Lecture: How to Adapt Your Examination for a Low Vision Patient

During this live webinar, how to adapt your clinical environment and tests to address the visual concerns of low vision patients are discussed.

Lecturer: Dr. Nicole Ross, OD, MSc, FAAO


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DR ROSS: Hi there. I’m Dr. Nicole Ross, and today we’re going to be talking about the low vision examination and integrating low vision care into your practice. So first I kind of want to spend the first part of the talk going over: What is low vision? And what are the characteristics of the patients worldwide that have low vision? And then next we’re gonna go over some of the different diagnoses that these patients have, and also most importantly how we manage low vision, and I do want to mention that we do have another webinar on the topic of low vision devices specific to management on February 22nd, in which we’ll develop on this point further. So what is low vision? These are patients who have permanently impaired vision in both eyes that causes functional limitations. So these are patients who have been through all necessary medical treatment and medical interventions, and their best corrected vision still remains reduced from that perfect 6/6 or 20/20. Many efforts have been made to define visual impairment and low vision, in different categories, depending on the visual acuity in the better seeing eye. And it may also in some cases be defined in terms of field loss, in patients who have peripheral vision loss, but may have good acuity. So we think of patients with 6/6 as having normal or perfect vision. In most places in the world, 6/12 allows you to drive without restrictions, and then between 6/12 and 6/60, we consider a patient being visually impaired or low vision, and at 6/60 and beyond, we start getting into the definitions of legal blindness, either the US and North American definition, or the WHO definition, which is 6/150, or 20/500. So in terms of how the WHO groups visual impairment and low vision categories based on acuity — it’s seen in this table here. And we think of grouping people in terms of moderate, severe, profound low vision, and the reason for grouping people this way is because these patients will have different visual needs and different functional demands. And so today the focus of our talk really stems from this highlighted box here of patients with moderate to total vision loss. That’s sort of our discussion today, incorporating and managing these patients in your practice. So low vision is really a consequence of an aging population in most places. Certainly in North America, in the US, if we look at citizens over the age of 40, the prevalence of low vision was estimated to be 2.4 million, and the annual incidence well over 4,000. And this is expected certainly to rise as estimates for 2020, which is only two years away. Worldwide, the picture is a little bit different. And this is from work from the WHO. And in work from the WHO, certainly they cite that cataract and uncorrected refractive error are leading causes of low vision and visual impairment, but really, when we’re focusing on the definition of low vision, we’re talking about uncorrected — we’re talking about corrected visual acuity, and we’re talking about visual impairment that cannot be corrected further. So I’ve actually taken uncorrected refractive error and also cataract out of the graph below. So here in this graph, we’re seeing causes of low vision from macular degeneration, glaucoma, diabetic retinopathy, trachoma, and other causes. And we can see we have our Asian countries grouped in red, our European and North American countries in green, Latin America in purple, and Africa in orange. And as you can see, in all of the countries, sort of surprisingly so, perhaps we have a higher incidence of macular degeneration causing moderate to severe low vision in North American and European countries, but overall there’s not too much variation, as we might have expected, between these categories. If we look at WHO’s definition of blindness, so these are patients whose best corrected acuity is 20/500 or worse, and looking at their diagnoses, again, I’ve taken out the uncorrected refractive error and cataracts, because those are really low vision cases. And just focused on the other ones. We again see that macular degeneration has a high prevalence in European and North American countries, but also surprisingly in Asia and parts of Latin America as well. And again, while these numbers are higher, it shows that across the globe we are all dealing with similar issues. So I want to answer and bring up this next poll question of: Which of the following cases meets the definition of severe low vision? So would that be uncorrected acuity of 6/12 or 20/40, as we say here in North America, in the better eye? Reduced acuity due to cataracts of 20/200? Reduced best corrected acuity of 20/200 in the better eye due to macular degeneration, or uncorrected visual acuity of 6/60 in each eye? Great. So I think we’re all on the same Page here. The corrected answer is indeed C. So again I just want to stress the point that we’re talking about cases of low vision where patients have permanently reduced vision that cannot be corrected further, and then we’re classifying them based on their best corrected visual acuity in their better eye. Oftentimes patients might say things like: Oh, I’m legally blind in my right eye without glasses, and really, that doesn’t quite meet the definition of low vision that we’re discussing. And that’s a common misnomer. So if we look at a provision of low vision services worldwide, and this was done by Jo Keith and her group — and we can see that areas in white have no coverage of low vision services. Some of the light dots have really limited services provided. In hashed areas, less than 10% of the population has access to service. In the cross, between 11% and 50% have service. And that certainly is our case in the US. Certainly we’re aiming to increase access to low vision services, which is a huge priority. And then we have more of the darker shaded areas, which is my home country of Canada, with greater than 50% coverage, and one nation in Europe there as well. If we look at who provides low vision