Intellectual disability is a term describing limitations in a person’s cognitive abilities, combined with other limitations in skills. There are many causes for developmental delay. A small percentage of intellectual disability cases are linked to genetic and hereditary causes, and in some cases the cause is unknown.
The functioning abilities of people with intellectual disabilities vary according to classification. People with mild have a good chance of leading an independent life, while people with more severe intellectual disability need significant support systems throughout their lives.
Classification of intellectual disability
Intellectual disability is commonly classified in four categories: mild, moderate, severe, and profound. This classification is based in IQ level determined in standard tests. People with mild intellectual disability have an ID of 50 – 70. Moderate intellectual disability correlates to an IQ level of 50 – 35. Severe intellectual disability is defined by an IQ of less than 35.
What are the causes of intellectual disability?
There are several causes for developmental delay. In many cases, the cause is unknown. Intellectual disability can be related to hereditary diseases, non-heritable genetic disorders, infections before birth, trauma during pregnancy or birth (such as fetal alcohol syndrome – a common cause of intellectual disability worldwide), birth defects affecting the brain and nervous system, hormonal abnormalities, and more.
Genetic evaluation of intellectual disability
Among the causes of developmental delay are genetic and hereditary causes. Two of the most common genetic causes of intellectual disability are Down syndrome and Fragile X syndrome.
Down syndrome is caused by a chromosome abnormality. People with Down syndrome have three copies of chromosome 21 instead of two. This disorder includes a wide range of health problems, among them intellectual disability that can manifest in different severity levels and is sometimes very mild.
Down syndrome can be detected during the pregnancy via amniocentesis or CVS (chorionic villus sampling), or through a simple karyotype test. Since the risk for Down syndrome increases with maternal age, women over the age of 35 are recommended tests for detecting this disorder. In addition, anatomy scans and prenatal screening tests in the first trimester can help get an early diagnosis.
Fragile X syndrome
Fragile X syndrome is another common cause of genetically related intellectual disability. This disorder is caused by many repeats of the X chromosome. The number of repeats increases with each generation and the disorder manifests when the number of repeats reaches a certain level.
Since this disorder is related to the x chromosome, it is more prevalent in men (who have only one x chromosome) and is inherited from their mothers. Fragile X syndrome can be detected in prenatal genetic testing, that are recommended to pregnant women as part of regular tests.
Other genetic disorders
There are other genetic disorders that can cause developmental delay. These include Klinefelter syndrome (an extra X chromosome in men), Neurofibromatosis, hereditary thyroid disorders, Phenylketonuria, Prader–Willi syndrome, and more. Most disorders have other symptoms aside from intellectual disabilities. These diseases can be detected through genetic tests.
In many cases, there is no known cause for the intellectual disability, and it cannot be detected in routine genetic tests. These cases require a more thorough analysis. These testing procedures do not usually benefit those already suffering from intellectual disabilities but can help give genetic counselling to families planning another pregnancy.
Advanced genetic technologies such as CGH (Comparative genomic hybridization) can detect small genetic changes that could not previously be identified and could be linked to intellectual disabilities. Interpreting these tests must be done with great care, since the meaning of the genetic defect found are not always entirely clear.