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Introduction

The term intellectual and developmental disability (IDD) is internationally accepted to describe limitations in cognitive and adaptive functioning. The evolution of the terminology from earlier diagnostic labels with negative connotations is a reflection of a better understanding of the concept of cognition and cognitive deficits within the scientific and socio-cultural contexts and, more importantly, the constant striving to provide this group of individuals with the respect and dignity they rightly deserve. Although IDD is initially identified in infancy and early childhood years, it has life-long implications for growth and development, education, independence, health care, employment, and community living.

Definition

According to the American Association on Intellectual and Developmental Disabilities (AAIDD), IDD “is a disability characterized by significant limitations both in intellectual functioning and in adaptive behavior as expressed in conceptual, social, and practical adaptive skills” [1]. The assessment of intellectual functioning and adaptive behavior must take into consideration the expectations based on individual’s age and culture as well as the influence on cognitive assessment of sensory, motor, communication, or behavioral factors, which shouldbe taken into consideration in the administration of assessment instruments, and interpretation of the results of the tests.

In the United States, a widely used definition is taken from the Individuals with Disabilities Education Act, which defines IDD as “significantly sub-average general intellectual functioning, existing concurrently with deficits in adaptive behavior and manifested during the developmental period that adversely affects a child’s educational performance.”

According to the Diagnostic and Statistical Manual of Mental Disorders [2], IDD is defined as an intelligence quotient (IQ) of approximately 70 or below on an individually administered standardized test of intelligence, concurrent with deficits in adaptive functioning in two of the following areas: communication, self-care, home living, social or interpersonal skills, use of community resources, self-direction, functional academic skills, work, leisure, health, and safety. All definitions stipulate that the onset of disability must occur before the age of 18 years.

It is generally agreed that, although not perfect, appropriately measured IQ provides the best estimate of intellectual functioning. Based on the mean value for IQ of 100, the upper limit of 70 represents the value that is two standard deviations below the mean. Because there is a five point standard error of measurement, it is argued that a range of 70–75 should be considered as the upper limit of IQ as the cut off value for intellectual disability. Theoretically, therefore, an individual with an IQ score of 75 with significant adaptive deficits will be considered to have an ID, whereas an individual with no adaptive disability and an IQ score of 70 or even 65 may not be considered to have an ID.

The severity of IDD is further categorized based on intellectual functioning, adaptive functioning, and intensity of supports needed [1] (see Table 1.1). When the level of intellectual disability cannot be reliably assessed, but there is a high level of confidence based on clinical judgment, a diagnosis of IDD is made without necessarily specifying the severity. Ultimately, however, the designation is a practical one and relates to ability of the individual to function independently within a relatively typical context in a family and in a community.

Table 1.1 Classification of level of intellectual disability [2]
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Epidemiology

The reported prevalence of IDD reflects consideration of the definition used, method of ascertainment of the data, and the characteristics of the population studied. Based on the typical bell- shaped distribution of intelligence in the general population, then 2 standard deviations below the mean, which is approximately 2.5 % of the population, is expected to have IDD. Most epidemiological studies consider those with an IQ score of 50 or less as having severe IDD, and those above that as having mild IDD. Using these designations, 85 % of individuals with ID will have mild levels of disability, 10 % will have moderate ID and the remainder will have severe or profound levels of ID. The prevalence of severe IDD has remained constant over several decades at 0.3–0.5 % of the general population in the United States. Based on the United States National Center for Health Statistics 1997–2003 National Health Interview Survey, the prevalence of IDD among children ages 5–17 years is estimated to be 7.5 per 1000 [3].

Intellectual disability is reported to be twice as common in males compared to females. The recurrence risk of IDD in families with one previous child with severe IDD is reported to be between 3 % and 9 % [3].

