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

Characteristics of Low Vision Patients

Okay. So what are the characteristics of this low-vision population? In order to find this out, we had a network put together, a collaborative network of low-vision centers throughout the country (slide 20). There are 28 of these centers or 30 of these centers that are pooling from the patients they’ve seen. And about 60 percent of the centers are associated with academic medical centers. The other 40 percent are parts of group practices or private practices. So all of these low-vision specialists are combining their data together into a single database and this is an ongoing project. So what I’m giving you is a snapshot based on close to 1,000 patients that have been selected by this network. And it’s just sequential – it’s a series of patients.

So there’s obviously a selection bias, because obviously, patients have to be willing to join. So it shouldn’t be treated as an epidemiological study, but this will give you a good picture of the kinds of people who are seeking services in low-vision clinics.

Age and Gender

Okay. With all due respect to the youngsters in this room who have low vision, most people with low vision are old. The median age is 75 [- 76] years (slides 21 and 22). So half the people with low vision are older than that.

And here is the – this is just a histogram showing the distribution of number of patients in the sample who – by age who are presenting to these low-vision clinics (slide 22). And it’s heavily skewed to the older age – it’s a long tail that goes off to age 18 being the lowest age, and the median, as mentioned, being 76. The mean is 72 – standard deviation is about 16.

So for the most part, 80 percent of the people with low vision who are visiting low-vision clinics – and it agrees with the epidemiologic studies as well – are over age 65. Most people with low vision are women – two-thirds of them are women (slides 23 and 24). Why? Women live longer. See what you have to look forward to, ladies?

Okay. The age distribution of low vision among women is very similar to the age distribution among men – except there are just more women out there. The percentages – the distribution percentages are the same.

Visual System Disorders

Most people who have low vision have a central vision loss (slide 25). And this is a pie chart showing the various diagnoses of patients who come into these clinics seeking low-vision services (slide 26). 

Macular Disease

More than half – the ones that have some shade of yellow – are due to a macular disease. That’s a disease of your central vision, which is your acuity. Around 10 percent have glaucoma coming in seeking services. Diabetic retinopathy, which represents somewhere in the neighborhood of around 10-12 percent of the retinal vascular diseases is included in here. And stroke is about 2 percent. So by far, most of the people who are seeking services have some type of macular disease. The dry form of macular degeneration is most common, followed by the wet form – inherited forms.

Reduced Visual Acuity

The major causes of functional limitations from low vision or reduce visual acuity (slide 27). And looking at the distributions of acuity in the sample (slide 28), about 35 percent of the people who present to a low-vision clinic have acuity in the range of 20/20 to 20/60. So these people feel their vision’s bad enough that they’re seeking services. Close to 40 percent are the range of 20/60 to 20/200. About 20 percent are 20/200 to 20/500 and about 6 percent are worse than that. Somewhere in the neighbor of about 26 percent are legally blind.

But if you ask these people how’s your vision rate on a scale of zero being poor to four being excellent, the average rating is between poor and fair – no matter what your visual acuity when you come in (slide 29). And you’ll notice even those with the range of 20/20 to 20/60 are included. To be fair, some of these people have glaucoma. Glaucoma can shrink your visual fields way down and not necessarily have to do with central vision. A lot of these people also have dry forms of macular degeneration, which can produce what’s called foveal sparing. The center can still read the eye chart, but it’s like looking through a keyhole. And surrounding that would be a blind area so that they could still have very poor vision, even though they might be able to read quite far down on the eye chart.

Question by [Participant]: Are those corrected numbers – the visual acuity?

Response by Bob Massof: I’m sure that they’re not coming in for refract records. So these are people who have best-corrected visual acuity on presentation. I should say they’re wearing their individual correction. If they needed correction, they don’t get into the database. Now, if they get corrected and back to normal, they’re happy. You’ve cured their low vision. Same with [those who] come in with a cataract and you say, you came into the wrong department. You have to go over here and get the cataract done. They don’t come back. They not in these databases. So these are people (slide 29) who stayed in the database.

Visual acuity is a measure of blur. My daughter’s gotten over it, but I haven’t. And that the – now, I’m just simulating 20/200 visual acuity (slide 30). So if the only thing that was occurring was blur, the picture on the left would [represent] 20/20, the picture on the right is what 20/200 would look like. That’s the difference. And you can overcome that blur with magnification (slide 31). And that’s the main trick that’s used in a low vision clinic: to provide people with telescopes and magnifiers to compensate for the loss of acuity. You can read the bigger letters on the charts and make everything bigger and you can make the compensation.

Reduced Contrast Sensitivity

But the other major cause of functional limitations is reduction in contrast sensitivity (slide 32). We don’t hear about that as much. And information in the image is defined by contrast (slide 33). Whereas, if visual acuity is lost, it looks blurred. When contrast sensitivity is lost, it goes away. You don’t see at all.

