A mother from Swansea is facing a bill of almost £3 million to save her five-year-old son's life, after the first gene therapy for his rare form of dementia was approved in the US - and priced accordingly.

Tate McDaid was diagnosed with Sanfilippo Syndrome when he was four. The condition, a form of dementia that affects just over 100 children in the UK, is neurodegenerative and causes the gradual loss of abilities like walking and eating. It's often described as "childhood Alzheimer's" or "childhood dementia," which is the kind of phrase that should make you sit down.

In September, the first gene therapy to treat the condition in children was approved in the US. His mum, Tammy, said she spent "months praying" for that approval - only to discover the treatment costs £2.95 million per child, with it entirely unclear how families outside the US are supposed to fund it.

"I was excited when the treatment initially got approved but then reality hit hard the next day," Tammy said. That was when she read the US price for the one-time treatment had been set at $3.95 million.

Tammy contacted Ultragenyx, the company behind the treatment, hoping to understand how families outside the US - you know, the ones without American health insurance - are meant to cover the costs. "We understand what they are charging for American citizens, but they've got health insurance," she said. "Not even just the UK, but everyone else in the world looking for this treatment."

Asked how the US-approved drug, FAYUVI, would work for families outside the US, Ultragenyx offered a statement that manages to be both accurate and profoundly unhelpful: "There is not a single estimate we can provide for the total cost of care for an international patient, as those costs would depend on the patient's individual medical needs and the treating institution."

The Society for Mucopolysaccharide Diseases, a charity supporting children with Sanfilippo Syndrome, says about 140 children in the UK have the condition. There is currently no cure for Sanfilippo Syndrome Type A, also called Mucopolysaccharidosis type III (MPS III), but the new drug could delay its onset and allow Tate to remain active for longer.

That's the crux of it: it can progress more rapidly after the age of five, which is why Tammy says "it's really a battle of time" to raise enough money for a treatment not yet available in the UK. "If a millionaire turned around tomorrow and gave us the money, we'd go to the US straight away," she said.

With Tate turning six this month, Tammy doesn't believe time is on their side. "Tate is already shown signs of cognitive decline," she added. "The older he gets the less the treatment will work and the more ability he will lose in the process. I feel like we are at a turning point and I'm petrified."

She describes him as "the happiest little man" who is "blissfully unaware" of his problems - and explains how important his mobility is to him. "He doesn't speak, he's never spoken. But his mobility is everything to him and that is why I'm fighting for it," she said.

Tammy began raising money before the new drug was approved by the US Food and Drug Administration and now has more than £160,000. That covers only about 5% of the total estimated cost. "Unfortunately, we're in a world where we need millions, not hundreds of thousands," she added. "I'm just really hoping by some miracle the pharmaceutical company eventually come back to us and say they can do it for a little bit cheaper, but it's still going to be in the millions."

Ultragenyx, for its part, says it is "pursuing a global regulatory strategy" and that FDA approval is "an important first step in our long-term goal of bringing this treatment option to families around the world." The company added that it understands "the urgency families affected by Sanfilippo syndrome Type A face in identifying treatment options and why some may consider travelling to the US to access gene therapy," and recommends families "work with their child's healthcare provider to understand what potential access pathways may exist before assuming this is their only option."

Meanwhile, staff from the Noah's Ark Children's Hospital in Cardiff are discussing the situation with officials in the US on the family's behalf, and health officials from Wales are talking to their US counterparts to see what potential access pathways exist for Tate. Which is diplomatic language for: everyone is trying to figure out how a five-year-old gets a drug that costs more than a house.