Pervasive developmental disorders
This group encompasses autism and childhood schizophrenia. The former is
characterized by its early onset, usually before 30 months of age, whereas childhood
schizophrenia presents later. They are conditions that represent profound adaptive
problems in thinking, language, and social relationships. Autism in particular has the
distinctive feature of restricted and stereotypical behaviour patterns. Most children
score below normal on IQ testing and thus experience significant developmental
delay. The more severely delayed children seem oblivious to their parents or carers,
express themselves minimally, show a low level of interest in exploring objects, avoid
sounds, and engage in ritualistic behaviour. Children with Asperger's syndrome
display some of the features of autism but may also possess a level of skill in some
areas well above the average for their peers.
These features need to be taken into consideration when attempting dental care, and
underlines the particular importance of acclimatization and familiarity of routine
(rituals) as part of that process.
The causes of autism are unknown but are thought to be prenatal and not social in
origin. Much interest was generated in a possible link with MMR vaccine as a
possible aetiological factor but this evidence has since been discredited. A major
malformation in the cerebellum has recently been implicated as a possible causative
factor. The prevalence of autism ranges from 0.03% to 0.1% with fluctuations that
may point to an environmental cause.
Learning difficulties
Learning difficulty is associated with dyslexia, minimal brain damage, attention-
deficit disorder, and hyperactivity. All these categories are controversial, mainly
because they have been overextended.
Historically, a child with a learning difficulty has been defined as one whose
performance in one academic area is more than 2 years behind the child's ability. Thus
the impairment is restricted in its range and there is a discrepancy between academic
performance and tested general ability. In these two ways a learning difficulty differs
from mental retardation because the latter is characterized by general delay and
academic performance is usually at the level expected from ability. In practice,
learning difficulty has been used to characterize any child with a learning problem
who cannot be labeled mentally retarded, no matter how broad the range of
impairment or the discrepancy from the tested ability level. This overextension of the
definition has not only increased the apparent prevalence of learning disability but has
also made the whole area rather confusing.
In general, the prevalence of learning difficulties is estimated on average to be about
4.5%. There is overlap between learning difficulties and other problems, for example,
higher levels of classroom behavioural problems and an increased risk of delinquency.
In part, this accounts for the greater predominance of males in groups with intellectual
impairment as they are more likely than females to be disruptive at school and thus be
referred for assessment by educational psychologists.