Child and Adolescent Psychiatry

236 Chapter 28

Specific behavioural patterns
Some organic causes of intellectual disability are particularly associated
with specific patterns of psychiatric problems. The Lesch-Nyhan syndrome,
for example, is much more likely to lead to severe self-injury than other
organic disorders resulting in equally low IQs. When the organic syndrome
is genetic or chromosomal, the common behavioural characteristics are
referred to as thebehavioural phenotypeof the disorder. Other examples
include the social anxiety and gaze avoidance associated with the fragile X
syndrome; and the insatiable overeating associated with the Prader-Willi
syndrome. Non-genetic syndromes can also have associated behavioural
features, and these, too, are sometimes referred to as behavioural pheno-
types. Thus congenital rubella is associated with features of autism, while
the fetal alcohol syndrome is associated with ADHD.
Box 28.1 portrays four possible causal pathways that could account
for the observed association between intellectual disability and psychiatric
disorder. For some psychiatric disorders, the evidence supports possibility
B, namely that the same biological factors that cause intellectual disability
also, and independently, cause the psychiatric problems. Take autism, for
example. A child with an IQ of 40 and tuberous sclerosis is at high risk
of autism, whereas a child with an IQ of 40 and cerebral palsy is at much
lower risk. Thus, IQ cannot account for this difference, which is almost
certainly related to the different biological substrates of the two sets of
disorders.

Box 28.1Why are intellectual disability and psychiatric

disorder associated?

Harder to cope

Less success

More frustration

More stress

More teasting

Biological

factors

Low IQ

Psychiatric

problems

Organic factors

Polygenes

Social factors

Low IQ Psychiatric problems

Possibility A

Possibility B

Social

factors

Low IQ

Psychiatric

problems

Possibility C

Psychiatric

problems Low IQ

Possibility D

For some other psychiatric disorders, however, the evidence favours
possibility A, namely that low intelligence, whatever its cause, predisposes
a child or adolescent to psychiatric problems. It is certainly plausible
that low intelligence and poor academic attainments often undermine an
individual’s self-esteem and result in teasing by others. In addition, less
intelligent individuals may find it harder to overcome everyday stresses,
and may be more prone to ‘act out’ when under stress. For all these