3. Etiology (Causes) of Intellectual Disability

The causes of intellectual disability are diverse and can occur at any stage from before conception to childhood. They are typically categorized based on the timing of the insult: before birth (prenatal), during birth (perinatal), or after birth (postnatal). In many cases, the exact cause remains unknown.

A. Prenatal Causes (Before Birth)

These are the most common causes of intellectual disability and involve factors affecting the fetus's development during pregnancy.

1. Genetic and Chromosomal Abnormalities:

• Down Syndrome (Trisomy 21): Caused by an extra copy of chromosome 21. It is one of the most common genetic causes and is characterized by distinct physical features and varying degrees of intellectual impairment.

• Fragile X Syndrome: Caused by a mutation on the FMR1 gene on the X chromosome. It is the most common inherited form of intellectual disability. The mutation prevents the production of a protein crucial for neural development.

• Inborn Errors of Metabolism: These are genetic disorders where the body cannot properly break down specific nutrients, leading to a toxic buildup that damages the brain.

  • 1. 1. Phenylketonuria (PKU): The body cannot metabolize the amino acid phenylalanine. If not detected and managed with a strict diet shortly after birth, it leads to severe intellectual disability.

       2. Tay-Sachs Disease: A fatal disorder where a harmful amount of a fatty substance builds up in the brain and spinal cord, causing profound neurological deterioration.

• Other Syndromes: Many other syndromes are linked to ID, such as Prader-Willi syndrome, Angelman syndrome, and Williams syndrome.

2. Maternal and Environmental Factors:

• Maternal Health and Nutrition:

  • Severe Malnutrition: A deficiency in essential nutrients like folic acid, iron, and iodine can impair fetal brain development.

• Teratogenic Exposures (Substances that cause birth defects):

  • 1. Fetal Alcohol Spectrum Disorders (FASD): Caused by maternal alcohol consumption during pregnancy. It is a leading preventable cause of intellectual disability and can cause distinct facial features, growth problems, and learning/behavioral issues.

    2. Smoking and Illicit Drug Use: Linked to low birth weight, prematurity, and developmental delays.

    3. Teratogenic Medications: Certain prescription drugs (e.g., for acne like isotretinoin, or anticonvulsants like valproate) can cause harm if taken during pregnancy.

• Maternal Infections (TORCH Infections): A group of infections that can cross the placenta and infect the fetus, causing serious developmental problems.

  • 1. Toxoplasmosis

    2. Other (e.g., Syphilis, HIV, Zika virus)

    3. Rubella (German Measles)

    4. Cytomegalovirus (CMV)

    5. Herpes Simplex Virus

• Physical Factors:

  • 1. Exposure to Radiation: High levels of radiation can cause microcephaly (abnormally small head) and intellectual disability.

    2. Complications of Pregnancy: Uncontrolled maternal diabetes, high blood pressure (preeclampsia), or placenta previa can compromise oxygen and nutrient supply to the fetus.

B. Perinatal Causes (During Birth)

These causes are related to complications occurring during the labor and delivery process.

• Birth Asphyxia (Oxygen Deprivation): A lack of oxygen to the baby's brain during a difficult or prolonged birth. This can be caused by umbilical cord problems, a detached placenta, or the baby being stuck in the birth canal.

• Prematurity and Very Low Birth Weight: Babies born very early (<32 weeks) or with very low birth weight (<1500 grams) are at significantly higher risk. Their organs, especially the brain, are not fully developed and are vulnerable to bleeding (intraventricular hemorrhage) and other injuries.

• Birth Trauma: Physical injury to the baby's brain during delivery, which can be caused by a prolonged labor, abnormal positioning, or the use of instruments like forceps or vacuum extraction.

• Neonatal Jaundice and Kernicterus: Severe, untreated jaundice (high levels of bilirubin) in the newborn can lead to kernicterus, a type of brain damage that causes cerebral palsy, hearing loss, and intellectual disability.

C. Postnatal Causes (After Birth)

These causes involve factors that affect the child after delivery, typically in the first few years of life when the brain is still developing rapidly.

• Infections: Serious infections that affect the brain directly.

  • Meningitis: Inflammation of the membranes surrounding the brain and spinal cord.

  • Encephalitis: Inflammation of the brain itself.

• Traumatic Brain Injury (TBI): Severe head injuries from accidents, falls, or unfortunately, physical abuse (Shaken Baby Syndrome). The extent of disability depends on the severity and location of the brain injury.

• Severe Neglect and Psychosocial Deprivation: A profound lack of stimulation, interaction, love, and learning opportunities in early childhood can severely impair cognitive and social development. The brain requires environmental input to develop properly.

• Toxic Exposure: Lead poisoning (e.g., from lead-based paint or contaminated water) is a classic example of an environmental toxin that can cause significant and irreversible brain damage and intellectual disability in young children.

• Seizure Disorders: Poorly controlled or prolonged seizures (e.g., status epilepticus) can cause brain damage over time.

• Malnutrition: Chronic, severe protein-energy malnutrition and a lack of essential micronutrients (like iodine and iron) in early childhood can permanently stunt brain development and cognitive function.