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Every individual's background is different, and there are numerous known causes to intellectual and developmental disabilities. Several of these known conditions are listed below. Click through the tabs to learn more about each diagnosis, as well as to find support resources/organizations affiliated with each condition.
Note: this is not a comprehensive list.
According to the Eunice Kennedy Shriver National Institute of Child Health and Human Development, cerebral palsy (CP) is "a group of neurological disorders that begin in infancy or early childhood and that primarily affect body movement but, in more severe cases, may also be associated with intellectual disabilities." (NICHD, 2021).
There are several types of cerebral palsy, including:
Individuals with spastic CP exhibit muscle stiffness, which leads to sudden, sometimes erratic movement. Spastic CP can impact different body parts depending on the area of the brain impacted. Subtypes of spastic CP include:
Individuals with dyskinetic CP are described to have "slow and uncontrollable jerky movements of the hands, feet, arms, or legs" (NICHD, 2021). Dyskinetic CP is not as associated with intellectual disabilities as other forms.
While dyskinetic and static CP impact movement and stiffness in muscle tissue, ataxic CP deals more with balance and depth perception. Individuals may experience instability in walking and in fine motor tasks.
Entails any combination of symptoms from the above forms of cerebral palsy.
Learn more about cerebral palsy below.
Down Syndrome, also referred to as Trisomy-21, is a genetic disorder that develops in the presence of an extra chromosome 21 (or additional piece of the 21st chromosome). This is the most common chromosomal cause of IDD (NICHD, 2018).
Recently, the National Institutes of Health began an initiative to conduct more research on Down syndrome and comorbidities with other medical conditions, including Alzheimer's/dementia, autism, cataracts, celiac disease, congenital heart disease, and diabetes (NIH, 2018). This research effort, known as the INCLUDE (INvestigation of Co-occurring conditions across the Lifespan to Understand Down syndromE) Project, is ongoing.
Fragile X Syndrome is a genetic disorder caused by mutations on the X chromosome. This is the most common inherited form of intellectual or developmental disability (NICHD, 2021). This syndrome can impact physical development throughout puberty, learning ability, behavioral capacities (including anxiety, aggression, attention, and shyness), speech and language acquisition, and sensory sensitivity.
Fragile X Syndrome, while X-linked, is present in both male and female populations.
Klinefelter syndrome, also referred to as XXY syndrome, is characterized by a male individual having 1 or more extra X chromosome(s). (NCHID, 2016). While not always affiliated with an intellectual delay or disability, some variants of Klinefelter syndrome (e.g. those with additional extra X chromosomes) can exhibit characteristics of IDD (Defendi, 2020).
Phenylketonuria (commonly shortened to PKU) is a genetic disorder caused by a deficiency in phenylalanine hydroxylase, an enzyme necessary for the routine processing of complex proteins (NICHD, 2016). If the condition goes untreated, the buildup of unprocessed phenylalanine can lead to intellectual and developmental disabilities. Hospitals in the United States screen for PKU in infants shortly after birth, and prenatal testing is also available to determine if the fetus has PKU.
Individuals with PKU must follow strict dietary limitations in order to prevent the accidental ingestion of proteins that could lead to phenylalanine buildup, including restricting the intake of meat, dairy, and other protein-rich foods.
Prader-Willi Syndrome (PWS) is a genetic disorder that impacts an individual's eating habits, behavior, mood, physical growth, and intellectual development (NICHD, 2021). PWS is frequently caught in infancy and childhood, as children affected show significant delays in meeting growth milestones (e.g. standing and walking) as well as excessive eating.
PWS is the most common genetic disorder responsible for childhood obesity, due to the symptom of hyperphagia (chronic overeating).
Learn more about Prader-Willi Syndrome below:
Individuals diagnosed with Rett syndrome show a peculiar pattern of development where, after following and successfully meeting several developmental milestones, there is a sudden plateau, regression, or (in extreme cases) a complete loss, of previously acquired skills.
A classic diagnosis of Rett syndrome tends to follow the following pattern (NICHD, 2022):
Developmental stalls begin - in some cases, skill development may stop completely.
Skills begin to quickly regress. This is typically seen in many areas, including in speech and motor abilities (fine and gross).
Skill regression begins to slow to a halt: there is potential for skill redevelopment and improvement. Some individuals may experience seizures in this phase.
Changes begin in motor capacity - this can vary in presentation, such as loss of muscle tone, stiffness, scoliosis, and even immobility.
Learn more about Rett syndrome below.
While there are several genetic disorders and predispositions an individual can have to developing an intellectual disability, genetic factors are not the exclusive cause for IDD. In some cases, traumatic brain injury (or TBI) can lead to downstream impacts and effects on an individual's cognitive functioning.
Because TBI is so subjective, it's best to observe individuals on a case-by-case basis. Variations depend on the severity of the injury, the location of the injury, and the type of injury (concussion, for example; NICHD, 2020).
TBI can (depending on the injury) impact a lot of skills and abilities, including (NICHD, 2020):
Learn more about TBI by consulting the resources below.
Turner syndrome, another genetic disorder that can lead to intellectual or developmental disability, is characterized by a damaged or missing X chromosome in genetically female individuals (XX). Turner syndrome is also referred to as "Monosomy X" or as "XO syndrome" (NICHD, 2021).
Depending on the extent to which an X chromosome is damaged (whether it is partially missing or completely absent), the impacts on an individual can vary. Typical areas affected include appearance, stature, primary and secondary sex characteristics, and even organ function (e.g. heart, kidneys, bone structure). Individuals have higher susceptibility to osteoporosis, type 2 diabetes, and thyroid issues (NICHD, 2021).
Learn more about Turner syndrome below.
Stemming from a mutation that leads to the loss of genetic material on chromosome 7, Williams syndrome is another rare genetic disease that can lead to developmental delay and intellectual disability (MedlinePlus, 2022). In addition to altering intellectual functioning, this disorder presents with distinct personality characteristics and facial features.
Personality trends of individuals include social proactivity and having an outgoing disposition. There are also common comorbidities with psychological diagnoses such as ADHD, phobias, and anxiety.
Most notably, individuals with Williams syndrome tend to display facial similarities, including "a broad forehead, puffiness around the eyes, a flat bridge of the nose, full cheeks, and a small chin" (MedlinePlus, 2022).
Learn more about Williams syndrome by reviewing the resources below.