CAUSES Of INTELLECTUAL And DEVELOPMENTAL

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.CAUSES ofINTELLECTUAL ONS andPREVENTIONUpdated January, 2015

Edward A. Polloway, Ed.D.Rosel H. Schewel Chair of EducationLynchburg College in VirginiaJ. David Smith, Ed.D.Professor EmeritusUniversity of North Carolina, GreensboroJacqueline Lubin. M. Ed., Ed.D. (candidate)Ministry of Education, St. LuciaKarian N. Antoine, M. Ed.Ministry of Education, St. LuciaAntonia Charles, M. Ed.Ministry of Education, St. LuciaJanuary, 2015Note: This chapter was updated for publication in a planned revision (eighth edition) of Beirne-Smith et al.‘s book on intellectual disabilities.However, that book was never published and so this revision is the property of the authors as a non-published (commercial) document. Pleaseaddress all inquiries to the senior author.2

OverviewThis document is intended for educators and/or students with interest in intellectual anddevelopmental disabilities. It discusses disabilities under four main categories- genetictransmission, chromosomal abnormalities, cranial malformations, and other congenital factors. Ineach section, several examples of disabilities are explained with emphasis on causation,diagnosis, and treatment. The table below summarizes the disabilities discussed in thisdocument.CategoryIntellectual/Developmental DisabilitiesGenetic TransmissionDominant Transmission Neurofibromatosis Tuberous Sclerosis Huntington’s DisorderRecessive Transmission Phenylketonuria (PKU) GalactosemiaSex-Linked InheritanceChromosomal Abnormalities Lesch-Nyhan Syndrome Fragile XAutosomal Disorders3 Down Syndrome Prader-Willi Syndrome Williams Syndrome Cri du chat

CategoryIntellectual/Developmental DisabilitiesSex Chromosome AbnormalitiesCranial MalformationsOther Congenital Factors Klinefelter Syndrome Turner Syndrome Microcephaly HydrocephalusMaternal Disorders Rubella (German Measles) Rhesus DiseaseSubstance Exposure Fetal Alcohol Spectrum DisorderWithin this paper, pre-to-post natal concerns are discussed. Included is an explanation ofstrategies that can be used to prevent intellectual and developmental disabilities. These strategiesrange from those that can be implemented within the preconception stage to early childhood.There is an explicit description of ethical considerations that may arise in the effort to preventintellectual disabilities. The paper concludes with final thoughts and a summary of key points.The information provided in this document can be used for general knowledge, as part ofcourse content in special education classes and/or reference material. However, please note thatall information is the property of the authors and so full credit must be given to authors wheneverany material is used. Please address all inquiries to the senior author.4

ObjectivesAfter reading this chapter, the student should be able to: provide an overview of causation discuss the basic principles of genetics identify and discuss the major biological causes of intellectual and developmentaldisabilities identify areas in which treatment implications are indicated discuss various ways that intellectual disability can be prevented identify and discuss selected ethical issues facing the fieldThe task of sorting out the many causes of intellectual disability is formidable. From thebeginnings of the study of intellectual disability in the earliest part of recorded history to themore advanced efforts in the 21st century, the search for causation has been challenging. Thegoal of this chapter is to provide a foundation for understanding the complexities in the causes ofintellectual and developmental disabilities.Causes of intellectual and developmental disabilities have traditionally been divided intotwo categories: biological (or physiological) and environmental (or psychological andsociological). Such a taxonomic grouping of causes might be thought to create clear dichotomyof specific causes. However, factors from both of these domains are often relevant in individualcases of these disabilities.Although hundreds of specific factors have been identified as causative agents ofintellectual disability, the number of cases of with unknown causes are still as large as those thatare known and specifiable; that is, in 50% or more of cases can a specific cause (i.e., biological5

