Aetiology of learning disability

This contribution discusses the aetiology of learning disability. Learning disability, termed ‘mental retardation’ in ICD-10, is defined as significantly below average general intellectual functioning (IQ < 70), accompanied by deficits or impairments in adaptive behaviour, and with an onset in childhood. It is important to make a diagnosis of the aetiology of a learning disability if possible, for a number of reasons. These reasons can be broken down into psychological, physical, psychiatric and genetic counselling. As with most aspects of medicine, the causes of learning disability can be divided into genetic, biological and environmental. These variously occur before, during or after birth. Down syndrome is the most common specific cause of learning disabilities, followed by fragile-X syndrome. In 30–50% of people with learning disability, however, no cause is found, although the aetiology is more likely to be identified in the more severe cases. It is important to realize that in people with learning disability the cause will be multifactorial. Antenatal factors such as chromosomal and genetic factors, congenital infections, teratogens, malnutrition, radiation or unknown factors can all cause learning disability. Perinatal factors can cause a learning disability; these include asphyxia/hypoxia at birth, mechanical birth trauma, hypoglycaemia and prematurity. Postnatal factors can cause learning disability and there has been a recent emphasis on more research in this area. The obvious causes are injury due to trauma, anoxia or infection (meningitis/encephalitis). The environment is also an important factor in contributing to learning disability.