232 Chapter 28
Several syndromes, including the fragile X syndrome and the fetal alco-
hol syndrome, have already been discussed in Chapter 1. Other relevant
syndromes include:
Down syndrome: affects up to 1 in 800 live births, with older mothers
at much greater risk. This is the commonest single cause of marked
intellectual disability, accounting for around a sixth of all cases: 95%
are due to an extra chromosome 21 resulting from non-disjunction,
something which more commonly affects older mothers; 4% are from
translocations, which are familial; and 1% are mosaics. Physical features
include: small head; round face; upslanting eyes; epicanthic folds; large
fissured tongue; low-set simple ears; short stature; single palmar crease;
incurved little fingers; and hypotonia. Cardiac and gastrointestinal mal-
formations are common. There is an increased risk later in life of
deafness, leukaemia and Alzheimer’s disease.
Single gene disorders: there are many rare genetic disorders that sometimes
or always cause intellectual disability. As a rule of thumb, you can
assume that these disorders are autosomal recessive unless you know
otherwise. There are a few exceptions: the Lesch-Nyhan and Hunter
(but not Hurler) syndromes are sex linked; and tuberous sclerosis and
neurofibromatosis are autosomal dominant.
Sex chromosomal anomalies: individuals with the common anomalies (XO,
Turner syndrome, XXY, Kleinfelter syndrome, XXX and XYY) are usually
of normal or low-normal intelligence, though there is some excess of
mild and marked intellectual disability.Diagnostic assessment of intellectual disability
Children with marked intellectual disability are usually referred to a paedi-
atrician because of associated physical abnormalities or slow development,
either noted by parents or picked up on developmental screening. Mild
intellectual disability may not be noticed until learning difficulties become
apparent in school. Parents and teachers are usually fairly accurate judges
of a child’s ability level. If asked, they are often able to give a good estimate
of the mental age. Nevertheless, even experienced parents and teachers
sometimes seriously misjudge intelligence. Thus, a child with autism and
normal intelligence (as judged by non-verbal tests) may be thought to have
a marked intellectual disability on the basis of his poor performance in ver-
bal tests and his lack of commonsense. This kind of misjudgement may lead
to an inappropriate placement in a school for marked intellectual disability.
Even more commonly, children and adolescents with mild intellectual
disability are believed by their teachers to be of near average ability, with
poor academic performance being attributed to lack of effort, emotional
problems, or social disadvantage. Once again, the misjudgement leads
to inappropriate academic provision and pressure. Given this, it is often
sensible to supplement parent and teacher reports with formal psycho-
metric testing. Besides measuring IQ reliably, a detailed psychometric