Mental Retardation
The conceptualization of mental retardation includes deficits in cognitive abilities, as well as in behaviors required for social and personal sufficiency, known as adaptive functioning. Wide acceptance of this definition has led to the consensus that an assessment of both social adaptation and intelligence quotient (IQ) is necessary to determine the level of mental retardation. Measures of adaptive function assess competency in performance of everyday tasks, whereas measures of intellectual function focus on cognitive abilities. Evidence shows that individuals with a given intellectual level do not all have the same adaptive function, yet it is likely that IQ contributes an upper limit or ceiling to adaptive accomplishments.
CLASSIFICATION
According to the text revision of the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR), mental retardation is defined as significantly subaverage general intellectual functioning resulting in, or associated with, concurrent impairment in adaptive behavior and manifested during the developmental period, before the age of 18 years. The diagnosis is made regardless of whether the person has a coexisting physical disorder or other mental disorder. Table 34-1 presents an overview of developmental levels in communication, academic functioning, and vocational skills expected of persons with various degrees of mental retardation.
General intellectual functioning is determined by the use of standardized tests of intelligence, and the term significantly subaverage is defined as an IQ of approximately 70 or below or two standard deviations below the mean for the particular test. Adaptive functioning can be measured by using a standardized scale, such as the Vineland Adaptive Behavior Scale. This scale scores communications, daily living skills, socialization, and motor skills (up to 4 years, 11 months) and generates an adaptive behavior composite that is correlated with the expected skills at a given age.
Approximately 85 percent of persons with mental retardation fall within the mild mental retardation category (IQ between 50 and 70). The adaptive functions of persons with mild mental retardation are effective in several areas, such as communications, self-care, social skills, work, leisure, and safety. Mental retardation is influenced by genetic, environmental, and psychosocial factors; previously, the development of mild retardation was often attributed to severe psychosocial deprivation. More recently, however, researchers have increasingly recognized the likely contribution of a host of subtle biological factors, including chromosomal abnormalities, subclinical lead intoxication, and prenatal exposure to drugs, alcohol, and other toxins. Furthermore, evidence is increasing that subgroups of persons with mental retardation, such as those with fragile X syndrome, Down syndrome, and Prader-Willi syndrome, have characteristic patterns of social, linguistic, and cognitive development and typical behavioral manifestations.
The DSM-IV-TR includes in its text on mental retardation additional information regarding the etiological factors and their association with mental retardation syndromes (e.g., fragile X syndrome).
EPIDEMIOLOGY
The prevalence of mental retardation at any one time is estimated to range from 1 to 3 percent of the population. The incidence of mental retardation is difficult to calculate because mild mental retardation sometimes goes unrecognized until middle childhood. In some cases, even when intellectual function is limited, good adaptive skills are not challenged until late childhood or early adolescence, and the diagnosis is not made until that time. The highest incidence is in school-age children, with the peak at ages 10 to 14 years. Mental retardation is about 1.5 times more common among men than among women. In older persons, prevalence is lower; those with severe or profound mental retardation have high mortality rates because of the complications of associated physical disorders.
COMORBIDITY
Prevalence
Epidemiological surveys indicate that up to two thirds of children and adults with mental retardation have comorbid mental disorders; this rate is several times higher than that in the community samples of those without mental retardation. The prevalence of psychopathology seems to be correlated with the severity of mental retardation; the more severe the mental retardation, the higher the risk for other mental disorders. A recent epidemiological study found that 40.7 percent of intellectually disabled children between 4 and 18 years of age met criteria for at least one psychiatric disorder. The severity of retardation affected the type of psychiatric disorder. Disruptive and conduct-disorder behaviors occurred more commonly in the group with mild retardation; the group with more severe retardation exhibited psychiatric problems more often associated with autistic disorder, such as self-stimulation and self-mutilation. In contrast to the epidemiology of psychopathology in children in general, age and sex did not affect the prevalence of psychiatric disorders in this study. Those with profound mental retardation were less likely to exhibit psychiatric symptoms.
