Oct
2004

Lens Choices

By Robert J. Lee, OD

A patient in her mid-sixties complains her glasses don’t provide the vision she needs anymore. As you continue to probe her past visual history she says her near vision has been getting progressively worse; she has difficulty reading the newspaper and mail. She takes a multi-vitamin but no other OTC or prescription medications. Despite a family history of macular degeneration, she has not had an eye examination in several years. Her present glasses are self-prescribed +2.50D reading glasses providing 20/200 near acuity OD and OS at a testing distance of 40cm.

How can you determine if this prescription meets the patient’s needs? What should the next steps be?
Asking these basic questions is the first step toward assessing the needs of the low vision patient. The eyecare practitioner (ECP) should then conduct a thorough low vision examination that includes taking a comprehensive case history, distance and near needs analysis, systemic and visual/ocular health evaluation, and finally a treatment plan including optical and non-optical devices and training and appropriate referral when indicated.

Now is a good time for ECPs to develop expertise with low vision patients. Over the next decade, the market for low vision products and services is expected to grow significantly. As more Baby Boomers enter their sixties, many of them will need this specialized form of vision correction.

Case History
To determine the best prescription for a patient, the eyecare practitioner (ECP) needs to take a detailed case history. Begin by reviewing the patient’s chief complaint and list their activities of daily living (ADL). These should include the type of work they may do, hobbies, sports and social needs.

Using this list as a guide, ask the patient what goals they would like to attain if their vision was better; i.e., “What are the one or two things you miss most due to your decreased vision?”

Be sure to ask the patient if they have ever been prescribed a low vision device. If so, find out how well the device worked and if the patient is still using it.

It’s important to also ask lifestyle-related questions.

Reciprocal of Vision
Because the above patient’s goals are to read the newspaper and mail, the ECP’s first task is to arrive at a starting near prescription. That’s where the reciprocal of vision (RV) formula comes in. The (RV) formula assists in determining the amount of plus power needed to achieve the patient’s visual acuity goal.

The formula works like this. After a careful refraction, take the denominator of the best corrected visual acuity and divide it by the denominator of the goal acuity. (Newspaper print has a demand of approximately 20/40.)

Best corrected visual acuity at 40cm.
 = 20/200 thus, 200/40 = 5
 Goal visual acuity = 20/40

In order for the patient to achieve their goal acuity of 20/40, they will need to bring their reading material five times closer than the reference distance of 40cm, or 8cm. Recall that the accommodative demand at 40cm is 2.50 diopters. Since your new working distance is now five times closer or 8cm, the accommodative demand also is multiplied by a factor of five. Thus, this patient needs +12.50 diopters in order to clearly focus on reading material at 8cm.

Lens Options
There are several options besides conventional glasses that may be considered. Let’s start with the least invasive (most natural) option first. Since the patient is accustomed to wearing glasses this would be a good first choice.

High Add Powers
We tend to prescribe bifocal adds of +2.50 diopters on a daily basis to our absolute presbyopic patients (absolute meaning the patient has zero accommodative ability left to contribute for reading). Add powers of four, six and eight diopters are not uncommon for the patient with low vision. Here are some points to keep in mind when prescribing these higher adds:
• The turnaround time is usually longer to process these glasses. Patient education and expectations are important.
• These prescriptions cost more. Adjust your fees accordingly.
• As the add power increases, the flat-top ledge protrudes from the carrier lens more. To minimize glare off the ledge, a light tint is often helpful.
• Round 28 bifocal segments are offered in add powers up to 10 diopters. These may be cosmetically more acceptable to patients because they are less conspicuous compared to flat tops.

Prism Half Eyes
Prism half eyes are a viable option for those patients who do not require a distance prescription. In our above example the lens power required would be +12.50 diopters. Close working distances create high convergence demands.
Ready-made prism readers incorporate base-in prism (s) to minimize the convergence demand. Typically, the amount of prism is two diopters greater than the lens power i.e., +8.00 D, 10s; 12.00 D, 14s.

