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Industry: Email Alert RSS FeedIdentification and Evaluation of Mental Retardation
American Family Physician, Feb 15, 2000 by Donna K. Daily, Holly H. Ardinger, Grace E. Holmes
Mental retardation in young children is often missed by clinicians. The condition is present in 2 to 3 percent of the population, either as an isolated finding or as part of a syndrome or broader disorder. Causes of mental retardation are numerous and include genetic and environmental factors. In at least 30 to 50 percent of cases, physicians are unable to determine etiology despite thorough evaluation. Diagnosis is highly dependent on a comprehensive personal and family medical history, a complete physical examination and a careful developmental assessment of the child. These will guide appropriate evaluations and referrals to provide genetic counseling, resources for the family and early intervention programs for the child. The family physician is encouraged to continue regular follow-up visits with the child to facilitate a smooth transition to adolescence and young adulthood. (Am Fam Physician 2000;61:1059-67,1070.)
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The diagnosis of mental retardation in young children is frequently missed. The three most common errors made by clinicians who overlook the possibility of mental retardation are (1) concluding that a child does not "look" retarded, (2) assuming that a child who is ambulatory is unlikely to be retarded and, (3) if retardation is actually considered, concluding that it is not possible to test young children.1
Prevalence
Mental retardation is present in about 2 to 3 percent of the population. It can be defined as cognitive ability that is markedly below average level and a decreased ability to adapt to one's environment. The onset of the condition occurs during the developmental period, i.e., gestation through age 18 years.
Mental retardation comprises five general categories: borderline, mild, moderate, severe and profound. Categories are based on scores obtained through use of age-standardized tests of cognitive ability (Table 1).2 Mental retardation may occur as part of a syndrome or broader disorder but is most commonly an isolated finding.
The inclusion of concurrent related limitations in two or more adaptive skill areas was added to the definition of mental retardation in 1992 by the American Association on Mental Retardation.3 Because standardized testing in very young children is less predictive of future cognitive outcome, the term "developmental delay" has been used to characterize the developmental status of children under age three.
Etiology
A number of environmental, genetic or multiple factors can cause mental retardation. Unfortunately, in approximately 30 to 50 percent of cases, the etiology is not identified even after thorough diagnostic evaluation.4,5 Some persons have a congenital malformation of the brain; others had damage to the brain at a critical period in pre- or postnatal development. Acquired causes of retardation include near-drowning, traumatic brain injury and central nervous system malignancy.
Prenatal causes of mental retardation include congenital infections such as cytomegalovirus, toxoplasmosis, herpes, syphilis, rubella and human immunodeficiency virus; prolonged maternal fever in the first trimester; exposure to anticonvulsants or alcohol; and untreated maternal phenylketonuria (PKU). Complications of prematurity, especially in extremely low-birth-weight infants, or postnatal exposure to lead can also cause mental retardation.6
Metabolic disorders are another possible cause of mental retardation. In some cases (e.g., PKU, hypothyroidism), retardation is preventable with early treatment. Other disorders (e.g., mucopolysaccharidosis, sphingolipidoses) are less responsive to early intervention. Molecular medicine has made it possible to diagnose a number of conditions referred to as mitochondrial cell diseases.7
A number of single-gene disorders result in mental retardation. Many of these are associated with atypical or dysmorphic physical characteristics. Such conditions include fragile X syndrome, neurofibromatosis, tuberous sclerosis, Noonan's syndrome and Cornelia de Lange's syndrome. A complete list of syndromes associated with mental retardation is beyond the scope of this article, and the reader is referred to the reference list.8,9
As many as one fourth of persons with mental retardation have a detectable chromosome abnormality. Children with Down syndrome (trisomy 21) usually have highly recognizable physical characteristics, but features associated with other chromosomal abnormalities, such as Klinefelter's syndrome (47,XXY), may not be as obvious to family members or the physician. Other children may have a small deletion or duplication of a particular chromosome that is rarely reported; thus, the phenotype is still undetermined. Some chromosomal abnormalities are inherited from a parent but most occur de novo. Many previously described clinical syndromes have been found to have an associated chromosomal abnormality (e.g., DiGeorge, Prader-Willi, Angelman and Williams syndromes).
The following case report highlights the importance of early diagnosis, in planning therapy for the child and in providing family planning information to the parents.
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