In all neurodevelopmental disorders, possible or certain causes can be categorised according to when the brain injury/disorder originally arose. There is usually a distinction between the prenatal period (the period before birth), the perinatal period (the first week, including the birth itself) and the postnatal period, where the injury/disorder occurred after the perinatal period. Genetic factors dominate among prenatal causes and include both inherited conditions and de novo (new) mutations. This group includes a large number of known syndromes. The term syndrome refers to several simultaneously occurring and clinically observable signs or symptoms that collectively indicate an underlying medical condition.
As with other neurodevelopmental disorders, medical causes of intellectual disability can usually be traced to the prenatal period. However, there are many children with mild intellectual disability for whom we cannot define the exact medical cause. The underlying medical cause is much more frequently identified for children with more severe forms of intellectual disability.
Various genetic/chromosomal syndromes can cause mild intellectual disability, for example DiGeorge syndrome and Klinefelter syndrome (boys with XXY chromosomes). Thanks to CGH (comparative genomic hybridisation) arrays, capable of detecting very small genetic abnormalities (like deletions and duplications), genetic diagnostics have improved even further. One study was able to identify such a genetic abnormality as the underlying cause in 21% of children with mild intellectual disability.
Given the current rate of development in genetic testing, we will be able to map out more and more causes in the future. Among acquired prenatal causes, foetal alcohol spectrum disorder (FASD) is the most widely known. Perinatal causes can be related to risks associated with the immature brain, i.e. if the child is born very or even extremely prematurely (earlier than 32 weeks or 27-28 weeks, respectively).
Mild intellectual disability can sometimes also be an expression of the variation in human abilities and aptitude relative to society’s demands.