Up to one child in every classroom may have a visual impairment which stops them fully seeing the world around them, prompting a Kiwi to come up with a unique app to detect the issue.

Nicola McDowell was just 16 when she suffered a major brain haemorrhage which left her without the ability to see large chunks on her right side.

She lived with it for 17 years before a chance encounter with a paediatric ophthalmologist saw her diagnosed with CVI – Cerebral Visual Impairment. It's an issue caused by damage or injury to the brain as opposed to the eyes. While there is no cure it can be managed with support and vastly improved vision and quality of life.

"It completely changed my life when I was diagnosed with CVI at age of 32," she said. "Beforehand, I couldn't do things and I didn't know why. I thought I was just weird and useless, whereas now I know why."

International research has found 3.4% of children have CVI-related issues, with the majority having difficulty in school. Many are undiagnosed as it's not picked up in current childhood eye screening.

Paediatric ophthalmologist professor Gordon Dutton said "it is very poorly understood and very poorly recognised internationally, and yet it is common".

While working in education, McDowell met a boy named Austin living with cerebral palsy and low visual acuity. She soon realised he was struggling with the same visual issues as her, sparking the idea to create a way to screen for CVI.

Building on her own experience, she came up with a simple activity of matching cards - which she called the "Austin Assessment" - and has since developed into an app – a world first.

"My aim is that no child should ever experience when I did and go undiagnosed and unrecognised with their visual issues," she said.

It's proving a success.

The Austin Assessment correctly identified that 14-year-old Ella Bartley had CVI. She'd suffered a brain tumour at the age of two.

"I can't see my lower visual field," she said, "which is very hard because I trip over a lot of stuff."

Since being diagnosed, there's been a significant change in her grades and mental health.

"It's changed my perception on everything. I don't doubt myself as much anymore and I can understand myself better.

"I always felt dumb," she said, "so it was a big relief to find out there's an actual reason for that, and that I'm not just not smart."

Trials of the app are going well, with plans to develop it for public use both in New Zealand and abroad.

Dutton, who's based in Scotland, said "it is a unique test and nothing like it has ever been invented to my knowledge in the past".

McDowell is now trying to raise $650,000 to make the app more widely available.