prescribing for presbyopia
Amsterdam's New Concept in RGP Bifocal Design
BY DAVID W. HANSEN, OD
Presbyopia correction is a hot topic in the eye care profession. There is rarely an ophthalmic journal or "vision" public information story that doesn't mention the aging population. Presbyopia is also also universal, so research from other countries is valuable.
While the North American contact lens industry has focused mainly on hydrophillic design, Europe has a long tradition of using rigid materials to design bifocal and multifocal contact lenses. Presbylite, a new bifocal concept from Amsterdam, is being distributed in the United States by Lens Dynamics, Inc.
Presbylite is a translating bifocal design with a "wedge" sector near zone and small "triangle" intermediate zone. A large distance viewing area allows up to 30 degrees of nasal rotation without the diplopia, ghosting or distortion of some translating designs.
The lens is not truncated, but ballasted with 1.5 diopters of prism. This keeps the lens comfortably thin, while providing an alternating translation of the lens from the distance through the intermediate to the bifocal zone.
The lens is made in most powers (-20.00D to +20.00D) with near add powers of +1.50D to +3.00D. The typical starting diameter is 9.3mm, but 9.0mm and 9.6mm lenses are also available. Lens Dynamics, Inc. currently manufactures the lens in Boston ES and Boston EO materials.
Many practitioners have concerns about needing trial lenses for their presbyopic patients. "Here we go again with another trial set" is the slogan of many that have gone down this road before. The European experience has been very successful through trial lens fitting with this lens because it greatly reduces the practitioner's chair time. Loaner trial sets are available from the laboratory. With most specialty lenses, a trial diagnostic lens helps to achieve the maximum fit.
The Presbylite bifocal lens is fit similarly to single vision designs that have an even cornea-to-lens relationship and an alignment fluorescein pattern along the horizontal meridian, but Presbylite utilizes fitting on mean K. Look for peripheral clearance and vertical lens movement without obstruction during the blink cycle. Centration is important, and avoid high riding or superior lid attachment lenses.
Position the near segment at or slightly above the inferior pupil margin in normal room illumination. Use an ophthalmoscope to view the alignment of the segment with the pupil. Nasal rotation of 30 degrees is acceptable, but compensate for temporal rotation in the final lens order. The intermediate sector will not interfere if the near zone is positioned appropriately.
Once the lens is fit with the optimal translation, use a trial frame over-refraction to help determine the final distance and near prescriptions. A trial lens is preferred to a refractor because it simulates a natural head position without near segment interference.
We thank our friends worldwide for inventive bifocal designs that will benefit presbyopes.
Presbylite's new design allows up to 30 degrees of nasal rotation.Dr. Hansen, a diplomate and fellow of the American Academy of Optometry, is in private practice in Des Moines, Iowa.