CVI Prevalence Paper (CVI Project Findings)

Paper: Cerebral visual impairment-related vision problems in primary school children: a cross sectional survey. Cathy Williams et al Link at end of page.

How many children have CVI?

We don't know for sure, but this research project is the first major study of its kind seeking to find out.

Several mainstream primary schools in England, where children are aged between 5-11 years, participated in this research project, as well as one Special Needs primary school.

2298 mainstream school children, and 14 special needs school children were involved in the study.

The average age was 8 years.

As this study gave a one-off "snapshot" of each child, not a full review, it talks about "CVI related vision problems". This is not the same as being diagnosed with CVI. However it is very likely that some or even many of the children with CVI-related vision problems would be diagnosed with CVI if they were to be seen in a suitable clinic.

In addition to seeking the prevalence of CVI related vision difficulties, part of the study also sought to establish whether there were any groups of children who were more likely than others to have such difficulties. So they investigated three distinct groups:

1. A Red Flag Group
A 'red flag' is where a child has already been identified as needing support with their learning and/or (the child) has been given a high score on a behaviour questionnaire filled in by teachers and some parents (suggesting problems). The red flags are listed in Table 1 of the paper, and range from a child receiving extra help in class, to a teacher or parent reporting a child's difficulties.

2. A Random Group
A random group of children who were not being given extra help, was selected to separately seek out any differences between the Red Flag children and those who were thought to be managing their school work effectively.

3. A "Special School" Group
The children from the Special School became the third group.

So, we have three groups of children who were studied in detail. Only just under half the children who were invited to an assessment came with a consent form so could be included.

The final participating numbers were:

  • 1. Red Flag - 207 children
  • 2. Random - 41 children
  • 3. Special School - 14 children.

The total in the mainstream school group is 248, made up of 207 red flag children and 41 random children. This group is not meant to be representative of a typical class, it is very heavy on red flags.

Mainstream School Results

Red Flag
Of the 207 Red flag children:

  • 35.3% (73) had CVI related vision problems

Of the 41 Random children

  • 12.2% (5) had CVI related vision problems

Red Flag & Random Combined
Of the 248 mainstream children:

  • 31.5% (78) had at least one CVI related vision problem
  • 10.5% (26) had multiple CVI related vision problems

The 78 children with at least one CVI related vision problem, as a percentage of the full study of 2298 mainstream school children = 3.4%.

So, in mainstream primary school children, where a typical class has thirty children, the evidence shows that on average, at least one child per class will have CVI related vision problems.

That 'at least' is important.

  • Participating = 2298 children
  • Assessed = 248 children.
  • Not Assessed 2050 children.

Of the 248 who were assessed we know 78 had CVI related vision problems.

Some children had one CVI related vision problem, some had two and some had three.

We don't know how many of the 2050 unassessed children might have CVI vision related problems, but it is highly like some would have done, so this 3.4% estimate is a minimum and may well be an underestimate.

In the full participating group of children, 20% were found the have 'red flags'. Of the remaining 80% it may be the case that there are children who also had 'red flags', but these were not picked up through the early questionnaires. When it came to selecting 5% to form the Random group for assessment, to compare findings of prevalence of CVI related visual difficulties, to the children with 'red flags', several children were removed from that group because more detailed investigations found they also had red flags.

Of the random children, the prevalence of CVI related visual difficulties was found to be 12.2%.

  • We don't know if the random group would deliver a prevalence result of 12.2% if more children had been tested, particularly as this was only a small group (of 41 children, 5 had CVI related vision problems). The CVI related vision problems of four out of five of these children related to only one CVI. A fifth child in this group had multiple CVI-related vision problems, and it emerged they were born prem. There may well be unidentified 'red flag' children within the untested group of random children.

Of the red flag children, the prevalence of CVI related visual difficulties was found to be 35.3%.

  • We don't know if the red flag group would have delivered a prevalence result of 35.3% if more had been tested, although as this was a bigger group, this result can be considered statistically more robust. Of the children in this group who were assessed, many had a diagnosis of a neurodevelopmental disorder, but in the unassessed red flag group, these were a minority. This suggests that if the whole of the red flag group had been assessed, that 35.3% might have been less.

There is a lot we don't know, and we need further information.

What this research does tell us, is that it is likely that many more cases of CVI would have been identified in the remaining 2050 children if they had been assessed, and that the 3.4% figure would go up quite a lot. But this has yet to be evidenced.

It also tells us that when a 'red flag' is identified for a child, that child needs to be investigated for CVI, and if CVI is found, appropriate measures need to be put in place.

This paper shows us the direction for more research, particularly into which groups of children are more likely to have CVI related vision difficulties, and how they can best be helped when CVI is identified.

Scotland has one of the most sophisticated systems for recording visual impairments in children in the world. Approximately 0.07% of children in Scotland have a CVI diagnosis. That means, from these findings, that for every child in Scotland with a CVI diagnosis, there are likely at least fifty who remain undiagnosed. From this study, we also learn that of these undiagnosed fifty, 40 (80%), are likely to be struggling at school.

Our results suggest that on average, there would be at least one affected child in every class of 30 children.

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This is a massive deal!

In mainstream schools CVI is likely to be greater than three times more common than autism.

Special School Results

Special School
Of the 14 special school children:

  • 57.1% (8) had CVI related vision problems.

In the special school the percentage of children with CVI was 57.1%, but this was only a small sample, comprising only 14 children. The only other published research we are aware of where such a group of children was assessed for indications of CVI associated visual processing difficulties was the study from Northern Ireland we have already featured (Eyecare & Behaviour Improvements in Special Schools Paper) which reported just under a quarter (23.5%) as likely having CVI. Children in special schools cover a huge range of abilities and disabilities, but these two figures of 23.5% or 57.1% suggest to us that CVI related accommodations should be present in every class in every special school.

Two things we LOVED!

  • Someone thought ahead about how children singled out as a 'red flag' might feel stigmatised, so they made a point of being clear that there was a wide group being tested.
  • This research included the views of parents and carers. Parents are the world experts on their child, and as we said in our coverage of the CVI (with Optic Ataxia) in Autism Paper, "It might not please the statisticians, but for research to reflect what is real, parents need to be included."

This paper is discussed in our Newsletter 30, including considerations about the wider implications of many of the important points it raises, and the relevant links.

The Paper:

This paper is very easy to read. Some of the number crunching around the statistics and analysis can get a little complex, but overall it is simple to follow and understand.

Paper: Cerebral visual impairment related vision problems in primary school children: a cross-sectional survey. Cathy Williams et al. Developmental Medicine & Child Neurology. 3 February 2021.

Watch CVI Project leader Dr Cathy Williams discuss these findings, from 2.43.00Video Link:


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