prescribing for presbyopia
Today's Soft Bifocals: A New Level of Sophistication
BY DAVID W. HANSEN, OD
November 1998
For years, the contact lens industry has proclaimed a 25 percent success rate for soft multifocal contact lenses. Designed to present a simultaneous image, these lenses have traditionally created a ghosting or doubling effect with reduced visual acuity. Pressures from the presbyopic demands of the growing baby boomer generation and from practitioners who have been frustrated with past soft multifocal experiences have fostered the development of more sophisticated designs and improved materials. Today's soft multifocal patients also have the convenience of frequent replacement.
Zoning Out to Improve Vision
Whether a soft or RGP multifocal lens design, adequate translation, or movement, of the lens is important to achieve visual acuity in different ranges of focus. Since most soft lenses don't translate excessively, zone sizes and positions substitute for the translation process. Traditionally, bifocal lenses are either near center or distance center designs. Changing the sizes of these zones accentuates the near or distance visual acuity. Manufacturers have provided us with both options.
The key to fitting multifocal lenses is determining which area of vision is most important to the patient -- near, distance or intermediate. Pupillary dynamics and illumination considerations will turn the lens selection process from a scientific forum to a trial-and-error office visit. Designing a good lens depends on acquiring good diagnostic information. It's often advisable to send patients home with the lenses so they can adapt to the unique vision in the real world environment.
Distance Center Zone
Most soft multifocals use the distance center philosophy, manufactured with either an anterior or posterior aspheric surface to generate the change in prescription from the geometric center of the lens to the edge.
Some lenses emphasize correcting the distance power to provide maximum distance acuity; others enhance the intermediate and near zones. For example, the PSD lens from Blanchard Contact Lens corrects the dominant eye for distance, while the company's PS design uses progressive power for the intermediate and near ranges. The new UltraVue lens from Acuity One, LLC, a similar system, includes the P lens, made in 2.3mm, 2.6mm and 2.9mm sizes for the dominant eye, and the C lens, with a near central zone of 1.7mm, 2.0mm and 2.3mm sizes to emphasize the near vision. UltraVue employs a "20/20 and 20/40 philosophy," which describes expectations for distance and near visual acuities. Is this monovision? Not really, since you're using bifocal lens designs to cover most usable areas of focus.
Aspheric designs may produce distance blur if the pupil is too large (>5.0mm). You may need to use either Lifestyle Xtra (The Lifestyle Co.) or Specialty Progressive (Specialty Ultravision Inc.) lenses, which have a distance zone, surrounded by a near zone, then an intermediate zone, then another distance zone. These lenses have adds of +1.00D, +1.50D, +2.00D and +2.50D. The Acuvue Bifocal (Vistakon) is the newest entry in the center distance category.
Near Center Zone
Lenses manufactured with a near center zone include Unilens (Unilens Corp. USA), Horizon 55 Bi-Con (Westcon Contact Lens Co.), Abcon N (World Optics Inc.), and Multifocal Profile A and B (Sunsoft). Sunsoft has also introduced Additions (C, +2.50D add), a planned replacement option. No adjustments to spectacle sphere power are required for the Additions, other than the typical vertex distance. CLS
Dr. Hansen, a diplomate and fellow of the American Academy of Optometry, is in private practice in Des Moines, Iowa.