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Mental Retardation

Judith D. Laval, MD


Mental retardation is characterized by limitations in performance that result from significant impairments in measured intelligence and adaptive behavior. It also confers a social status that can be more handicapping than the specific disability itself. Because the boundaries between "normality" and "retardation" frequently are difficult to delineate, the pediatric identification, evaluation, and care of children with cognitive difficulties and their families require a considerable level of both technical sophistication and interpersonal sensitivity.

In 1992, the American Association on Mental Retardation revised its official definition to formalize the paradigm shift from viewing mental retardation as an individual trait to thinking of it as an expression of the interaction between a person with limited intellectual functioning and the environment. Consequently, categories of mild, moderate, severe, and profound retardation have been replaced by a classification system that specifies four levels of support systems needed for daily functioning (i.e., intermittent, limited, extensive, and pervasive). Four assumptions were articulated as essential to the appropriate application of the new definition as follows: (1) valid assessment considers cultural and linguistic diversities; (2) limitations in adaptive skills occur within the context of community environments typical of age peers and indexed to individualized needs for supports; (3) adaptive limitations coexist with strengths; and (4) with appropriate and sustained supports, the life functioning of individuals with mental retardation will generally improve.


The determinants of competence in any individual are complex and multifactorial. Regardless of his or her level of performance, each child's abilities are influenced by both the integrity and maturational status of the nervous system and by the nature and quality of his or her life experience. Some children sustain significant neurologic insults and develop normal skills. Others manifest severe cognitive impairment despite the absence of recognizable focal neurologic findings or historical evidence of significant risk factors for central nervous system dysfunction. The neurobiologic roots of mental retardation may be found among such diverse factors as structural malformations of the brain, metabolic abnormalities, and central nervous system deficits related to infection, malnutrition, or hypoxic-ischemic injury. The experiential precursors of retardation may be identified in histories of dysfunctional caregiving related to parental psychopathology, extreme family disorganization, or economic hardship. Children who live in poverty are particularly susceptible to the cumulative burdens of both social stress and the greater biologic vulnerability related to a higher prevalence of such risk factors as perinatal complications and nutritional deficiencies.


Approximately 3% of the general population has an intelligence quotient (IQ) less than two standard deviations below the mean. It has been estimated that 80-90% of persons with mental retardation function within the mild range, whereas only 5% of the population with mental retardation is severely to profoundly impaired. The prevalence of mild retardation varies inversely with socioeconomic status,

TABLE 40-3 -- Potential Contributing Factors in the Pathogenesis of Mental Retardation
Preconceptual Disorders
Single gene abnormalities (e.g., inborn errors of metabolism, neurocutaneous disorders)
Chromosomal abnormalities (e.g., X-linked disorders, translocations, fragile X)
Polygenic familial syndromes
Early Embryonic Disruptions
Chromosomal disorders (e.g., trisomies, mosaics)
Infections (e.g., cytomegalovirus, rubella, toxoplasmosis, human immunodeficiency virus)
Teratogens (e.g., alcohol, radiation)
Placental dysfunction
Congenital central nervous system malformations (idiopathic)
Fetal Brain Insults
Infections (e.g., human immunodeficiency virus, toxoplasmosis, cytomegalovirus, herpes simplex)
Toxins (e.g., alcohol, cocaine, lead, maternal phenylketonuria)
Placental insufficiency/intrauterine malnutrition
Perinatal Difficulties
Extreme prematurity
Hypoxic-ischemic injury
Intracranial hemorrhage
Metabolic disorders (e.g., hypoglycemia, hyperbilirubinemia)
Infections (e.g., herpes simplex, bacterial meningitis)
Postnatal Brain Insults
Infections (e.g., encephalitis, meningitis)
Trauma (e.g., severe head injury)
Asphyxia (e.g., near drowning, prolonged apnea, suffocation)
Metabolic disorders (e.g., hypoglycemia, hypernatremia)
Toxins (e.g., lead)
Intracranial hemorrhage
Postnatal Experiential Disruptions
Poverty and family disorganization
Dysfunctional infant-caregiver interaction
Parental psychopathology
Parental substance abuse
Unknown Influences

whereas moderate to severe disability occurs with equal frequency across all income groups. Because a diagnosis of mental retardation relies on an assessment of adaptive behavior and not solely on IQ, the epidemiology varies with the life cycle. The reported incidence of retardation increases initially with age, the numbers rising sharply in the early school years and then declining in late adolescence as individuals with mild impairments leave the formal education setting and are assimilated into the "normal" adult world. Identification of children with mild retardation in the preschool period is most commonly precipitated by concerns about the development of language.


