Proceedings of: Workshop on Improving Building Design for Persons with Low Vision

Introduction and Definition of Terms

Thank you very much. Well, I want to thank Vijay for inviting me to participate – exciting and stimulating. I’ve been in the field long enough to have to explain what low vision is each time I get up and talk. It’s just great progress not to have to do that – to actually have a meeting organized for people outside the small circle of those that work on this problem all the time. To organize a meeting devoted to the subject says a lot about how important it’s become.

Vijay asked me to talk about – kind of introduce your client to you, to talk about the low-vision population, what the characteristics of low vision are, and how we define it. So that’s most of what I’m going to talk about. So my apologies to Dr. Alibhai, Dr. Siemsen and Dr. Brabyn who would probably do a better job giving this talk, but too bad.

Low Vision and Blindness

So let’s start with a definition of low vision. And I’ve heard a lot of definitions. Before I read what’s on the slide, low vision’s a medical term. And when ophthalmologists measure your vision, they measure visual acuity. And visual acuity is expressed as a ratio of 20/20, 20/200. So the larger the number in the denominator, the worse your vision is. And of course, the smaller the fraction – 20/20, is one; 20/200, point-one.

Operational Definitions

So if your vision is low, you have low vision acuity. So low vision is kind of a shorthand notation for low visual acuity. However, the use of the term “low vision” has gone well beyond just talking about visual acuity. In fact, it refers to any chronic visual impairments that cause functional limitations or disability (slide 3).

We say it’s chronic – by definition, it can’t be corrected with medical or surgical interventions. If you can take out the cataract, we don’t call it low vision; if we can give you glasses, we don’t call it low vision. We correct that. So low vision is after everything has been done that can be done and you’re still left with a bit of impairment that interferes with daily activities.

Visual impairment can refer to loss of visual acuity – I’ll explain what that is; loss of contrast sensitivity – I’ll explain what that is; loss of peripheral vision; blind spots and a number of other types of visual bursts, color vision deficiencies and so on, which – and usually, visual impairment is defined in terms of some type of clinical test that gives us a score that we can compare to norms and say whether or not there’s an impairment there. Some impairments cause more problems with daily functioning than other types of impairments and we’ll emphasize those that are most –have the biggest effect on daily functioning.

Functional limitations is a term that refers to an increased difficulty in being able to do certain functions that require vision. Reading is one of the functions we single out; mobility; visual motor activities – this is eye-hand coordination type things, using your hands under visual control; and interpreting visual information. You know, if you’re watching a reality TV show, in order to know what’s going on, you have to be able to interpret what you’re seeing. So an awful lot of what we do every day involves visual information processing and interpreting visual information.

So functional limitations usually imply that you have an increased difficulty doing it, but it’s not impossible to do. Whereas disability usually refers to the activities themselves that are impacted. So if you’re unable to perform your usual customary activities because of your visual impairment, we say you’re suffering a disability. And especially with the new [term] –– International Classification Function, disability has come to be defined strictly in terms of activities. So the two more or less go hand in hand, but you’re not necessarily disabled if you have a functional limitation.

Official Definitions

Okay, the official definition of low vision usually goes hand-in-hand with blindness, and low vision and blindness are often in the same sentence. And there’s a kind of commonsense definition of blindness. We think we have no useful vision, that you can’t use your vision at all. But then the technical definition of blindness is usually defined in terms of some level of visual impairment and it’s usually even visual acuity or visual fields.

The World Health Organization defines blindness as corrected visual acuity (slide 4). By corrected I mean wearing your glasses, getting the best vision you can. The corrected visual acuity that’s less than or equal to 20/400 in your better-seeing eye – and I’ll explain what these numbers mean – or your maximum diameter of your visual field is under 10 degrees. Then you qualify for the term blindness, according to the World Health Organization criteria.

World Health Organization finds low vision and its corrected visual acuity that’s less than 20/60, but it’s greater than or equal to 20/200. So if you’re in that range, your vision’s impaired, but still is useful to you, then use the term “low vision”. And they also include a visual field definition that if the maximum diameter of the visual field is 10 degrees or greater, but less than 20 degrees, then you earn the term low vision from the World Health Organization.

In the United States, blindness is defined as part of the Social Security Act for purposes of defining disability for disability benefits (slide 5). And blindness is – today is defined as corrected visual acuity in the better-seeing eye that is less than 20/100. It used to be less-than or equal to 20/200. The reason for the change is that we got new eye charts. The eye charts used to be 20/100, 20/200 was the next line. We got new eye charts and added a 20/160 line in between. Subsequently, we had a lot of people who could read 20/160; they lost their blindness-related benefits, because of 20/160.

So people like Dr. Siemsen and Dr. Alibhai had the old Snellen charts in the other room. And if you had to come in for a disability test, they’d take out the old Snellen chart and checked to see if – and finally, commonsense prevailed and said what you really mean is they can’t read the 20/100 line, they can read the 20/200. So the cut really is 20/100. So that’s the new definition of blindness for the disability insurance.

