prescribing for presbyopia
Location, Location, Location = RGP Bifocal Success
BY DAVID W. HANSEN, OD, FAAO
The real estate profession has known it for years. The success of commercial businesses are enhanced by the physical location, and one of the top priorities of most homebuyers is the location. The success of fitting RGP bifocals and multifocals is also dependent on the position of the lens on the cornea. I now believe after years of prescribing different bifocal designs, the most important factor is location of the lens. Location of the lens for proper distance viewing, the location of the lens for intermediate acuity, and the location of the lens for near seeing.
Factors Influencing Centration
Corneal size and shape, eyelid interaction, the lacrimal lake and the contact lens design are major factors influencing the final success of this modality. This is the reason why the success of empirical bifocal fitting is compromised. Diagnostic trial lenses will aid the fitting success.
Corneas need to be measured (with corneal topography, I hope) for shape of the cornea, position of the highest peak, unusual flat or steep areas, potential for diseases, and areas of dryness, scars, infiltrates, dystrophies and degenerations. Peripheral corneal and conjunctival topography abnormalities can alter the currents of the tears. A bifocal contact lens will move differently when any or all of the above factors are outside the normal configuration. Modifying the contact lens or selecting the appropriate design for these characteristics will assist the practitioner.
Eyelid interaction significantly influences on the position of the lens and changes with aging. It is imperative to measure and record the position of the eyelids with relationship to the center of the pupil and monitor the effect caused by corneal change, systemic conditions, medications, former contact lens "after effects," and eyelid tonicity and ptosis conditions. Blepharoplasty and surgical intervention may be needed to complete the successful bifocal fit.
Good lacrimal volume and tear physiology consistency, free of dryness, are essential for good lens translation. Assess tear volume and tear break-up time.
Figure 1. This aspheric multifocal lens decenters nasally.
Positioning Bifocal Designs
All RGP bifocals and multifocals must translate, but they also must center. If the patient was formally fit with a high-riding single vision design with superior lid attachment and you see superior corneal molding, you may need to steepen the base curve to drop the lens into central alignment. Usually steepening the lens 0.50D to 0.75D will center the lens. Also, try reducing the diameter 0.3mm to 0.5mm. If a translating design still rides superiorly with eyelid capture, you may need to double truncate the lens to divorce it from the lids.
If the former single vision RGP lens positioned inferiorly causing inferior corneal surface change, the lens will need modifications to raise it to facilitate viewing through the distance portion. Flattening the base curve with a larger diameter or steepening the base curve may help.
If the lens doesn't center, the patient will complain of ghosting, blurriness, time lag with far-to-near focusing, poor edge comfort or hazy vision. These symptoms are generally the result of poor lens positioning or pupillary interference with the different optical zones. Minimize the compli- cations with precision fitting.
Dr. Hansen, a diplomate and fellow of the American Academy of Optometry, is in private practice in Des Moines, Iowa.