factor) be identified (Dykens, Hodapp & Finucone, 2000; van Karnebeek et al., 2005). A keyproblem is that causes may be undetermined for the many persons identified as having a “mild”disability (which constitute perhaps 60% of persons with intellectual disabilities; PCPID, 2007).A traditional perspective of causation is the two-group model that includes a group withorganic (i.e., specific biological) causes and a second group presumed to be socio-cultural or,historically cultural familial (Spinath, Harlaar, Ronald, & Plomin, 2004). The first groupconsists of known and specifiable biological causes that often classified as pathological and/orclinical. Although such causes may result in intellectual disabilities at all levels, most attention inthe past was drawn to their etiology of more significant disabilities. Biological pathology can beidentified in from 60% to 75% of cases IQs falling below 50 (McLaren & Bryson, 1987) orhigher (Heikura et al., 2005). But the traditional association of a single, organic cause just withsignificant disabilities is too simplistic; many individuals with high incidence disabilities (i.e.,mild intellectual disabilities) may also be affected because of physiological factors.The other traditional assumption—that mild disabilities are the result of multiple,unspecifiable environmental events—has also given way to the fact that this is only a “broadbrush” distinction (Moser, 2000) and thus estimates over the past two decades have concluded:up to 40% of all cases of mild disabilities may have a specific identifiable cause (Harris, 2006;McLaren & Bryson, 1987); 10-50% of cases of mild intellectual disabilities are related to geneticetiologies (Dykens et al., 2000); and only 33% are unknown (Heikura et al. 2005). Further theassumption that sociocultural factors are all environmentally-based has been challenged bystudies of possible genetic influence (e.g., Spinath et al., 2004).Clearly these data are influenced significantly by state prevalence rates (see USDOE,2007), in that states with high rates of identified intellectual disabilities (for ages 6-17) (e.g.,6

West Virginia, 2.47%; Wyoming, 2.25%) are likely to have a larger number of students withmild disabilities and an associated lower percentage of identified biological causes than would astate with a lower prevalence (e.g., New Jersey, 0.34 %; California, 0.34 %) (see Polloway,Lubin, Smith, & Patton, 2008). Despite these data, it is nevertheless important to recognize thatmany individuals with mild disabilities are also affected by genetic and other biological causesand that psychological and social influences are equally important in cases of severe disability.Finally, there is increasing confidence that the cause of virtually all cases of intellectualdisabilities may become identifiable (Moser, 2000).Given this confounding complexity, why should educators, psychologists, and otherbehavioral scientists spend time studying causation of intellectual disabilities? Kolstoe (1972), inhis classic work, noted that familiarity with these causative factors facilitates multidisciplinarycommunication, is an essential element of professionalism, and is important in enablingprofessionals to make accurate information available to parents. Clearly, parents have the right toknow information that may be related to inherited forms of intellectual disabilities and alsoshould be well-informed about preventable environmental hazards that might otherwise beassociated with disabilities (Percy, 2007). Furthermore, in most situations, etiologicalinformation from educators and childcare professionals can contribute to a more accuratediagnosis. The role of teachers, for example, may include monitoring the effects of ongoing orprogressive disorders that may hinder daily performance, preventing future occurrences throughparent counseling, or facilitating immediate change (e.g., intervention in a case of child abuse).Finally, research is beginning to identify certain educational and psychologicalintervention strategies that may be etiology-specific. Therefore, an understanding of causationmay ultimately lead to alternative approaches to curriculum and instruction (e.g., Powell,7

Haughton, & Douglas, 1997; Hodapp, 1997). Hodapp (1997) suggested that the field hasgenerated conflicting views of the relationship between etiology and behavior; on the one hand,the position is that, for example, specific genetic disorders have no specific effects on behaviorwhile an alternative is that genetic disorders are regularly and consistently associated with adistinctive behavioral pattern. He recommended a compromise between these two positions:a few different genetic disorders show an identical behavior among thoseaffected, and this behavior differs from the behavior of individuals with othertypes of mental retardation. Thus, it is not as if all genetic disorders haveidentical effects on behavior or that each genetic disorder has unique effects.Instead a few disorders show similar effects, which are, in turn, not shared bymixed etiological groups (p. 70).Dykens (2001) noted that the value of etiological information continued to grow in partbecause:Behavioral researchers are playing a pivotal role in this ground swell ofsyndromic research. One long-term goal of this line of work is to examinelinks between genes, brain, and behavior . Simply put, behavioral expertsare needed to solve the behavioral half of these gene-behavior puzzles. In theshort-term, behavioral data can be put to immediate good use as guidepostsfor treatment and intervention, and such syndrome-specific recommendationshave already been made for fragile X, Down, Prader-Willi, Williams, and othersyndromes (p.1).With specific regard to the question of whether genetic disorders matter, it is interesting to notethe points summarized by Dykens, Hodapp, and Finucane (2000) who spoke to three specific8