The mental disorders that occur among persons with mental retardation appear to run the gamut of those seen in persons without mental retardation, including mood disorders, schizophrenia, attention-deficit/hyperactivity disorder (ADHD), and conduct disorder. Those with severe mental retardation have a particularly high rate of autistic disorder and pervasive developmental disorders. About 2 to 3 percent of persons with mental retardation meet the criteria for schizophrenia; this percentage is several times higher than the rate for the general population. Up to 50 percent of children and adults with mental retardation had a mood disorder when such instruments as the Kiddie Schedule for Affective Disorders and Schizophrenia, the Beck Depression Inventory, and the Children’s Depression Inventory were used in pilot studies, but because these instruments have not been standardized within this population, these findings must be considered preliminary.
Table 34-1 Developmental Characteristics of Persons with Mental Retardation | ||||||||||||||||||||||||||||
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Highly prevalent psychiatric symptoms that can occur in persons with mental retardation outside the context of a mental disorder include hyperactivity and short attention span, self-injurious behaviors (e.g., headbanging and self-biting), and repetitive stereotypical behaviors (hand-flapping and toe-walking). Personality styles and traits in persons with mental retardation are not unique to them, but negative self-image, low self-esteem, poor frustration tolerance, interpersonal dependence, and a rigid problem-solving style are overrepresented. Specific causal syndromes seen in mental retardation can also predispose affected persons to various types of psychopathologies.
Neurological Disorders
Comorbid psychiatric disorders are increased in individuals with mental retardation who also have known neurological conditions, such as seizure disorders. Rates of psychopathology increase with the severity of mental retardation; thus, neurological impairment increases as intellectual impairment increases. In a recent review of psychiatric disorders in children and adolescents with mental retardation and epilepsy, approximately one third also had autistic disorder or an autistic-like condition. The combination of mental retardation, active epilepsy, and autism or an autistic-like condition occurs at a rate of 0.07 percent in the general population.
Psychosocial Features
A negative self-image and poor self-esteem are common features of persons with mild and moderate mental retardation, who are well aware of being different from others. They experience repeated failure and disappointment in not meeting their parents’ and society’s expectations and in falling progressively behind their peers and even their younger siblings. Communication difficulties further increase their vulnerability to feelings of ineptness and frustration. Inappropriate behaviors, such as withdrawal, are common. The perpetual sense of isolation and inadequacy has been linked to feelings of anxiety, anger, dysphoria, and depression.
ETIOLOGY
Etiological factors in mental retardation can be primarily genetic, developmental, acquired, or a combination of these. Genetic causes include chromosomal and inherited conditions; developmental factors include prenatal exposure to infections and toxins; and acquired syndromes include perinatal trauma (e.g., prematurity) and sociocultural factors. The severity of the resulting mental retardation is related to the timing and duration of the trauma as well as to the degree of exposure to the central nervous system (CNS). The more severe the mental retardation, the more likely it is that the cause is evident. In about three fourths of persons with severe mental retardation, the cause is known, whereas the cause is apparent in only half of those with mild mental retardation. A recent study of 100 consecutive children with mental retardation admitted to a clinical genetics unit of a university pediatric hospital reported that in 41 percent of cases, a causative diagnosis was made. In general, etiological classifications used included genetic, multifactorial, environmental,
and unknown etiology. No cause is known for three fourths of persons with borderline intellectual functioning. Overall, in up to two thirds of all persons with mental retardation, the probable cause can be identified. Among chromosomal and metabolic disorders, Down syndrome, fragile X syndrome, and phenylketonuria (PKU) are the most common disorders that usually produce at least moderate mental retardation. Those with mild mental retardation sometimes have a familial pattern apparent in parents and siblings. Deprivation of nutrition, nurturance, and social stimulation can contribute to the development of mental retardation. Current knowledge suggests that genetic, environmental, biological, and psychosocial factors work additively in mental retardation.
and unknown etiology. No cause is known for three fourths of persons with borderline intellectual functioning. Overall, in up to two thirds of all persons with mental retardation, the probable cause can be identified. Among chromosomal and metabolic disorders, Down syndrome, fragile X syndrome, and phenylketonuria (PKU) are the most common disorders that usually produce at least moderate mental retardation. Those with mild mental retardation sometimes have a familial pattern apparent in parents and siblings. Deprivation of nutrition, nurturance, and social stimulation can contribute to the development of mental retardation. Current knowledge suggests that genetic, environmental, biological, and psychosocial factors work additively in mental retardation.