services, there certainly is a range of providers that are providing the services. Some are optometrists like myself. But others may be from the educational field, ophthalmology, rehab teachers, et cetera. But even with including all of these providers, we still have reduced access to care that is less than ideal worldwide. If we look at some of the barriers of access for patients with low vision, some cite cultural barriers worldwide. Cost, in terms of devices and different visual aids that would help assist patients with the visually mediated tasks that they need to do. Certainly there’s this notion that nothing further can be done, and certainly while nothing further might be able to be offered surgically, the patient gets the message that there’s nothing else out there to help, and so that is a barrier that I think all of us health providers can help address. You know, distance in travel for patients is a huge issue. Because many of these patients cannot drive themselves, et cetera, because of their reduced vision, and are relying on others and family members to take them to appointments. If we look at the common presenting complaints of just our low vision patients, this was a review that we did while I was at Wilmer Johns Hopkins, looking at all the low vision patients and their primary visual complaints on entering the low vision appointment. Reading was by far the number one complaint, followed by others. And this certainly is true worldwide. While it might not be reading specifically, there’s a high prevalence of difficulties with near tasks. Whether it’s sewing, crochet, and other near tasks. That tends to be the number one complaint. So in terms of assessing a low vision patient, certainly our visual measures that we do in every ophthalmological exam are important, of visual acuity. I would add contrast sensitivity to that. Knowing where scotomas or blind spots are on the visual field has a tremendous impact on a patient’s functional ability. And refractive status is obviously important, to make sure that we do know in fact what that best corrected vision is, even in a patient with a lot of pathology and ocular disease. We want to also look at more functional measures of reading fluency, and really a goal-based case history. The goal of low vision rehabilitation is not necessarily to improve the vision, but we’re improving the patient’s ability to do visually mediated tasks. So we’re looking for a lot of different workarounds, whether that be training, whether that be rehab teaching, whether that be a low vision device. To assist them. But we’re really sort of helping them with visually mediated tasks. So we need to know what tasks are important to them, because any prescribed therapy or device has to be not only appropriate in terms of the amount of magnification and contrast enhancement and lighting provided, but it also has to be practical for that patient to do the task that they need to do. And a lot of self-reported ability and talking with the patient and counseling is required in really getting that history. There’s also cognitive measures that we have to sort of consider, especially in our older population. We won’t get into that much in today’s talk, but there are a lot of different screening measures that are available, because certainly the cognitive capacity of the patient is going to affect your rehab plan and what you suggest for the patient. So when I am asking a low vision case history, it’s a little bit extended than the typical case history of an eye exam. I think about the five sort of functional domains of a person, and how vision affects those domains. So I ask questions with respect to reading or near tasks, I ask questions about visual information or sort of general seeing. So this would be, for example, questions about facial recognition, and things like this. Mobility is a tremendously important question. We know that in reduced visual acuity, mobility and fall risk is a concern. I ask questions about their activities of daily living, or ADLs, as we commonly abbreviate this. So cooking, cleaning, managing dressing and so on. And also driving, if appropriate, if the patient was previously a driver or is a driver. And so what do I ask with respect to reading? There’s a lot of variety in terms of near tasks and reading, to ask a patient, that you would really want to know. You would really want to know what the favorite reading materials is. Are these large print magazines? Is this a small printed bible that they’re trying to read? Was the patient previously an avid reader? Is continuous reading the goal? Or do the patients simply want to make out a food package and what it is? Was this the sodium-free Campbell’s soup or not? What about the habitual reading distance? Is the patient adamant that they must read at 40 centimeters with the newspaper on the desk? Or are they flexible? Are they willing to hold things closer? Are they willing to adjust that distance? For all patients, the ability to complete spot reading tasks — about the microwave dinner packages, instructions, medicines, et cetera — is important. So we always have to address those. And then the question is: Do we also have more complex needs in the workplace? Or for pleasure reading, that we also have to manage? So visual information — we’re asking questions about: Can the patient recognize faces? This is a particularly sensitive point, as many of my low vision patients are fearful answering the doorbell, because they’re not sure who is at the door. They can’t recognize the person. Or they get into awkward social situations, where they feel that they might be ignoring someone that they know, who has entered the room, because they can’t readily recognize the face too quickly. Watching TV is another area in question. You know, how big is the TV? How close do they sit to it? Coincidentally enough, studies show that many low vision patients actually watch more TV than their normally sighted counterparts. So TV is an important question. Glare is also an important question. And in a lot of ocular pathology, light scatter is a problem. So glare sensitivity can be a problem. Indoors, outdoors — and it’s important to tease that out. With respect to mobility, we want to know: Does the patient use any mobility aids currently? Do they