Traditional options for magnification include higher adds, high-plus reading glasses, magnifiers, and telescopes. (Magnification is discussed in Chapter 1). Other practical solutions to difficulties include modifying a task, such as by moving closer to distant targets; using larger formats, such as large-print checks; or substituting with audio, such as digital audio books available through libraries or online. There is increasing interest in devices such as e-readers, cell phones, computers and electronic magnification as excellent options for patients with low vision. Other technologies include head-worn devices, devices that read text aloud, detect objects, or assist patients with low vision or blindness by giving audio directions. Implantable devices include the implantable miniature telescope and retinal prosthesis.
Higher adds The simplest intervention for a patient with moderate low vision is to increase the reading add. Ophthalmologists should be strongly encouraged to consider +4.00 adds or simple over-the-counter readers, such as +4.00 readers, as long as patients can learn to maintain the closer focal distance and use supplemental lighting if beneficial. This can be attempted prior to referral for comprehensive vision rehabilitation. Patients will often accept higher adds, such as readers in powers from +6.00 to +12.00 with appropriate base-in prism and the appropriate focal distance. A +10.00 reading add will require a reading distance at the focal point of the lens, which will be 1/10 m (ie, 10 cm, or 4 in). In binocular patients, prism is required to assist convergence and relax accommodation. The recommended prism strength is 2 prism diopters (∆) 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 sphere with 10∆ BI; OS: +8.00 sphere with 10∆ BI. Prism is not required in adds up to +4.00 D.
Readers with prism are available ready-made in powers from +6.00 D to +14.00 D and allow a wide field of view. Monocular aspheric spectacles, available from +6.00 D to +32.00 D, are used less often. Computer glasses with intermediate-strength add powers are useful and can be prescribed as a bifocal with the intermediate power in the upper segment. In general, reading glasses allow hands-free magnification and a large field of vision; however, there is a shortened working distance, and supplemental lighting is required.
Historically an add was calculated as the inverse of the visual acuity (Kestenbaum rule: the inverse of the visual acuity fraction is the add power, in diopters, to read 1 M type—about 8 points, corresponding to the 20/50 line on a standard near-vision card calibrated for use at 14 inches), and this calculation may provide a general starting point; however, it is now appreciated that many other factors influence reading fluency, such as fixation location, scotomas, perceptual span, crowding, and contrast sensitivity. The Kestenbaum rule would estimate that a patient with 20/200 acuity would require 200/20, or 10 D of add. For fluent reading, patients with 20/200 acuity may actually require higher add than calculated.
Magnifiers Handheld and stand magnifiers are available in illuminated or nonilluminated formats. A simple handheld or illuminated stand magnifier in low-to-medium power can allow continuous text reading for patients with mild-to-moderate vision loss. Patients with tremors or difficulty holding a magnifier may find success with a stand magnifier that rests directly on the page (Fig 9-10). Stand magnifiers are set in a frame that sits on the material and keeps the lens at the appropriate distance. Higher-powered magnifiers (higher than 20 D) have a limited field of view and allow one to read short text, but are seldom comfortable for extended reading of continuous print because of the requirement of continuous movement along the line of text. Low-powered magnifiers (+5 to +12 D) with light-emitting-diode illumination are the most common handheld magnifiers (Fig 9-11).
Figures 9-10 An illuminated stand magnifier placed flat against the page provides magnification, illumination, and stability. As with all magnifiers, the field of view decreases with increased magnification.
(Courtesy of Mary Lou Jackson, MD.)
Figure 9-11 Illuminated hand magnifier.
(Courtesy of Mary Lou Jackson, MD.)
The “power” or “magnification factor” or “enlargement ratio” of a magnifier is usually specified in terms of the relative angular size of the magnified image compared with the angular size of the original object at a standard reading distance (see Chapter 1). Most commonly, the reference distance is taken as 25 cm. In general, the maximal magnification will be obtained when the object to be viewed is placed at the anterior focal point of the magnifier. When the magnifier is used this way, the magnification factor is equal to the dioptric power of the lens divided by 4 (the dioptric equivalent of the reference distance of 25 cm). For example, the power of a +24 D magnifier is 6× (24 D/4 D); however, magnifiers are used in different manners and held at different distances. Simple low-power magnifiers (eg, +4 D) are rarely used by holding the object at the anterior focal point of the magnifier, because it is difficult to hold a lens steady so far from the page; therefore, the previous magnification factor convention is no longer appropriate. Magnifiers are often labeled with a “trade magnification” power. Trade magnification is calculated as the (diopter power of the hand magnifier)/4 +1, so the trade magnification of +4 D handheld lens is 2×.
Telescopes There are 2 types of telescopes; astronomical (also called Keplerian) telescopes and Galilean telescopes (optics of telescopes are discussed in Chapter 1). Their features are compared in Table 9-1. Telescopes are much less commonly used than magnifiers, as tasks that require magnification for distance viewing are less-frequent goals than those for near viewing. Handheld monoculars, binoculars, and spectacle-mounted telescopes allow the benefit of magnification at a greater distance, with the drawback of reduction in field of view, a narrow depth of field, and reduced contrast. (Fig 9-12A). The latter is particularly limiting in patients with significant loss of contrast sensitivity. Autofocus telescope models are available. A simple telescopic spectacle without a casing has become popular, as it is lightweight and relatively inexpensive (Fig 9-12B).