myths. The first myth that syndromes occur rarely was questioned because approximately 1/3 orall persons with intellectual disabilities may result from genetic causes. Second, the question ofwhether syndromes only provide labels that are unnecessary was refuted in part by the fact thatthe effects of genetic disorders can sometimes be modified such as through intervention efforts(see the succeeding discussion on PKU and dietary controls). Third, biological syndromes havebeen questioned as not mattering for practical concerns but, as in the example of Prader-Willisyndrome, syndrome identification can matter significantly in this case in terms of eating habitsand intervention. Cutting and Denckla (2004) noted that “with advances in behavioral,neuroimaging, and genetic methodologies, the next decade should yield a deeper understandingof the complexities of cognition and different types of disabilities, with the end goal of producingeffective, school –based treatments for students with an array of cognitive weaknesses” (p.174).Such research reflects the study of behavioral phenotypes which focuses on determiningrelationships between specific genetic, chromosomal, or neurodevelopmental disorders and themental, intellectual, and behavioral features that are causally related to the specific condition.Harris (2006) noted that “behavioral phenotypes are stereotypical patterns of behavior that arereliably identified in groups of individuals with known neurodevelopmental disorders and are‘not learned’, . . . this does not mean that the behavior is present in all instances but theprobability of its incurrence is increased” (p.195).While a general awareness of causative factors is necessary for all professionals, themechanisms of specific causes require multidisciplinary involvement. Input from variousdisciplines (e.g., biology, medicine, epidemiology, social work, psychology, psychiatry) andspecial education often is essential to determine whether a cause can be specified or is even9

relevant to treatment and/or education. That many causes of intellectual disabilities cannot becurrently identified also serves as a stimulus for continuing research.Finally, while considering information on causation, it is important that the reader notlose sight of the fact that behind these terms are people responding to special challenges in theirlives. As Blatt (1987) cautioned, “Treatises that deal with etiological conditions rarely recognizethe human being [in] the superficially unattractive trappings of the condition” (p. 128). Readersmust not overlook the fact that we are talking about real people who happen to have a givendisability.Our discussion of causes begins with attention to terminology and then focuses ongenetics, other biological causes, and environmental influences. It concludes with attention toprevention and related ethical issues.TERMINOLOGYAs noted earlier, the task of understanding the causes of intellectual and developmentaldisabilities is challenging, and translating specific terms for known causes into usefulinformation can be particularly difficult. This section offers ways to understand some of thelabels ascribed to selected causative factors.First, it is helpful to consider the causes of intellectual disabilities through adevelopmental lens. This can be conceptualized as a series of concentric circles representing(from inner to outer) that relate to: inherited traits: factors related to inheritance; conditions thatrefer to all influences through the time of conception; congenital conditions also inclusive offactors taking place during pregnancy; and constitutional considerations that encompass allbiological influences (Polloway & Rucker, 1997).10

Much of the focus of attention related to the causation of intellectual disabilities has to dowith specific syndromes. As Percy et al. (2007) noted, “syndrome” derives from the Greek wordssyn, meaning “together with” and drome, referring to “to run”. As a result, syndromes focus onconsiderations that “run together”, and thus that represent “a complex of concurrent things” (p.229).The terminology used to identify various syndromes comes from three sources: (a)conventional wisdom or practices related to a specific historical era, (b) names of persons whoinitially identified or described the condition, and (c) biomedical terms describing the cause orthe resultant disabilities.Several examples illustrate how historical names have been associated with intellectualdisabilities for syndromes. Perhaps best known is the certainly archaic (and offensive) termmongolism, which was coined by J. Langdon Down in 1866, two decades after Seguin’s initialidentification of the condition (Menolascino & Egger, 1978). For 100 years, this term, which wasassigned simply because of Down’s inaccurate observation that one frequent characteristic of thesyndrome was facial similarity to Asians, prevailed in medical and psychological circles. Jordan(2000, p. 325) summarized the context within which Down made his observations:‘the inescapable observation that the expanding centers of manufacturingcontained a population of overcrowded, chronically ill, malnourished Britonsled to an alarming conclusion, namely, that the population was increasinglyunhealthy and their retrogression in health and habits would be increased witheach succeeding generation.’ Down noted that ‘the largest proportion of idiocyis to be found among the lower orders, how can it be expected but that themother . . . should propagate an enfeebled race?’ In short, the British raceappeared to be deteriorating at a rapid rate. In them, the appearance of features11

