by Paul Chous, MA, OD
"Low Vision" refers to persons who have sub-normal vision as a consequence of congenital or acquired eye disease, as well as to the eye care and rehabilitative specialty that helps those individuals maximize whatever remaining vision potential they may have. The causes of low vision as a clinical entity are varied and numerous; the leading causes in the relatively affluent Western World are age-related macular degeneration, followed by glaucoma, followed by diabetic retinopathy.
Diabetic retinopathy is the leading cause of new blindness in Americans under age 74, with 12,000 to 24,000 cases of new blindness annually. If all manifestations of diabetic eye disease are considered (retinopathy, cataract, glaucoma, ischemic optic neuropathy and retinal vascular occlusion in particular), diabetes accounts for perhaps three times this number. (It is important to note, however, that these statistics pale in comparison to the worldwide incidence of preventable and/or curable blindness due to conditions like Vitamin A deficiency, River Blindness, cataracts, and trachoma.)
The clinical practice of low vision is based upon the twin strategies of magnification and contrast enhancement, but also depends upon cognitive, behavioral and emotive integration with the activities of daily living. Magnification can be achieved three different ways: relative distance magnification entails moving the patient closer to objects of visual interest (for example, by moving closer to a street sign, or holding reading material closer to the eyes with the assistance of lenses having the appropriate focal length for a closer "working distance"); optical magnification works by using telescopes (for distance viewing), magnifying lenses, microscopes or video magnifiers (for near vision activities like reading or drawing insulin into a syringe) to achieve magnification without moving physically closer to objects; physical magnification depends upon enlarging materials for use (e.g., large-print books, numbers on a telephone touch pad or clock). A combination of magnification strategies is often employed to achieve the necessary amount of total magnification. An example may be useful.
A patient with 20/400 best corrected vision due to diabetic retinopathy (the big E on the Snellen acuity chart) must, by definition, receive 10x magnification to achieve a functional 20/40 level of acuity, for example, to see a road sign (the Snellen denominator, 400, divided by the desired Snellen denominator, 40, equals 10). To accomplish this, she can: stand 10 times closer to the sign than somebody with 20/40 acuity; use a low vision telescope with 10x magnification; or stand twice as close to the sign in tandem with using a 5x telescope (2x relative distance magnification times 5x optical magnification equals 10x total magnification). Similar calculations and combinations are used for near low vision devices. From a practical standpoint, patients typically respond best to low vision magnification techniques when their best corrected visual acuities are 20/800 or better, since higher degrees of magnification would increasingly diminish the user's field of view.
Contrast enhancement improves visual acuity independently of magnification, by making objects "stand out" more. Much of our visual world has poor contrast (e.g., gray colored cars on gray pavement on a cloudy day), and persons with low vision typically have tremendous deficits in contrast sensitivity as well (see Figure 1 for an example of low and high contrast objects).
Contrast can be enhanced by improved lighting (especially closer, incandescent lighting), improved print quality, employing stark color differences between commonly used items and their surrounding/background environment, and use of colored filters.
< Here, in text, was a simple figure demonstrating the affect of different backgrounds upon "apparent" contrast>
A host of adaptive low vision products are available, including large print and talking materials. For persons with diabetes, both large-display and talking glucose monitors are available, as are insulin pumps with audible bolus selection, syringe magnifiers, and tactile syringe-filling aids. Patients typically function best with a variety of low vision aids, and with a course of professional evaluation, demonstration and follow-up by low vision specialists, including eye care providers, mobility specialists, rehabilitation psychologists and occupational counselors when indicated. There is always something that can be done to help visually impaired or blind persons--knowledge is power, and getting help is a matter of knowing where to look. (For a list of low vision resources, the reader may wish to consult my web site at www.diabeticeyes.com, or Chapter 15 of my book on diabetic eye disease.)
As might be anticipated, clinical depression is common among patients with vision loss, and diabetes itself compounds this tendency. Far more insidious are the stereotypes and prejudices of sighted persons, including health care providers, about the abilities, cognitive and emotional status of persons with reduced or no eyesight. An address before the National Federation of the Blind, by its president, Marc Maurer, makes a compelling case against sight discrimination (a lengthy and eloquent speech previously published verbatim in the Voice); we would all do well to remember that common noun descriptors such as "blind," "handicapped," "disabled" and even "diabetic" are, in some sense, existentially diminutive, and that persons are not and should not be treated merely by such labels mainstream culture gives them via language. Not only is knowledge power, but language is, as well. (Perhaps this discussion may give all of us some sense of what it might mean to be "blind" in a predominantly sighted society, or "black" in a predominantly Anglo society.)
Unfortunately, many people have suffered eye complications from diabetes, including severe loss of vision. More tragically, many more will probably do so, in spite of all we know about prevention and effective treatment. With the aging of our population, the prevalence of serious visual impairment is expected to rise dramatically, and virtually all of us have family members or friends who are or will be affected. Hopefully, through effective patient education and the efforts of dedicated health care professionals and researchers, far fewer will suffer vision loss and other disability. Hopefully, cures for diseases like diabetes, and the eye complications it causes are just around the bend.
Until that time, though, it is critical that patients with vision loss be aware of what can be done, right now, to help them cope with that loss. For those who haven't experienced vision loss from diabetes (or any etiology for that matter), some familiarity with low vision services might best be viewed as an "insurance policy" of sorts; hopefully, this information will never be needed, but it's nice to have "just in case."