Genetic Etiological Factors in Mental Retardation
Abnormalities in autosomal chromosomes are frequently associated with mental retardation, whereas aberrations in sex chromosomes can result in characteristic physical syndromes that do not include mental retardation (e.g., Turner’s syndrome with XO and Klinefelter’s syndrome with XXY, XXXY, and XXYY variations). Some children with Turner’s syndrome have normal to superior intelligence. Agreement exists on a few predisposing factors for chromosomal disorders—among them, advanced maternal age, increased age of the father, and X-ray radiation.
Down Syndrome.
The description of Down syndrome, first made by the English physician Langdon Down in 1866, was based on the physical characteristics associated with subnormal mental functioning. Since then, Down syndrome has been the most investigated, and most discussed, syndrome in mental retardation. Children with this syndrome were originally called mongoloid because of their physical characteristics of slanted eyes, epicanthal folds, and flat nose. Despite a plethora of theories and hypotheses advanced in the last 100 years, the cause of Down syndrome is still unknown.
The problem of cause is complicated even further by the recent recognition of three types of chromosomal aberration in Down syndrome:
Patients with trisomy 21 (three chromosome 21 instead of the usual two) represent the overwhelming majority; they have 47 chromosomes, with an extra chromosome 21. The mothers’ karyotypes are normal. A nondisjunction during meiosis, occurring for unknown reasons, is held to be responsible for the disorder.
Nondisjunction occurring after fertilization in any cell division results in mosaicism, a condition in which both normal and trisomic cells are found in various tissues.
In translocation, a fusion occurs of two chromosomes, usually 21 and 15, resulting in a total of 46 chromosomes, despite the presence of an extra chromosome 21. The disorder, unlike trisomy 21, is usually inherited, and the translocated chromosome may be found in unaffected parents and siblings. The asymptomatic carriers have only 45 chromosomes.
The incidence of Down syndrome in the United States is about 1 in every 700 births. In his original description, Down mentioned the frequency of 10 percent among all patients with mental retardation. For a middleaged mother (>32 years of age), the risk of having a child with Down syndrome with trisomy 21 is about 1 in 100 births, but when translocation is present, the risk is about 1 in 3. These facts assume special importance in genetic counseling.
Mental retardation is the overriding feature of Down syndrome. Most persons with the syndrome have moderate or severe retardation, with only a minority having an IQ greater than 50. Mental development seems to progress normally from birth to 6 months of age; IQ scores gradually decrease from near normal at 1 year of age to about 30 at older ages. The decline in intelligence may not be readily apparent. Infantile tests may not reveal the full extent of the defect, which may become manifest when sophisticated tests are used in early childhood. According to many sources, children with Down syndrome are placid, cheerful, and cooperative and adapt easily at home. With adolescence, the picture changes: Youngsters may experience various emotional difficulties, behavior disorders, and (rarely) psychotic disorders.
In Down syndrome, language function is a relative weakness, whereas sociability and social skills, such as interpersonal cooperation and conformity with social conventions, are relative strengths. Most studies have noted muted affect in children with Down syndrome relative to children of the same mental age who do not have retardation. Those with Down syndrome also manifest deficiencies in scanning the environment; they are likely to focus on a single stimulus and have difficulty noticing environmental changes. A variety of mental disorders occurs in persons with Down syndrome, but the rates appear to be lower than those in other mental retardation syndromes, especially autistic disorder.
The diagnosis of Down syndrome is made with relative ease in an older child but is often difficult in newborn infants. The most important signs in a newborn include general hypotonia; oblique palpebral fissures; abundant neck skin; a small, flattened skull; high cheekbones; and a protruding tongue. The hands are broad and thick, with a single palmar transversal crease, and the little fingers are short and curved inward. Moro reflex is weak or absent. More than 100 signs or stigmata are described in Down syndrome, but rarely are all found in one person. Life expectancy was once about 12 years; with the advent of antibiotics, few young patients die from Down syndrome, but many do not live beyond the age of 40 years. Life expectancy is increasing, however.