And when patients experience the loss of contrast sensitivity, the way they will describe it is glare. Okay, and here’s a simulation of what it looks if you lose contrast sensitivity (slide 34). The only difference between these two images – well, the two differences – one is this is blurred to 20/200, the same as the other one was. But in addition, the contrast of the image is uniformly reduced, as if you lost contrast sensitivity by about – I guess it would be on the order of about 28 log units.

The way contrast sensitivity is measured is with an eye chart, but the letters are all the same size. It’s called the Pelli-Robson Chart (slide 35). All the letters, [which] are set up between the viewing distance and the size of the letter, [are] the equivalent of about 20/400 or 20/800 letters, they try to make them as big as possible. But what varies as you move around the chart is the contrast of the letters.

If you have perfectly normal contrast sensitivity, you should have no trouble reading the letters right here [pointing to bottom of chart]. What you do is just see how far down the track you can read. And each contrast varies in a tenth of a log for each triplet of letters (slides 36 - 42). So you specify contrast sensitivity really as a ratio of the light-to-dark. If the letter’s absolutely black, there’s no light coming off of it, the contrast will be 100 percent, no matter what the background is, as long as it’s not black too.

Contrast is not the same thing as brightness. You can’t improve contrast simply by turning up the intensity of the light. It’s the ratio of the light to the dark. If you turn up the intensity of the room lights, you’re going to reflect the same ratio from the two things. What improves as you turn up the light is your sensitivity to contrast. At low light levels, contrast sensitivity is worse for everybody, whether you have low vision or not; at higher light levels, your contrast sensitivity is better. There are many ways to specify that ratio (slide 43). We won’t go into it. It’s just a light-to-dark ratio which formally you want to use. And it’s important to remember that it’s – those of you who are illuminating engineers – reflectance determines the contrast not the overhead light.

Contrast Correlates to Visual Acuity

In this population [i.e., patients], contrast correlates with visual acuity. If you lose your central vision and you’re forced to use your peripheral vision to do everything you used to do with your central vision – everybody’s contrast sensitivity is worse in the periphery than it is in the center; everybody’s visual acuity is worse in the periphery than it is in the center, so the two tend to go down together more or less (slide 44). But there are people who can lose more contrast sensitivity way out of proportion to the visual acuity loss and vice versa. So even though they correlate, they do have independent effects. I’ll show you those.

Interpreting the Interactions

Now, if we ask: What is the actual distribution of contrast sensitivity in people who have visual acuity loss? Here you can see a normal contrast sensitivity is about 1.3. That’s the lower limit of normal. You get above 1.3, we won’t get too concerned about, but below 1.3 or lower. People in the 20/20, 20/60 range, the average contrast sensitivity is in the range of 1.2 and you can see contrast sensitivity just declines with each of the acuity tabs (slide 45). That’s just a restatement of what we saw before.

However, within these groups, there is a distribution (slide 46). There are people who have in this range – 20/20 and 20/60 – that have contrast sensitivity that is in the pits. They can’t see anything; everything is just totally washed out, even though they have relatively good acuity. And acuity, by the way, is measured with the highest possible contrast you can get. On the visual acuity chart are close to 100 percent contrast. And you can see there are a lot of people – about 40 percent of the people in the range between 20/20 and 20/60 have contrast sensitivity that would be considered moderately impaired.

Those of you who do illumination engineering probably know that as you go to higher spatial frequencies in the images, contrast falls. People who do remote sensing and that type of thing tell us that there’s almost a linear relationship between the contrast in terms of power versus the spatial frequency level. So the more detail there is, the bark on the tree will show more contrast than the tree; what gets lost is the detailed information, the pattern, the herringbone. The things that you designers are paid so much to put into the environment – they can’t see it, so you’re wasting somebody’s money.

And again, if you ask about contrast sensitivity – whether the contrast sensitivity tests out to be in the normal range, or they all say of course, they all came to a low vision clinic, so we kind of expect them to (slide 47). But then there are – it’s kind of a combination of an appearance.

The third thing that occurs is glare (slide 48). And what we find – and most of the glare [does not] come from eye disease in the retina, [but from] the cataract that didn’t get taken out, because if you have low vision, finding a cataract surgeon to take out that cataract can be pretty difficult, because they’ve been burned a few times where they’ve taken out the cataract and the vision hasn’t changed, because the person had a macular degeneration or something and they’re very unhappy. So you become a poor candidate [for cataract surgery], in technical terms, if you have low vision. And so a lot of the glare is coming from the cataract.

Effects of Lighting and Daylighting on Performance

Most patients self-report their vision quality is quite poor – in poor ranges, over 90 percent (slide 49). And important to you is when we ask them, what kind of affect does lighting have on your performance activities? Close to 70 percent say it has a major effect. Lighting is one of the biggest problems. Over 20 percent say moderate effect and only 11 percent say it has no effect. So lighting is a very important consideration.

Question by [Participant]: Day lighting?

Response by Bob Massof: Yeah, daylight – both too much and too little. Too much light and they tend to wash out from this glare. Too little light their sensitivity is reduced and what might look like mood lighting to the average person can be a funhouse.