from other ethnic groups, in particular ‘the great Mongolian family,’ as Downphrased it, led him to see the degeneracy problem writ large on Britain’spopulation.Realization that Down syndrome is found in all racial groups (including persons from Mongolia)eventually aided in the much-needed elimination of this racist term from the vocabulary of mostprofessionals. Its use, however, unfortunately persists, particularly in some popular media.A second, more direct way to identify a clinical syndrome is to attach to it the name ofthe researcher who contributed in a major way to its understanding. For instance, professionalsnow identify as Down syndrome the chromosomal condition disorder that J. Langdon Downoriginally described as mongolism. Other relatively well-known syndromes so named includeTay-Sachs, after the British and American physicians who described the characteristics of thecondition in the late 1880s, and Lesch-Nyhan syndrome, named for two of the three researcherswho first identified this disorder in 1964.The third source of syndrome labels is biomedical terminology. Although some of theseterms are frequently used by laypersons, their meanings are often obscure, in spite of theirgrounding in common forms. Thus, many of the labels convey primary features of the disorder,either causal or characteristic.Although labels are only an attempt to refer to complex phenomena, simply beingfamiliar with the derivatives can be of assistance in understanding the nature of these disordersand the terms related to them. Several specific terms illustrate the system. For example,toxoplasmosis indicates a condition (-osis) of poisonous (toxo-) blood (-plasm). Although theclinical definition of toxoplasmosis is much more specific, the word, when analyzed, gives a fairsuggestion of what the condition is about. Another example is hydrocephalus. The term refers to12

a disorder resulting from a blockage of cerebrospinal fluid, but breaking the word down into“water” (hydro-) and “head” or “brain” (cephalo-) provides a descriptive, if admittedlysimplistic, picture of the condition. A third example is the disorder called myelomeningocele. Asthe term suggests, this condition is characterized by a saclike mass (-cele) on the spinal cord(myelo-) containing membrane tissue of the central nervous system (-meningo-).GENETIC CONSIDERATIONSGenetics is the study of heredity and its variations. As such, its scope is enormous and itscomplexities great. Advances in genetics over the past 70 years rival those in any area of science.The contributions of geneticists to understanding the causes of developmental disabilities areparticularly noteworthy. In the last three decades, this knowledge base has mushroomed due toresearch on gene mapping and DNA sequencing; most of it done under the auspices of theHuman Genome Project (see Box 1).BOX 1The Human Genome ProjectIn 1990, a huge international research endeavor . . . was officially launched. This project is involving manyscientists and is equivalent in scope and cause to putting man on the moon. Its objective is to determine thesequence of all the DNA in the human genome . . . This work will enable all of the genes in the human body . . to be identified. It also will aid in the identification of defects causing virtually every known geneticdisorder. As genes causing genetic disorders are identified, it will become possible to test individuals forincreasing numbers of inherited disorders, including intellectual disabilities associated with brainmalfunction. The identification of genetic causes of specific intellectual disabilities will become the searchfor specific treatments, preventions, and cures. . . The human genome project is also examining the ethical,legal, and social implications of human genetic research and is developing guidelines to help society dealwith such issues.Source: Percy, M., Lewkis, F.Z., & Brown, I. (2007). Introduction to genetics and development. In Brown, L.,Percy, M. (Eds). A Comprehensive Gide to Intellectual & Developmental Disabilities (p.88). Baltimore,MD:Paul H Brookes.13 page

Note: This chapter was updated for publication in a planned revision (eighth edition) of Beirne-Smith et al.‘s book on intellectual disabilities. However, that book was never published and so this revision is the property of the authors as a non-published (commercial) document. . Lubin, Smith, &

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