5b - Reduced Visual Acuity

Posted by Helen St Clair Tracy in Level 5 - CVIs Basic (Occipital)
Published: 28/01/2020, 12:23am | Updated: 24/02/2020, 7:44am

Video Link: https://vimeo.com/391942489

Sight Test

Before considering CVI as a cause of low vision it is important to understand that up to date eye tests are essential to check for eye disorders, and to ensure significant refractive errors (near or long sight, and astigmatism) are corrected with spectacles or contact lenses. Conditions of the eye causing reduced vision need different approaches to help the person. For the person who has additional support needs or learning difficulties, sight tests need to be carefully planned. The UK charity SeeAbility has a lot of useful information on their website (click here).

Reduced visual acuity means that the measured level of visual acuity is less than typical. Typical visual acuity is sometimes referred to as '6/6' or '20 20 vision'.

Visual acuity is explained in Lesson 3b, please review this lesson if needed.

There are different ways of measuring visual acuity, depending on where you are, and what you are doing, for example the science and research communities tend to use LogMAR, while the USA uses Imperial measures. The conversion table (above) shows the different measurements, and has an additional column called Line Width.

The line width relates to the minimum thickness a line needs to be for the person with that level of visual acuity to see it clearly, at 25-30cm.

That word clearly again. So something narrower than that line width doesn't mean it necessarily disappears, it just won't be seen so clearly.

But it is not just lines - there are lines and there are gaps.

The gaps separate things. If the gap is too small to be seen, then the image will be less clear.

For most people it should be relatively easy to recognise this image (below) as lots of bunches of flowers:

But would you have recognised the flowers from this image (below):

This image has been simulated to show what the flowers would look like for someone who has visual acuity measured at 6/60 (see conversion table above). That means that anything with a line width of less than 0.9mm (or just under a millimetre) is not seen clearly. But a millimetre is tiny!

How can such a tiny change make such an important difference?

The answer is simply that it makes the difference between seeing and recognising, but people often may not realise this, and when they think about making things big enough, they focus is on things like just font size, but what about everything else that needs to be recognised as well?

So, what's changed? Let us look at the two images side by side, with a difference of less than a millimetre in terms of line width...

Compare any of the bunches of flowers, and you will see that the detail has been lost - they are less clear, because the visual acuity is reduced.

What about these two images below:

On the left are letters from an eye test, on the right some text (taken from a blog a parent wrote for us).

On the eye test, look for the letter c on the third row from the top, second from the right (between the letters N and R).

On the text, look for the letter c on the third line from the top, in the second word 'including'.

Both letters c are a similar line width, although slightly different shape.

The c on the eye chart has a very narrow gap where the two ends of the letter nearly meet - if that gap was much narrower people might not be able to see it and mistake it for the letter o (this is what we mean when we say the gap is as important as the line width).

Imagine you are in the optician's clinic, the lights are off and in a quiet room you are reading the letters you can see. Think of what your brain is doing - recognising single letters in a quiet, uncluttered (as it is usually dark) environment.

Now imagine you are in a classroom, reading the passage from the Parents Blog, and think again what your brain is doing. Possibly processing clutter and noise (depending on the classroom), reading multiple symbols and putting them together into words which are recognised and then the words put together in sentences which are further recognised and the meaning extracted, all combined with the meaning of the previous sentence, so as to extract further deeper meaning, possibly also feeling the pressure of time limitations.

So, we ask - are the two letters c seen equally clearly just because they are a similar line width? Or not?

A person's measure of visual acuity is a useful tool, and provides an indication of how big things need to be, to be seen clearly. But this is a minimum standard, and sometimes, due to many factors, their visual acuity might be even more reduced than the level formally measured.

In children it is relatively rare to have an isolated deficit in visual acuity due to the brain, and it is most commonly a part of a combination of CVIs including reduced visual attention due to simultanagnostic vision, introduced in Lesson 3g. Where this is the case, if you make something bigger, you may be making it harder to see.

Consider these two toys (below)

For many with reduced visual acuity, things are just made bigger, but what if the person also has reduced visual attention due to simultanagnostic vision?

Making something bigger, might mean that less of it is seen, so it can be harder to make sense of what is being looked at.

Looking at the images above, which makes more sense? The very small image on the left where the detail is less clear, or the image on the right, where the image is bigger but only a part of it can be seen? We call this the Visual Acuity / Simultanagnostic Vision Problem and will consider it further with suggested approaches in future lessons.

So, visual acuity is about line width, but not just that. Visual acuity is also about what the brain is doing, and importantly, needs to be understood along with any other conditions or difficulties the person may be affected by, including other CVIs.

Targeted Support

Make things big enough to be clearly visible*.

*Unless other CVIs are present, this will be explained in a future lesson with an alternative approach for targeted support where reduced visual acuity is accompanied by simultanagnostic vision.

Visual Behaviours

Reduced Visual Acuity is one of the most recognised CVIs and there are many different tests for this, appropriate for different ages and abilities. Even with a profoundly disabled person, it should be possible to measure their visual acuity.

The ability to see clearly does vary, and this might cause, or contribute towards reading difficulties, and reading may become more difficult when:

  • In a cluttered or noisy environment
  • Where the person feels pressure or is not enjoying what they are reading
  • Where the text / words / lines and paragraphs are very close together
  • Certain decorative fonts or handwriting

This can further reduce the functional visual acuity, because the person may be feeling worried or frustrated, using up even more brain power - and so a downward spiral is created.

In social Interactions, reduced visual acuity may cause or contribute towards:

  • Clumsiness, for example knocking things over can cause difficulties. Seeing less clearly affects how accurately we reach for things and move through the world. This can make the person feel embarrassed.
  • Picking up the wrong things
  • Not recognising people
  • Apparent forgetfulness / absent mindedness - where things have not been seen as clearly as expected, including by the person themselves. This can make the person feel as if they are stupid.

Self Referencing

Where a person has known and measured reduced visual acuity, someone should ensure things are big enough to be seen clearly. Often this is applied to school or work, through reading and writing, but what about other areas in life, for example:

  • A subtle change in a facial expression may be missed by someone with reduced visual acuity.
  • Everything in a cluttered busy environment is going to be slightly more difficult to see and find, and extra effort is needed to ensure everything is still clearly visible enough


Before you move onto the next lesson, please check you understand the following:

  • The importance of an eye test
  • What reduced visual acuity is
  • The difference between line width and the gap that separates things
  • That visual acuity varies
  • You can name many factors that can cause visual acuity to vary
  • That visual acuity is uncommon in children on its own, and is usually part of a number of CVIs
  • That sometimes making things bigger can make them less easy to understand, called the Visual Acuity / Simultanagnostic Vision problem

Next lesson: Level 5c CVIs Basic (Occipital) - Reduced Contrast Sensitivity

Further reading is not necessary to proceed, but if interested, you may enjoy the following:

What Is CVI? Understand / The Image

What Is CVI? Assess / Functional Vision

What Is CVI? Assess / The Technical Stuff / Visual Acuity

Simulated Images

Ulster University has an excellent free resource section for parents, with a lot of information about visual acuity, click here.

Visual Acuity / Free Resources


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