Intellectual Disability in Older Adults

Definition and Causes

Intellectual disability (ID) is the current term used to describe what in the United Kingdom has been known as learning disability and in the United States as mental retardation. The World Health Organization's International Classification of Diseases (ICD-10) still uses the term mental retardation , and its report on healthy aging in this population uses the term intellectual disabilities . In Australia the 1986 Victorian Act of Parliament defines intellectual disability in the following way:

Intellectual disability in relation to a person over the age of five years means a significant sub-average general intellectual functioning existing concurrently with deficits in adaptive behavior and manifested during the developmental period. (Intellectually Disabled Persons Services Act, 1986)

The threshold at which general intellectual functioning is considered “subaverage” is often fixed at an IQ of 70, two standard deviations below the mean IQ. Controversially in 1992 the American Association on Intellectual and Developmental Disabilities (AAIDD) loosened this threshold to include people with IQs in the range of 70 to 75. The AIDD also required deficits in 2 out of 10 assessed areas of adaptive functioning. This definition was adapted by the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders , fourth edition (DSM-IV). In 2002 the AAIDD reinstated the IQ 70 threshold and required deficits in conceptual, social, and practical adaptive skills to be present. Areas covered by these skills include communication, personal care, home life, social skills, community utilization, self-governance, health and safety, functional academic skills, work, and leisure activities. These changes in definition have implications for epidemiologic data collection, but the key concept of ID remains. An IQ less than 70 is necessary but is, in itself, inadequate for the diagnosis to be made. For the diagnosis to be made, there must be evidence of both a developmental disorder (with onset during childhood) and deficits in adaptive behavior.

Further classification of ID can be made within the broad definition. The Diagnostic Criteria for Psychiatric Disorders for Use with Adults with Learning [Intellectual] Disabilities (DC-LD) describes the mental health of a person with ID in terms of ID severity, ID causes, and related mental disorders (developmental disorders, psychiatric illness, personality disorders, problem behaviors, and other disorders). Severity is grouped according to IQ: 50 to 69, mild ID; 35 to 49, moderate; 20 to 34, severe; and less than 20, profound. The Swedish model of ID classification developed by Kylen is often helpful in clinical situations where IQ is not known:

• Severe: Communication is based on simple nonverbal signs, no verbal communication, no concept of time or space. Equivalent to IQ less than 10.

• Moderate: Limited verbal skills. Limited understanding of local space. Can structure thoughts in relation to individual experiences. Equivalent to IQ 10 to 40.

• Mild: Basic literacy and mathematical skills present. Can rearrange, structure, and perform concrete cognitive operations. Equivalent to IQ 41 to 70.

Severity may also be broadly estimated in terms of functional abilities:

• Mild: Social and work skills adequate to work at a minimum wage

• Moderate: Requires significant support to be able to work in a protected environment

• Severe: Can partially contribute to his or her economic support with total supervision

In addition, the DC-LD includes appendices that relate to medical factors influencing health status and contact with health services. The latter are highly relevant because developmental disorders that affect the brain, giving rise to ID, often affect other body systems also.

The cause of ID is frequently unknown in older adults but can be considered along conventional lines of external causes (infection, injury, poisoning), internal disorders (endocrine, metabolic), perinatal insults, and congenital conditions (chromosomal abnormalities, gene mutations). The latter are of particular relevance to the health of older adults with ID as specific syndromes are associated with risk of particular physical disorders and diseases. Common syndromes seen in older adults include Down syndrome (DS), Angelman syndrome, fragile X syndrome, Klinefelter syndrome, Turner syndrome, and Williams syndrome. Table 57-1 provides a brief description of these. It is worth noting that, given the preceding definition of ID, by no means does everyone with one of these syndromes fulfill the diagnostic criteria for ID; this is particular true of women with Turner syndrome, who have a tendency for nonverbal cognitive deficits but are often of average intelligence.

