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Synonyms and keywords: General learning disability; intellectual disability; unspecified intellectual disability

# Overview
Intellectual disability (ID) belongs to neurodevelopmental disorders that affect children and adolescents during the developmental period. It is categorized into four subclasses that determine the need for support. This condition is characterized by impairments in both intellectual and adaptive functions. Research involving ID has found that genes that encode cognitive abilities play an essential role in its pathophysiology. An interplay of environment and genetics can cause ID, and comprehensive screening is done in these cases. More importantly, ID has existing co-morbid conditions, which makes treatment and care a challenge.

# Historical Perspective
Intellectual disability traces its roots back to ancient civilizations. The Egyptians focused on treating disabilities and other ailments, while Greek and Roman Civilizations negatively viewed disability, killing those with disabilities. During the Middle Ages, intellectual disability revolved around religion and superstitions. The Church became a refuge for the individual with disabilities by providing shelter. The Restoration period associated "idiocy" and mental illness with immortality for which having a disability is a punishment. 
In the 17th century, John Locke differentiated intellectual disabilities from physical ones, where both mental and emotional deficits characterized intellectual disabilities. Oxford Philosopher Willis pinpointed various etiologies for a mental disability such as heredity, trauma, other diseases, and spirits. 

# Classification
The DSM 5 Classification of Severity for Intellectual disability has veered away from IQ scores and now considers adaptive functioning as the basis for classification. 

# Pathophysiology
Intellectual disorders with intact cortex have found that most of the known genes influencing cognitive abilities are X-linked. These genes code for different proteins and some are involved in neuronal connectivity and synapse formation and activity. 
Recent progress in unraveling the pathophysiology of ID involves defects in synaptogenesis and synaptic activities, including neuroplasticity.  
An important finding that illustrates the importance of synapses in the occurrence of ID involves the FMRP protein that is absent in Fragile X syndrome. The FMRP protein is normally detected in the nucleus, body, and dendrites. It is upregulated by glutamate-mediated stimulationthe specific knockout of Fmr1 results in abnormal morphology of dendrites in Purkinje cells in the cerebellum. Therefore, defects in synaptic structure and overall neuronal connectivity impairs proper information processing.  

# Causes
In less than 50% of individuals with mild ID, a specific cause is identified, and it increases to 75% in those with severe ID. 

## Genetics
Down syndrome (Trisomy 21) is the most common genetic cause of ID, while Fragile X is the most common inherited cause of ID.

## Environmental Factors
Alcohol exposure during pregnancy, lead and other heavy metals, iodine deficiency, brain infections, congenital rubella syndrome, and cytomegalovirus infections, as well as hypoxic-ischemic injury, and periventricular hemorrhages all cause brain injury resulting in disability. "Intellectual Disability and Mental Health Problems : Evaluation of Two Clinical Assessment Instruments, Occurrence of Mental Health Problems and Psychiatric Care Utilisation"..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"\"""\"""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{display:none;font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}

# Differentiating Mental retardation from other Diseases
- Neurocognitive disorders  there is loss of cognitive functioning in these cases.
- Specific learning disorder and language disorders  compared to individuals with ID, these individuals have deficits in communication and learning aspects but with normal intellect and adaptive function.
- Autism spectrum disorder  social development and language deficits are the hallmark of autism spectrum patients and have normal motor development.
Possible hearing and visual impairments should be ruled out in diagnosing intellectual disability.  

# Epidemiology and Demographics
The prevalence is 1% in the general population, with 6 per 1000 persons having a severe mental disability. In the United States, individuals with a severe intellectual disability are at 0.3-0.5%, while worldwide prevalence is at 16.41 per 1000 people in developing countries and 9.21 per 1000 people in developed countries. Males are more likely to have a mental disability with a ratio of 2:1, and families with one child with severe mental disability have a recurrence risk of 3% and 9%.  

# Risk Factors
Prenatal causes 
- Genetic syndromes
- Inborn errors of metabolism
- Brain malformations
- Maternal disease
- Environmental factors such as the history of alcoholism, teratogens, and other drugs
Perinatal causes include events during labor and delivery that ultimately lead to ischemic injury to neonates' brains. 
Postnatal causes 
- Hypoxic-ischemic injury
- Traumatic brain injury
- Infections
- Demyelinating disorders
- Infantile spasms
- Severe and chronic social deprivation
- Heavy metal poisoning
- Toxic metabolic syndromes

# Screening
Evaluation is dependent on age at onset, the severity of signs and symptoms, and the need to determine the underlying etiology of ID. A comprehensive screening includes clinical assessment paying particular attention to prenatal and perinatal history and family pedigree, psychological testing, karyotyping and metabolic screening, as well as neuroimaging tests.  "www.aaidd.org" (PDF).

## Intelligence assessment
Intelligence Quotient (IQ) is the standard for estimating intellectual function. Standardized tools such as the Wechsler scale is administered to children 6-16 years old, while a brief assessment tool]] such as the Kauffman Brief Intelligence Test is an alternative if the Wechsler test is not possible. The mean value of IQ is 100, and 70-75 represents the upper limit of two standard deviations below the mean. Several factors may influence intelligence assessment, and this includes measurement error, Flynn effect, practice effects, outliers, and test selection, to name a few.  

## Adaptive function assessment
The Vineland Adaptive Behavior Scale evaluates communication, ability to perform activities of daily living (ADL), motor and socialization. Another assessment tool is AAIDD's Diagnostic Adaptive Behavior Scale (DABS), administered to individuals 4-21 years old. This tool focuses on the "cut-off" area for ruling in a diagnosis of ID to determine eligibility for special education services, social security benefits, and home and community-based waiver services.  

## Genetic and Metabolic Testing
Newborn screening programs screen for inborn errors of metabolism with a yield of < 1%, and chromosomal analysis is required in children with unknown causes of ID with a yield of 12%. While genetic tests help discern the prognosis and treatment plan, it should be taken into account that these are expensive, and the findings may not reflect the phenotype of an individual. 

# Natural History, Complications and Prognosis
ID often is accompanied by other mental, medical, and physical conditions like epilepsy and cerebral palsy. The most common comorbid conditions are attention deficit hyperactivity disorder (ADHD, depression, bipolar disorder, anxiety disorder, autism spectrum disorder, and stereotypical movement disorder. 

# Diagnosis
The DSM 5 Diagnostic Criteria specifies that all of the three criteria must be satisfied: 
- Individuals have difficulty in executive functioning, academic learning, and experiential learning confirmed by standard assessment tools and clinically.
- Social communication skills are challenging and practical skills that impair independence (performing activities of daily living) and interaction with other people.
- It happens during the developmental period.
The classification of ID must also be specified.

# Treatment
Treatment should address the underlying cause of ID, treatment of comorbid physical disorders that may further impair functioning, such as pharmacologic treatment for behavioral disorders in Fragile X patients, and institution of special education, rehabilitation, and psychosocial interventions.  "Clinical Characteristics of Intellectual Disabilities - Mental Disorders and Disabilities Among Low-Income Children - NCBI Bookshelf".