Common Causes
Common Types
Exams
Rehabilitation
Devices

Overview
Few people are totally without sight. Most individuals today classified as "blind" actually have remaining sight and, thanks to developments in the field of low-vision rehabilitation, can be helped to make good use of it, improving their quality of life.

Anyone with non-correctable reduced vision is visually impaired and can have a wide range of problems. The World Health Organization uses the following classifications of visual impairment based on vision in the better eye with best possible glasses:

20/30 to 20/60: Mild vision loss or near-normal vision

20/70 to 20/160: Moderate visual impairment or moderate low vision

20/200 to 20/400: Severe visual impairment or severe low vision

20/500 to 20/1,000: Profound visual impairmen or profound low vision

Less than 20/1,000: Near-total visual impairment or near total blindness

No light perception: Total visual impairmet or total blindness

There are also levels of visual impairment based on visual field loss (loss of peripheral vision).

In the United States, any person with vision that cannot be corrected to better than 20/200 in the best eye, or who has 20 degrees or less of visual field remaining, is considered legally blind.

Visual impairments take many forms and exist in varying degrees. It's important to understand that visual acuity alone is not a good predictor of the degree of problems a person may have.  Someone with relatively good acuity (e.g., 20/40) can have difficulty functioning, while someone with worse acuity (e.g., 20/200) might not be having any real problems.
[back to top]

Common Causes of Low Vision
Some of the more common causes of low vision are macular degeneration, diabetic retinopathy, retinitis pigmentosa (RP), cataracts and glaucoma.

Other common causes include:

Retinopathy of prematurity (ROP): Previously known as retrolental fibroplasia, this occurs in infants born prematurely and is caused by high oxygen levels in incubators during the critical neonatal period.

Retinal detachment: This may result in total impairment in the detached area of the affected eye. It involves the retina separating from its underlying layer. Causes are holes in the retina, eye trauma, infection, blood vessel disturbance or a tumor. Through early diagnosis, most detached retinas can be surgically re-attached with partial to complete restoration of vision if surgery is performed in a timely fashion.

Acquired (traumatic) brain injury: Vision can also be lost or damaged as a result of head injuries, brain damage and stroke. Signs and symptoms can include reduced visual acuity, visual field, or contrast sensitivity, blurred vision, eye misalignment, poor judgment of depth, glare sensitivity, confusion when performing visual tasks, difficulty reading, double vision, headaches, dizziness, abnormal body posture, and balance problems.
[back to top]

Common Types of Low Vision

Loss of central vision: Creating a blur or blindspot, this makes it difficult to read, recognize faces, and distinguish most details in the distance. Mobility, however, is usually unaffected because side (peripheral) vision remains intact.

Loss of peripheral (side) vision: Sometimes referred to as "tunnel vision", this is 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 directly ahead. Typically, loss of peripheral vision may affect mobility, and if severe, can slow reading speed as a result of seeing only a few words at a time.

Blurred vision: This results in objects both near and far apperaing out of focus even with the best conventional spectacle correction possible.

Generalized haze: This is the sensation of a film or glare that may extend over the entire viewing field.

Extreme light sensitivity: When standard levels of illumination overwhelm the visual system, the result is a washed out image and/or glare disability. People with extreme light sensitivity may actually suffer pain or discomfort from relatively normal levels of illumination.

Night blindness: This is in inability to see outside at night under starlight or moonlight, or in dimly lighted interior areas such as movie theaters or restaurants.
[back to top]

Exams for Low Vision
When providing an exam for a low-vision patient we'll ask for a complete personal and family general health and eye health history. In addition, we'll 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 color vision and contrast sensitivity. Measurements will be taken of the person's visual acuity using special low vision test charts, which include a larger range of letters or numbers to more accurately determine a starting point for determining the level of impairment. Visual fields may also be evaluated. A specialized refraction must be performed and each eye will be thoroughly examined.

We may prescribe various treatment options including low-vision devices, as well as assist the person with identifying other resources for vision and lifestyle rehabilitation.
[back to top]

Rehabilitation for Low Vision
Each type of low vision problem requires a different therapeutic approach. A thorough examination will result in the development of a treatment plan which may include prescription of glasses, specialized optical systems, therapeutic filters, non-optical options, and/or video magnification, and the prescription of rehabilitation therapy to effectively maximize visual functioning for activities of daily living. Therapy may also be prescribed to enhance remaining visual skills.

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.

As of October, 1999, both the American Optometric Association and the American Academy of Ophthalmology have called for Medicare coverage of low vision rehabilitation services. Many Medicare carriers now have policies in place that cover some of the vision rehabilitation services so be sure to ask about this type of coverage.
[back to top]

Devices for Low Vision

Spectacle-mounted magnifiers: A magnifying lens is mounted in spectacles (this type of system is called a microscope) or on a special headband. This allows use of both hands to complete the close-up task, such as reading.

Hand-held or 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: These can serve as supplements to other specialized systems. They are convenient for short term reading such things as price tags, labels, and instrument dials. Both types can be equipped with lights.

Video magnification: Table-top (closed-circuit television) or head-mounted systems enlarge reading material on a video display. Some systems can be used for distance views tasks. These are portable systems, and those that can be used with a computer or monitor. Image brightness, image size, contrast, and foreground/background color and illumination can be customized.

In addition, there are numerous other products to assist those with a vision impairment. This includes large-type books, magazines, and newspapers, books on tape, talking wristwatches and self-threading needles.
[back to top]