Tillie has 'childhood dementia'. Now, her parents fear she's been given a death sentence.

Tillie Mae Mawdsley is just nine years old, and is not expected to live to see her teenage years.

When she was just a toddler, Tillie was diagnosed with a rare genetic condition called Sanfilippo, which causes the progressive breakdown of the central nervous system.

The life-limiting illness means Tillie will lose the ability to walk, talk and eat. The condition is likened to a childhood form of dementia.

"It's memory loss, the disease attacks the brain," Tillie's mum, 36-year-old Michala Mawdsley told The Sun of her daughter's condition.

"It's horrible, it really is like a childhood dementia."

For the last six years, Tillie has been a part of a clinical trial offered to just six children in the UK by an American pharmaceutical company. The enzyme replacement therapy involved Tillie and her family making a 640km round trip from their home in Hertforshire to Manchester Children's Hospital every four weeks.

Now, the drug trial has been stopped without warning, stating that patient's weren't improving.

Tillie's parents believe she has now been handed "a life sentence".

"We feel like the rug has been pulled from beneath us... It's difficult to accept there's nothing we can do for her now," mum Michala said.

"Tillie’s treatment gave her a quality of life. The trial was making her better and I believe that drug was working."

Michala and her 40-year-old husband Paul believe the drug had stopped Tillie Mae's symptoms from worsening.

Without treatment, Tillie will regress back to being a toddler, 'un-learning' all the skills she has acquired while on the drug trial.

"We were grateful that she had the opportunity but she stayed the same and that was enough for us," Michala said.

"They wanted to see a change in children and argued the drug wasn't causing any cognitive improvement. They believed it was going to be the next big thing but then one day the trial just stopped and they never really gave us any real reasons why."

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Michala said that Tillie's older sister, 10-year-old Lexie, was devastated to hear her sister's trial had ended.

"When...Lexie found out that the medicine had stopped she said 'Tillie Mae is going to die now', it was awful hearing it," she said.

Now, Tillie's family is focused on taking things "day by day" and making memories to last a lifetime with the limited time she has left.

Tillie's parents are now fundraising to raise awareness to help other children suffering from the same illness.

"It would be a dream to find a cure for Sanfilippo. People need to know what it is just like they know what meningitis and Down's Syndrome is," Michala said.

"Hopefully in time we can help to find a cure."