50 Chapter 4
social withdrawal and behavioural disturbance, but is not usually hard to
distinguish from an ASD.
Intellectual disability without features of autism
Language and pretend play will be absent if mental age is under 12
months. Simple stereotypies are common. These children are socially
responsive in line with their mental age.
Intellectual disability with some features of autism
Many children with intellectual disability have a ‘triad’ of impairments
affecting (1) social interaction; (2) communication; and (3) play, as well as
varying degrees of repetitive and restricted behaviours. Only some of these
children meet the full diagnostic criteria for autism, but many more can be
diagnosed as having atypical autism (though not all clinicians think this is
a useful thing to do).
Rett syndrome
This syndrome is usually due to a mutation in the MECP2 gene on the X
chromosome, but occasionally results from a mutation in other genes. The
mutation is typically new (that is, not present in either parent), but may
be inherited from a phenotypically normal mother who has a germline
mutation. Male fetuses with the mutation usually die before birth since,
unlike females, they lack a second normal X chromosome. As a result,
recognised cases of Rett syndrome occur almost exclusively in girls (affect-
ing about one in 10,000 live-born females). The features of the syndrome
may be confused with autism. There is global developmental regression
with loss of acquired abilities at about 12 months of age, accompanied
by: deceleration of head growth; characteristic ‘hand washing’ stereotypies
and restricted hand use; episodic over-breathing and unprovoked laughter;
and progressively impaired mobility. Most children with Rett syndrome
are appropriately socially responsive once allowance is made for their low
mental age and physical disabilities. The disease is progressive and affected
individuals are usually in wheelchairs by their late teens and die before 30.
Neurodegenerative disorders with progressive dementia
These need to be considered when a period of normal (or nearly normal)
development is followed by the loss of skills and the emergence of features
of autism. With time, frank neurological impairments emerge and the
affected individual eventually dies. There are many such genetic disorders,
all of which are fortunately rare. Examples include adrenoleucodystropy,
juvenile Huntington disease and Batten disease. HIV encephalopathy is
probably the commonest cause of childhood dementia worldwide.
Disintegrative disorder
Also known as disintegrative psychosis or Heller syndrome, this very rare
condition (with a prevalence of roughly 1 in 50,000) involves entirely
normal development for two to six years, followed first by a phase of