prescribing for presbyopia
Distance- vs. Near-Centered Adds
BY DAVID W. HANSEN, OD
Sunshine, bright skies, light trade winds, minimal dust and small pupils! Wouldn't it be nice to practice where bifocal contact lenses were always successful and pupil size wasn't an important selection factor in your decision?
Environment and heritage have a huge impact on our lives. Living in the Midwest with patients who have origins from Western Europe and Scandinavia, I sometimes fantasize about fitting bifocal contact lenses on the ideal candidate. Since we can't change the environment or our patients' legacy, we must carefully select bifocal designs that give the optimal prescription with less than ideal conditions.
Contact lens manufacturers and clinicians have pondered where best to place reading and intermediate prescriptions. Traditionally, translating lenses have been designed with the distance power in the superior portion of the lens and the near vision segment towards the bottom.
Simultaneous (including aspheric) strategies generally place the distance viewing area in the center of the lens. These lenses are made by a variety of RGP labs. The anterior and posterior surfaces can be manufactured with spherical or aspheric curves to obtain the prescription changes from the distance to the near powers.
The third category includes variations of these two designs, including multiple rings with different prescriptions for different viewing areas. By changing the size of zones, the visual acuity quality of the different viewing distances can be emphasized.
Distance-centered adds. Concentric (or annular) designs have been tried with either the distance or near prescriptions in the center, or visa versa in the peripheral areas of the contact lens. Manufacturing the distance power in the center allows these lenses to translate for optimal tear exchange. When these designs don't move, corneal physiology may be compromised. If they move excessively, decreased visual acuity or ghosting due to the interference with multiple prescription zones will be problematic.
Aspheric options, either RGP or soft, require the distance power in the center. Clinically, aspheric designs have provided acceptable visual acuity. Woods et al studied the clinical performance of back surface multifocal RGP lenses. Patients rated distance visual acuity performance 83 percent, with near visual acuity at 73 percent. Some 86 percent of the subjects chose this design as their preference over their previous design.
Near-centered adds. Near centered adds reduce the frustration of soft lenses which don't traditionally move to gain translation into the reading zone. By changing the near center zone size (taking into account the pupil size), the practitioner can fit the soft lens patient without worrying about lens movement needed for the translation.
Combination adds. The soft lens industry has recently utilized variations of distance- and near-centered adds. These lenses are distance centered on one eye and near centered on the other eye with rings of intermediate areas.
Which Lenses to Use
You have many options when fitting bifocal contact lenses, but analyzing the pupil size is very critical for success. Not to oversimplify bifocal fitting, but I have found that near-centered adds work better with large pupils (> 5mm) and distance-centered adds are advantageous for smaller pupils (<5mm). If pupil size is not taken into consideration, the selection process may become complicated.
Bright lights, small pupils, and distance-centered designs will assist with multifocal success.
Dr. Hansen, a diplomate and fellow of the American Academy of Optometry, is in private practice in Des Moines, Iowa.