Braille Monitor November 2006
by Robert W. Massof
From the Editor: Dr. Robert Massof is professor of ophthalmology and neuroscience and director of the Lions Vision Research and Rehabilitation Center, Wilmer Eye Institute, Johns Hopkins University School of Medicine in Baltimore, Maryland. Thursday afternoon, July 6, he delivered the following remarks to the convention. Dr. Massof has addressed the convention before. He is remarkable for his accurate understanding of low vision and blindness and how best to help those losing sight.
Many of us find ourselves from time to time having to answer questions about low vision or advising about where to seek help. Dr. Massof here provides the primer we need in order to be of intelligent assistance to those with a bit of residual vision. This is what he said:
In this presentation I will talk about low vision and its relation to blindness. "Low vision" is a medical term that is defined as chronic disabling visual impairments that cannot be corrected with glasses, contact lenses, or medical or surgical treatment. Most people who consider themselves blind would be included in this broad-reaching definition of low vision. An estimated 3.5 million Americans have low vision. Out of that group approximately two million have mild low vision, which mainly affects driving and reading ability, and about one million meet the legal criteria for blindness. Eighty percent of the people with low vision are over age sixty-five, and most of these people developed low vision late in life. The prevalence of low vision in our population is about one in a hundred for people in their sixties and rapidly increases to one in five for people in their nineties. Across all age groups the incidence of more severe forms of low vision is about 250,000 new cases per year. For the older population the death rate almost matches the rate of new cases of low vision. Consequently the total number of cases of low vision would be expected to increase very slowly. But, because the population is aging, over the next twenty years both the total number of low vision cases and the number of new cases per year will double. Many of the services for the blind in this country target children and working-age adults. But, because of their huge numbers, there is an immediate and rapidly growing need for services that target the senior population.
Historically the term "low vision" comes from references to low visual acuity. Ophthalmologists and optometrists, the two health care professions concerned with the eye and vision, measure visual impairments by asking their patients to read letters on a vision chart. This single measure of vision is called visual acuity. Most people equate normal vision with a visual acuity of 20/20 or better. This number has made its way into our everyday speech with colloquialisms such as "20/20 hindsight"; ABC has a news show called 20/20; and 20/20 is commonly used in promotion and marketing to refer to a clear vision or goal. At the other extreme, a statutory definition of blindness is visual acuity of 20/200 or worse in the better eye. Although most people are familiar with the meaning of 20/20 and 20/200, it is probably safe to say that many do not know what the numbers represent. Visual acuity is reported as a ratio. The top number in the ratio is the distance at which the letters on the eye chart are read. For both 20/20 and 20/200, the 20 in the numerator denotes a test distance of twenty feet. The bottom number in the ratio refers to the size of the letters on a particular line of the vision chart. The denominator also is in units of distance; it denotes letter size as the farthest distance at which a person with 20/20 visual acuity can read the letters. Thus a visual acuity of 20/200 means that the smallest letter read at twenty feet can be read at two hundred feet by a person with normal visual acuity. An object at two hundred feet will appear one-tenth the size that it appears at twenty feet. Thus the 20/200 letters on the vision chart are ten times larger than the 20/20 letters.
Visual acuity can also be expressed as a single number: the ratio 20/20 is equal to one, and the ratio 20/200 is equal to 0.1. Thus low values of visual acuity mean that the letters must be larger in order for the person to read them--the larger the letter needs to be, the lower the visual acuity. Ophthalmologists and optometrists frequently use the word "vision" to mean visual acuity. So, if a person has low visual acuity in both eyes, we say that person has "low vision." Although we apply the term "blind" to people with visual acuity less than or equal to 20/200, approximately 80 percent of people who are blind have some vision that can be measured in terms of visual acuity. Technically they too could be and often are described as having low vision. But before we consider changing the name of this august organization to the National Federation of Low Vision, we must note that the term low vision has come to mean more than just low visual acuity--it also implies a strategy of functioning based on the use of remaining vision. Similarly the term "blind" has come to mean more than the lack of vision--in contrast to low vision, it implies a strategy of functioning based on alternatives to vision. From this perspective, the difference between blindness and low vision is not just one of degree of visual impairment; rather it is a difference in the methods and strategies used to function in everyday life.
