An experimental autism therapy that helps parents communicate with their children has had a record level of success in reducing core symptoms of the condition.
A trial involving daily 20 to 30-minute sessions of planned communication and play activities showed benefits that lasted six years after the treatment ended.
It is the first time long-term effects of such an early intervention have been demonstrated.
The follow-up study was designed to look at long-term outcomes from the Pre-school Autism Communication Trial (Pact), which involved 152 autistic children aged two to four.
In the new analysis, 121 or the original participants were re-assessed after six years. Of these, 59 had received the Pact therapy.
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Symptoms in the treated children were found to be less severe, with improvements in social communication and a reduction in repetitive behaviour.
Although no changes were seen in other areas such as language, anxiety and challenging conduct, experts regarded the results as highly significant.
Study leader Professor Jonathan Green, from the University of Manchester, said: "This type of early intervention is distinctive in being designed to work with parents to help improve parent-child communication at home.
"The advantage of this approach over a direct therapist-child intervention is that it has potential to affect the everyday life of the child.
"Our findings are encouraging, as they represent an improvement in the core symptoms of autism previously thought very resistant to change.
"This is not a 'cure', in the sense that the children who demonstrated improvements will still show remaining symptoms to a variable extent, but it does suggest that working with parents to interact with their children in this way can lead to improvements in symptoms over the long-term."
The findings are published in The Lancet medical journal.
