SYDNEY, Feb 14 - A team of paediatricians and researchers at the Telethon Kids Institute assessed and diagnosed youth aged 10-17 incarcerated at the Banksia Hill Juvenile Detention Centre at Canning Vale, Western Australia.

Lead researcher Professor Carol Bower says they uncovered "unprecedented" levels of severe language and cognitive impairment and FASD - a neurodevelopmental disorder caused when an unborn child is exposed to alcohol in the womb.

"Of the 99 young people who completed full assessments we found 36 of them - more than one in three - had FASD," Bower said.

"Of this 36, only two had been previously diagnosed."

According to Bower, this is the highest known prevalence of FASD in a custodial/corrective setting worldwide, and almost double the previous highest Australian estimate in a non-custodial setting.

Just as worryingly, 89 per cent of the sentenced young people had at least one severe neurodevelopmental impairment, whether they had FASD or not. 

The sorts of difficulties experienced included problems with planning, memory, cognition, motor skills and attention.

"Almost half the young people had severe problems with language, how to listen and understand and how to reply and explain what they think," Bower said.

Published in the British medical journal BMJ Open, the findings have prompted calls for every child to enter the Australian justice system to undergo neurodevelopmental assessment.

Paediatrician Dr Raewyn Mutch says many of these kids would have just been written off as "naughty children".

She says is likely their "socially unacceptable" behaviour has arisen from a brain that just isn't working properly.

"Some of these young people were profoundly impaired, yet for many this was the first time they had received a comprehensive assessment to examine their strengths and difficulties, despite attending schooland, in many cases, despite their prior engagement with child protection services and the justice system," Mutch said.

"These are missed opportunities for earlier diagnosis and intervention, which may have prevented or mitigated their involvement with justice services - and more importantly may have permitted alternative community care with targeted health and educational interventions and rehabilitation."