A specific etiology can only be identified in less than half of individuals who have mild degrees of ID, while an underlying biological or neurological etiology can be identified in more than two-thirds of individuals who have severe IDD [3]. Etiological factors are predominantly prenatal in origin and include chromosomal anomalies, genetic syndromes, congenital brain anomalies, congenital infections, inborn errors of metabolism, neurodegenerative disorders. Perinatal factors include prematurity or birth injury, while postnatal etiologies include CNS infections such as meningitis or encephalitis, brain injury from accidental and non-accidental causes, including child abuse like “shaken baby” syndrome and environmental factors such as lead toxicity. However, by far the most common potentially preventable causes are directly or indirectly related to socioeconomic factors which effectively ‘shift the population distribution curve’ (see chapter “The Sphingolipidoses”).

Clinical Features

At this point it is appropriate to use the term to IDD, because the discussion becomes inclusive of all conditions rather than focusing exclusively on the cognitive measures. Children who have IDD can present with a wide range of initial clinical symptoms and signs depending up on the underlying cause and severity of the disability [4–9]. Children who have delays in motor milestones will present early while children with learning disabilities or behavioral symptoms and are identified relatively later. Common presentations of IDD by age are summarized in Table 1.2. The age at which IDD is recognized also depends on its severity (see Table 1.3).

Table 1.2 Common presentations of IDDa by age
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Table 1.3 Key elements of history
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Other mental disorders (see Table 1.4) are 3–4 times more common in children with IDD [10]. Some children may present with behavioral symptoms which may dominate the clinical picture. The term dual diagnosis is used in the situation of a co-morbid mental/behavioral disorder with IDD.

Table 1.4 Selected standardized instruments
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Diagnosis

For individuals with IDD there are multiple associated developmental domains and often other co-occurring conditions, therefore an interdisciplinary approach is strongly indicated. Diagnosis is made based on etiological considerations, dominant presenting problems, severity of the condition and co-occurring disorders. It is critical to obtain a complete and comprehensive history (see Table 1.5) followed by a complete and detailed general physical examination, with a focus on physical features and neurological examination. A work up should obviously include compete audiological and vision evaluation in all children [4–9]. Technically, a diagnosis of IDD requires individualized cognitive and adaptive testing by qualified examiners using standardized instruments (see Table 1.6). Standardized testing should be age appropriate, take into account develop mental age of the child, and the cultural background which can significantly influence the child’s ability to perform in the test; it should be remembered that whatever test is used for this purpose should be specific to a language and culture, for example, for English speaking children in the USA or in the UK, or for French speaking children in France. Care must be taken, therefore, to be sensitive to the background, language, culture, education and life experiences of the child (or adult) who is being evaluated. Appropriate accommodations should also be made for any motor, behavioral, sensory disorders or other complicating factors.

Table 1.5 Factors that may guide decision to pursue etiological diagnosis
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Table 1.6 Reasons offered in support of pursuing an etiological diagnosis
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While it is important to try to establish the etiology for the IDD, this may not always be possible. Etiological diagnosis should be pursued if there are specific developmental, physical or medical characteristics and particularly, if there are unusual physical or medical characteristics and critically if there is any loss of function or failure to grow or develop which could indicate an inborn error of metabolism or neurodegenerative disorder. Factors that might guide the decision to pursue etiological diagnosis are summarized in Table 1.5 and some reasons offered by those who favor such an approach are summarized in Table 1.6.

Approach to Management

Children with IDD are best supported through an interdisciplinary team approach in the setting of a medical home [11]. Early identification and early intervention, with appropriate therapies and family support, are critical to establishing the framework for long term services and support, and for promoting optimal development for the child as well as providing families with an understanding of the therapeutic needs of their child. Such an approach further sets the stage for families’ awareness of the social network that is destined to be part of their lives for the foreseeable future. The physician should provide general medical care according to established guidelines. Specific health maintenance guidelines for children are published by the American Academy of Pediatrics for several conditions e.g., Down Syndrome, and can be accessed at www.aap.org. It is critically important that the clinician have a good working knowledge of medical and community resources that may be required and should have ongoing communication to facilitate and coordinate the child and family’s needs in the spirit of the Medical Home (see chapter on Medical Home).

