False: Administrative prevalence is defined as ‘the numbers for whom services would be required in a community which made provision for all who needed them’. It is roughly half to a third of the true prevalence (Gelder et al 2006, p. 707).

False: Antenatal infection with cytomegalovirus, rubella, toxoplasmosis, herpes simplex and syphilis can cause learning disability as all these agents can cross the placenta (Craft et al 1985, p. 133; Gelder et al 2006, p. 714).

True: In Prader–Willi syndrome, around 70% have a microdeletion of the paternally derived chromosome 15, about 28% have a maternal disomy and the rest have imprinting centre mutations or are carriers of a chromosomal translocation.

In Angelman syndrome, about 70% have a microdeletion of the maternally derived chromosome 15, about 5% have a paternal disomy, 3% have an imprinting centre mutation and 5% have a mutation in the UBE3A gene. The others have no identifiable defects on chromosome 15 and the locus may be elsewhere on the genome.

Thus in those where uniparental disomy is the cause of the syndrome, Prader–Willi is associated with a maternal disomy and Angelman with a paternal disomy (Gelder et al 2000, p. 1958; Johnstone et al 2004, p. 557).

True: (Gelder et al 2000, p. 1729).

False: Cri du chat syndrome is caused by a deletion on the short arm of chromosome 5, i.e. 5p15.3 and 5p15.2. It is the commonest human deletion syndrome. It accounts for around 1% of the profoundly learning disabled population. Severe learning disability, microcephaly and a high-pitched cat-like cry are prominent features. Patients may survive to adulthood (Gelder et al 2000, p. 1960).

False: Down’s syndrome affects approximately 1 in 1000 live births. The incidence increases with maternal age as follows:

False: There is no loss of skills. Milestones develop fairly quickly in the first year and then more slowly (Gelder et al 2006, p. 717).

False: Bipolar affective disorder (BPAD) is not common in Down’s syndrome. However, previous reports that BPAD does not occur at all in Down’s syndrome have been proved wrong. The frequency and clinical features of BPAD in Down’s syndrome are the same as in non-Down’s syndrome learning disability (Fraser & Kerr 2003, p. 279; Johnstone et al 2004, p. 544).

True: Autoimmune thyroid dysfunction, especially hypothyroidism, is common in Down’s syndrome. Hypothyroidism occurs in 20% of adolescents and adults with Down’s syndrome. Hypothyroidism is a differential diagnosis in behavioural disturbance, depression and dementia in Down’s syndrome (Johnstone et al 2004, p. 542).

True: Fragile X syndrome is an X-linked disorder. It shows an X-linked dominant inheritance with incomplete penetrance. It is the most common LD syndrome caused by mutation of a single gene. It is the most common inherited cause of LD. It accounts for 50% of all cases of X-linked LD (Craft et al 1985, p. 123; Gelder et al 2000, p. 1953).

True: Gaze avoidance is shared by Fragile X syndrome as well as autism. It may be a manifestation of defective non-verbal communication (Gelder et al 2000, p. 1954; Johnstone et al 2004, p. 552).

False: One per 30 000 live births in the UK and one per 62 000 births in the USA (Craft et al 1985, p. 97; Sadock & Sadock 2002, p. 1172).

False: Hurler’s syndrome is an autosomal recessive mucopolysaccharide disorder. There is no generally accepted effective treatment for mucopolysaccharidoses. Allogenic bone marrow transplantation is being investigated (Gelder et al 2000, p. 1959; Harris 2000, p. 358).

False: XXY karyotype and male hypogonadism are the key features of Klinefelter syndrome. Diagnosis often occurs at puberty with varying development of secondary sexual characteristics, small testes or infertility. Even though the adults are usually around 4 cm taller than average, the length of the penis is normal or moderately short. Testes are normal until puberty but thereafter small and firm (Craft et al 1985, p. 126; Johnstone et al 2004, p. 547).

True: Many fetal disorders – cytogenetic, biochemical, molecular and neural tube defects – can be diagnosed prenatally at 15–18 weeks of gestation by amniocentesis. Lesch–Nyhan syndrome can be diagnosed prenatally with amniocentesis or chorionic villus sampling.

True: Niemann–Pick disease is a group of four autosomal recessive disorders. Types A and B result in accumulation of sphingomyelin. Types C and D have defective neuronal cholesterol transport.

False: Phenylketonuria is caused by the deficiency of phenylalanine hyroxylase, which converts phenylalanine to tyrosine. Phenylalanine hydroxylase deficiency results in the accumulation of phenylalanine to toxic levels causing severe LD, epilepsy and microcephaly. Elimination of phenylalanine from the diet from birth drastically reduces the severity of LD (Fraser & Kerr 2003, p. 36; Gelder et al 2000, p. 1960; Johnstone et al 2004, p. 527).

True: The affected girls develop normally for 6–28 months. Thereafter, there is deterioration in speech and motor abilities. Abnormal involuntary stereotyped movements, particularly involving the hands and fingers, e.g. hand flapping and hand wringing, appear later (Gelder et al 2000, p. 1955; Johnstone et al 2004, p. 552).

True: Tay–Sachs disease is an autosomal recessive disorder resulting in increased lipid storage. The features include progressive blindness and deafness, spasticity, cherry red macular spot and mental retardation. There are several different clinical forms. The commonest is the infantile form. Death occurs most commonly between 2 and 4 years (Craft et al 1985, pp. 37, 95; Fraser & Kerr 2003, p. 37; Gelder et al 2006, p. 715).

False: One-third have average intelligence. Two-thirds have mild to severe LD (Craft et al 1985, p. 114; Gelder et al 2000, p. 1958).

True: West syndrome consists of the triad of infantile spasms, severe LD and hypsarrhythmia. Hypsarrhythmia is an EEG pattern of a chaotic mixture of high amplitude slow waves with variable spikes and sharp waves. Onset is in infancy (Lishman 1997, p. 242).

True: Wilson’s disease or hepatolenticular degeneration is a rare, autosomal recessive disorder of copper metabolism. The pathogenesis is the absence or dysfunction of the copper-transporting P-type ATPase coded on chromosome 13q14. It causes liver dysfunction, personality and cognitive changes, tremor, dysarthria and wing-beating tremor, i.e. arms beat up and down at the shoulders (Johnstone et al 2004, p. 336; Mitchell 2004, p. 171).

True: The IQ may be very slightly lower than normal. They also have behavioural features that increase the likelihood of antisocial acts (Gelder et al 2000, p. 1861; Johnstone et al 2004, p. 548).

True: The risk of active epilepsy in various IQ groups is as follows:

Severe, intractable epilepsy can cause LD. Non-epileptic seizures are more common in LD than in the general population. In addition, patients with true epilepsy are the most likely to have non-epileptic seizures (Johnstone et al 2004, p. 571).

True: People with mild LD have a higher prevalence of brooding, somatization and sleep disturbances than the general population, especially when depressed or anxious (Johnstone et al 2004, p. 566).