Anyone with reduced vision is visually impaired, and can have problems functioning, ranging from minor to severe difficulty. There are two general classifications of low vision in use today:
- partially sighted – visual acuity that with conventional prescription lenses is still between 20/70 and 20/200 (a person with 20/70 eyesight must be 20 feet away to see clearly an object that a person with 20/20 eyesight can see clearly from 70 feet away);
- legal blindness – visual acuity that cannot be corrected to better than 20/200 with conventional lenses and/or the patient has a restricted field of vision less that 20 degrees wide. (Note that some definitions of “partially sighted” include the legally blind.)
Low vision impairments take many forms and exist in varying degrees. It is important to understand that the visual acuity alone is not a good predictor of the degree of the problem a person is having. Someone with relatively good acuity (e.g., 20/40) can be having a very hard time functioning, while someone with worse acuity (e.g., 20/200) might not be having any real problems doing the things that they want to do.
The common types of low vision are:
Loss of Central Vision – the center of the person’s view is blurred or blocked, but side (peripheral) vision remains intact. This makes it difficult to read or recognize faces and most details in the distance. Mobility, however, is usually unaffected because side vision remains intact.
Loss of Side Vision – typified by an inability to distinguish anything to one side or both sides, or anything directly above and/or below eye level. Central vision remains, however, making it possible to see what is directly ahead. Typically, loss of side vision affects mobility and slows reading speed because the person sees only a few words at a time. Sometimes referred to as “tunnel vision.”
Blurred Vision – objects both near and far appear out of focus, even with the best conventional spectacle correction possible and even when the target is very large.
Generalized Haze – the sensation of a film or glare that may extend over the entire viewing field and may produce various patterns or areas of relatively severe vision loss.
Extreme Light Sensitivity – exists when standard levels of illumination overwhelm the visual system, producing a washed out image and glare disability. People with extreme light sensitivity may actually suffer pain or discomfort from relatively normal levels of illumination.
Night Blindness – inability to see outside at night under starlight or moonlight, or in dimly lighted interior areas such as movie theaters or restaurants.
Doctors of Optometry who specialize in low vision care are skilled in the examination, treatment and management of patients with visual impairments not fully treatable by medical, surgical or conventional eyewear or contact lenses. Each type of low vision problem requires a different therapeutic approach. A thorough examination by an optometrist, which will also include tests to determine the patient’s current vision status, may also include a vision rehabilitation program to enhance remaining vision skills.
The low vision specialist will ask for a complete personal and family general health and eye health history. In addition, the optometrist will discuss the functional problems with the patient, including such things as reading, functioning in the kitchen, glare problems, travel vision, the workplace, television viewing, school requirements, and hobbies and interests.
Preliminary tests may include assessment of ocular functions such as depth perception, color vision, contrast sensitivity, and curvature of the front of the eye. Measurements will be taken of the person’s visual acuity using special low vision test charts, which include a larger range of letters to more accurately determine a starting point for gauging low vision. Visual fields are usually evaluated, and each eye will be examined.
The optometrist may prescribe various treatment options, including low vision devices, as well as assist the person with identifying other resources for vision and lifestyle rehabilitation.
There is a wide variety of optical devices and adaptive products available to help people with low vision live and/or work more productively and safely. Most people can be helped with one or more of them. Unfortunately, only about 20-25 percent of those who could benefit have been seen by a low vision specialist and had treatment options, including low vision devices, prescribed specifically for them. The more commonly prescribed devices are:
- Spectacle-mounted magnifiers – A magnifying lens is mounted in the individual’s spectacles or on a special headband. This allows use of both hands to complete the close-up task, such as reading.
- Spectacle-mounted telescopes – These miniature telescopes are useful for seeing longer distances, such as across the room to watch television, and can also be modified for near (reading) tasks.
- Hand-held and stand magnifiers – Serve as supplementary aids. They are convenient for reading such things as price tags, labels, and instrument dials. Both types can be equipped with lights.
- Electro-optical aids – Closed-circuit television (also called CCTVs) enlarge reading material on a video screen. Some are portable, while some can be connected to a computer. The user can adjust the image brightness, size, contrast and background illumination.
In addition, there are numerous other products to assist those with low vision, such as large-type books, magazines, and newspapers, books-on-tape, talking wristwatches, self-threading needles, and more.
If you, or someone you love, suffers from low vision, your optometric low vision specialist can provide the help and resources needed to gain back the independence and freedom that once seemed lost. People with low vision can be taught a variety of techniques to perform daily activities with what vision remains. There are government and private programs that offer educational and vocational counseling, occupational therapy, rehabilitation training, and more.