High-plus reading glasses, magnifiers top list of commonly prescribed low vision devices

Issue: March 2005

Reading glasses, magnifiers

Dennis W. Siemsen, OD, MHPE, FAAO: I believe every primary care optometrist should be proficient in prescribing spectacles, high-plus reading glasses, hand magnifiers and stand magnifiers. However, knowing which to prescribe — and when — is the important part. When I lecture around the country, here is how I describe the process:

First, determine the amount of power needed to reach the visual goal. For example, say the patient wants to read the newspaper. Newsprint is about 1 M in size. He or she can already read large print with the +2.50 add. Large print is 2 M in size. Therefore, we must make the newsprint appear two times larger. To do this with a bifocal, we increase the add to +5.00, moving the image from 16 inches (40 cm) to 8 inches (20 cm).

High plus readers, particularly those with base-in prism, can be very useful by helping counteract the effects of increased convergence demand. Take the total amount of add plus the distance prescription, and calculate the total power of the readers. To determine the approximate amount of prism needed, take the power of the add (in this case, +5.00) and add 2 prism diopters base-in (+5.00 + 2 pd BI = 7 pd BI). I find the prism especially useful in diabetics and small central scotomas.

What about hand magnifiers? Using our first example, finding a +5.00 D magnifier (don’t go by the “X” printed on the magnifier) and using it with the existing add, patients will get the same +5.00 effect when they hold the magnifier at 20 cm from the spectacles.

For a stand magnifier, find a magnifier with an actual enlargement ratio (again, not just the “X” marking) of at least 2. When used with the +2.50 add, this will again achieve the +5.00 D equivalent.

Dennis W. Siemsen, OD, MHPE, FAAO [photo]

Dennis W. Siemsen, OD, MHPE, FAAO, can be reached at Low Vision Service, Division of Optometry, Mayo Clinic, 200 First St. SW, Rochester, MN 55905; (507) 284-3726; fax: (507) 284-4612; e-mail: siemsen.dennis@mayo.edu.

High-plus lenses, magnifiers

Joseph Hallak, OD, PhD, FAAO: First, I look to see if there is adequate binocular vision. I will then try the Fonda’s base-in prism readers (+4 to +14). Otherwise, I will work with high-plus lenses and hand magnifiers favoring the better seeing eye. What power do I start with?

I use the so-called Kestenbaum rule. The inverted ratio of the best-corrected visual acuity (BCVA) in Snellen form will give the expected add for near. With this add, the patient should theoretically be able to read 20/50 or 1 M in metric notation (newspaper print).

For example, a BCVA of 20/200 inverted equals 10; with a +10-D lens, the patient should be able to read 20/50 print held at a distance of 10 cm. I do not hesitate to use the Clear Image II lenses (Designs for Vision, Ronkonkoma, N.Y.). They are specially designed for an aplanetic (flat) field from edge to edge. This is particularly helpful if the patient is using eccentric fixation, such as in macular degeneration.

Hand magnifiers are rated according to their magnifying power. The relative magnification is F/4, the power of the lens divided by 4. For example, starting from 20/200 and aiming to read 20/50, one needs 4X magnification or +16 magnifier.

When BCVA is worse than 20/400, it is time to consider the closed-circuit TV (CCTV) for its higher magnification and contrast manipulation.

For distance correction and with BCVA of better than 20/400, I consider simple Galilean telescopes, preferably spectacle mounted (opera glasses type). They are inexpensive and lightweight; the adjustment and centering is more forgiving than their Keplerian counterpart, albeit their magnifying power range is more limited. They can also be used in reverse to allow for field expansion. A 5° concentric field will become 15° with a reverse 3X telescope, but the acuity will also decrease by threefold.

Joseph Hallak, OD, PhD, FAAO, is in private practice in Hicksville, N.Y. He can be reached at 183 Broadway #308, Hicksville, NY 11801; (516) 935-0717; e-mail: drjhallak@aol.com.

Determine patient's specific needs

Alfred A. Rosenbloom Jr., OD, FAAO: I do see a role for the primary care optometrist in low vision rehabilitation. Most important, determine the patient's specific visual goals. Then measure visual acuities accurately with appropriate test charts, as well as the refractive state utilizing modifications in standard techniques.

I recommend using less complex forms of magnification, including high-addition lenses with the appropriate working distances, illuminated hand and stand magnifiers and low-power telescopes. The primary care optometrist should prescribe filters and glare-control lenses as needed and also work with the patient to assess the advantages of increased task lighting.

All optometrists should educate themselves about closed-circuit TVs (video magnifiers), specially designed software and other electronic devices. Also, be knowledgeable about non-optical products, such as talking watches, clocks and calculators; support canes; and check writing and signature guides.

When necessary, the optometrist should refer to a colleague actively practicing low vision care as well as to agencies offering low vision services for people who are blind or visually impaired.

Alfred A. Rosenbloom Jr., OD, FAAO [photo]

Alfred A. Rosenbloom Jr., OD, FAAO, is chair emeritus of the Low Vision Service at the Chicago Lighthouse and a Primary Care Optometry News Editorial Board member. He can be reached at 1850 West Roosevelt Rd., Chicago, IL 60608-1298; (312) 666-1331; fax: (312) 243-8539; e-mail: aarlovis@voyager.net.