Intellectual disability

Intellectual disability, also known as general learning disability, and formerly as mental retardation, is a generalized neurodevelopmental disorder characterized by significant impairment in intellectual and adaptive functioning that is first apparent during childhood. Children with intellectual disability typically have an intelligence quotient below 70 and deficits in at least two adaptive behaviors that affect everyday living. According to the DSM-5, intellectual functions include reasoning, problem solving, planning, abstract thinking, judgment, academic learning, and learning from experience. Deficits in these functions must be confirmed by clinical evaluation and individualized standard IQ testing. On the other hand, adaptive behaviors include the social, developmental, and practical skills people learn to perform tasks in their everyday lives. Deficits in adaptive functioning often compromise an individual's independence and ability to meet their social responsibility.
Intellectual disability is subdivided into syndromic intellectual disability, in which intellectual deficits associated with other medical and behavioral signs and symptoms are present, and non-syndromic intellectual disability, in which intellectual deficits appear without other abnormalities. Down syndrome and fragile X syndrome are examples of syndromic intellectual disabilities.
Intellectual disability affects about 2–3% of the general population. Seventy-five to ninety percent of the affected people have mild intellectual disability. Non-syndromic, or idiopathic cases account for 30–50% of these cases. About a quarter of cases are caused by a genetic disorder, and about 5% of cases are inherited. Cases of unknown cause affect about 95 million people as of 2013.

Signs and symptoms

Intellectual disability becomes apparent during childhood, and involves deficits in mental abilities, social skills, and core activities of daily living when compared to peers of the same age. There are often no physical signs of mild forms of ID, although there may be characteristic physical traits when it is associated with a genetic disorder.
The level of impairment ranges in severity for each person. Some of the early signs can include:

Delays in reaching, or failure to achieve milestones in motor skills development
Slowness learning to talk, or continued difficulties with speech and language skills after starting to talk
Difficulty with self-help and self-care skills
Poor planning or problem-solving abilities
Behavioral and social problems
Failure to grow intellectually, or continued infant childlike behavior
Problems keeping up in school
Failure to adapt or adjust to new situations
Difficulty understanding and following social rules

In early childhood, mild ID may not be obvious or identified until children begin school. Even when poor academic performance is recognized, it may take expert assessment to distinguish mild intellectual disability from specific learning disability or emotional/behavioral disorders. People with mild ID are capable of learning reading and mathematics skills to approximately the level of a typical child aged nine to twelve. They can learn self-care and practical skills, such as cooking or using the local transit system. As individuals with intellectual disabilities reach adulthood, many learn to live independently and maintain gainful employment. About 85% of people with ID are likely to have mild ID.
Moderate ID is almost always apparent within the first years of life. Speech delays are particularly common signs of moderate ID. People with moderate intellectual disabilities need considerable support in school, at home, and in the community in order to fully participate. While their academic potential is limited, they can learn simple health and safety skills and to participate in simple activities. As adults, they may live with their parents, in a supportive group home, or even semi-independently with significant supportive services to help them, for example, to manage their finances. As adults, they may work in a sheltered workshop. About 10% of people with ID are likely to have moderate ID.
People with Severe ID, accounting for 3.5% of persons with ID, or Profound ID, accounting for 1.5% of people with ID, need more intensive support and supervision for their entire lives. They may learn some ADLs, but an intellectual disability is considered severe or profound when individuals are unable to independently care for themselves without ongoing significant assistance from a caregiver throughout adulthood. Individuals with profound ID are completely dependent on others for all ADLs and to maintain their physical health and safety. They may be able to learn to participate in some of these activities to a limited degree.

Co-morbidity

Autism and intellectual disability

Intellectual disability and autism spectrum disorder share clinical characteristics which can result in confusion while diagnosing. Overlapping these two disorders, while common, can be detrimental to a person's well-being. Differentiating between these two disorders will allow clinicians to deliver or prescribe the appropriate treatments, though comorbidity between ID and ASD is very common; roughly 30% of those with ASD also have ID. Both ASD and ID require shortfalls in communication and social awareness as defining criteria.
In a study conducted in 2016 surveying 2816 cases, it was found that the top subsets that help differentiate between those with ID and ASD are, "impaired non-verbal social behavior and lack of social reciprocity, restricted interests, strict adherence to routines, stereotyped and repetitive motor mannerisms, and preoccupation with parts of objects". Those with ASD tend to show more deficits in non-verbal social behavior such as body language and understanding social cues. In a study done in 2008 of 336 individuals with varying levels of ID, it was found that those with ID display fewer instances of repetitive or ritualistic behaviors. It also recognized that those with ASD, when compared to those with ID, were more likely to isolate themselves and make less eye contact. When it comes to classification ID and ASD have very different guidelines. ID has a standardized assessment called the Supports Intensity Scale ; this measures severity on a system built around how much support an individual will need. While ASD also classifies severity by support needed, there is no standard assessment; clinicians are free to diagnose severity at their own judgment.