use a support cane? Do they use a walker? Do they use a long white cane? Do they have a relevant fall history? And how many falls have they had? How often? What are the details of what happened? Was curb detection a problem? Was it stairs? Was it uneven pavement? When we’re talking about activities of daily living, we’re really talking about the home safety of the person. We really want to identify any home safety issues. Such as burning themselves or the food when preparing meals, being able to manage their medications appropriately. Not taking too much of something or not taking medicines at the appropriate time, as instructed. What their living situation is. Do they have support from family members? Who can help with some of these activities of daily living? Or are they on their own? Do they use a phone or computer? You know, mobile phone use is highly prevalent worldwide. Are they able to dial the phone in an emergency? Are they able to connect with loved ones? Do they have any other safety measures, such as a LifeAlert or something that, if they fell or had an accident, that they could reach out to somebody? Driving — there’s lot of important questions here, and certainly driving regulations really vary wildly worldwide, as to the vision requirements needed for driving. And some countries have no stipulated vision requirements for driving. But as their doctor and professional, we want to give them good advice with respect to driving. So we really want to know when, where, and why they’re driving, if they had a motor vehicle accident history, do they have active licensure, or not? Assessing visual acuity in patients with low vision — there’s certainly a lot of different choices. You have your traditional sort of electronic and Snellen projector chart. You have the ETDRS chart, or the Feinbloom chart. All may be appropriate at different times. But I would say my strong preference, if at all possible, is for a LogMAR chart. And the reason being is that this chart has uniform size progression. And it’s a logarithmic progression in metric, and as a Canadian, I like metric. And most of you worldwide probably do acuities in metric. So it’s easier to handle. And also convert to our diopteric measurements, when we’re calculating magnification. Each line on a LogMAR chart is 0.1 log unit smaller than the previous line. And you can adjust it. I have mine on a rolling cart, so I know if I move the chart from 2 meters to 1 meter, I expect the patient to improve by three lines, and if I vice versa move the chart from 3 meters to 4 meters, I expect them to perform 3 lines worse. The chart allows you to use 2 meters, 3 meters, and 1 meter. Those are commonly used. So you can measure vision up to the 20/600 level. If the vision is worse than that, then we unfortunately do have to go to a Feinbloom chart, which is pictured here, which can be done at any distance, as long as you are consistent. The LogMAR charts typically work well with European or Eastern European letters. The Sloan letters were used to calibrate this chart. For other languages, what is commonly used are the Landolt C rings on the LogMAR chart, to get the same calibrated measurement. Refractive error, after measuring acuity — we obviously want to know what the best corrected acuity is, after getting and entering acuity. And many of my students and interns ask me the question: Well, the patient has a lot of pathology here. Is it worthwhile doing the refraction for the patient? And so in order to give an insightful comment to their question, this excellent retrospective review was published by Janet Sunness. And she looked at over 700 of her low vision patients, and she was looking at which patients improved and benefited from a refraction, despite their ongoing pathology. And she found improvement of two lines or more of visual acuity in about 81 patients, or 11%. 3% of her patients improved four lines or more. And when she stratified things by diagnosis, looking at who were the patients that improved the most with the refraction, these were patients with macular degeneration, myopic degeneration, which perhaps makes sense, in progressive myopia, and also patients status post retinal detachment, especially if they had a scleral buckle, which then would elongate the eye, making the eye more myopic. So those were the patients who benefited most from a refraction. In doing refraction in low vision patients — and I know Dr. Sara Woznik has a webinar planned up ahead to discuss trial frame refraction more specifically — we really want to use large changes that the patient can notice. So I always start refraction with a retinoscopy, even if it’s just over the habitual glasses, to know how far off the mark are those habitual glasses. And I also have to think about the patient’s pathology, how likely there’s gonna be a change in refraction, and maybe if there’s other reasons to repair the glasses, I especially want to make sure I do a refraction. And so with low vision patients, you’re going to do a loose lens refraction in a trial frame. Which worldwide is more common than the phoropter anyway. But you’re really going to use large lens changes, to make sure the patient knows a difference between choice one and choice two. And obviously you’re going to use larger differences between lenses when the acuity is much poorer. And the common rule to pick our bracket sizes that we use is the just-noticeable difference rule. So this rule implies that we take the denominator of the Snellen fraction, and that becomes the just-noticeable difference. So for a 20/100 patient, they have a just-noticeable difference — they’ll notice differences of 1 diopter. So I want to choose two lenses that are 1 diopter part. So in this case, that would be the plus or minus 50, would be 1 diopter apart. And then for a 20/200 patient, conversely, that would be a just-noticeable difference of 2 diopters. And so we’re showing the patient a +1 and a -1 lens, and that is 2 diopters apart. And similarly, when we’re testing for astigmatism, we want to keep in mind that just-noticeable difference in the same way. So we’re going to use a handheld Jackson-Cross Cylinder, and we’re going to pick the power of that Jackson-Cross Cylinder based upon the power of that just-noticeable difference calculation. The