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).

VA and Macular Field Integrity

So, I’ve been emphasising that single-letter high-contrast visual acuity correlates poorly with visual function in a pathological eye. But it’s still worth measuring. In low vision work, the VA ‘journey’ can tell you as much as the destination.

Partly that’s because if we get a very poor VA then that’s something useful to know — a very poor VA certainly does correlate with a high level of vision impairment. But when we get a reasonably good VA, what I’m interested in is not so much exactly how far down the chart they get, but how they got there, because that gives me some pretty good hints as to how intact their macular field is.

Aside: I use a chart at 2.4 metres (8 feet), but I record all my VAs as corrected to the equivalent in 6 metres (20 feet) notation. Yes, I know, from a pure scientific point of view I should be recording my VAs as 2.4/xxx. But I don’t, because it’s hard for non-experts to understand, and I value clear communication more than anything. When I communicate with other vision professionals, doctors or agencies helping the patients rehabilitation, if I say the patient’s VA is 2.4/38 they have no idea what I’m talking about. But if I tell them the 6/95 (which is the equivalent), then everyone understands. The same is true if anyone later needs to access my patient’s records (I work in a hospital clinic, so it’s not uncommon for ophthalmologists and other optometrists to be looking at my consultation records without me being there). So I’ve marked my chart with the 6 metre equivalent, and that’s what I record. The only thing I need to be careful of is to take +0.50D off all my refractions, to allow for the viewing distance.

How Patients Read the Chart

The red area here indicates the lines that are just too small for the patient. The orange is the lowest one they could read, their threshold of discrimination — so that’s their VA.

The two lines in yellow are above threshold, but they’re still a bit challenging, so the patient might even make a mistake or two. But the lines in green are well above threshold, and the patient will read them easily and accurately. Note that the green starts at three lines above threshold (three lines between the red and the green, indicated by the two red arrows).

So the normal journey is that the patient reads down the chart quickly and accurately, then they start to slow down as they find it difficult, and then they can’t go any further. We’re all used to this.

Even if they patient has quite a lot of refractive blur, they’ll still do the same journey — it’s just that the threshold is higher up. Still, three lines above threshold they’ll find the letters easy.

In contrast, when we deal with an eye with some sort of pathology, we often get a different journey. Notice that the patient is badly struggling with the last three lines before absolute threshold, and still having some difficulty with reading even the one five lines above threshold. In this patient’s case, there is a six line gap between threshold and fluent easy reading.

Why is it different? Obviously something is interfering with their ability to accurately detect/recognise letters that are larger than their foveal threshold. The most common reason is patchiness of their macular field — that is, the macula immediately around the fovea (parafoveal macula) has at least some significant areas of field loss, which disrupts the ability to recognise larger letters/patterns. But be alert for other possibilities too, such as dry eye/tear instability, and saccadic inaccuracy (from parkinsons disease, for instance).

How should you record this VA? I record it as a standard VA, but with a notation of something like “6/12, but struggles from 6/38”.

Importantly, this may be your first indication that you have a patient for whom magnification might not be quite as effective as the theory says it should be. On a LogMAR chart, three lines difference means twice as big (that is, 6/12 is three lines above 6/6, and 6/24 is three lines above 6/12). In an eye with no pathology, going three lines up from threshold (so twice as large) should mean the patient finds the letters very easy to read. But with the patient shown above, three lines up is… well, it’s an improvement, but still quite challenging, so don’t expect a 2x magnifier to be the magic answer for them. Indeed, they have to be six lines up to read those single letters comfortably, which is four-times larger — twice as large, and then twice as large again — so they’re going to need at least 4x magnification to read threshold text comfortably.

And then, word acuity is often more difficult than single-letter acuity, so they might need even more than that… but more on that later.

The pattern above implies that this patient has a dense macular scotoma just to the right of fixation. Some patients might realise that there really should be five letters on each line, and keep reading all the way over, but often people are used to VA charts that have fewer letters on the large lines so they might not realise anything’s wrong. If the scotoma comes right up to fixation then they might miss the right letter even down to threshold, but often they reach a point where they start reading all five letters (I interpret this as indicating that there’s a little bit of residual vision just next to the fovea so they become able to detect the presence of another letter as the gap between letters narrows).

Lots of macular conditions can cause this, but start thinking about the possibility of a right hemianopia. It’s amazing that a lot of people with even total hemianopias show no awareness that they have lost so much vision, especially if there is some degree of neglect from a stroke. A patient with a total hemianopia will miss the right letters all the way down to threshold, whereas if there is macular sparing they will tend to see the whole line on the smaller letters.

Tip: One that often gets missed is a right macular hemianopia. They’ll pass confrontation tests easily, but they have a dense scotoma right up to fixation. Macular hemianopias aren’t always picked up on standard field tests either, as it only takes a little fixation instability for the patient to see at least some of those points. Hemianopias can come about from posterior strokes that happen ‘quietly’, without the normal signs we associate with strokes, so the patient may not be aware they’ve ever had a stroke. A full hemianopia will normally mean the patient is tending to bump into things more, but a macular hemianopia can have very little effect… until they try to read, that is. Sometimes they might be diagnosed with stroke-induced alexia. But (very) high magnification shouldn’t help someone with alexia, whereas it will usually help a patient with a macular hemianopia.

I call this ‘Goldilocks’ vision (after the story of Goldilocks and the Three Bears, in which Goldilocks finds that the porridge has to be not too hot, not too cold, but juuuust right). This patient really struggles with even the top letters. But as they go down the chart they start to speed up, before then slowing down again. This strongly implies a ring scotoma — an area of foveal sparing surrounded by dense field loss. Above threshold the letters start becoming easier, but at a certain point the letters are getting too large to fit within the foveal area, so they become harder to recognise.

This is actually quite a common presentation in patients who have dry AMD, as the fovea is quite frequently spared until last. If you don’t understand what’s going on, it can be exceedingly confusing. I’ll talk more about ring scotomas later — they deserve their own special discussion. But for now, just remember — when a patient can’t see anything on the top line, give them a couple of seconds before you take the occluder away, because sometimes they’ll notice that they can read some of the letters lower down.

Some Notes on VA in Low Vision

It can be tedious to get patients to always read from the top of the chart, but I recommend you do it anyway. Someone with reasonably good VA might decide to start half-way down, but then you miss out on all that information. Another thing to be aware of is that some patients will stop as soon as they reach the first line they find difficult. It’s worth pushing them gently to ‘give it a go’. Sometimes they’ll read another five or six lines!

Sometimes patients bob their heads around as they’re reading the chart. I used to think that was them trying to get their other eye out from behind the occluder, but I began to realise that it’s often actually a sign that they have a scotoma just next to fixation. Especially in AMD, the scotoma can seem externalised — there’s often a profound sense that there is ‘something in the way’, and patients feel that if only they could look around the blockage they could see. Sometimes patients even ask if I have a special lens that could bend the light around “that thing that’s in the way”.

Another behaviour is patients contorting their bodies to look at the chart in extreme side-gaze (“I need to look at it out of the corner of my eye.”) That’s a sign that they have foveal impairment, and they’re forcing their eye into eccentric fixation to get better VA.

In both of these situations, I pause and have a discussion with the patient about how they can use eccentric fixation in a more controlled way.