(Title with apologies to Public Enemy).
If there’s one thing I want you to understand right away, it’s this:
You can usually trust a poor VA. But you should never trust a good VA.
High-contrast single-letter VA is a good measure of visual function in a healthy-but-blurred eye, which is why we use it all the time in routine optometric practice. But it’s a particularly poor measure of visual function in a pathological eye.
Indeed, high contrast VA is often the last thing to go. This can be quite distressing for patients, who can tell that there is something wrong with their vision, but get repeatedly told by their optometrist or ophthalmologist that they are still seeing very well. Rather than being reassured, it can be quite distressing, as the patient then can’t reconcile what they are seeing with what their vision expert is telling them. Are they imagining things? Or is their vision ‘expert’ not as expert as they say they are?
If the scotoma does go across the fovea, you will certainly get a greatly decreased VA. If your patient is only reading 6/120 (20/400), you can really tell they have low vision. But we frequently see patients with very substantial loss of visual function who are still seeing 6/12, 6/9, sometimes even 6/6 (20/20). Even when the VA is moderately impaired (say 6/48), bear in mind that they might have impaired function well in excess of what you’d expect from that VA.
Three Zones of Vision
We’re trained to consider the visual field as having two zones — the small macular
field (detail vision) and the peripheral
field (all the rest). The job of the peripheral field is to provide a rough outline, and then we shift fixation to bring the macula to bear on points of interest in order to fill in detail.
If you’re going to understand low vision (especially dry AMD), it’s more useful to consider three zones. Yes, the job of the peripheral field is to guide large saccades of the macular field. But in turn the job of the macular field is to guide very fine saccades of the tiny foveal field. This distinction is particularly relevant when people have geographic atrophy, which often preserves the foveal field until late in the disease.
You need to remember that a good VA indicates the foveal field is intact, but it says nothing about the integrity of the macular field.
I’m in Australia, so we use metric VA, based around a six-metre viewing distance. For the sake of American readers I’ll try to remember to throw in some twenty-foot figures as well.
Causes of Vision Impairment
Impairment of visual function in low vision is primarily due to three-and-a-half factors:
- Loss of foveal vision (and therefore decreased VA) — of course it happens, but it’s only one possible factor.
- Impaired low contrast vision — takes the richness and texture out of what people see.
- Patchy macular fields — may leave the fovea intact (and therefore retain good VA), but badly impair ‘integrative’ tasks that use the larger macular field, such as recognising faces and reading fluently.
- Impaired luminance (low and/or high) vision — which I’m counting as only a half a factor, as it kind of goes hand in hand with the low contrast impairment (more — much more — on that later). This often manifests as the macula field dropping out in low luminance conditions, and greatly underperforming even in lighting levels which we normally think of as perfectly adequate. On the other side it can also result in heightened glare sensitivity. Having both a low luminance deficit and a heightened glare sensitivity can be particularly difficult to manage.
As we go on through these pages, I’ll be considering low vision rehab from the perspective of trying to manage all of these factors, not just acuity.
As a clinician, you should certainly be looking to investigate and quantify all of these factors. But the most important part of your examination is your history
. Talk to the patient. Listen to what they are telling you. I find these questions very helpful:
Do you have any trouble recognising faces? Be alert, sometimes patients will answer by saying “No, I can generally recognise people quite well” but then add “…because I know what they are wearing” or “…because I’m good at recognising voices.” But this is a really good question to get an idea of whether they have significant macular field impairment.
Do you have any trouble pouring yourself a glass of water? This is a common low contrast task, being able to see the level of a clear liquid in a glass, or while pouring a kettle into a mug.
Do you find things easier to see if you take them over to a sunny window? Good for picking up a luminance deficit. Even though we all do this sometimes, patients will a luminance deficit will often give a much stronger positive response.
Do you have trouble finding things in your pantry, or in crowded cupboards? They’re usually dim, so again a good question for picking up a luminance deficit.
Do you have trouble reading? You generally don’t need to ask this, it’s one of the most frequent presenting complaints. Be aware that a negative answer doesn’t necessarily mean there is no issue — not everyone is much of a reader anyway, and if your patient is illiterate they might not even notice it as a problem. And some people have a very adaptable personality type where if they run into problems with reading they just switch straight on to doing something else instead, so they might deny having any reading ‘problem’.
So, if high contrast VA is a poor measure of low vision, what should we make of all these treatment studies that use VA as their major (or only) measure of effectiveness? Clearly, you have to interpret them with extreme caution. As I write this, there is a lot of work going on exploring possible treatments for dry AMD, and that’s particularly problematic as the fovea is frequently spared until the very final stage of the disease.
What would be a better measure? I’m not sure. Low contrast vision is certainly better, but has its own limitations. I doubt there is any one traditional clinical measure of vision that correlates really well with pathologically-impaired visual function. So perhaps that’s the answer, using repeated application of validated questionnaires that measure practical vision impairment.