Children with physical findings suggestive of recognizable syndromes that are associated with mental retardation should be identified at birth or during early infancy. Down syndrome and primary microcephaly are examples of such conditions. These disorders, however, represent a small percentage of the population of youngsters with intellectual impairment. The overwhelming majority are identified because of their failure to meet age-appropriate expectations.

Delayed achievement of developmental milestones is the cardinal symptom of mental retardation. Although youngsters with severe impairment show marked delays in psychomotor skills in the first year of life, children with moderate retardation typically exhibit normal motor development and present with delayed speech and language abilities in the toddler years. Mild retardation, on the other hand, may not be suspected until after entry to school, although participation in a nursery school or child care program can highlight discrepancies in the performance of a preschooler with significantly subaverage abilities.

The natural history of mental retardation is highly variable and dependent on the availability of appropriate educational and therapeutic experiences as well as on neuromaturation and the presence of associated disabilities. Although many youngsters may experience transient "plateau periods" during which measurable progress may be minimal, most individuals with mental retardation acquire new skills and continue to learn throughout their lifetimes. The ability to formulate specific prognoses, except for children who manifest severe to profound retardation, is quite limited, especially during the preschool years. Generally speaking, children within the relatively mild range of retardation who receive appropriate education can achieve 4th to 6th grade reading levels and may be able to function relatively independently as adults. Individuals with more significant intellectual limitations require greater degrees of supervision, depending on their range of adaptive abilities. Children with histories that suggest a loss of previously acquired skills represent an important subgroup for whom the diagnosis of a progressive rather than a static neurologic disorder must be investigated. In such cases, developmental deterioration is rarely reversible, yet a precise diagnosis is important for genetic counseling and informed family support.

A thorough pediatric history is essential to identify relevant contributing factors as well as to document the evolving pattern of the child's developmental skills over time. The product of the history should be a comprehensive inventory of risk factors (both within the child and within his or her environment) that increase the likelihood of developmental dysfunction as well as protective factors that may contribute to more adaptive functioning. Common protective factors include good physical health, a normal rate of growth, healthy parent-child attachment, and a cohesive family unit within a supportive social network.

A systematic physical examination may reveal findings that help to explain the etiology of the child's disability or that identify particular treatment needs. Table 40-4 lists a number of atypical physical features that have been associated with a

TABLE 40-4 -- Atypical Physical Features That May Be Associated with Increased Incidence of Mental Retardation
Double whorl
Fine, inable, prematurely gray or white locks
sparse or absent hair
Upward-and-outward or downward-and-outward slant
Inner or outer epicanthal folds
Coloboma of iris or retina
Brushfield spots
Eccentrically placed pupil
Low-set pinna
Simple or abnormal helix formation
Flattened bridge
Small size
Upturned nares
Increased length of philtrum
Hypoplasia of maxilla or mandible
Inverted V shape of upper lip
Wide or high-arched palate
Short 4th or 5th metacarpals
Short, stubby fingers
Long, thin tapered fingers
Broad thumbs
Abnormal dermatoglyphics (e.g., distal triradius)
Transverse palmar crease
Abnormal nails
Short 4th or 5th metatarsals
Overlap of toes
Short, stubby toes
Broad, large big toes
Deep crease leading from angle of 1st and 2nd toes
Abnormal dermatoglyphics
Ambiguous genitalia
Large testicles
Cafe-au-lait spots
Depigmented nevi
Evidence of abnormal enamelogenesis
Abnormal odontogenesis

higher incidence of mental retardation. In some cases, a particular cluster of phenotypic characteristics may suggest a specific syndrome related to a chromosomal abnormality or known teratogenic effect. It should be emphasized, however, that many of these features are found in children without developmental disabilities, some tend to be familial, and several appear with greater prevalence among specific ethnic groups.


