Psychiatry
Intellectual Disability
Intellectual disability for MBBS and NEET-PG: definition, intelligence quotient, grades of severity, Down and fragile X syndromes and preventable causes, mapped to NMC codes PS15.
MedNext Academy | 3 min read
Intellectual Disability
Intellectual disability for MBBS and NEET-PG: definition, intelligence quotient, grades of severity, Down and fragile X syndromes and preventable causes, mapped to NMC codes PS15.
This chapter covers intellectual disability: its definition through intellectual and adaptive impairment with childhood onset, the measurement of the intelligence quotient, the grades of severity, and the wide range of genetic, prenatal, perinatal and postnatal causes. It emphasises multidisciplinary support and prevention.
High-yield: Intellectual Disability
- Intellectual disability is defined by significantly below-average intellectual functioning together with impaired adaptive behaviour, with onset before the age of eighteen.
- The intelligence quotient is the ratio of mental age to chronological age multiplied by a hundred, with an average of about a hundred.
- Severity is graded as mild, moderate, severe and profound using the intelligence quotient and the level of support required.
- Mild intellectual disability, with an intelligence quotient of roughly 50 to 69, is the most common category and often allows independent living with support.
- Genetic causes include Down syndrome, fragile X syndrome and phenylketonuria.
- Down syndrome from trisomy 21 is the commonest chromosomal cause of intellectual disability.
- Fragile X syndrome is the commonest inherited cause of intellectual disability.
- Phenylketonuria and congenital hypothyroidism are preventable causes detectable by newborn screening.
- Prenatal causes include intrauterine infection, alcohol exposure and iodine deficiency; perinatal causes include birth asphyxia; postnatal causes include meningitis and head injury.
- Iodine deficiency remains an important preventable cause of intellectual disability in some regions.
- Assessment combines a developmental history, standardised intelligence testing and measures of adaptive functioning.
- Management is multidisciplinary, centred on special education, skills training and family support rather than on a curative drug.
- Psychiatric and behavioural disorders are more common in people with intellectual disability and are often under-recognised.
- Prevention through antenatal care, newborn screening and immunisation is the most effective public health approach.
Causes and grades
- **Definition:** Low intellectual functioning plus impaired adaptive behaviour, onset before age eighteen.
- **Commonest causes:** Down syndrome (chromosomal); fragile X syndrome (inherited).
- **Preventable causes:** Phenylketonuria, congenital hypothyroidism, iodine deficiency, birth asphyxia.
- **Management:** Multidisciplinary special education, skills training and family support.
NMC competencies in this chapter
- **PS15.1:** Aetiology and magnitude of intellectual disability
- **PS15.2:** Intelligence quotient and its measurement
- **PS15.3:** History, clinical examination and assessment
- **PS15.4:** Psychosocial interventions and treatment
Frequently Asked Questions
How is intellectual disability defined?
It is defined by significantly below-average intellectual functioning together with impaired adaptive behaviour, with onset before the age of eighteen. Both deficits are needed for the diagnosis.
What is the commonest cause of intellectual disability?
Down syndrome, from trisomy 21, is the commonest chromosomal cause, while fragile X syndrome is the commonest inherited cause.
How is the intelligence quotient calculated?
It is the mental age divided by the chronological age, multiplied by a hundred, with an average value of about a hundred. It is used with adaptive measures to grade severity.
How is intellectual disability managed?
Management is multidisciplinary and centres on special education, skills training and family support, with treatment of associated psychiatric and behavioural problems, rather than a curative medication.
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