Causes of intellectual disability (aetiology)

Aetiology is the study of causes. In at least 50% of cases, the specific cause of intellectual disability in individuals has been determined ²⁹ , ³⁰ . There may have been an infection during pregnancy, problems at birth, postnatal problems, congenital abnormalities or a non-congenital brain injury. But even after clinical genetic testing, the cause of an intellectual disability often remains unknown. Recent developments in clinical genetics, however, would certainly merit performing additional assessments. In the majority of children with a developmental delay, a specific cause of intellectual disability can be found.
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It is important to determine the root cause of the intellectual disability for several reasons:

• If a syndrome has caused the intellectual disability, it is important to determine whether the behaviour is linked to that syndrome. Many syndromes have a characteristic pattern of behaviour, the 'behavioural phenotype' ³³ ,  ³⁴ . It is important to identify this behavioural phenotype, as this knowledge may further clarify causal factors. It may also help in determining and executing appropriate interventions. 
• Many syndromes are associated with physical problems ³⁵ , ³⁶ ,  ³⁷ , ³⁸ . Periodical health monitoring programmes have been drawn up for an increasing number of syndromes: monitoring takes place regularly, for example on an annual basis. Possible syndrome-related health problems can then be identified and treated in a timely fashion. However, healthcare providers may not be aware of the conditions associated with a syndrome. This makes it more likely that somatic conditions will be missed.
• Brain damage due to perinatal problems (such as hypoxia) is another possible cause of intellectual disability. But brain damage itself can also directly lead to spasticity, swallowing disorders and recurrent respiratory tract infections. 
• In clients with a metabolic disease (another cause of intellectual disability) , impaired motor and/or cognitive skills can cause health and behaviour problems.
• In a client with a non-congenital brain injury (also a possible cause of intellectual disability), challenging behaviour may be associated with this injury. ³⁹