Brain changes in those with Huntington's disease have been detected 24 years before noticeable symptoms emerge, raising hopes that doctors can intervene before sufferers become ill.
In a groundbreaking study by University College London researchers, subtle increases in the protein known as neurofilament light (NfL), a product of nerve cell damage, was recorded in the spinal fluid of those with the incurable hereditary illness.
Until now, it was unknown when the first signs of damage to the brain begin to appear in patients.
It is hoped this breakthrough will help researchers ascertain the ideal time to intervene and start treatment for the disease, with the aim of prolonging the onset of symptoms for as long as possible.
Professor Sarah Tabrizi, who led the study, said it was a "significant advance" in understanding the earliest phases of the disease and the best time to begin treatment.
"Ultimately our goal is to deliver the right drug at the right time to effectively treat this disease," she said. "Ideally we would like to delay or prevent neurodegeneration while function is still intact, giving gene carriers many more years of life without impairment."
Co-author of the study, Dr Paul Zeun, added: "We have found what could be the earliest Huntington's-related changes, in a measure which could be used to monitor and gauge effectiveness of future treatments in gene carriers without symptoms."
The researchers studied 64 young people with the genetic mutation found in Huntington's disease and a control group of 67 others without the genetic mutation.
They found that 47 per cent of those with the genetic mutation had more of the NfL protein in their spinal fluid than those in the control group.
It was predicted, based on the participants' ages and genetic factors, that it would be 24 years before other symptoms associated with the disease would emerge.
These include changes in thinking, behaviour or involuntary movements.
The study was published in The Lancet Neurology.