As the child grows, so too do his or her needs change from the early intervention services to school-based services, and then to transition to the needs of a young adult and through the adult years on to old age and geriatric services. Advances in medical care and in social services have played a part in the increased lifespan of individuals with IDD resulting in the need to support the families as well as the individuals with IDD. It is important to remember at all times, that the individual with IDD lives in the context of a family, of a community and of society at large.

Public Services

In the United States, several Federal and State laws provide the framework and funding for intervention programs and educational services for children with IDD (see section “Reaching the Unserved and Underserved: Medical Care in Rural and Urban Settings” of chapter “The Rights of Individuals with Disability to Parenthood”). The mainstay of support for infants from birth three years of age and their families is early intervention services provided by local community agencies through the development and implementation of the Individualized Family Service Plan. For children and adolescents between the ages 3–22 years the main focus is on meeting the child’s educational needs, the Individualized Education Plan (IEP) which is developed and implemented by the student’s school district. Between 14 and 16 years of age an Individualized Transition Plan (ITP) is developed that addresses the student’s transition to adult services, vocational preparation, and independent living. After completion of high school, the individual is supported by the Individualized Habilitation Plan (IHP) that provides support services for adults. The intensity of support services needed depends up on the severity of the IDD (see Table 1.1). The adult outcomes and functioning of individuals with IDD are summarized in Table 1.7 [12].

Table 1.7 IDD and adult reference age functioning
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Transition to Adult Services

Puberty and adolescence bring hormonal, behavioral and social changes that announce imminent adulthood. Almost all the laws described above are specifically for children, particularly early intervention programs and special education services, and children also have the benefit of pediatricians and developmental pediatricians who have known them since infancy, as well as many specialty pediatric programs. Unfortunately, there are limited services for adults with IDD, and these are heavily dependent on state and local services, many of which are privately developed by family members and often funded by charitable organizations and businesses. It becomes critically important to begin to prepare the young teenager for a meaningful and productive life after graduation from high school with anticipatory planning (see section “Transitioning Youth to Adult Health Care: A Person-Centered and Culturally Competent Approach” of chapter “Siblings of Children and Adults with Intellectual and Developmental Disability”). At this stage, questions emerge about post graduate education, vocational training, exploration of employment opportunities, the preparation for the options for independent living and the need for adult oriented health services. Often neglected is the important consideration for a social life. While in school a child wakes up every morning with a place to go where there are people and a schedule of activities in an active nurturing and social environment. After graduation, if there are no plans in place, there is nowhere to go in the morning and nothing to do all day, except watching TV and perhaps minor sporadic outings. This sedentary life can contribute to depression and obesity, so it is very important to begin to plan early and to develop the necessary skills and set up the infrastructure for relative independence as well as for a fulfilling and satisfying life.

Adult Life and Aging

Programs and services for adults are not as varied, fulfilling or plentiful as for children, but with available federal and state programs, creative local programs and with resourcefulness, individuals with IDD can go on to lead satisfying, and productive lives, each consistent with his or her own potential abilities.

Improvements in healthcare for adults with IDD has resulted in increases in lifespan over the last century, which are substantial and worthy of celebration. Guidelines for the management of adults with IDD are emerging with major contributions from The Netherlands and from Canada (see sections “Canada” and “Service Organization and Support for People with Intellectual and Developmental Disabilities in the Netherlands” in chapter “Reaching the Unserved and Underserved: Medical Care in Rural and Urban Settings”), and approaches in the management of older individuals with IDD have been developed with integration into generic geriatric services (see chapters on aging....).

Conclusions

IDD is a term used to describe a segment of a population who have limited intellectual abilities and other characteristics that begin in childhood and have a lifelong affect their ability to function independently in society. Over the past century significant knowledge and resources have developed to meet the needs of children and their families and, also for adults and the aging population which has resulted in an improvement in quality of life and in the lifespan of all individuals with IDD. It is therefore important for all clinicians involved to have an understanding of the etiology, presentation and characteristics of each individual as well as the services available to help that individual reach his or her full potential within the context of the family, the community and society. Although we have made substantial progress, there is still much to learn and develop to assure that each individual leads a fulfilling and satisfying life and assure that there is full integration and inclusion for all people with IDD into all aspects of society.