next step, really, in the exam, after we’ve tested acuity, we’ve now got a best corrected acuity and refraction, is really to test contrast sensitivity. I really like the MARS chart, and this is available in numbers, in addition to letters. It’s easy to store. It’s done at 50 centimeters. And as you can see, each letter on the chart decreases in contrast. If you don’t have a contrast chart available, you still can get a sense of the patient’s contrast sensitivity function with some really pointed history questions. So patients with poor contrast sensitivity will have a lot more difficulty seeing facial detail, because the face tends to be low contrast. They’ll often comment about difficulty seeing stairs or steps, especially if the edges are not marked. They’ll often comment about tripping on uneven ground. They may report there’s an overall haze in their vision. And they also may report that they can’t read any digital displays or analog clocks. They’re very difficult, when you have very poor contrast sensitivity and very washed-out vision. So you can get a sense if the patient does have severe contrast sensitivity loss through those pointed questions. This is a table that I particularly like, in terms of quantifying contrast sensitivity loss. So for an older patient, between 1.6 to 1.48 log contrast sensitivity is really near-normal. When we get around 1 log unit, we’re talking about moderate contrast sensitivity loss. And then less than 1, we’re talking about a very severe loss, reduced by a factor of 6 or more times, compared to normal. And then, at 0.48, this is really profoundly reduced contrast sensitivity. If we talk about reading, specifically, there are three measures that are really important to gather from your patient, to help with their reading or near task goals. Which tends to be the number one complaint of patients. That will be your visual acuity measurement, knowing the best corrected acuity, because that will determine how much magnification is required for that patient to read. Contrast sensitivity is probably the second most important measurement, because that gives you a sense of how much lighting or contrast enhancement is required. And then also a measure of central scotomas or visual field. Because if you have a blind spot in the center vision, like many patients with macular holes or macular disease, then you need more magnification than you would anticipate, based on their acuity alone, because they have to read around this blind spot, and it’s going to cover up part of the word. So when we talk about estimating magnification for a near task, I often use continuous text reading cards, like the MNREAD. Because that really will give me a better sense of how the patient is managing, especially if they have central scotomas. That they have to read around. Because a patient with a central scotoma will have better letter acuity at near than they will have a reading acuity. So let’s kind of talk about principles of magnification, now that we’ve kind of gone over those three essential elements of the exam that you need to do in order to address a near complaint, which is what we’re talking about today, since it’s the most common complaint for low vision patients. So there’s options for magnification. Certainly we’ve all thought about relative size magnification, in taking the size of something and making it bigger. Reading a large print book or putting something in the photocopier, and enlarging it 200 times. Or 200%, so two times. We also talk about relative distance magnification. So these are patients that move quite close to objects, in order to make them bigger or to produce a bigger retinal image. So that is also another form of magnification. And if we talk about relative distance magnification, this is also relatively easy to calculate. We’re comparing two sizes. So the original distance to a new distance. So if a patient was reading at 40 centimeters, and couldn’t read or manage something, but then moved it to 20 centimeters, we would say that they are producing or getting an effect of 2X magnification. So relative distance magnification is always compared to some standard distance. When we talk about… Well, how do I figure out what low vision aid to prescribe for near tasks? I tend to veer away from the term “magnification”, and tend to think about requirements in diopters. I’m an optometrist. I like diopters. And also, there’s tremendous variation between manufacturers of magnifiers, in terms of how they label things. So a 5-diopter magnifier could be labeled as 3X, and it could also be labeled as 2X, or even as 1X, depending on the manufacturer. So really looking at the diopteric need of the patient, and the diopteric requirement for the magnifier tends to be a much easier approach to manage. And there really are several approaches to calculating this. So these are the four rules that we think about, when we think about determining equivalent power or Feq, as it determines — of a magnifier that’s needed. We can use Kestenbaum’s rule, we can use the inverse of the near acuity method, we can consider the acuity reserve rule, or we can consider my personal favorite, which is the inverse of the critical print size. So let’s go over these briefly. So Kestenbaum’s rule dictates that the required equivalent power needed for a patient is determined by their best corrected acuity at distance of their better eye, and taking the reciprocal of that, to get a diopteric value. And this would be the diopteric value required for the patient reading 1M letters on a near card. And this is equivalent to most countries’ newspaper print size. So regardless if you do Snellen or if you do the metric acuity, you’re basically inverting the acuity. So if we have 6/30, we would divide 30 by 6, and we would get 5 diopters as the equivalent power required for near tasks for that patient. Now, there have been many critics of Kestenbaum’s rule, and the primary critique is that many patients have pathology, whether it be diabetic retinopathy or macular degeneration or some other pathology. And those patients tend to do better on a distant chart than a near chart, and they tend to do better on a near chart than a continuous text chart, which is why the last card is my favorite. But the near acuity method