Magnification is an obvious compensatory strategy for low vision. If the smallest letter a person can read is ten times larger than the smallest letter that can be read by a person with 20/20 vision, then 10X magnification should make everything that is resolvable to the person with 20/20 vision resolvable to the person with 20/200 vision. There are only three ways to magnify. The first is called "linear magnification," which is achieved by increasing the size of the object. Examples of linear magnification include large print, large-screen display, computer-text and image-enlarging software, and closed-circuit television magnifiers or projectors, which present larger versions of the object to view on a screen. The second type of magnification is called "distance magnification," which is achieved by moving closer to the object. To achieve 10X magnification, we would have to reduce the viewing distance by 90 percent. But, like the need for a macro lens on a camera, when you get very close to an object, you will need stronger optics to keep the image in focus. Children have the ability to change the eye's focus over a large range, so they can get extremely close to an object and still keep it in focus. But adults lose that ability because of hardening of the eye's lens with age. Consequently adults require strong glasses or hand-held lenses, called magnifiers, to focus on closely viewed objects.
The third type of magnification is "angular magnification," which is achieved with optical instruments like telescopes and telemicroscopes. In the case of angular magnification, the size of the image is enlarged without modifying the object or changing the viewing distance. The enlarged image is an optical illusion created by changing the convergence angle at the eye of the light coming from the object.
Magnification is an effective strategy for people with mild and even moderate low vision, that is, in cases where visual acuity is 20/100 or better. This level of visual acuity requires only 5X or less magnification. However, for lower visual acuity the required magnification is greater, which invokes diminishing returns that make magnification less effective. Magnification requires a trade-off between image size and field of view. Larger images take up more space; therefore less of the object can be seen when it is magnified. Also magnifying devices limit the field of view further because of the aperture or viewing window of the telescope, lens, or video display. Another trade-off, in addition to field of view limitations, is that magnifying devices also magnify image motion. At higher magnifications the image being viewed appears to move if the telescope or magnifier moves, and the object's motion is magnified by the same factor as the magnification of size. For example, if 30X magnification is achieved with a CCTV, a one-inch-per-second movement of the object translates to a thirty-inch-per-second movement of the image on the CCTV screen. Thus in the case of magnification more is not always better. Just because magnification works well for the person with 20/70 visual acuity, we cannot assume that magnification will work for the person with 20/400 visual acuity. At lower visual acuities magnification is less effective for many tasks because of greatly magnified image motion and a very limited field of view.
Most often magnification is used for reading. Normally the average reader has a reading speed of approximately two hundred words per minute, a little faster than the highest speed the average person can talk. The top speed that can be achieved with a magnifying device is about 120 words per minute. But that speed drops with increasing magnification, primarily because of field-of-view limitations and magnified-image motion. It is not uncommon for low vision readers to top out around twenty to fifty words per minute, and many, because of blind spots in their central vision, cannot exceed ten words per minute--that is an agonizingly slow pace of one word every six seconds. Studies have shown that these same people can achieve reading speeds in excess of 200 words per minute using speech-output reading machines.
In addition to the trade-offs required by magnification, we also must take into consideration the fact that people with low vision usually have other problems with their vision besides low visual acuity. Most eye diseases that cause low vision also cause a loss of contrast sensitivity, dimming of vision, and blind spots in the field of vision. None of these problems can be helped by magnification. To a limited extent compensations can be made for contrast-sensitivity loss by exaggerating contrast in the environment. For example, the side of the bathtub can be made more visible by simply hanging a contrasting towel over the edge. Also to a limited extent task illumination can be used to compensate for dim vision. But outside the home environmental adaptations such as these are not practical solutions.
Low visual acuity has its greatest impact on reading and seeing fine detail, but contrast-sensitivity losses, dimming of vision, and blind spots have their biggest impact on mobility. Many people who would describe themselves as having low vision need mobility training, and most are not getting it. Falls and hip fractures are significantly higher in the elderly low vision population than in the age-matched sighted population. Seniors with low vision tend to be home-bound. They decrease their physical and social activities, they become isolated, and more than one-third are clinically depressed, and ultimately they spiral into frailty and poor health.
