After years of being told seizures were "just in her head", a young Australian girl has finally learned what was causing her mystery illness.

Key points: A DNA test was able to pinpoint Teresa's connective tissue disorder

A DNA test was able to pinpoint Teresa's connective tissue disorder The diagnosis means her condition can be treated and is now under control

The diagnosis means her condition can be treated and is now under control Health experts say DNA sequencing technology could help other patients with undiagnosed genetic conditions

DNA sequencing technology was able to confirm 17-year-old Teresa's condition was due to a defective collagen gene.

But for years a number of doctors had diagnosed her with "mild cerebral palsy". They didn't think she had epilepsy and even suggested her attacks "might be psychological".

Not satisfied with that explanation, she went to see Professor Sam Berkovic at the Austin Hospital's Epilepsy Genomics Clinic in Melbourne.

"Teresa had been told that her problem was largely psychological, which neither she nor her mother believed, and that's always difficult when you go from doctor to doctor and hear that," he said.

The teenager said not having a proper diagnosis made her life difficult.

"I was angry because I wanted answers to it all because I was getting left out of everything," she said.

Professor Berkovic and his team thought Teresa did have epilepsy, and that there might be a genetic cause.

He found out she had a family history of developing cataracts at a young age.

"That led us to consider a family of genes regarding connective tissue, the tissue that sews all our tissues together," he explained.

Professor Berkovic said they took a blood sample and sequenced all of Teresa's genes, looking at different genetic regions until they found the right one. Researchers discovered their hunch was correct.

"It turns out Teresa has a change in a particular collagen gene," Professor Berkovic said.

This genetic change was causing her epileptic seizures and her family history of cataracts.

Since the finding, Teresa has been taking a small dose of epilepsy medication and her seizures have stopped.

"Being on the right medication has changed everything for me," she said. "Now I go out to the movies with my friends and go shopping."

"I got to do a massive fashion parade with TAFE and that was a big deal for me."

Genetic sequencing could help others

Health experts said DNA sequencing technology could help many other patients with undiagnosed genetic conditions.

Professor Berkovic said studies had shown that upwards of 25-50 per cent of patients with mystery illnesses may be solved through genetic sequencing.

"Until recently, we were only able to study a small number of genes in each individual largely for reasons of cost," he said.

"Now we do what's called whole genome sequencing where we can look at all our genes in one fell swoop."

Victorian Health Minister Jill Hennessy said personalised diagnosis and treatment was the future of medicine.

"For patients like Teresa, their life is completely changed by getting a diagnosis," she said.

"It fundamentally changes the landscape of diagnosis and treatment, and I am very excited by the potential of this technology."