Persons with Down syndrome tend to exhibit marked deterioration in language, memory, self-care skills, and problem solving in their 30s. Postmortem studies of those with Down syndrome older than the age of 40 years have shown a high incidence of senile plaques and neurofibrillary tangles, as seen in Alzheimer’s disease. Neurofibrillary tangles are known to occur in a variety of degenerative diseases, whereas senile plaques seem to be found most often in Alzheimer’s disease and in Down syndrome. Thus, the two disorders may share some pathophysiology.
Fragile X Syndrome.
Fragile X syndrome is the second-most-common cause of mental retardation. The syndrome results from a mutation on the X chromosome at what is known as the fragile site (Xq27.3). The fragile site is expressed in only some cells, and it may be absent in asymptomatic males and female carriers. Much variability is present in both genetic and phenotypic expression. Fragile X syndrome is believed to occur in about 1 of every 1,000 males and 1 of every 2,000 females. The typical phenotype includes a large, long head and ears, short stature, hyperextensible joints, and postpubertal macroorchidism. The mental retardation ranges from mild to severe. The behavioral profile of persons with the syndrome includes a high rate of ADHD, learning disorders, and pervasive developmental disorders, such as autism. Deficits in language function include rapid perseverative speech with abnormalities in combining words into phrases and sentences. Persons with fragile X syndrome seem to have relatively strong skills in communication and socialization; their intellectual functions seem to decline in the pubertal period. Female carriers are often less impaired than males with fragile X syndrome, but females can also manifest the typical physical characteristics and can have mild retardation.
Prader-Willi Syndrome.
Prader-Willi syndrome is postulated to result from a small deletion involving chromosome 15, usually occurring sporadically. Its prevalence is less than 1 in 10,000. Persons with
the syndrome exhibit compulsive eating behavior and often obesity, mental retardation, hypogonadism, small stature, hypotonia, and small hands and feet. Children with the syndrome often have oppositional and defiant behavior.
the syndrome exhibit compulsive eating behavior and often obesity, mental retardation, hypogonadism, small stature, hypotonia, and small hands and feet. Children with the syndrome often have oppositional and defiant behavior.
Cat’s Cry (Cri-du-Chat) Syndrome.
Children with cat’s cry syndrome lack part of chromosome 5. They have severe retardation and show many signs often associated with chromosomal aberrations, such as microcephaly, low-set ears, oblique palpebral fissures, hypertelorism, and micrognathia. The characteristic cat-like cry caused by laryngeal abnormalities that gave the syndrome its name gradually changes and disappears with increasing age.
Phenylketonuria.
PKU was first described by Ivar Asbjörn Fölling in 1934 as the paradigmatic inborn error of metabolism. PKU is transmitted as a simple recessive autosomal Mendelian trait and occurs in about 1 of every 10,000 to 15,000 live births. For parents who have already had a child with PKU, the chance of having another child with PKU is 1 in every 4 to 5 successive pregnancies. Although the disease is reported predominantly in persons of North European origin, a few cases have been described in blacks, Yemenite Jews, and Asians. The frequency among institutionalized patients with retardation is about 1 percent. The basic metabolic defect in PKU is an inability to convert phenylalanine, an essential amino acid, to paratyrosine because of the absence or inactivity of the liver enzyme phenylalanine hydroxylase, which catalyzes the conversion. Two other types of hyperphenylalaninemia have recently been described. One is caused by a deficiency of the enzyme dihydropteridine reductase and the other by a deficiency of a cofactor, biopterin. The first defect can be detected in fibroblasts, and biopterin can be measured in body fluids. Both of these rare disorders carry a high risk of fatality.
Most patients with PKU have severe retardation, but some are reported to have borderline or normal intelligence. Eczema, vomiting, and convulsions occur in about one third of all patients. Although the clinical picture varies, typical children with PKU are hyperactive; they exhibit erratic, unpredictable behavior and are difficult to manage. They frequently have temper tantrums and often display bizarre movements of their bodies and upper extremities, including twisting hand mannerisms; their behavior sometimes resembles that of children with autism or schizophrenia. Verbal and nonverbal communication is usually severely impaired or nonexistent. The children’s coordination is poor, and they have many perceptual difficulties.
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