Epidemiology 37What is common?
Most epidemiological studies show that disruptive behavioural disorders
(oppositional-defiant disorder and conduct disorder) are the commonest
group of disorders, affecting roughly 5–10% of the population, closely
followed by anxiety disorders, affecting roughly 4–6% of the population.
Depression is also common in adolescence, when it affects about 2% of the
population. The ICD-10 criteria for hyperkinesis are stricter than the DSM-
IV criteria for ADHD, and typical prevalence estimates are in the region of
1–2% for the former and 3–6% for the latter.
Comorbidity
Many children and adolescents with psychiatric disorders meet the criteria
for more than one psychiatric diagnosis. For example, a child who meets
the criteria for generalised anxiety disorder commonly also meets the crite-
ria for other anxiety disorders too, including specific phobias, social phobia
and separation anxiety disorder. Similarly, a child who meets the criteria
for ADHD commonly also meets the criteria for oppositional-defiant or
conduct disorder. For some disorders, comorbidity is the rule rather than
the exception. Depression, for instance, is usually accompanied by an
anxiety or behavioural disorder. There are several possible explanations
for comorbidity. First, the current psychiatric classification may have erred
too far in the direction of splitting rather than lumping. If we labelled ‘sore
throat’ and ‘runny nose’ as separate disorders, many individuals would
be comorbid for the two. Second, one disorder may be a risk factor for
another. Consider the association of depression and behavioural problems.
Perhaps the severe irritability that sometimes accompanies depression
leads to aggressive outbursts. Or maybe conduct disorder leads to isolation
and criticism, and this, in turn, results in depression. Another possibility
is that the same risk factor might simultaneously predispose an individual
to several disorders. For instance, a genetic problem with mood regulation
could manifest both as behavioural and affective disorders (via irritability
and depression respectively).
Most disorders go untreated
Even when children and adolescents do have psychiatric symptoms that
result in significant social impairment or distress, only a minority of them
are in contact with specialist mental health services. Referral for specialist
help is most likely when the problems are a substantial burden to their
parents. Conversely, if parents do not feel burdened, their children are
unlikely to receive specialist mental health care for their disorders. Some
of the children and adolescents with psychiatric problems who are not
being seen by specialist mental health services do get help from elsewhere
in the health sector, or from education or social services. However, around
half of affected individuals get no professional help at all in high-income
countries – with even lower rates of getting appropriate help in low- and
middle-income countries.