The surgery took just 30 minutes, but follow-up care and check-ups meant the family stayed in Ohio for three months with their daughter who was aged about five months at the time.

The initial results looked promising and the family travelled back recently for a six-month evaluation.

Infants with Canavan may show poor head control, listlessness and later develop feeding difficulties, seizures and life-threatening health issues.

Previously, getting a diagnosis for Canavan disease could be difficult and parents would then be told there was no treatment.

This is something Dr Lee Coffey, a lecturer and molecular biologist at South East Technological University in Waterford, experienced when his twin brother had two children who received the devastating diagnosis.

“It took months of persistence for them to finally find out, and in the meantime the symptoms were accumulating,” Coffey recalls. They had to pay privately for their own genetic sequencing.

The family live in Melbourne, Australia and the oldest child is now about 10 years of age so it is sadly unlikely that the new therapy will help.

The faulty enzyme leads to a toxic build-up of a compound in the brain, which causes the child’s brain and health to deteriorate. There was no treatment offered for Coffey’s nephews after their diagnosis.

Coffey changed the direction of his research as a direct result of seeing what his brother and his family were going through.

“I had never heard of it [Canavan disease]. I was Googling like everyone else,” Coffey recalls.

He began collaborating with US scientists who were working on viruses to deliver a gene therapy by intravenous injection. This research was led by scientists at the University of Massachusetts in Boston, a separate group to the Ohio trial.

Coffey aimed to make a “super enzyme” in his lab by developing variations of the natural enzyme. In animal studies, his modified enzyme was more active than the natural version and therefore could be considered for a future therapy.

It could mean a lower dose of the therapy and fewer copies of the modified enzyme gene achieving the same effect – potentially lowering costs and minimising any side effects.

The earlier you can get a working copy of the gene into the brain cells of patients the better.

This will result in production of the functioning enzyme, called aspartoacyclase (Aspa), which does the job of clearing away N-acetyl-aspartic acid (NAA).

NAA is a natural compound in the brain but without the working enzyme it piles up to harmful levels and impacts the insulation (myelin) around nerve cells. This insulation is crucial for the speed and integrity of nerve signals.

A child’s health will then deteriorate as they age and the condition is often fatal by the age of 10. Gene therapy aims to land a working gene into enough brain cells so that adequate quantities of the Aspa enzyme are made for the child’s life. Stopping damage to the brain as early in life as possible is the best scenario.

Back in Ohio, neurosurgeon Dr Rob Lober at Dayton’s Children Hospital carried out the procedure on the Irish baby. He was satisfied with how it went.

“Within the first month she was laughing in our clinic and there were changes that the family could see,” he says. There were also positive signs of reduced swelling on brain scans.

The surgeon had been apprehensive after the very first procedure. “I sat there thinking: ‘Oh God, what have I done? I’ve just injected trillions of viral particles into this brain,’” Lober recalls.

Our small community hospital probably has the most experience in the world in Canavan disease, because we’ve seen so many patients. But you don’t want to give false hope. This is still an experiment

—  Dr Rob Lober, neurosurgeon at Dayton’s Children Hospital, Ohio

Patients’ families have travelled from Ireland, Chile, Argentina and Slovakia and stayed in Dayton for months to allow for follow-up.

“I didn’t expect to see much of a result for the first one to three months,” says Lober. “What we’re seeing is that within a couple of weeks they’re brighter, they’re more alert.”

It is still uncertain what dose to give – whether one injection will be enough or when children might be too old to benefit as they deteriorate over time, accruing damage to their brains that might not be recoverable.

“Our small community hospital probably has the most experience in the world in Canavan disease, because we’ve seen so many patients,” says Lober. “But you don’t want to give false hope. This is still an experiment.”

The family, who have asked not to be named, say they were delighted with the care provided by the medical team at Dayton’s Children’s Hospital.

“While it is still early days, we are already beginning to see some positive impacts of the treatment and we can say without doubt that we have a happier, more content little baby,” the family told The Irish Times.

The therapy was developed by Prof Paola Leone, a neuroscientist at Rowan-Virtua College of Medicine and Science in New Jersey.

“Canavan disease is an ideal candidate for gene therapy. It is caused by a single gene and it affects a single organ,” says Leone.

She spent three decades working on a therapy for the disease, after publishing a small study on the condition in the 1990s. This led to parents showing up at Yale University lab carrying their three-month-old baby who had just been diagnosed with the disease.

“I was a junior researcher at the time and I didn’t even have funding,” she recalls. “Canavan found me, rather than me finding Canavan.”

Her group last year reported positive results from eight children with the condition, including the baby from Ireland, in the scientific journal Nature Medicine. The procedure can be more straightforward with babies, since they have a natural soft spot on their head that a surgeon can sneak a catheter through.

The biotech company Myrtelle, based in Wakefield, Massachusetts, is leading the trial.

Jordana Holovach, head of communications and community at the firm, says it plans to submit a biologics licence application to the US Food and Drug Administration this year, with potential approval in 2027.

Holovach set up a non-profit organisation to fund research and raise awareness of Canavan after her first child, Jacob, was diagnosed with the condition.

Lober admits he had some worries about the experimental procedure. “I came to this late. I didn’t really know what to expect,” says Lober.

“You walk this tightrope as a surgeon, with the stress of potentially causing adverse side effects. But it’s been an amazing experience.”

Aside from Canavan disease, there are other rare genetic disorders called leukodystrophies that also cause damage to the oligodendrocytes – the support cells around nerves that assist with critical white matter insulation.

It is hoped the gene therapy techniques being developed for Canavan babies may also prove helpful in treating children with these other conditions.