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1. What Is Intellectual Disability?
Intellectual Disability (ID) refers to a condition characterized by limitations in intellectual functioning and adaptive behavior, manifesting before the age of 18, which necessitates support in daily and social life. Diagnosis is based on international standards such as DSM‑5 and ICD‑11, typically requiring an IQ of 70 or below and onset before age 18.
ID is more than just a low IQ; it is a multifaceted condition involving impairment in:
- Conceptual skills (e.g., reading, writing, arithmetic),
- Social skills (e.g., communication, interpersonal relationships),
Practical skills (e.g., self-management, financial skills).
2. Causes of Intellectual Disability
The causes of ID are diverse, generally classified into three categories:
① Genetic and Chromosomal Factors
Down Syndrome (Trisomy 21): Caused by an extra copy of chromosome 21. Features include developmental delay, intellectual impairment, characteristic facial features, and congenital heart defects. Occurs in approximately 1 in 700 births.
Fragile X Syndrome: A genetic intellectual disability resulting from CGG repeat expansion in the FMR1 gene on the X chromosome. Characteristics include moderate to severe intellectual disability, ASD-like features, facial traits (long face, prominent ears), and an X-linked dominant inheritance pattern, more severe in males.
Williams Syndrome: Arises from a deletion at 7q11.23. Features include cardiovascular abnormalities (e.g., aortic stenosis), mild intellectual disability, musical talent, a highly social personality, and distinct “elf-like” facial appearance.
Other genetic disorders include Trisomy 13 (Patau Syndrome), Trisomy 18 (Edwards Syndrome), 5p deletion (Cri du Chat syndrome), 22q11.2 deletion (DiGeorge Syndrome), Angelman syndrome, Prader–Willi syndrome, mitochondrial diseases, and single-gene disorders (e.g., Tay–Sachs disease, cystic fibrosis). Many of these can be detected in utero via NIPT from around 10 weeks of pregnancy.
② Perinatal Environmental Factors
- Viral infections during pregnancy (e.g., rubella, cytomegalovirus)
- Exposure to radiation or alcohol (fetal alcohol syndrome)
Hypoxia or intracranial hemorrhage during birth
③ Postnatal Acquired Factors
- Head trauma
- Central nervous system infections (e.g., encephalitis, meningitis)
- Severe abuse or neglect
(Source: National Center of Neurology and Psychiatry, Japan)
3. Relationship Between Down Syndrome and Intellectual Disability
In Down Syndrome, having three copies of chromosome 21 affects brain development, metabolism, immunity, and endocrine function—particularly impacting brain structure and function, leading to mild to moderate intellectual disability at birth.
Typical characteristics:
- IQ generally ranges between 35–70 with wide variability.
- Language development is often delayed; comprehension may be easier than expression.
- Difficulties often present in memory, attention, and abstract thinking.
- Strengths may include visual learning and memory for routines.
- Frequently co-occurs with congenital heart defects and gastrointestinal anomalies.
- Neuroimaging shows delayed development of the cerebral cortex and hippocampus.
- Alzheimer’s disease risk is significantly higher in adulthood; about half may develop it after age 40, due to the APP gene being on chromosome 21.
However, with early intervention, language training, and special education, individuals can develop functional living skills. Many can achieve employment and community integration.
4. Differences from and Overlaps with Neurodevelopmental Disorders
Neurodevelopmental Disorders include:
- Autism Spectrum Disorder (ASD): Characterized by challenges in social communication, restricted interests, and repetitive behaviors; often begins in early childhood and may coexist with intellectual disability or language delay.
- Attention Deficit Hyperactivity Disorder (ADHD): Exhibits inattention, hyperactivity, and impulsivity beyond age-appropriate levels. Usually diagnosed during school age and can impair academics and relationships.
- Learning Disabilities (LD): Difficulty with reading, writing, or arithmetic despite average intellectual ability. Includes dyslexia, dysgraphia, and dyscalculia. These are processing disorders unrelated to sensory deficits or educational deprivation.
While ID and developmental disorders are distinct, they often co-occur—about 30–40% of individuals with ASD also have intellectual disability.
5. Severity Classification of Intellectual Disability
According to WHO and Japan’s Ministry of Health:
| Severity | IQ Range | Level of Required Support |
| Mild | 50–69 | Minimal support; capable of employment |
| Moderate | 35–49 | Needs ongoing support |
| Severe | 20–34 | Requires extensive daily support |
| Profound | ≤20 | May require medical-level care |
6. NIPT and Intellectual Disability
Non-Invasive Prenatal Testing (NIPT) analyzes cell-free fetal DNA from maternal blood to screen for chromosomal abnormalities (e.g., trisomy conditions) and partial deletions/duplications. It’s non-invasive, has no miscarriage risk, can be done after around 10 weeks of pregnancy, and usually returns results within a week.
Primary targets include Trisomy 21 (Down Syndrome), Trisomy 18 (Edwards Syndrome), and Trisomy 13 (Patau Syndrome). NIPT offers very high sensitivity and specificity (typically ≥99%), but it’s a screening tool, not diagnostic. Positive results should be followed by confirmatory tests such as amniocentesis. Expanded NIPT options are available, covering sex chromosome anomalies, microdeletions, and even hundreds of autosomal recessive disorders, offering a broader testing scope.
Summary
Intellectual disability can arise from genetic, chromosomal, or environmental factors, making individualized support essential.
Down syndrome is the most representative chromosomal abnormality and can be accurately detected through NIPT.
Prenatal testing requires not only medical judgment but also ethical and social considerations.
A society built on accurate knowledge and strong support systems is vital to uphold both new life and the choices of families.
