The first Alzheimer's patients to test pioneering gene therapy are proof of the treatment's promise, say doctors.
The surgery takes around 11 hours
Between 2001 and 2002, surgeons at San Diego's University of California placed genetically modified tissue into the brains of eight Alzheimer's patients.
It is designed to boost a naturally occurring protein that stops cell death and stimulates cell function.
Now six patients are showing signs that the implants have successfully slowed their disease, Nature Medicine reports.
The surgeons decided they were ready to begin trials in humans after getting promising results in primates.
In the animals, the therapy restored old, shrinking brain cells back to near-normal size and quantity, as well as connections essential for communication between the cells.
Initially, the surgeons carried out the operation while the patients were awake but lightly sedated.
However, two of the patients moved as the therapy, grown from skin cells taken from the same patients, was being injected into the brain, which caused bleeding.
One of these patients died five weeks later. As a result of the bleeds, the surgeons changed the way they carried out the operation and all of the subsequent surgeries were performed under general anaesthesia without any complications.
Professor Mark Tuszynski, the neurologist who led the research, said their latest follow up of these six patients suggested the treatment was working.
Memory tests suggest the gene therapy has slowed cognitive decline by as much as 50%.
Brain scans also show that the patients' brains are more active than before.
When they carried out a post-mortem on the patient who died, they found some of the brain tissue that had been dying off through Alzheimer's had started to rejuvenate.
These areas were around the sites where the injections had been given.
Professor Tuszynski said that although it was still relatively early days, if the findings were confirmed it would be the first treatment that doctors had to actually prevent cell death in people with neurological diseases.
"If validated in further clinical trials, this would represent a substantially more effective therapy than current treatments for Alzheimer's disease," he said.
Their studies so far have been to check that the technique is safe.
Now that has been shown, further studies can be done to determine how effective the treatment is.
Harriet Millward, deputy chief executive of the Alzheimer's Research Trust, said the research was very exciting, but cautioned that it would not be a complete cure for Alzheimer's.
She said they were currently funding research into drugs that mimic the action of nerve growth factor (NGF), the name given to the gene therapy involving stimulating cell function.
"In principle, if you can get the NGF into the brain successfully, this could offer a way of slowing down the decline of mental abilities in patients with Alzheimer's disease," she said.
Professor Clive Ballard, director of research at the Alzheimer's Society, said: "Although very preliminary, the findings are consistent with previous studies in animals, and offer an extremely exciting possibility of a novel therapy.
"We very much look forward to further studies."