Dear reader: This page is part of a series of articles written for vision professionals. If that’s not you, it might not make much sense. If you’d like to learn all about eyes, vision impairment and what you can do about it, I strongly recommend you start by reading the article series I wrote for everyone, which starts here (click).

Don’t. Don’t Don’t. Don’t Believe the VA.

With apologies to Public Enemy (Don’t Believe the Hype)

If there’s one thing I want you to understand right away, it’s this:

You can usually trust a poor VA. But when there’s pathology, never trust a good VA.

High-contrast single-letter visual acuity 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? It can kind of feel like gaslighting.

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.

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.

But 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 broader macular field, which is often where the action is at.

Note re imperial vs metric units: 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 translations as well.

Causes of Vision Impairment

Impairment is primarily due to one or more of these 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.
• 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.
• Patchy macular fields — may leave the fovea intact (and therefore retain good VA), but badly impair ‘integrative’ tasks that use the larger macular field. That means things like recognising faces and reading fluently.

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.

Key point: 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. Seeing the level of a clear liquid in a glass, or while pouring a kettle into a mug. Patients will often overflow the glass, or have to put their finger in to judge the level (which really isn’t great with hot liquids!)
• 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. I used to think that it must be due to the pupil getting smaller, so the depth of field was better. I mean, that works for me, when I’ve forgotten my reading glasses. But these are patients who are already wearing their reading specs, so the page was already in focus — increased depth of field isn’t going to bring them any extra benefit. The benefit is simply because their eyes function better with more light.
• Do you have trouble finding things in your pantry, or in cupboards and drawers? 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, since it’s one of the most frequent presenting complaints. But be aware that a negative answer doesn’t necessarily mean there is no issue — not everyone is much of a reader anyway, so 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.

What would be a better measure? There’s no perfect answer. Low contrast vision is certainly better, but has its own limitations. There isn’t any one traditional clinical measure of vision that correlates really well with pathologically-impaired visual function.

The main thing is:

Listen to what your patient is telling you. Even if their VA is good, they might be having significant vision difficulties.

If you really need a formal measure, the best would be using validated questionnaires that reliably measure levels of overall practical vision function and impairment (https://www.cera.org.au/pro-questionnaires/).