Single-letter high-contrast visual acuity correlates poorly with visual function in a pathological eye. But it’s still worth measuring.
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 get there, because that gives me some pretty good clues as to how intact their visual field is.
When we’re measuring the VA of a person who has some degree of optical blur (myopia, etc), this is what we get:
The lines marked in red here are those the patient cannot read at all. The line marked in orange is the patient’s threshold, the last line they read, in which they might miss some of the letters. (This is the line we call the patient’s Visual Acuity).
The two lines in yellow are above threshold, but they’re still a bit challenging, and 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).
This pattern holds true for a normal blurred eye no matter what the eventual VA is. Here’s a patient with a reduced VA due to optical blur:
Notice that the VA is worse, but there is still the same three line gap between threshold and the letters that are read easily.
In contrast, when we deal with an eye with some sort of pathology, we often get a different journey. Here’s an example.
Notice that the patient is badly struggling with the last three lines before absolute threshold, and still having some difficulty with the line even 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 (often caused by Parkinsons disease).
It can be tedious to get patients to always read from the top of the chart, but that’s what I recommend. Someone with reasonably good VA might decided 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 struggle with. It’s worth pushing them gently to ‘give it a go’. Sometimes they’ll read another five or six lines!
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 our vision actually works.
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”.
There are other patterns to watch for too. For example, this patient keeps missing letters over on the right side of the chart:
This pattern implies a quite dense macular scotoma just to the right of the patient’s fovea. If the patient’s got good insight, they might twig that there really should be five letters on each line, and keep reading all the way over, but often patients are used to VA charts that have less letters on the large lines so they don’t 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.
This can also indicate 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.
And what do you make of this patient?
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.
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.
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