Thomas is recovering well say his mum and dad and Dr Jean-Pierre Lin
A five-year-old boy has become the smallest patient to undergo deep brain stimulation at a London hospital.
Thomas Melville-Ross has had electrodes inserted in his brain as a treatment for dystonia - a condition which causes involuntary muscles contractions.
At just 12.6kg (2 stones) - around the same as a toddler - his size meant the operation was only possible due to the development of a new small implant.
His twin sister, Alice, is also due to have the operation later this year.
Dystonia is a painful condition which causes affected parts of the body to develop abnormal movement or postures.
In severe cases it can be massively disabling.
Thomas and his sister, who live in Buckinghamshire, have the condition as a result of being born 16 weeks prematurely.
Deep brain stimulation is the only treatment in severe cases and is done through a surgically implanted medical device similar to a pacemaker.
The implant delivers controlled electrical pulses to affected areas of the brain to block out the signals which cause the disabling movements.
Previously implants have been too large to use in very young children.
But doctors believe that the years before the age of eleven are the most important in terms of neurological development.
A team at Guy's and St Thomas's and Kings College Hospitals said the new implant also has battery that can be recharged from outside the body, meaning it can last for almost a decade rather than only a couple of years.
Surgeons were keen to do the operation on Thomas as soon as possible as his dystonia makes it impossible for doctors to fit him with a cochlear implant he needs to cure his profound deafness.
Dr Jean-Pierre Lin, consultant paediatric neurologist at Guy's and St Thomas' said although the implant takes three months to work there were already signs of improvement.
"Premature babies like Thomas are often affected by dystonia but because they develop smaller physically it has been impossible to give them early treatment.
"As a result they have gone on to suffer a poor quality of life and also a number of knock on problems such as spinal and hip injuries.
"This new device means we can try and help manage their conditions from a far earlier age."
Mr Richard Selway, the surgeon who did the operation on the 30 December, said dystonia could be "disastrously disabling"
"In addition to being smaller, the fact the new device is rechargeable is a massive benefit to the patient.
"This offers exciting possibilities and we anticipate being able to treat a lot more young children in the coming years."
James Melville-Ross, Thomas's father, said it was a big decision to put him forward for major surgery at such a young age but it was necessary.
"It is still early days, just over a week since the operation, but we are hopeful this will improve his long-term quality of life."
Philip Eckstein, chief executive of the Dystonia Society, said it was an "exciting development".
"The painful and uncontrollable muscle spasms of dystonia can be devastating to the child and the child's family.
"The fact that the operation can now be performed at a much earlier age means that there is less chance of pain and permanent muscloskeletal deformity and a much better chance that the child can have a good quality of life like their brothers or sisters."