Epilepsy and intellectual disability

Around 22% of individuals with ID have epilepsy. The incidence of epilepsy is associated with level of ID; epilepsy affects around half of individuals with profound ID. Proper epilepsy management is particularly crucial in this population, as individuals are at increased risk of sudden unexpected death in epilepsy. Nonetheless, epilepsy management in the ID population can be challenging due to high levels of polypharmacy prescribing, drug interactions, and increased vulnerability to adverse effects. It is thought that 70% of individuals with ID are pharmaco-resistant, however only around 10% of individuals are prescribed anti-seizure medications licensed for pharmaco-resistant epilepsy. Research shows that certain ASMs, including levetiracetam and brivaracetam, show similar efficacy and tolerability in individuals with ID as compared to those without. There is much ongoing research into epilepsy management in the ID population.

Causes

Among children, the cause of intellectual disability is unknown for one-third to one-half of cases. About 5% of cases are inherited. Genetic defects that cause intellectual disability, but are not inherited, can be caused by accidents or mutations in genetic development. Examples of such accidents are development of an extra chromosome 18 and Down syndrome, which is the most common genetic cause. DiGeorge syndrome and fetal alcohol spectrum disorders are the next most common causes. Some other frequently observed causes include:

Genetic conditions. Sometimes disability is caused by abnormal genes inherited from parents, errors when genes combine, or other reasons like de novo mutations in genes associated with intellectual disability. The most prevalent genetic conditions include Down syndrome, Klinefelter syndrome, Fragile X syndrome, neurofibromatosis, congenital hypothyroidism, Williams syndrome, phenylketonuria, and Prader–Willi syndrome. Other genetic conditions include Phelan–McDermid syndrome, Mowat–Wilson syndrome, genetic ciliopathy, and Siderius type X-linked intellectual disability as caused by mutations in the PHF8 gene. In the rarest of cases, abnormalities with the X or Y chromosome may also cause disability. Tetrasomy X and pentasomy X syndrome affect a small number of girls worldwide, while boys may be affected by 49, XXXXY, or 49, XYYYY. Jacobs syndrome is not associated with significantly lowered IQ though affected individuals may have slightly lower IQs than non-affected siblings on average.
Problems during pregnancy. Intellectual disability can result when the fetus does not develop properly. For example, there may be a problem with the way the fetus's cells divide as it grows. A pregnant woman who drinks alcohol or gets an infection like rubella during pregnancy may also have a baby with an intellectual disability.
Problems at birth. If a baby has problems during labor and birth, such as not getting enough oxygen, they may have a developmental disability due to brain damage.
The group of proteins known as histones have an essential part in gene regulation, and sometimes these proteins become modified and are prevented from working properly. When the genes responsible for the development of neurons are affected, it affects the brain and behavior in the individual.
Exposure to certain types of disease or toxins. Diseases like whooping cough, measles, or meningitis can cause intellectual disability if medical care is delayed or inadequate. Exposure to poisons like lead or mercury may also affect mental ability.
Iodine deficiency, affecting approximately 2 billion people worldwide, is the leading preventable cause of intellectual disability in areas of the developing world where iodine deficiency is endemic. Iodine deficiency also causes goiter, an enlargement of the thyroid gland. More common than full-fledged congenital iodine deficiency syndrome, as intellectual disability caused by severe iodine deficiency is called, is mild impairment of intelligence. Residents of certain areas of the world, due to natural deficiency and governmental inaction, are severely affected by iodine deficiency. India has 500 million people with a deficiency, 54 million with goiter, and 2 million with congenital iodine deficiency. Among other nations affected by iodine deficiency, China and Kazakhstan have instituted widespread salt iodization programs. But, as of 2006, Russia had not.
Malnutrition is a common cause of reduced intelligence in parts of the world affected by famine, such as Ethiopia and nations struggling with extended periods of warfare that disrupt agriculture production and distribution.
Absence of the arcuate fasciculus.
Furthermore, lack of stimulation of sensory pathways in infants can also cause developmental and cognitive delays.