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- Intellectual and Developmental Disabilities (IDD)
- Learning disabilities
- Autism Spectrum Disorder (ASD)
- Downs Syndrome
- Fetal Alcohol Syndrome (FAS)
- Cerebral Palsy
- Attention-Deficit Hyperactivity Disorder (ADHD)
- Phenylketonuria (PKU)
- Hypoxia
- Fragile X Syndrome (FXS)
- Rett Syndrome
- Developmental Dyslexia
- Auditory Processing Disorder (APD)

Examples of developmental disorders

Autism Spectrum Disorder is a neurodevelopmental disorder that affects communication and behavior. It displays symptoms such as impaired communication, lack or impairment in social skills and interactions, and repeated patterns of behaviors that interfere with an individual's ability to cognitively function. These symptoms are almost always present in early development, and are considered to be on a 'spectrum' because of the differences in severity of these symptoms.


Although autism can be diagnosed at any age, it is said to be a “developmental disorder” because symptoms generally appear in the first two years of life. Boys tend to be diagnosed more often than girls by approximately 4x. There is also an overrepresentation of transgender or gender nonconforming people with ASD.

Autism Spectrum Disorder

Phenylketonuria (PKU) is a recessive hereditary disorder, meaning that two copies of the mutated gene implicated in PKU disease processes must be inherited in order for an individual to display symptoms of this disorder. It’s one of the most common inherited metabolic disorders. The mutated genes cannot produce the enzyme that normally metabolizes the compound phenylalanine, found in certain food products, into tyrosine. The build-up of phenylalanine in the brain can become toxic, leading to intellectual disability when the build-up interferes with myelination in the central nervous system. If a baby with PKU ingests food with phenylalanine, the build-up during the post-birh period stunts brain development. Because of its preventable nature, and because the test is inexpensive and accurate, many governments have passed laws for mandatory PKU testing shortly after birth, and this has prevented many cases of intellectual disability.
PKU can be prevented by regulating the level of phenylalanine intake, especially early in life. After the post-birth brain development period has passed, the low phenylalanine diet can be relaxed somewhat since the danger to brain development has passed. 
For pregnant women with PKU, however, this isn’t the case. When a fetus is in utero, it’s protected by its mother’s metabolism. For non-PKU having mothers, this means the baby is especially protected during this time. However, if the mother has PKU and ingests a diet with typical or high levels of phenylalanine - while it won’t damage HER brain - it will cause improper brain development in the fetus. 

Phenylketonuria (PKU)

Hypoxia describes a deficiency in oxygen. If newborns experience birth complications such that they experience a lack of oxygen at birth, they are more likely to have an intellectual disability, or significant difficulties with intellectual functioning and adaptive behavior. 

Hypoxia

Fragile X syndrome is perhaps the most common genetic form of intellectual disability. It results when the trinucleotide repeats (i.e., CGG repeats) in the long arm of the X chromosome number more than 200 repeats, as opposed to the normal 6-50 repeats. This extension of repeats leaves this region of DNA fragile, or prone to breakage. Specifically, the overexpansion causes methylation and transcriptional repression of the Fragile X Mental Retardation 1 gene (*FMR1*), which encodes an important RNA-binding protein that inhibits RNA translation into proteins known as fragile X retardation protein (FMRP). The condition affects 1 in 3600 males and 1 in 8000 females. The reason for this large discrepancy between incidences in biological sexes is due to the fact that genetic females have two X chromosomes rather than one X like genetic males do. Generally speaking, females have one normally functioning *FMR1* on the non-mutated chromosome in each cell, which allows their symptoms to be milder since the amount of abnormal *FMR1* is more variable than in males.

Fragile X Syndrome

Down syndrome is a developmental disorder that causes varying degrees of physical and cognitive disability. It is the most common genetic condition and is caused by a chromosomal abnormality in which there is an extra copy of chromosome 21. 

Down Syndrome

Fetal Alcohol Syndrome is a specific diagnosis within the broader Fetal Alcohol Spectrum Disorder. It occurs when a mother consumes alcohol during pregnancy, especially during the critical period of the 3rd or 4th week of gestation. This exposure damages the developing nervous system, leading to distinct anatomical and behavioral abnormalities.

Fetal Alcohol Syndrome (FAS)

Learning disabilities are cognitive disorders that impact specific areas of cognition, such as language or reading. They are classified as specific neurological impairments, distinguishing them from global intellectual or developmental disabilities. A key characteristic is that they often affect individuals with average to above-average intelligence, meaning the disability is confined to a particular cognitive area and is not an indicator of overall intellectual capability.

Learning Disabilities

Attention-Deficit Hyperactivity Disorder (ADHD) is characterized by inattention and hyperactivity-impulsivity and is divided into three subtypes:
- primarily inattentive
- primarily hyperactive-impulsive
- combined

Attention-Deficit Hyperactivity Disorder (ADHD)

Cerebral Palsy is the most common motor disability in children, and it is caused by abnormal development or damage to the brain. This results in an impaired ability to move and maintain posture. In mild cases, individuals may walk a bit awkwardly while severe cases may need a caretaker and special equipment.

Cerebral Palsy

Rett Syndrome is a rare X-linked disorder usually caused by spontaneous (de novo) mutations in the methyl-CpG-binding protein 2 (*MeCP2*) gene. It affects almost exclusively genetic females (1 in 10,000 females). This is because genetic males only have one X chromosome and therefore don’t have dosage compensation occurring, lessening the amount of the mutated gene circulating. Genetic males with the syndrome either die in utero or in infancy due to the severity of the mutation phenotype. For 6 - 18 months persons with Rett Syndrome experience normal development, hitting the usual developmental milestones, until said development suddenly stops and regression begins. There’s usually a mild improvement in some social skills at around age 5, but regression (mental and physical) will continue for the rest of the person’s life.


Rett Syndrome

Individuals with Intellectual and Developmental Disabilities

IDD 

In a foundational 1943 paper, psychiatrist Leo Kanner detailed a unique neurodevelopmental condition he termed "early infantile autism." His observations in a group of children highlighted key characteristics, including an inability to form close emotional bonds, abnormalities in speech and language, repetitive behaviors, and a strong intolerance for minor changes in their environment or routines.

Leo Kanner's 1943 Description of Early Infantile Autism

A 10-year-old student consistently scores in the high-average range on intelligence tests that measure problem-solving and abstract reasoning. However, the student struggles significantly with reading, frequently confusing letters and having difficulty sounding out words, which is well below the expected level for their age and overall intelligence. Which of the following best categorizes this student's specific challenge?

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