The primary care pediatrician is strategically situated to identify young children with possible mental retardation through routine developmental surveillance in the context of general pediatric care. Parental report of a child's typical skills and behaviors in conjunction with the use of in-office screening procedures are important complementary sources of information. For young children involved in a program outside of the home (e.g., child care or preschool), the impressions of the caregiver or teacher also are valuable. Concerns raised by parents, nonparental caregivers, or teachers or by direct observation of the child require systematic investigation. The extent to which a comprehensive developmental assessment can be performed within the primary care setting depends on the expertise of the physician and his or her office staff.

Ultimately, the diagnosis of mental retardation requires confirmation of significantly subaverage general intellectual functioning (i.e., an IQ standard score of 70-75 or below) in association with deficits in two or more of the following 10 adaptive skill areas: communication, self-care, home living, social skills, community use, self-direction, health and safety, functional academics, leisure, and work. Screening instruments (e.g., the Denver Developmental Screening Test) and nonstandardized developmental scales are unacceptable substitutes for validated and reliable diagnostic measures (e.g., the Bayley Scales of Infant Development, the Stanford-Binet Intelligence Scale, or the Wechsler Scales). After the psychometric diagnosis of mental retardation has been confirmed, a comprehensive medical evaluation is necessary to complete the assessment process.

A range of laboratory studies must be considered in the medical evaluation of a youngster with mental retardation. Table 40-5 lists important studies and the indications for their

TABLE 40-5 -- Indications for Laboratory Assessment of the Young Child with Mental Retardation
Chromosomal Karyotype
Unusual number or character of atypical physical features
History of maternal exposure to a teratogen
Major congenital malformations
Abnormal genitalia
Family history of unexplained retardation
Serum Amino or Organic Acids
Unexplained seizures in early infancy
Failure to thrive with neurologic regression
Unusual smell of urine or skin
Unusually light-colored hair
Unexplained acidosis
Family history
Urine Mucopolysaccharides
Coarse facial features
Short extremities
Short trunk
Cloudy corneas
Impaired hearing
Short stature
Stiff joints
Urine-Reducing Substances
Plasma Ammonia
Episodic vomiting and metabolic acidosis
Urine Ketoacids
Short friable hair
Blood Lead
History of pica
Serum Zinc
Serum Copper and Ceruloplasmin
Involuntary movements
Kayser-Fleischer rings
White Blood Cell Lysosomal Enzyme Analysis or Skin Biopsy
Loss of motor or cognitive milestones or functions
Optic atrophy
Retinal degeneration
Recurrent cerebellar ataxia
Coarse loose skin
Enlarged head beginning after 1 yr of age
Urine Vanilmandelic Acid
Episodic vomiting
Poor suck
Symptoms of autonomic dysfunction
Serum Uric Acid
Rage attacks
Plasma Very Long Chain Fatty Acids
Atypical phenotypic features
Early seizures and hypotonia
Hearing loss
Retinal degeneration
Renal cysts
Aberrant bone calcification
Blood Lactate and Pyruvate Plus Special Mitochondrial Studies
Metabolic acidosis
Myoclonic seizures
Progressive weakness
Retinal degeneration
Recurrent strokelike episodes
Viral Titers for Congenital Infection
Sensorineural hearing impairment
Neonatal hepatosplenomegaly
Neonatal petechial rash
Intracranial calcifications
Suspected seizure disorder
Severe receptive language impairment
Cranial Computed Tomography or Magnetic Resonance Imaging
Progressive enlargement of head
Tuberous sclerosis
Suspected gross malformation of brain
Focal seizures
Suspected intracranial mass

use. Growing appreciation of the frequency of fragile X syndrome has prompted some clinicians to suggest routine karyotypes to rule out this diagnosis in all children with retardation of unknown causes. Furthermore, advances in the technology of chromosome analysis have led some to recommend a repeat study of the karyotype of children with significant retardation who had normal findings several years previously.

