Syndromes causing intellectual disability 

Psychiatrists working with people who have intellectual disability (mental retardation) need expertise in the diagnosis and treatment of associated neuropsychiatric disorders. This entails knowledge of the causes of intellectual disability, and especially knowledge about those syndromal (often genetic) causes that are associated with neuropsychiatric manifestations. Such manifestations include vulnerability to behavioural and emotional disorders, epilepsy, and particular patterns of cognitive strength and weakness. This chapter provides an introduction to some such disorders and a discussion of the concept of behavioural phenotypes. For a detailed account of conditions causing intellectual disability texts such as Jones should be consulted. The concept of behavioural phenotypes is discussed in detail in O’Brien. The genetic aetiologies of intellectual disability include chromosomal abnormalities (trisomy, deletion, translocation, etc), single-gene defects, and the effect of interactions between several genes. The last is thought to account for a substantial proportion of people with mild intellectual disability by setting a ceiling on possible cognitive attainment (life experiences, nutrition, education, and other factors then determining the extent to which potential is fulfilled or thwarted). This chapter discusses the concepts of syndromes and behavioural phenotypes, then describes the clinical features of a number of syndromes that cause intellectual disability. Down syndrome, fragile-X syndrome, sex chromosome anomalies, and foetalalcohol syndrome are described in some detail. This is followed by a briefer alphabetical list of less common conditions.