Before we can talk about types of vision loss, we need to define legal blindness and what that means functionally – as opposed to what it means to the government.

The government’s definition comprises individuals whose central visual acuity does not exceed 20/200 in the better eye with correcting lenses – in the fields of vision such that the widest diameter of visual field encloses an angle no greater than 20 degrees.

When you get your eyes checked and you can see the big E on the top line, you are reading what is called the 20/200 line.

If you can’t see that line in the better eye AND regular glasses can’t correct your vision, you are considered “legally blind.”

Someone with perfect vision, 20/20, should be able to read

the big E at 200 feet, and, conversely, someone with 20/200 vision should be able to read the big E at 20 feet.

The second part of the definition has to do with our peripheral, or side, vision. Normally, we see at an angle of 90 degrees from each eye, for a total of 180 degrees of vision to our sides.

If the angle in the better eye is less than 20 degrees, you are considered legally blind. So, even with legal blindness a person can see pretty well.

However, as far as the government is concerned, that’s enough. There are all sorts of benefits

that a person with legal blindness can qualify for. In our profession, however, we care about functioning.

For example, a person may see perfectly well in the right eye, but be totally blind in the left. Legally blind? No, but functionally, he will walk into things on the left or not see a car coming from the left, and reading may be difficult as the person will have a hard time finding the next line on the page.

Therefore, the eye report gives us a starting point. Can we expect night blindness? Can we expect the need for extra lighting or contrast? Can we expect that magnification will help? We thoroughly interview each person to find out the individual issues and goals, and develop a plan from there. By the way, you don’t have to be legally blind for The Iris Network to work with you.

There are vision losses that can affect the central vision, peripheral vision or both. Each particular loss presents its own set of problems. Let’s tackle them one at a time.

Central loss: The vision that we use for detail is in the central part of our eyes. We use this vision for most of our daily tasks, especially in near work such as reading or sewing and for identifying things such as faces. When we lose this part of our vision, it is those everyday routine things that become most difficult.

Most commonly, the eye disease that is associated with this is called macular degeneration. The macula is the tiny spot on our retina that gives us the clearest vision. For some reason not yet understood, something happens to this part of our eye, usually as part of the aging process, and we can no longer do the kinds of tasks described above.

Fortunately, unless something else happens to the eye, this eye disease does not progress beyond central vision loss.

With training from our rehabilitation teachers, people with central vision loss can learn adaptive techniques to continue functioning independently. The use of low-vision devices and low-vision training is usually very helpful for people with this condition.

Using a technique called eccentric viewing, a person can learn to locate the spot on the eye where they see best and learn to look around their blind spot through that point. Contrast and lighting are also important, along with the other “tricks of the trade” that rehabilitation teachers use in teaching.

Peripheral loss: Peripheral vision is our side vision – and our up-and-down vision. It allows us to see best at night and to see out of the “corners” of our eyes.

Loss of this part of our vision affects our ability to see people coming at us from the side,

to see how wide an area we are walking through, and to see low-hanging objects such as tree branches and low-lying objects such as curbs.

Loss of peripheral vision clearly affects our mobility to a greater degree than central loss, both during the day and at night. Retinitis pigmentosa is an eye disease typical of this kind of loss.

RP is a genetic disease. There is no way to predict its final outcome. It is typical, but not guaranteed, that the disease runs a similar course within the same family. Training in mobility, especially in the earlier stages of the disease can be critical. As the disease progresses, training in other areas becomes important too.

General vision loss: Some eye diseases cause an overall vision loss that might be described as a blurring of vision. Typical of this might be cataracts – the clouding up of the lens, or diabetic retinopathy – caused by weakening blood vessels in the eye. With this kind of vision loss, training in all the areas described last month is helpful to the individual.

Next month: Cane vs. guide dog – how does a person decide?

Ruth Mlotek is director of Rehabilitation Services, South. If you or someone you know can benefit from these services, call the Iris Network at (800) 715-0097. The Iris Network has outreach offices throughout the state. If there is a specific topic you’d like to learn about, call and leave a message.

The Maine Audio Information and Reading Service is a program of the Community Services Department of the Iris Network. Maine AIRS broadcasts local stories, obituaries and ads from the Bangor Daily News and other Maine daily and weekly newspapers from studios in Brewer. If you know of an individual who is blind, visually or print-impaired who would benefit from these broadcasts, call 989-0058. Les Myers is director of Maine AIRS.