dictates that you take a near acuity with the appropriate near habitual — near addition, to allow the patient to see the card, if they’re presbyopic. And you would invert that, and that would give you the diopteric equivalent power needed for reading 1M print. So, for example, if the patient read 2M on this New England College of Optometry near card, at 40 centimeters, we would record their near acuity as 0.4/2.5M. And we would invert that near acuity, and get a value of 6.25 diopters that is required for near magnification. And that can now be the tentative new add. So you can prescribe them much stronger reading prescription for this patient. So rather than a standard near addition, you could perhaps prescribe the +6.25 near addition to allow them to read small print. As I mentioned, this method has also been criticized. It is better than Kestenbaum’s rule. It gets you closer to a good starting point in working with your patient with a near magnification solution. But many have criticized it, that it’s using a threshold acuity. And not very many patients like to read at their threshold. So Lovie-Kitchen and others proposed that the acuity reserve for peak reading efficiency is usually two times the threshold for most people. And so using the acuity reserve rule, they propose that you take the value that you get from these other two rules, of Kestenbaum, or even the near acuity method, and you double that value. So in the last example, the near acuity method dictated that we made it 6.25 diopters for reading 1M print. And the acuity reserve rule dictates that, well, to read 1M print comfortably and fluently, if fluent reading is the goal, then really 12.5 diopters is required. So double that initial value. But I think that really, ultimately, the best way, if you’re really — if the goal really is continuous reading — the best way to assess this and get an accurate calculation is using a continuous reading card. And Gordon Legge developed this card, the Minnesota Reading Test. It is available in multiple languages. English, Spanish, French, Italian, Portuguese. It’s been calibrated by many studies. It goes from very large print sizes to very small print sizes. And it’s also available as an app, if you’re a tablet user, on the iTunes Store. You can use this link to purchase the app, and as more languages come online with the Minnesota Reading Tests — they are looking at more languages every month — they will be available through the app. So using a continuous reading card, how do I determine equivalent power required for a near task, to read 1M print? Well, really, I have to determine what the critical print size is of the patient. That is, what is the smallest print that this patient can read at their maximum reading speed? And so really the following steps are required: I have the patient wear an appropriate spectacle correction for the test distance. So if the card is at 30 centimeters, I make sure they have a +3 near addition. If they’re presbyopic, if it’s a child, then whatever their best corrected glasses for distance are appropriate. I ask the patient to hold — where the card is in best focus — I have the patient start reading from the very, very large print sizes, and I note what their maximum reading speed is. Sort of even if it’s just sort of a general sort of sense. Whether it’s slow, moderate, or fast. And then I really listen to: When is the last print size that they read fluently? And when they started to slow down? And that really is their critical print size. Is the last print size that the patient read fluently. And then I also note their threshold. And so when calculating the equivalent power using this method, I’m really using the critical print size, and inverting that. So if the critical print size, the smallest print that that patient read fluently, at a test distance of 30 centimeters, is 2.5M, I divide 2.5M by the 30 centimeters, and that would give me a requirement of about 8 diopters for that patient to read 1M. And so once I got this number, regardless of the method you use, that’s really your starting point. And now you have to decide: Are we going to prescribe a near spectacle that’s quite a bit stronger, maybe, than the typical near spectacle prescription? Are we going to prescribe a hand magnifier? Or are we going to prescribe a stand magnifier-type system? So this is my next poll question for you. Just to make sure we’re all following along. So this is a 30-year-old patient with an incoming diagnosis of oculocutaneous albinism, presenting to your office, complaining of reading complaints. You do the MNRead test, and you find that the patient is holding the card quite close, at 15 centimeters, but they are able to accommodate to that distance, as they’re only 30 years old. And they rapidly read most of the card, but they start to slow after 1.3M, and then they’re unable to read any text past a 0.5M. So what is that critical print size from the above reading assessment? All right. So yes, the critical print size is 1.3M, but actually, to record an acuity, that actually is the task distance over the print size. So it actually would be C. The people who picked B were eager to get on to the next step, which is calculating the equivalent power required from this value. So they already were a step ahead of the question. The other thing that’s really important to determine during a reading assessment is really optimal lighting and its criticality. I usually select an initial lighting level based on the patient’s history usually with a gooseneck lamp that’s about 2 to 3 feet away, in front of the shoulder of the better-seeing eye. And then I sort of — once I’ve determined the critical print size, I ask the patient: Is it easier with the lamp really close to the page, at 3 feet, overhead? And you’ll have some information from the contrast sensitivity test as well, as to how critical lighting is. Obviously the lower the contrast sensitivity, the more critical it is. But the point I want to make here is that often patients will ask me: Based on my contrast sensitivity test, should I be replacing all the lightbulbs in my house? Should I have LED lighting only? And really, the bulb isn’t the most important piece. But it’s really the distance that you’re holding the bulb from the page. And that has to do with the physics of the inverse square law. So if I bring a light source three times closer, it’s going to be nine times brighter. And so really the intensity and the distance is the most important factor. And that’s really what you want to communicate to the patient. So now that we’ve sort of got a handle on how to determine the equivalent power, we now have to make some decisions about what is going to be the best choice for a low vision aid for reading, which is what we’ll focus on today. So these are sort of our three main optical categories of choices, and that is: Glasses, a hand magnifier, or a stand magnifier. So in this example, I have a patient here with their slightly hyperopic refractive error. The better eye is about 20/40. Unfortunately on the reading assessment, their critical print size was quite large print. 