There was no increase in the
incidence of obsessive-compulsive disorder over the same period, however, the investigators reported.
"Although the reasons for
the observed common pattern of change in reported cumulative incidence in Tourette's syndrome, hyperkinetic disorder & autism spectrum disorder can't be addressed with these data,
it's clear that the number of children with neuropsychiatric disorders & their families in need of support & services
has been growing in recent years," the authors wrote.
"Furthermore, while the search
for causes should proceed unabated, the ultimate value of these data are their contribution to the growing awareness of child
neurodevelopment problems in general & understanding of the resources needed to ensure optimal development for all children,"
they continued.
The authors proposed two models
to explain the observed increases. One possibility is that the pattern of increase is coincidental & that independent
disease factors are causing the rise in incidence of each condition.
Alternatively, there may be
one or more shared factors, such as genetics, environment, diagnostic shift, or a combination of these factors.
The authors drew on Denmark's
renowned national health care data banks, linking information from the Danish Medical Birth Registry with data from the Danish
National Psychiatric Register. The latter database included diagnoses recorded by psychiatrists using standardized diagnostic
codes (ICD-10).
They looked at records on
all 669,995 children born in Denmark from 1990 thru 1999. The primary study outcome was the cumulative incidence proportions
of each of the targeted disorders:
- Autism spectrum disorders
- Childhood autism (a subset of autism spectrum disorders)
- Tourette's syndrome
- hyperkinetic disorder
- Obsessive-compulsive disorder.
The data were stratified by
age & year of birth for each disorder, grouped into birth cohorts spanning 2 years.
The investigators identified
4,376 children who received a total of 4,637 diagnoses.
The diagnoses
of hyperkinetic disorder, Tourette's syndrome & autism spectrum disorder all showed statistically significant tests for trend.
"For hyperkinetic
disorder , an increase in cumulative incidence was observed across all cohorts
such that each successive birth cohort had a significantly higher cumulative incidence than the previous cohort (P<0.001),"
the authors wrote.
"For example, at 5 years of
age the cumulative incidences of hyperkinetic disorder for the 1994-1995 cohort, 1996-1997
cohort, & 1998-1999 cohort were 26%, 100% & 200% higher, respectively, than the cumulative incidence for the 1992-1993
cohort."
Similarly, the
1994-1995 birth cohort had a significantly higher cumulative incidence of Tourette syndrome than either the 1990-1991 cohort (P=0.005) or the 1992-1993 cohort (P=0.006), although there was no significant
change from children born in 1990 thru 1993 in cumulative incidence of Tourette's.
Looking at autism spectrum disorders overall, they found that the 1998-1999 birth cohort had significantly higher cumulative
incidence proportions than the 1994-1995 birth cohort (P=0.004), but there were no other significant differences
among the various birth cohorts.
Looking only at those with
a diagnosis of childhood autism (38% of all children
with autism spectrum disorders), the investigators found that the cumulative incidence
proportion was significantly higher for children in the 1998-1999 birth cohort than those born either from 1994-1995 (P<0.001)
or 1996-1997 (P=0.02).
There was no significant
change in autism incidence, however, between the 1994-1995 & 1996-1997 birth
cohorts.
The authors didn't find any
significant difference across the various birth cohorts in the cumulative incidence proportions for obsessive-compulsive disorder.
"It's difficult to explain
why obsessive-compulsive disorder was the only disorder displaying another pattern; the reason may be etiologic, due to non-etiologic diagnostic differences,
or due to the relatively short follow-up," they wrote.
Their results
support earlier findings of time trends in the increase of autism & suggest that if
"the debate surrounding explanations for the increase in autism incidence should also consider
the evidence of a more widespread epidemiologic phenomenon across different diagnostic conditions," the investigators stated
in their conclusion.
