The Definition Of Mental Retardation

Eventually, Down's conceptualization of congenital MR would become more restricted to identifiable genotypes, whereas his notion of developmental MR would expand to incorporate familial forms. As mentioned earlier, Zigler and his colleagues also differentiated between familial MR (also referred to as cultural-familial MR) and organic MR (49,50). His two-group approach to MR has its roots in Down and Ireland's classificatory schemas. Zigler defined individuals with organic MR as those who sustain organic damage from inborn or prenatal causes or postnatal trauma, whereas those with no known organic etiology would be categorized as cultural-familial

MR (50). The cultural-familial form of MR is also distinguished from organic causes by the greater severity and syndromal nature of the latter. Earlier, Dingman and Tarjan noted an excess in the frequency of individuals with MR based on the normal distribution of IQ scores in the general population and two standard deviations below the mean as the criterion for MR (IQ scores <70), and proposed a two-group approach to MR also (54). U.S. census data have been used to estimate the prevalence of MR, and that proportion has varied from 0.5% to 3% of the general population (12). The 3% estimate is typically adopted by other researchers (e.g., 55) and used to justify the two-group approach. However, Reed and Reed (56) examined subsequent generations of families of persons institutionalized with MR. Of the 289 probands whose IQ scores were below 70 and who showed no signs of organicity, 20% of their children were also MR, much higher than expected if only cultural-familial factors were involved. Their data suggest that the two groups are not necessarily distinct from one another, an argument made recently by (57).

As research into the causes of MR has expanded and advocates for those with MR have become more vocal, the definition of MR has also evolved. Unfortunately, the current definition does not take into account the possible differences between cultural-familial and organic MR, nor its overlap, although earlier versions of the Diagnostic and Statistics Manual (DSM) provided subcategories for organic causes (58). The Association for the Advancement of Mental Retardation (AAMR) proposed that a diagnosis of MR can be given if

1. An individual's intellectual functioning is approx 70-75 or below.

2. There are significant disabilities in two or more adaptive skill areas.

3. The age at onset is below 18 years.

The AAMR definition of MR is essentially that of the American Psychiatric Association's DSM-IV diagnostic criteria (59):

1. Significant subaverage intellectual functioning: an IQ of approx 70 or below on an individually administered IQ test (for infants, a clinical judgment of significantly subaverage intellectual functioning).

2. Concurrent deficits or impairments in present adaptive functioning in at least two of the following: communication, self-care, home living, social/interpersonal skills, use of community resources, self-direction, functional academic skills, work, leisure, health, and safety.

3. Onset before 18 years.

Although the current definition of MR does not differentiate between the organic and cultural-familial forms, it does oblige the diagnostician to employ both a psychometric instrument to test cognitive ability and a means by which to evaluate adaptive behavior. Other, neuropsychological measures have also been recommended to obtain a more comprehensive profile of strengths and weaknesses (60).

3.1. Genetics and Mental Retardation: The One and the Many

Despite differences in etiology between cultural-familial and organic MR, defining MR in this fashion compels researchers to think about MR as a single disorder with degrees of severity based upon IQ score: mild (55-69), moderate (40-54), severe (25-39), and profound (<25). Using his two-group approach to MR, Zigler (49,50) demonstrated that children with cultural-familial MR performed as well as mental age equivalent children without MR on a broad array of Piagetian tasks (61). However, children with cultural-familial MR perform less well on tasks involving learning and memory (62). On the other hand, children with organic MR do not perform as well on Piagetian tasks as do mental age equivalent children without MR (61).

There not only are differences between children with cultural-familial MR and those with organic MR, but among the different genetic disorders that produce cognitive impairment as well. Hodapp et al. (63) found that individuals with fragile X syndrome (FXS) exhibit sequential processing deficits on the K-ABC, while individuals with Down syndrome (DS) do not. Children with Williams syndrome (WS) seem to show remarkable expressive language skills given their typical level of MR (64), although other researchers have found similar cognitive-behavioral profiles between children with WS and age-matched children with FXS, despite the obvious phe-notypic differences between the two genetic disorders (65).

Differences between groups of individuals with different genetic disorders manifest themselves both cross-sectionally and longitudinally. It has been shown that IQ scores among children and adolescents with DS decline as these individuals age (66-67). More recently, other researchers have found that both IQ and adaptive behavior scores in children and adolescents with FXS also exhibit longitudinal decreases (68,69). However, IQ scores do not show longitudinal declines among individuals with other genetic disorders. For example, children and adolescents with Prader-Willi syndrome show no significant differences between test and retest IQ scores (70), nor are there longitudinal changes in IQ scores in children with FRAXE, a nonsyndromal X-linked disorder (71,72). These results strongly suggest that each genetic disorder should be examined comprehensively and individually at the cognitive-behavioral level employing as many of the same assessment tools in each case for comparative purposes (60).

There are types of cognitive impairment that exist at the edges of MR, other than those described. Some cognitive impairment has been described as "borderline MR" in which IQ scores range from 70 to 79 (4). Other forms of dysfunction exhibit markers for syndromes that tend to form a cluster of clinical behaviors—hyperactivity, distractability, impulsivity, and learning performance below expected levels of achievement. Originally, these symptoms were designated minimal brain dysfunction (MBD) but most recently have been cast under the general heading of learning disabilities (LD).

LD was defined by the federal government as part of the law (PL 94-142) passed by the U.S. Congress in 1975, and corresponds to a significant discrepancy between "ability" (as measured by psychometric testing) and "achievement" (as measured by accomplishment in school). Since the definition of a significant discrepancy between these two measures varies from one school district to another, prevalence estimates can range from 1% to 30% (73). According to one recent survey (74), as many as 50% of all children requiring special education were classified as LD. Interestingly, many school districts will label a child as LD instead of MR because of the stigma attached to MR. To date, there is no single psychometric instrument developed to evaluate LD. Indeed, the factor-analytic approach of psychometric testing used to evaluate individuals with LD or organic MR may also be of limited utility, especially for those who are most severely affected with MR.

Early studies of MBD suggested, but were unable to demonstrate, genetic factors associated with dysfunction (75,76). More recently, however, as the syndromes have become more clearly delineated phenotypically and geno-typically, genetic causes for LD have been found. For example, among individuals with NF1, 40-60% exhibit some form of LD (77). Besides NF1, evidence for a genetic basis for LD has been obtained from concordance studies of monozygotic and dizygotic twins, which strongly suggest a genetic factor or factors related to reading disabilities (78). Linkage analysis has also provided statistical support for a gene or genes producing dyslexia on chromosome 15 (79), and developmental dyslexia on chromosomes 6 and 15 (80).

3.2. Organic Bases for Cognitive Impairment

Cultural-familial MR notwithstanding, a large proportion of the general population is affected by genetic disorders that produce some form of cognitive-behavioral dysfunction. Broman et al. (81) estimated that neurobio-logical disorders caused about 0.5% of the incidence of MR in the population. However, Fisch (82) noted that the proportion cited by Broman et al. excluded many currently known genetic disorders, for example, the fragile X mutation, Williams syndrome, velo-cardio-facial syndrome, as well as the many nonsyndromal XLMR types. The prevalence of these disorders combined is probably about 0.3%. As result of the advances made in molecular genetics in the past 20 years, the number of genes identified as producing cognitive-behavioral disorders is increasing at an exponential rate. One might then expect that organic causes of MR or LD may well exceed 1% of the general population.

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