Calls have been made for more visual specialists in schools across Wales, to ensure children with Irlen Syndrome receive a diagnosis.
Irlen UK said children are “being turned off education” because they are not aware they have the condition.
It affects the brain’s ability to process visual information and can cause difficulty reading and writing.
Education Secretary Kirsty Williams said all pupils have an eye test by an optician when starting school.
Irlen UK said about 15% of people are thought to be affected by the syndrome.
Specialist tests – Irlen UK said it cannot be diagnosed by an optician – can determine whether a person has it, followed by an assessment to identify which coloured overlays, either as glasses or lenses, should be worn.
But it is not yet recognised as a medical condition, meaning assessment and treatment is not funded by the NHS.
People with the disorder said they believed earlier intervention would have helped them.
Jennifer Owen, 28, from Merthyr Tydfil, found out she had the disorder in 2012 and now campaigns to raise awareness of the condition.
She did not gain any GCSEs and believes she would have done better if she had been screened in school.
“I couldn’t read or write. My teachers thought I wasn’t trying,” she said. “There’s a risk of children falling through the net.”
Sarah Chambers, 45, from Cardiff, who has Irlen, said her daughter has recently been diagnosed aged eight.
She claimed teachers were not trained to properly identify when a child might have it, meaning they “go through school thinking they’re failing”.
Ms Chambers added cost was another issue, as many families might not be able to afford the private assessments.
What is Irlen Syndrome?
- A perceptual processing disorder- it is not an optical problem
- Symptoms include distortion of words, brightness and glare, effects on reading and writing, poor motivation
- People with the condition may also have ADHD, dyslexia and behavioural problems
- Coloured overlays can be worn to filter out specific wavelengths of light
- It can cost up to £450 for a private assessment and coloured overlays
Stephanie Jamison, Irlen director and diagnostician for south Wales and south west England, said there are seven Irlen UK certified screeners in schools across Wales but more were needed.
While funding for testing is available at university level, she said children may struggle to reach further education without a diagnosis.
“They could have been turned off education because studying is so difficult for them,” she said.
“Sometimes children are being excluded from school for it.
“Irlen isn’t just a reading problem, it’s a light sensitivity issue which can have a wider impact like poor concentration and fatigue.”
Ms Jamison acknowledged budget constraints and said certified specialists could be shared between schools.
Ms Williams said: “The Welsh Government recognises the importance of putting in place early and effective interventions to ensure that any learner experiencing difficulties with their vision is not adversely affected in terms of their educational outcomes.”