analyzing a patient as a prospective presbyopic contact lens wearer. The patient's refractive error and corneal shape along with upper and lower lid position are all important considerations.
But what about the pupil? How closely do you look at pupil size and dynamics? If we consider its relationship to the lower lid, the pupil could be the most critical fitting factor for presbyopic contact lenses.
Checking Pupil Size
What we want to identify is whether a patient has an average pupil or an extremely large or small pupil.
First, let's look at how to take pupil measurements. One is to simply place near the lid margin a millimeter ruler (with or without hemispherical markings) to estimate size. Another is to use a small vertical beam with the slit lamp while measuring through the reticule or with the beam control. Alternatively, consider using an infrared pupillometer or a topographer to gather very accurate pupil data (Figure 1).
Figure 1. Topographical maps demonstrating how different pupil size measurements can be.
Keep in mind that the measurements should be made under both mesopic and scotopic conditions to simulate daytime and nighttime situations.
Pupil Size and Design Selection
Most soft lens designs use a center-near concept for one or both eyes. Thus, a large pupil may not constrict enough during reading to gain the full effect of the near zone. Conversely, if the pupil is too small, it may not allow enough light to enter around the near zone when viewing distance objects.
Aspheric GP multifocal designs usually have a set posterior surface configuration that accommodates the central distance prescription and rapidly evolves into the near portion. Although some simultaneous vision occurs with these designs, they still must translate to provide optimal vision at near. If a patient's pupil is too small to reach the area of the multifocal add during lens translation, the full add effect won't be achieved. Patients who have large pupils will experience blur at distance and intermediate areas with these designs.
Some GP multifocal manufacturers employ a front-surface design treatment to enhance distance vision while maintaining excellent intermediate and reading vision. It's important to know whether a multifocal lens you use has this design treatment because you may be able to order it smaller or larger for patients who have an unusual pupil size or who describe halo or blur symptoms.
Taking a close look at pupil size will improve your presbyopic fitting success. Combining this with knowledge of zone dimensions in the soft and GP presbyopic designs you use will improve your success even further. 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.