Intellectual Disability – An Introduction

An intellectual disability, also known as mental retardation, is a condition connected with the IQ of an individual and affects many facets of their life.

What is Intellectual Disability?

The term ‘intellectual disability’ (ID), previously known as mental retardation refers to a group of disorders which cause below average intellectual functioning and impaired adaptive skills before a person is 18 years of age. Those suffering from an intellectual disability have an IQ of 70 or less. Impaired adaptive skills include limitations in self-care, living at home, communicating, social skills, academic scores, safety and in areas of work, leisure health and safety.

The prevalence of ID at any time is about 2-3% of the general population. School children have the highest incidence. It is 1 ½ times more common in males than in females.

The Intelligence Quotient (IQ) is calculated by the formula: mental age divided by chronological age multiplied by 100. In mild ID, IQ is 51-70; in moderate ID it is 36-50; in severe ID it is 21-35 and in profound ID it is 0-20. All levels of children with ID are educable and trainable to some extent.

Predisposing Factors:

It is difficult to pinpoint the exact cause of ID but certain conditions predispose a person to it. These are:

1. Low socio-economic strata: Poor medical care and inadequate nutrition to mother and baby, lack of immunization and treatment of infections and inadequate environmental stimulation are some factors to take into account.
2. Advanced age of the mother.
3. Consanguinity of parents (parents are blood relatives e.g. cousins).
4. Premature babies or those who have low birth weight.

Causes of Intellectual Disability:

Intellectual disability occurs due to a combination of medical, socio-cultural and psychological factors. The majority of cases are termed as idiopathic (unknown or unexplained). Common causes include:

1. Environmental factors like parental neglect, deprivation.
2. Genetic abnormalities and inherited metabolic disorders for e.g. phenylketonuria, galactosemia.
3. Chromosomal abnormalities like Down syndrome, Fragile X syndrome.
4. Developmental defects like microcephaly.
5. Infections like congenital rubella, HIV.
6. Toxic factors for e.g. mother abusing alcohol, cocaine and other drugs; lead poisoning
7. Malnutrition or iron-deficiency in mother.
8. Hormonal factors like cretinism (lack of growth hormone) and congenital hypothyroidism
9. Birth trauma like head injury, hypoxia (lack of oxygen to the brain before, during or after birth)
10. Unexplained factors – this accounts for the largest number of cases.

ID must be differentiated from:

1. Deprivation: Sensory deprivation like blindness or deafness, parental or emotional and environmental or cultural are examples of deprivation of different kinds.
2. Speech and hearing defects.
3. Motor system defects like paralysis.

Prevention of intellectual disability:

1. Counselling the mother – to give up alcohol, cocaine and other drugs during pregnancy.  Iodides and antithyroid medicines are also harmful.
2. Genetic counselling – It is important in consanguineous marriages to inform the couple about prenatal diagnosis (doing tests when the woman is pregnant). Also, when the woman’s age is above 35, likelihood of Down syndrome increases.
3. Good healthcare facilities for pregnant women, nutritional counselling and good obstetricians to prevent fatalities during delivery.
4. Routine screening of all infants for metabolic disorders and iron deficiency.

In the next article, we discuss signs and symptoms, investigations and treatment of ID.

Written by Dr Nisreen Nakhoda, General Physician

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