Just as there is a considerable overlap between congenital syndromes, such as DS and ID, there is a similar overlap between ID and autism. The diagnosis of autism depends on (1) abnormal social development, (2) communication deficits, and (3) restricted and repetitive interests and behavior. Approximately three quarters of people with autism have a nonverbal IQ less than 70 and hence also fulfill diagnostic criteria for ID, but in autism social and communication skills are worse than expected for any given nonverbal IQ.

Epidemiology of Intellectual Disability and Aging

Biologic Aging in Individuals with Intellectual Disability: Syndromic and Nonsyndromic

Even with recent improvements, the life expectancy of people with ID is considerably less than that of the non-ID population. This raises the question as to whether ID is associated with accelerated biologic aging. The measurement of biologic age usually depends on identifying suitable biomarkers of aging. Criteria for such biomarkers have been proposed :

• 1.

  They must reflect some basic biologic process of aging rather than disease.

• 2.

  They must have high cross-species reproducibility.

• 3.

  They must change independently of chronologic time.

• 4.

  They must be obtainable antemortem.

• 5.

  They must be measurable over a short period compared to the life span of the organism.

One such biomarker is telomere length. There is a paucity of data on telomere length in ID in general, but there is evidence for telomere shortening in DS though this appears to be downstream from cellular redox status as is also likely in the general population. In cri-du-chat syndrome there is often a deletion of the short arm of chromosome 5 where the telomerase reverse transcriptase ( hTERT ) gene is localized (5p15.33). Reconstitution of telomerase activity by ectopic expression of hTERT extends telomere length, increases population doublings, and prevents the end-to-end fusion of chromosomes. It may thus be one element contributing to the syndrome's phenotypic features. Whether this is the case or not, accelerated telomere shortening occurs with aging in this syndrome. At least a further 5% of ID is attributable to similar subtelomeric deletions or copy number variations and these, too, can influence telomere length. Telomere length is thus a potentially useful index of accelerated biologic aging in ID, but whether it contributes to aging itself or is only a correlate remains unclear. Moreover, telomere shortening is subject to syndrome-specific effects. Beyond the cellular level, various physiologic biomarkers are also affected in ID. Physiologic variables are long recognized as indices of biologic age. The limited data available indicate that people with DS have accelerated biologic aging but that people with nonsyndromic ID do not. In summary, the evidence suggests that syndromic biologic aging dominates over any accelerated biologic aging that might be associated with ID in general and that different ID syndromes are likely to have different aging profiles depending on the specific genetic changes underlying them.

Age-Related Disease: Syndromic and Nonsyndromic Patterns

People with ID exhibit considerable morbidity. A study of 346 people aged 20 to 50 years in North Sydney found they had a mean 2.5 major problems and 2.9 minor problems each with 42% of these undiagnosed prior to the study, and of the 58% already known, only 49% were being managed adequately. A study of 1371 adults aged 40 years and older in New York State found that increased age was associated with higher prevalence of cardiovascular disease, cancer, respiratory disease, musculoskeletal disorders, infections, and visual and hearing impairments; gastrointestinal disease was not associated with age but with being male, more severe ID, cerebral palsy, and obesity. Compared with the non-ID population, there was less cardiovascular disease and musculoskeletal disease, except osteoarthritis, and people with DS did not have solid neoplasias. However, these data may reflect underdiagnosis and lifestyle factors such as the low rate of cigarette smoking among people with ID. A similar age-related pattern of disease was found in southern Holland. The pattern may change as fewer people with ID live in institutions: there is evidence from the United Kingdom that poor diet, reduced physical activity, and obesity risk factors are greater in women with ID who are more able and independent. In addition to the general tendency for high levels of morbidity, which is associated with both age and degree of ID severity, specific syndromes carry their own particular risks ( Table 57-3 ). The most common syndrome in older adults with ID is DS, and discussion of the various problems in people with DS can provide a general approach to such problems in other syndromes.