3.2M. So if we invert that, we’re getting about an 8 diopter requirement for them to read 1M. And so if I am prescribing a near vision-only reading glass for this patient, 8 diopters is going to be my new near addition, and I’m going to, when I’m writing a near-only prescription, add 8 diopters to the distance refraction up here. And so this would be my near vision-only prescription. Now, at +8 near addition, that’s quite close for a near point. The patient is gonna have to bring the material quite close. And if the patient is reading binocularly, it will be hard for them to sustain convergence, to read quite close. You can even try it yourself. So you could consider some base in prism, to relieve that convergence demand. And typically we would prescribe 10 base in, for an 8 diopter near addition. Prism half eyes — if you don’t want to do sort of a custom near vision only glass, prism half eyes are available off the shelf by many low vision aid vendors. And they look like this. And certainly I would say that this allows the patient to read binocularly as much as possible by having that base in prism to relieve the near demand. But I would say that once we get to the very high powers of, like, +14, even with the prism, it’s hard to really sustain convergence. So at that point, the patient really is reading with their better eye. The important part in prescribing really strong glasses like this is the patient has to be aware of the reduced reading distance. That is now required with these very high near additions. And sometimes patients can be quite resistant to this adaptation, especially if they’re really used to reading at 40 centimeters. But one sort of benefit of this way is that glasses really will give the patient the widest field of view, because that magnifying lens is right at the spectacle plane. And they also, if they’re using eccentric viewing, can use that eccentric viewing port and sort of move the page along. So it is a good hands-free approach. So there’s advantages and disadvantages with all devices. Certainly the advantage here is that it’s sort of normal to use spectacles. The wide field of view. You can really customize this for your patient who has a lot of astigmatism and so on. But there’s also ready-made options available at a lower cost. Some of the disadvantages, obviously, are the close working distance, especially the higher the diopteric requirement — the closer the distance will be. And if the patient needs a light source, because of a reduced contrast sensitivity value, it can be hard to get a light in on the page and hold things close. And really, binocularity is limited, even with the use of base in prism, if we’re going stronger than +10. So here’s our next poll question for you. So you have a patient, Ms. Magnus, and she’s saying that she doesn’t want to use her magnifier anymore to read. After refraction, you conduct a reading assessment. Note the critical print size is about 2.5M at 33 centimeters. Calculate the equivalent power you would need to prescribe her magnifier. Excellent. So we’re all on the same track. So the Feq would be 7.5 diopters. Certainly there are not a lot of magnifiers out there that would be that specific. So certainly the people who picked 8 diopters — probably the 8 diopter magnifier would be the closest to that value that you would pick. Here’s our next question. Say that this patient actually wants prism half eyes. Ready-made, off the shelf near reading glasses, to read. So we calculated her Feq in the previous question. It’s 7.5 diopters. She’s got a slight hyperopic refraction of +50. What prism half eye glasses off the shelf would you assess primarily? Excellent. So the answer is B. So +8 with 10 base in. So typically off the shelf, prism half eyes — the amount of prism in them is usually a difference of 2 from the diopteric value. It’s sort of the cheat way I remember it. So +8 has 10 base in on each lens. +10 has 12 base in on each lens, and so on. So that’s sort of the typical pattern that we see from manufacturers. So now, given the spectacles that Ms. Magnus is wearing, the question is: What is the near point? So how close does she have to hold the paper from the spectacle plane when she’s using these glasses? Great. So one little caveat here. I meant to sort of catch and point out — when we’re prescribing glasses for the patient, so the patient is wearing +8 glasses. If she had no refractive error, 12 centimeters would be the near point. But since this patient does have a bit of refractive error at distance, just by a half diopter, the near point is actually equivalent to the near addition that that patient is getting. So it actually would be 13 centimeters, or slightly off from 12, in this case. So just sort of a reminder to consider the patient’s distance refraction, when we’re prescribing these off-the-shelf devices. So when we talk about magnifiers, handheld magnifiers are a very common approach to prescribe in low vision. Definitely understanding the goals of the patient will help you determine the most appropriate magnifier. Certainly we calculate Feq for 1M. But if the patient really wants to read a medicine bottle, that is much smaller print, then we sort of have to adjust our calculation. We also have to really consider the trade-off between field of view and the power of the magnifier. The stronger the magnifier, the less the field of view. And that’s where that calculation of equivalent power is handy, because