Patient education tips:
• As with high-add bifocals, a closer working distance is necessary. Unless the patient has had a high-add bifocal before, they may not be accustomed to holding their reading material at such a reduced distance.
• The base-in prism creates a very thick nasal edge to the lenses. Due to the thickness, adjustable nose pads may be limited on how wide the pads can be adjusted.
• As the lens power increases, the prism becomes cosmetically unappealing. It is recommended to show the patient a sample prism half eye before proceeding with the order.

Full Field Near Glasses
This lens design is not unlike full field reading glasses that you routinely prescribe for your patients. However the lens power may be higher, which has its drawbacks optically and cosmetically. Treat these lenses like any other high-plus prescription with goals of reducing weight and center thickness.

Prescribing tips:
• Limit the frame selection to round or oval frame shapes. Frames with sharp angles or aviator shapes will accentuate lens thickness and weight.
• If at all possible, attempt to have the frame PD equal to the patient PD to minimize lens decentration. 
• Lens materials such as high-index plastic, polycarbonate or Trivex are good choices. Trivex has the low specific gravity (light weight), an Abbe value approaching standard hard resin (good optics) and an impact resistance comparable to polycarbonate. 
• Aspheric lens designs due to their flatter peripheral curves contribute to reducing lens thickness. The flatter curves allow the lenses to be fit with a reduced vertex distance minimizing the unwanted magnification of the patient’s eyes.
• All high-index lenses transmit less light to the wearer’s eyes due to greater reflections off the lens surfaces. A multilayer anti-reflective coating can increase light transmission dramatically exceeding uncoated hard resin.

Hand Held Magnifiers
Hand held magnifiers can be used two ways. The simplest application is to look through the distance portion of the glasses while holding the magnifier above the reading material at its focal length. Select the power of the hand magnifier based on the Reciprocal of Vision times 2.5. The second application is to use the magnifier in conjunction with the spectacle add. The power of the magnifier and add combination is variable based on how far the magnifier is held from the spectacle add.

Prescribing tips:
• These are typically prescribed for short-term spotting needs such as viewing price tags while shopping, determining paper money denominations or reading short notes.
• These devices are contraindicated for tasks where both hands are needed or if the patient has difficulty with fine motor control (hand tremors). Glasses may be better in these situations.
• Hand held magnifiers require constant scanning over the reading material, which can be tiring for long-term use.

Stand Magnifiers
These devices are more difficult to prescribe compared to hand magnifiers because two factors are needed: The image location provided by the magnifier, which tells you what spectacle add powers can be used with the magnifier; the image magnification, which involves spectacle add power, image magnification and print size variables.

Prescribing tips:
• Long term scanning is easier compared to hand held magnifiers because the stand magnifier’s base is placed on the reading material. 
• Illuminated magnifiers are available for those with increased light level requirements.
• Due to their increased bulk and weight, these are perhaps better indicated for home or office use. Consider a hand held magnifier for portability.

Microscopes
Microscopes require the shortest working distance of all devices with similar magnification. Never the less, microscopes provide the largest field of view especially when prescribed as a single-vision full field or doublet microscope. When mobility is necessary, a bifocal form of microscope can be used, however, the field of view will be diminished. A smaller bifocal segment assists in mobility but also compromises the field of view.

Prescribing tips:
Prescribe the least amount of magnification to achieve the reading goal.
• Full field single vision provides the largest field of view but precludes mobility
• Bifocal or half eye designs assist with mobility but some field of view loss will be anticipated.

Knowing how to prescribe low vision lenses and other devices is a valuable skill for any eyecare practitioner, whether they specialize in low vision or incorporate it into a general eyecare practice. By understanding the basic features and benefits of these products, ECPs will be able to service a growing segment of the aging vision correction population while building their practice.

Robert J. Lee, OD is an assistant professor at the Southern California College of Optometry.

 

|