"The developing nervous system is sensitive to changes in levels of activity. Increased pain exposure in early life can produce changes that are not seen after the same injury later in life, and which can produce long-term changes in sensory thresholds and response to future pain."
Her research group is periodically checking on about 300 children born very prematurely - before 25 weeks and six days' gestation - and treated in an NICU. Interviews and tests of the participants, who are now 19 years old, are being done at present. When they were 11 it was found they were less sensitive to hot and cold than a comparison group drawn from their school classmates.
"Researchers in Canada have shown associations between pain exposure in neonatal intensive care and alterations in brain structure and pain response later in life," Dr Walker said.
"A number of studies show the higher your pain exposure in ICU, the more likely you are to have structural changes in the brain.
"Our laboratory research ... is investigating the mechanisms that cause long-term changes following neonatal surgical injury and evaluating which analgesics are best for preventing or modifying these effects."
It was important for doctors to recognise that patients coming in for follow-up surgery, such as children with a congenital heart defect first treated in an operation soon after birth, might suffer greater pain than new patients.
She said that although the clinical evidence for persistent changes to sensory responses following early-life pain and injury was growing, the reported effects varied in the impact, degree and direction - some studies found decreased pain sensitivity, others an increase - because of varying research designs.
Tot a tough little cookie, mum says
Three-year-old Georgia Delmonte has had three heart operations and appears to have a relatively high pain threshold, says her mother Sarah.
Georgia was born by emergency caesarean at Auckland City Hospital. Her doctor detected a heart murmur when she was 2 days old. Two days later, she had the first of a series of operations, at the Starship children's hospital, to correct an underdeveloped left side of her heart and related problems.
Her second operation was at six months and the third at 18 months and she is due for a fourth soon.
"The hospital pain management was absolutely appropriate and given at the right time," said Mrs Delmonte, of Mt Eden. "She wasn't consciously aware of a lot of pain and if she was it was remedied fairly quickly. The experience for us at Starship was great.
"She is 3 years old now and I would say she is - compared to my older daughter [Poppy, aged 5] who doesn't have any issues - less sensitive to pain.
"For example, she tripped over and she chipped a tooth. After the initial shock she was fine, but it turned out she had broken the tooth. A normal child you would think would be upset for quite a lot of time, but she wasn't."
Mrs Delmonte said Georgia is thriving. "She's doing great, she's doing really well. She's a very vibrant, happy, functional 3-year-old."