it sort of prevents us from overprescribing too much power. So many of my patients do have 2 to 3 different hand or stand magnifiers, of different powers, and I often sort of, in the office, work with one that’s right at that Feq value. One a little stronger, and one a little weaker. To really tease out: You know, what is this patient using the magnifier for? And how do I have to adjust my goal target print size for that? So in this patient that we had previously, with their refractive correction, you know, we determined that the Feq required for this patient was 8 diopters. So in the previous example, we went down the glasses and spectacle route. Here we’re gonna go down the hand magnifier route. And so I would start, and with this case, the Feq I calculated was 8 diopters. I would start with an 8-diopter handheld magnifier. So handheld magnifiers, as we know, these are plus lenses that are used mostly for brief spot reading tasks. Most patients are not going to read War and Peace with a handheld magnifier. And certainly there’s a huge range of powers available by a lot of low vision vendors worldwide. And you can have them illuminated for patients with contrast sensitivity loss, and also non-illuminated versions. The most important point to impart on the patient here is that they have to hold the object or the paper one focal length away from the magnifier. So the stronger the magnifier, the closer they’re gonna have to hold that magnifier to the page. And so, you know, I do have some videos available on this point. And so if you’re interested, please feel free to contact me, and I can share these videos with you that you can use as educational pieces. Hand magnifiers certainly — it’s familiar. But it does require a lot of good hand control. And does have a smaller field of view, compared to spectacles. A stand magnifier is another approach. These are magnifiers that are held flat on the page. Because you’re placing the plus lens at a fixed distance on top of the print, some add or accommodation is required for these. Commonly, we think about paper weight or dome magnifiers. These are prescribed a lot in children. And mainly because they have very short image distances, so patients can use a lot of accommodation, if they’re used to using a lot of relative distance magnification, to use these. You know, stand magnifiers, again, they’re great because you can drag them along. They don’t require a lot of hand control. But the amount — the distance that a patient can be from their hand magnifier depends on what glasses you’re prescribing. Now, when you’re prescribing stand magnifiers, and we’ll talk a bit more about this on February 22nd — so this is just sort of a teaser — the equivalent power that you get from using a stand magnifier is a combination of the enlargement ratio provided by the stand magnifier and the near addition you’re prescribing for the patient. You have to make sure that the near addition you’re prescribing is coincident with the image distance of the stand magnifier. And all low vision vendors — most of these actually list what the enlargement ratio is for the different stand magnifiers, so that you can look it up. If you find that your manufacturer in the region where you’re practicing does not list what the enlargement ratio is, I can show you, and I’m happy to email you about what the steps that you can take to make sort of a little bit of a lab bench to figure it out. So for our patient, we really are looking in this column here. This is from Eschenbach, one low vision vendor, and we’re looking at the ratio of these magnifiers. I want to briefly mention some of the electronic approaches. We talked about opticals today. A CCTV, which is a video magnifier, are prescribed for all ranges of visual impairment nowadays. And there’s many different features. One of the most common features is: It does provide a very wide field of view of the screen that you can’t do with optics. And it does, as shown here, really enhance the contrast of the print on the screen. So here, it’s even inverting the print. So you have white text on a black background. Many of these have an X,Y table, so you can move the text on the table, and also allow for autofocus. If you’re thinking about equivalent power, this would be the same calculation that you would use for your stand magnifier. Again, we’ll go over this more on the 22nd. And typically I use sort of a two-ruler method to determine what the patient is doing. As they enlarge print on the screen. There are also portable versions of electronic magnifiers. And these are very popular, mainly because the cost is reduced, and also the footprint in the home is reduced. And many also have output. Will speak aloud the text as you’re reading it, in many different languages. So here you can get excellent stamina, a great field of view, not so great if you have motion sickness. And the high cost sometimes is prohibitive for patients. Many of my patients are also using Kindles and e-readers. These also can read aloud to you. Many Kindles can have large print up to 2.5M, and can invert the contrast. iPads, you can go a little bit higher. To 3.2M on these. You can enlarge print more. So you may want to prescribe maybe a higher near addition for these patients in glasses, but maybe not quite as high as if the requirement was 1M. Maybe you want to cut it back a little bit, give them a little extra reading power, so that they can read large print on their tablet. Disadvantage here is obviously the internet connection requiring Wi-Fi, and that many of the menu buttons might not be in large print, despite changing the settings. So how do you start? Here are some suggestions for some equipment to start practicing in low vision. I would say a LogMAR acuity chart, a contrast chart, an M notation continuous reading card, a measuring tape, so you measure what distance the patient is holding that card. Because as you can see, that affects your magnification requirements considerably, and a trial frame set. Some of the low vision aids — I would recommend having on hand some handheld magnifiers, stand magnifiers, prism half eyes. Next lecture, we’re gonna go over telescopes and filters in a bit more detail, and an electronic magnifier. So thank you so much, and I’ll take questions.

