WILL HE GROW OUT OF IT?

Many of us must have shuddered when a paper has been returned from a journal with the suggestion that the statistical method is flawed: for many of us, statistical thinking is far from intuitive, and we olten make statistically illogical assumptions. For instance, the Government in the United Kingdom recently appeared surprised that when reading tests were given to a population of 7-year-olds. 90% of the scores did not cluster around the mean: some children had the reading age of an average 9-year-old. while others had barely made a start at reading. In other words, there was a normal distribution of reading ages around the mean. Similarly. a professor of neurosurgery has pointed out that half

. the drivers on the road are of less than average intelligence. as if this were a matter for regret or surprise, I f we accept that most measured parameters will show a normal distri- bution. then of course half the population will be slower than average, 10% will be 'slow' (at less than the 10th centile) and 3% will be 'very slow' (or very small. such as infants who are small for their gestational age). Therefore, while 12% of children may be slow to speak and 4% may be very slow, possibly only a fraction of these'. amounting to 1 or 2% of children, have a true disease. Of the 10% of 5-year-olds with enuresis and 10% of school-age children with learning disorder, those with true pathological causes of a developmental disorder tend to get lost among the much larger number of children who are slow but are developing normally given their natural potential.

Therefore, the clinical difficulty with the group of children with speech. language and specific learning disorder is to discern whether a particular child has a disorder. or is a normal child who will probably always be at the lower end of the spectrum or is a normal child who simply has a slow biological clock and is maturing more slowly. In the last case the question is. 'Will he simply carry on developing longer and eventually catch up?' Similarly. some children who are labelled 'gifted' because they are above the 97tti . . centile are children who have matured more quickly and may reach full maturity early. but who by the time they reach secondary school. as their more slowly maturing class- mates catch up. may no longer appear to be the Mensa geniuses that they seemed to be in primary school.

Though children who are on the 3rd centile for height tend to follow that centile and become small adults, this pattern seemingly does not apply to all aspects of develop- ment. Children who walk at 10 months are not considered more likely to be Olympic

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athletes than those who walk at 14 months. Children who go through an early puberty at 9 years are not better lovers as adults than those who only reach the milestone at 15 years. Children who are slow to speak may later achieve honours in English langdage. A child psychiatrist colleague of mine, a keen rugby player, recognised good players at age 12 who did not make the 2nd XV at'age 18, and top-class players who at age 12 had seemed to have two left feet. Children will cross centiles, or a skill may reach a ceiling, so that once a certain level is achieved there is no further development.

In contrast to the child who catches up, there are many who do appear to have a life- long developmental 'disease', s o the child with slow phonological development may have acquired all his later consonants by age 7 years but not have made a start at reading; his reading may be much improved by 12 years while his spelling is still way behind, and this may never catch up to normal, as can be seen even in final exam papers for doctors, speech therapists and others. Many of these types of specific devcloptnental dis- order, such as developmental spelling dysgraphia, developmental dyspraxia or develop- mental dyslexia. have a genetic basis and are more severe and up to 5 times more common in males. with their slower left hemisphere development, than in females; that is to say. the title of this editorial is not sexist.

Brain damage will slow up the rate of development even if i t does not produce hard neurological signs. Preterm infants or infants surviving mild or moderate perinatal asphyxia may walk later than normal and may hop or jump later; they may show visu- ospatial abnormalities - for example, copying a square and a diamond later than normal; they may also be slower in their phonological development. Eventually. howevh, they appear to catch up. Children with cerebral palsy have slowed motor maturation which may be independent of the neurology. i.e. one cannot necessarily find a neurological dif- ference between the hemiplegia in children who walk at age I year and in those who do not walk until age 2.

The ability to see brain maturation on MRI scans means that we should be able to follow maturation after events that are known to slow down brain development. The studies of Evrard' on migration and the development of susceptibility to asphyxia have shown how abnormalities such as fetal alcohol syndrome, cocaine addiction in the mother or asphyxia at various fetal ages may potentially alter later learning and rates of brain development. The recognition of microdysgenesis and heterotopias, particularly in the planum tempora'le. has drawn attention to a possible pathological substrate of learn- ing disorders and qutism.

If we are to answer some of the questions in developmental neurology. we will need to follow up many children into their late teen yeais. and this may require sequential imaging and physiological as well as psychological studies. Genetic studies on well- documented families with developmental disorders may give us valuable insight into factors controlling regional brain development in the child.

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KEITH BROWN

Rifu-iwws I Evrard P. ( 1905) Migrational disorden: causes of cerebral palsy and learning disorders. Paper presented at the

49th Annual Meeting of the American Academy Tor Cerebral Palsy and Developmental Medicine, Philadelphia. 30th Septembcr 199s.

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