Kestenbaum rule, which states that the predicted add power, in diopters, to read 1 M type is the inverse of the visual acuity fraction. For example, a patient with 20/200 visual acuity may benefit from a +10 D lens (200/20 = 10), with the material held at the focal point of the lens, 1/10 m (ie, 10 cm, or 4 in). Very often, however, patients require different magnification from that predicted by the Kestenbaum rule (usually even stronger), including patients with poor contrast sensitivity, macular scotomas, or a requirement to read print that is smaller than 1 M (Clinical Example 7-1).
CLINICAL EXAMPLE 7-1
For purposes of vision rehabilitation, it is often convenient to describe the print size of reading materials using M notation. The M size of an optotype is the distance (in meters) at which the sample can be read by a person with normal acuity—thus, “1 M” print is normally legible at 1.0 m.
For example, a standard 20/20 optotype intended for viewing in a full-size refracting lane is approximately 6 M (20 ft ≈ 6 m). In practice, 1 M type is approximately 8 points, corresponding to a 20/50-equivalent letter on a standard 14-inch near card, and the 20/20equivalent optotypes on such a card correspond to 0.4 M. The Kestenbaum rule then follows directly from the definition of the diopter.
Bifocals or reading glasses can be prescribed in strengths greater than +3.00 D as long as the shortened working distance is understood and accepted by the patient. Such add powers are usually well tolerated binocularly up to approximately +4.50 D, and monocularly up to +16.00 D for the better-seeing eye. In binocular patients, prism is required to assist convergence and relax accommodation. The recommended prism strength is 2 prism diopters (Δ) more base-in (BI) than the numerical add power, in each eye. For example, if the distance prescription is plano OU and the appropriate add power for reading is +8.00 D, then the prescription should read as follows: OD: +8.00 sp with 10Δ BI; OS: +8.00 sp with 10Δ BI. Readers with prism are available ready-made in powers from +6.00 D to +14.00 D and allow a wide field of view. Aspheric spectacles, available from +6.00 D to +32.00 D, are suitable for monocular use and require a very short working distance, which may present difficulty in directing light onto the material.
Glasses with intermediate-strength add powers are important for tasks such as using a computer. The intermediate add power for viewing a computer monitor is often prescribed as the upper segment of a bifocal; some patients use a clip-on add lens. Single-vision glasses matched to the distance from the eyes to the screen may be better tolerated, and if the patient works mainly at a single computer workstation, these can be left there for use when needed.
Before reading glasses are prescribed, the patient should demonstrate proficiency with actual print material. In general, reading glasses allow hands-free magnification and a large field of vision; however, there is a shortened working distance, and they cannot be worn when walking or driving.
Nonoptical Aids
The armamentarium of tools to assist patients with impaired vision extends beyond optical devices. Many simple, practical, nonoptical devices are available through specialty stores, catalogs, and websites. These items include large-format watches, telephones, remote controls, playing cards, and
checks. Bold-lettered computer keyboards, needle threaders, dark-lined writing paper, and felt-tip pens with black ink are also useful. Patients should be made aware of, and trained to use, such aids as appropriate. Medicare and other health insurers in the United States cover costs for occupational therapists to train patients in the use of such devices to achieve their individual goals but rarely cover costs for the actual devices.
Some of the more sophisticated devices and techniques available to assist patients are discussed in the following sections.
Electronic Devices
Many of the most exciting solutions for patients with reduced vision are found in new technologies (Fig 7-38). Video cameras combined with screens (video camera magnifiers) are available in many formats. These components allow variable magnification, comfortable reading positions, and enhanced or reversed contrast—features not available with optical magnifiers. Computer accessibility options on both Windows and Macintosh computers provide magnification, modified contrast, and audio screen readers.
Figure 7-38 Electronic low vision aids. A, Desktop-model closed-circuit video magnifier. B, Same magnifier as in A, in high-contrast mode. C, Same magnifier as in A, in reverse-contrast mode. D, Handheld closed-circuit video magnifier. E, Tablet device used as a large-print e-reader. F, Smartphone used as a video magnifier. G, Tablet device used as a video magnifier. H, Standard-format Windows computer screen. I, Same computer screen as in H, magnified using screen
enlargement tools. (Courtesy of Scott E. Brodie, MD, PhD.)
A large-screen monitor is helpful in many cases. Screen-enlargement software (eg, AI Squared’s ZoomText; see Fig 7-38H, I) provides sophisticated utilities for enlarging screen text and graphics and for integrating text-to-speech capabilities. In addition, smartphones, e-readers, and audio books have made reading possible for many patients with vision loss or even blindness. Cell phones are very accessible, and global positioning system (GPS) technology increasingly facilitates navigation for severely visually impaired individuals. Refreshable Braille displays are available for computers and tablet devices. They have small moving pins that rise up or down to create Braille patterns that can be read tactilely. These displays are connected to a computer or other device with a USB or serial cable.
Lighting, Glare Control, and Contrast Enhancement
Proper lighting is important for patients with reduced visual function, especially when contrast sensitivity is reduced. It is important to direct light onto the task, for example, with a gooseneck lamp or a head-worn light. Optimal types of lighting and positions of lights should be determined to minimize glare from light shining directly into the patient’s face or reflecting off the page. Video magnifiers are important options for patients with decreased contrast sensitivity. Patients who experience glare from light reflected off the page may benefit from the contrast reversal feature available in many video magnifiers (Fig 7-38C). Tinted lenses may also enhance contrast for some patients.
Nonvisual Assistance
As visual loss becomes more profound, vision enhancement may become less effective, and the importance of “visual substitution skills,” such as tactile and auditory aids, increases. The following are some examples:
Tactile aids. These range from raised dots on a kitchen appliance control knob to the use of Braille for reading.
Refreshable Braille displays. As mentioned, these devices are available to connect to computers or portable electronic devices and create readable Braille output from electronic text.
Auditory aids. Talking watches, audio output on computers and e-readers, audio books, and audio newspaper services are included in this category. In addition, voice recognition software is readily available for computers, and screen readers are built into computer operating systems, as well as being available separately. Optical character recognition (OCR) allows any printed text to be converted to audio format, and both desktop and handheld stand-alone devices are available that make use of this technology.
Orientation and mobility training. This type of training offers instruction in using remaining visual cues, telescopes, white canes, and GPS devices for safe and independent ambulation.
Eccentric Viewing or Fixation Training
Eccentric fixation training can help patients improve eye–hand coordination, tracking, and scanning.