>> So Dr. Ross, we have two questions so far. If you want to stop sharing your screen.

DR ROSS: Sure.

>> It should be up top. If not, I can stop sharing for you. So if you open the Q and A box, there’s two so far.

DR ROSS: Great. So this question… Two questions from Dr. Shah. What are the devices for handicapped children who are studying? So when we’re talking about patients with multiple disabilities, the strategy for magnification becomes a little bit more complex. And oftentimes I find for children in this situation we’re looking to adapt the environment appropriately. So for these children, a lot of the time I’m recommending much larger print sizes. So I’m using that critical print size measurement or even a near acuity measurement and taking into account the acuity reserve rule to suggest print sizes for the patient. Or if the patient is not reading print, but they’re using — looking at pictures, they’re using Meyer symbols on a communication board, we want to make sure those symbols are large enough for them to appreciate. Depending on the ability of the patient and their ability to coordinate their hands, sometimes a dome magnifier can work quite well. Or if the patient doesn’t have much use of their hands, sometimes we can prescribe a higher reading addition, but we just have to make sure a caretaker or someone is with them, so that they can adjust what they’re looking at to the appropriate distance that it has to be. The next question is: A power calculation of near if glasses are +4 for near. I’m not sure… I think there’s a bit more detail to that question that I’m not sure I’m capturing adequately. But if the question is if the patient has a distance refraction of +4, what near glasses would you suggest? Well, I would basically suggest near glasses based on that Feq calculation. So if it determines that I need 4 diopters to read the 1M print size, then I have to add that 4-diopter requirement to my distance prescription, and so my near glasses would be a +8. The next question… I want to know your preferred technique of refraction in low vision, retinal degenerations, and corneal opacities? So my preferred refractive technique would be a trial frame refraction, especially for retinal degenerations. And using large steps, using a just-noticeable difference rule, showing the patient large enough differences that they can choose between lens one and two, and that you can kind of consistently have a trend there. For corneal opacities, trial frame refraction is still helpful if glasses are the ultimate goal. I think keratometry also can have tremendous value too, in determining refraction for a patient with a lot of corneal opacities. But ultimately, you may want, for a patient with corneal opacities, if they can tolerate a scleral or contact lens, you may want to do a trial frame refraction over an initial contact, if contacts down the line are something you’re thinking about. Now, the next question: For refinement, do you choose objective or subjective refraction? So most of the time, I’ll start with my objective refraction of retinoscopy, and then I’ll complete the subjective refraction in the trial frame. So, for example, if the patient with my objective retinoscopy findings is 20/200 or 6/60, I’m going to choose lenses that are 2 diopters apart. So I’ll probably choose a +1 and a -1, and show those two choices on my best sphere check. And then as the acuity improves, the lens bracket size will decrease. And similarly, when testing for astigmatism, I’m gonna pick a handheld JCC. Again, based on that just-noticeable difference rule. So by the time I get to testing their astigmatism, after best sphere, if they’re now 20/100, I would probably use the +1 or -1 JCC to refine things. Sorry, the plus or minus 50 JCC to refine things. The next question is: Asking again about the near prescription, and they’ve clarified that they’re talking about a patient who has no refractive error distance, but has a macular disease. You know, +4 for a reading prescription for a patient with macular disease may not be enough. It sort of depends on their acuity level. So I would measure this patient’s near acuity, if you can, with a continuous text reading card, or even just with a near card. And based on that acuity, I would invert it, and that would give me the diopters required for a reading prescription. Oh, one comment here. Requesting more lectures on clinical optics. So thank you for that feedback. And so on February 22nd, I’ll be sure to integrate more clinical optics, when we talk about all the different low vision devices and the different demands of patients. Next question is: What are we doing if nystagmus is on both eyes, and the vision was 6/60, and no refractive error? So I would take the same approach with a patient with nystagmus. It’s still reduced acuity. So they’re still gonna require a lot of — some magnification. The difference here is a patient — if it’s truly just congenital nystagmus, and there’s no other retinal finding that’s causing the nystagmus, their contrast sensitivity is gonna be quite good. So the criticality for lighting and other things isn’t going to be as high. So really you’re prescribing magnification based on those four approaches I outlined. Is glare testing like the bright acuity test a must for the low vision patient? That’s an excellent question. I think the BAT or the bright acuity test can be helpful for a low vision patient to sort of quantify the glare. But most of the time I think the assessment of glare and the treatment for glare is a fairly subjective thing. So I’ll ask the patient to describe to me what conditions they’re having most difficulty in. Is it bright sun? Is it indoors in fluorescent lighting? What is the situation? And then I’ll take the patient into that uncomfortable situation, and we’ll trial different filters of different tints and different densities. So I have in the clinic an assortment of sort of fit-over glasses that fit over top of glasses that control for glare, that have top shields and side shields, that are pretty similar to shades that are given to patients after cataract surgery, but just have different colors and a lot of different densities. The other thing I have in the clinic is I have trial lens rings of all of the different colors available, all the different tints, and at different densities. And we’ll also trial it that way. And I’m basically relying on the patient to give me feedback as to what tint and what density resolves their complaint, but still gives them enough vision to make sure that they can navigate properly. Because we don’t want to make things too dark, that now they can’t see well, to navigate. But excellent question.

>> All right, Dr. Ross. I think that’s a good place to stop, since there’s no more questions.

DR ROSS: Great!

>> Thank you again, and we look forward to your next lecture.

DR ROSS: Thank you so much.

>> Have a good day, everyone. Bye.

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February 8, 2018

Last Updated: October 31, 2022

1 thought on “Lecture: How to Adapt Your Examination for a Low Vision Patient”

  1. Maybe, The near point of MS. Mugus should be 11cm instead of 13.3cm, considering the far prescripition is +0.50D?


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