The maximum visual field [diameter of] less than 20 degrees. Remember, that’s the low-vision range for the World Health Organization; it’s blindness for the U.S.

Low vision is not a term that is used in that way; however, it is defined by Medicare. And to be eligible for payment for services that are provided for rehabilitation of your vision – receiving rehabilitation related to your visual impairment, Medicare defines low vision using ICD-9-CM codes – these are diagnostic codes (slide 5). And mild low vision, which they don’t pay for, is in the range of less than 20/40, but greater-than or equal to 20/60. Moderate low vision – they’ll pay for that – is less than 20/60 and greater than 20/200 and/or if your visual acuity is better-than or equal to 20/60, you have blind spots in your central field that interfere with your functioning. So if you can document that, you’re still classified as moderate, even though your acuity might not reach those particular [limits]. And severe low vision is anything less-than or equal to 20/200 – same definition as legal blindness.

The ICD-9-CM codes go onto to break up severe low vision into severe, profound, near-total blindness, total blindness, but in Medicare’s, those don’t make a difference. And so we’ve tended to just stop at the severe to be all inclusive after that.

Visual Acuity

So what is visual acuity? When we’re defining low vision on a basis of [Snellen] test, what is it we’re talking about? Well, visual acuity is just a measure of the limit of your vision resolution (slide 7).

And this is what is called the ETDRS chart (slides 7 and 8). It’s a new design of the eye chart. And if you visit any low-vision clinic or you participate in any type NEI-funded clinical trial, you’re probably familiar with this chart. Okay, 20/20 visual acuity means that the smallest letter that a person can identify is five arc minutes – and I know there are [engineers] in this room, so you know what that means and I’ll explain for those who aren’t (slide 9).

Five arc minutes of visual angle and size and has a critical detail of one arc minute. Okay, if you drop a triangle from the eye to the letters – so the letters at the base of the triangle point to the triangle at the eye. That angle at the eye is what we’re talking about – the size of the angle.

There are 360 degrees in a circle; there are 60 minutes in a degree. So one minute is pretty small. If stars are separated by one minute apart, you can see them as two stars, if you have 20/20 vision acuity. So this is the definition of limited resolution. If they’re closer together than one minute apart, it looks like one star. They blur together.

Okay. The absolute limit to visual acuity, if the optics of the eye were perfect, would be about 20-over-eight. And that corresponds to the distance between pixels in your retina. Okay, these are the photoreceptors. They’re sort of like – think of those like pixels in your camera. We used to say [film] in your camera. But nobody knows what we’re talking about. So each photo receptor we thought of as a pixel. So the limited resolution induced by the pixelized nature of the retina is about – would give you about 20-over-eight, whereas 20/20 is a more practical definition of normal vision as the optics of our eye are anything but perfect (slide 9).

Now, conventionally, we specify visual acuity as the minimum angle of resolution, which abbreviated is M-A-R – MAR (slide 10). And it’s the ratio of the distance to the letters on the chart divided by the size of the letters. And when we’re talking about Snellen notation, which is the 20/20, it is the Snellen notation that dates back to the 1800s (slide 11). He was a Dutch ophthalmologist who invented the eye chart and visual acuity measurements.

The standardized distance is 20 feet. Those of you who develop offices know that nobody has a 20-foot exam room, but we still specify the distance, as if we’re testing at 20 feet. Sometimes people who are purists will use mirrors to optically get the 20 feet, but for the most part, 20 feet is just an idea. So you measure the distance.

Twenty, the bottom number, the other 20, that’s the size of the letter. And the size is specified also as a distance. And it’s the distance at which that letter [transcends] five minutes of arc. So if it’s a bigger letter, you have to put farther away in order for it to be five minutes. So 20/20 says you can resolve this 5-minute arc letter at 20 feet and the size of the letter is five minutes of arc at 20 feet. Twenty-two-hundred means the letter is five minutes of arc at 200 feet, which means it’s 10 times bigger (slide 12). So, if 20/20 the smallest detail you can resolve is one minute of arc, in 20/200 the smallest detail you could resolve is 10 minutes of arc, okay? One-sixth of a degree. That’s pretty good vision, but you’re legally blind if that’s the best you can do. There are a lot of animals that would die to have 20/200 vision.

Okay, and the reason we have some problem with a small amount of visual acuity loss – small, relatively speaking – is because our whole society is built around normal vision. Newspapers, magazines are printed with print size that’s only three times the resolution limit of the average person – 20/20. It’s the size of 20/60 so it can be read comfortably. For most people who would have 20/40 acuity, they would struggle with 20/60, because it’s like trying to read the – if you have 20/20 vision – trying to read the back of the [one dollar] bill because the print size is too small.