A comprehensive history, physical examination, and laboratory evaluation often lead to identification of specific factors that contribute to the phenomenon of mental retardation. A thorough diagnostic formulation highlights those contributing factors that are amenable to specific treatments (e.g., hypothyroidism or an excessive lead burden), suggests associated problems that require intervention or continued surveillance (e.g., a seizure disorder or a sensory impairment), and provides a comprehensive data base that can inform ongoing management decisions. More commonly, however, the results of the medical evaluation are nonspecific and inconclusive. In such cases, when there is no evidence of a specific central nervous system insult, a family history of disability, or identifiable environmental problems, the retardation is presumed to be secondary to an unknown congenital influence on the development of the brain.


Management of a child with mental retardation is multidimensional and highly individualized. Although the potential need for a highly specialized multidisciplinary effort should be considered, not all children with mental retardation are served best by a complex array of services and professionals. Wise decisions about resource needs are most likely when they are informed by the development of an individualized plan, the goals and objectives of which flow from a careful consideration of the specific risk and protective factors inherent in the child and his or her family.

One of the critical and most demanding roles played by the physician involves the initial synthesis and presentation of diagnostic findings to the family. This process involves a highly sensitive interaction the details of which are often remembered and recounted verbatim by parents for many years thereafter. A skilled clinician provides complete and accurate information about what is known about the nature and possible causes of the child's disability, identifies areas of relative competence and adaptive behaviors, provides emotional support, works with the family to define specific goals and objectives and to formulate a strategy for further management, provides sufficient opportunities for parents to identify their own needs for further information, and responds honestly to unanswerable questions. When managed well, the initial informing interview can provide a strong foundation for ongoing parent-professional collaboration.

Specialized educational and therapeutic services are central elements in the multidisciplinary treatment of children with mental retardation. During the adolescent years, issues related to sexuality, vocational training, and community living become more prominent than at earlier stages. The role of the physician necessarily varies with the needs of the child and family. All children must be assured of routine health maintenance services including immunizations, monitoring of growth, and prompt treatment of minor illnesses. Specific medical complications that occur with greater frequency among children with developmental disabilities (e.g., seizure disorders, impairments of vision or hearing, and nutritional problems) require accurate diagnosis and prompt management. Ongoing health surveillance should be guided by knowledge of the relative risks of specific associated disorders (e.g., slowly progressive sensorineural hearing impairment in children with congenital CMV infection or the development of hypothyroidism, atlantoaxial instability, conductive hearing loss, or celiac disease in youngsters with Down syndrome). Finally, the physician has an important responsibility to ensure the provision of sophisticated genetic counseling whenever the diagnosis of a heritable disorder is considered.

Collaboration between the primary care physician and an early intervention service system is particularly important in the management of children with developmental impairments in the first years of life. Early identification and prompt referral ensure access to individualized therapeutic and educational services for the child in conjunction with flexible support services for the family. Such services are delivered best when they focus on the family as a dynamic system and view child and family adaptation as interdependent and mutually influenced by the environment in which they live. Although significant methodologic limitations compromise the ability to evaluate adequately the full range of effects of early intervention programs on young children with disabilities, a substantial body of research demonstrates the presence of positive short-term benefits on standardized developmental test scores. The long-term effects of early intervention on children's social competence and family adaptation are largely unknown.


Although most pathogenetic mechanisms remain unknown, an increasing number of disorders can be detected through prenatal diagnostic studies such as ultrasound, amniocentesis, or chorionic villous biopsy. The provision of complete information and sensitive medical management are therefore essential to ensure informed family decisions about all available prenatal intervention options, including experimental fetal surgeries (such as the placement in utero of an intracranial shunt) and the elective termination of a pregnancy. When specific early treatments are available for infants with metabolic disorders (such as phenylketonuria) or structural abnormalities (such as hydrocephalus), successful prevention requires prompt diagnosis and sophisticated management. In contrast, identifiable metabolic disorders for which specific therapies are not yet available, such as the mucopolysaccharidoses, must await further advances in molecular biology before effective prevention efforts can be realized.

The central theme common to all efforts to prevent mental retardation is the promotion of healthy brain development and the provision of a nurturing and growth-promoting environment. Because most of the population of mentally retarded individuals are mildly retarded and because mild retardation is disproportionately prevalent among lower socioeconomic groups, substantial prevention efforts must focus on the biologic well-being and the early life experiences of children living in poverty. In this regard, prenatal care, regular health supervision, and family support services represent major prevention strategies.