It Doesn't Last Forever
Eventually it gets everyone. That point in a patient's life comes when no matter how perfect their vision may have been as a young adult, they begin to notice changes. The newspaper becomes more difficult to focus or the fine print on the restaurant menu isn't quite as sharp. They will have to face it, they are becoming a presbyope. When it comes time to visit their vision specialist, you will tell them that they require a variable focus form of correction because their prescription requirements for distance vision are divergent from those required for reading. Typically, this would mean bifocals or even trifocals depending on the specifics of your patient's needs. The first time you recommend bifocals, you can almost hear what they are thinking "Oh my, now I am getting old!" The cosmetic issues associated with bifocals or trifocals are more profound than any other spectacle aesthetics issue. These lenses, with their seg lines, are deeply associated with age. There must be a way to correct the vision of these patients without advertising to everyone their special needs.
Don't Hand Me Any Lines
Progressive Addition Lenses (PALs), in their various styles, address the need for variable vision correction by employing a form of aspheric lens design. By combining curves, they blend differing optical zones together to provide specific dioptric corrections in specific areas of the lens, which correspond to the types of subject matter the eye will be acquiring through that area. For example, the traditional role of a bifocal segment is reading. The PAL possesses a progressive corridor 10-20 mm in length that runs vertically down the optical center of the lens. The bottom portion of this corridor is the reading power of the lens. The top of the corridor and above holds the distance and intermediate vision zones of the lens. The corridor is designed to gradually shift the eye from one power to the next over the length of the corridor as the eye would normally move from looking ahead in traffic to glancing down at the instrument panel of the car. The longer the corridor, the more gradual the transition. There are also specific refractions for the lens periphery for side-to-side or peripheral vision corrections. This is all accomplished without the obvious seg lines dividing the various zones.
Read Between the Lines
The only real downside to PALs is that these various optical zones have to meet somewhere and resolving their differing powers has been subject to much scrutiny. In all cases, PALs possess a degree of astigmatic error down either side of the progressive corridor. There are many available brands and types of PALs, each an effort to address the need to maximize the useable area of the lens and diminish the innate afocal areas which cannot be used. The distortions found between these zones play a significant role in the ability of the average patient to adapt to the routine use of PALs.
Adaptation - More than Just a Lens Issue
The ability of a patient to adapt to the use of PALs encompasses many issues above and beyond the particular style of the PAL prescribed. One of the most key issues is patient selection. Most patients adapt very well, but PALs aren't necessarily for every presbyope who walks through the door. PALs are not recommended to patients who are satisfied with their current multifocal lenses and have no interest in cosmetics, or those who have a history of poor adaptability. Most PALs are not available for add powers greater than 3.5D, as this patient segment is often not appropriate. The best candidates for use of PALs are new presbyopes who are confronting their first run at multifocal lenses. These patients are usually interested in avoiding seg lines and are often quite motivated. Additionally, due to the minimal add power requirements of a new presbyope, distortion is minimal and adaptation in fairly easy. Once adapted, increased add powers down the road are more readily accepted. Probably the most important factor in successfully fitting a PAL is ensuring that the patient had realistic expectations about their vision with this new lens style. Knowing the advantages and disadvantages up-front will go a long way toward heightened motivation and ease of adaptation.
Article provided courtesy of Optical Services International