Ophthalmologists and optometrists can be overheard saying that their visually impaired patients should not receive mobility training because they are not totally blind. Blindness has a stigma that doctors and their patients alike want to avoid. People with low vision often refuse to use a cane despite its many benefits because it identifies them as blind, and their service providers will reinforce that decision by concurring with their beliefs.
The major difference between low vision and blindness is the person's attitude toward his or her visual impairment. People who confidently identify themselves as blind have accepted blindness as a personal trait. People who identify themselves as having low vision tend to struggle with trying to function as if they were normally sighted. It is not unusual for people with low vision to try to conceal their visual impairments and avoid challenging situations in order to avoid appearing incompetent. They will avoid using technology and alternative methods in public that identify them as blind. Their service providers also continue to focus attention on the remaining vision and declare success when they solve the problems that can be solved with magnification. In such cases the unaddressed issues, which are usually much larger, are avoided.
We have interviewed over 1,500 patients entering our low vision clinic at Johns Hopkins using a questionnaire called the Activity Inventory. The Activity Inventory organizes everyday activities into a hierarchical framework. Within this structure tasks refer to very specific cognitive and motor activities. Examples of tasks include cutting food, reading recipes, setting stove and oven dials, and measuring ingredients. Goals refer to the reasons for performing tasks. For example, cutting food, reading recipes, measuring ingredients, etc. are all tasks that are performed with the goal of cooking daily meals. Goals, in turn, are organized under objectives. For example, activity goals such as cooking daily meals serve the objective of living independently. The Activity Inventory has three objectives: living independently, social interactions, and recreation. Fifty goals are listed under those three objectives, and 450 tasks are under the fifty goals.
Besides being listed under goals, tasks can be organized by required function. The four functions are reading, mobility, seeing (that is, acquiring and interpreting information through vision), and manipulation (that is, visually guided motor activities). Reading recipes is an example of a reading task, returning to your seat at a restaurant is an example of a mobility task, watching television is an example of a seeing task, and sewing a hem is an example of a manipulation task. If tasks are difficult or impossible to perform, we say the person has functional limitations. If goals cannot be achieved, we use the term disability, and if objectives cannot be achieved, we use the term handicap. Tasks can be made easier to perform by using assistive technology. Goals can be made easier to achieve by making subsidiary tasks easier to perform or by teaching the person alternative strategies for accomplishing the goal. Rehabilitation occurs at the task level, but the outcome of rehabilitation must be judged at the level of the goal.
We first ask the person to rate the importance of each of the fifty goals. If the goal is not important to the person, we ignore it. If the goal is of some importance to the person, we then ask him or her to rate the difficulty of achieving it. If the person says that the goal is not difficult to achieve, we again ignore that goal. In order for an activity goal to be the subject of rehabilitation, it must be both important to the person and difficult or impossible to achieve. If those conditions are satisfied, we then ask the person to rate the difficulty of performing each task that is listed under the goal, or indicate that the task is not applicable. Upon completion of the survey, we know what the person's disabilities are and what functional limitations cause those disabilities. Using these criteria, the goals most in need of rehabilitation by our low vision patients are shopping, correspondence, managing personal finances, traveling, attending spectator events, face-to-face communication, driving, and reading the newspaper. Reading function is included among tasks that serve these goals, but there are many more tasks that cannot be assisted by magnification only, and for many people magnification is not adequate. For example, if the top reading speed that can be achieved with magnification is twenty words per minute, then NFB-NEWSLINE® would be a better method of achieving the goal of reading the newspaper than trying to do it with a magnifier.
What is important to preventing disabilities is achieving activity goals, not the performance of tasks in a usual and customary manner. There are numerous paths to the same result. Efficiency and effectiveness of the methods employed should be the major consideration in choosing a strategy to achieve a goal, not the results of clinical tests of vision that are used to classify people. Low vision rehabilitation service providers need to be educated on methods employed successfully by the blind; the stigma attached to these methods must be overcome and eventually eliminated from society; and the reach of the NFB must be extended to embrace those who now identify themselves as having low vision.