Loupes (telemicroscopes) are spectacle-mounted close-focusing telescopes set to focus at near points. They allow a greater working distance than high-add reading glasses; however, as with all telescopes, the visual field is narrow and the depth of field small. Bioptic telescopes are spectacle-mounted telescopes set to focus at distance, mounted in the upper portion of the lenses of carrier spectacles. Many states, and the Netherlands in Europe, allow driving with bioptic telescopes. The telescopic portion of the spectacles is positioned superior to the line of sight and used only briefly to read signs or look into the distance. The rest of the time, the individual drives looking through the regular prescription portion of his or her spectacles. Driving with a bioptic telescope requires prescription of the device as well as device training, driver training and, in some states, on-road evaluation. Patients with good contrast sensitivity and intact central field are optimal candidates for bioptic driving.
Table 9-1 Comparison of Galilean and Astronomical Telescopes
Figures 9-12 Telescopic devices. A, This range of telescopic devices includes monocular and binocular telescopes. B, The spectacle telescope is lightweight and binocular and can be used for stationary distance viewing such as watching television.
(Courtesy of Mary Lou Jackson, MD.)
Electronic devices Electronic devices allow magnification, contrast enhancement, and text-to-speech conversion. Magnification using video magnifiers (video cameras combined with screens; also referred to as CCTVs), computers, or tablets is now used very extensively by patients with low vision (Fig 9-13). Video magnifiers are available in various formats, including handheld versions, desk versions, or devices worn on the head. They allow variable magnification, comfortable reading positions, and enhanced or reversed contrast, features not available with optical magnifiers. Desk and head-mounted video magnifiers can use optical character recognition to read text aloud. Computer accessibility options on Windows and Macintosh computers provide magnification, modified contrast, and audio screen readers. Large monitors and larger-format keyboards assist, and televisions are often used as large monitors. The major difficulties with electronic devices are cost and training requirements; however, in some jurisdictions, and for certain individuals, devices are provided at no or reduced cost. For example, devices are provided by some state societies and are also provided to veterans.
When the accessibility features of the computer operating system are not adequate, additional magnification, screen reading (eg, AI Squared’s ZoomText; see Fig 9-13I), or speech-to-text software can be considered. These require training. Smartphones provide very impressive utility for patients with vision loss as they allow not only magnification, but also text-to-speech and audio interaction for voice dialing phone numbers, searching the internet, and sending voice texts or email. Many additional cell phone applications can identify colors, currency, or objects. New and emerging voice-directed devices accomplish tasks such as setting thermostats or selecting music. Services available using cell phone applications can offer audible directions, object identification, or identification of objects in one’s path. Audio books, most often in DAISY digital format (Digital Accessible Information System) are used widely by patients with vision loss and are available free through the Library of Congress in the US, the Center for Equitable Library Access in Canada, and from online libraries such as Bookshare. These are extensive libraries. The Bookshare accessible library, for example, has more than 500,000 titles that can be accessed via audio, by listening while seeing highlighted text, by using digital braille, or converting to large print or paper braille format. Many applications and devices exist to play audio books.
Nonoptical aids The armamentarium of tools to assist patients with impaired vision extends beyond electronic and optical devices. Simple, practical devices include large-format watches, telephones, remote controls, playing cards, and checks. Talking clocks, scales and timers, bold-lettered computer keyboards, needle threaders, dark-lined writing paper, and felt-tip pens with black ink are a partial list of items that are often useful.
Figure 9-13 Electronic low vision aids. A, Desktop video magnifier. B, Same magnifier as in part A, in high-contrast mode. C, Same magnifier as in A, in reverse-contrast mode. D, Portable handheld video magnifiers. E, Tablet device used as a large-print e-reader. F, Smartphone accessibility features. G, Tablet device used as a video magnifier by placing on a glass desk. H, Standard-format Windows computer screen. I, Same computer screen as in part H, magnified using screen enlargement tools.
(Courtesy of Scott E. Brodie, MD, PhD; Mary Lou Jackson MD.)
Blind rehabilitation and sight substitution devices Patients with no or very limited vision, particularly those who lose vision quickly, will require blind rehabilitation with sight substitutes that may include electronic text-to-speech or braille. Refreshable braille displays can be connected to computers and tablet devices. They have small, moving pins that rise or lower to create braille patterns that can be read tactilely. Short-term residential blind rehabilitation services, available in some areas, can offer great benefit for patients faced with the daunting task of adjusting to sudden and profound loss of vision. Prosthetic retinal implants are being developed, and many groups around the world are working on subretinal, suprachoroidal, or epiretinal devices in addition to cortical visual prostheses that stimulate the brain directly. Currently, devices allow patients to see outlines or contrast.