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This is a corrected version of the article that appeared in print.

Am Fam Physician. 2000;61(4):1059-1067

See related patient information handout on mental retardation, written by the authors of this article.

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.

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


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.

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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.


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.

Illustrative Case

A 16-month-old boy was referred for developmental assessment because he was not yet talking.

He was born to a 30-year-old woman, gravida 2, para 2, living children 2. He was delivered at term by cesarean section with no prenatal, labor or postnatal complications. He was slightly blue at birth and required oxygen. Mother and infant went home in two days, and no other newborn problems were noted. Birth weight was 3.2 kg (7 lb, 1 oz). The infant was breast-fed for about 12 months. Solid foods were added at 10 months, but some difficulty was noted with chewing. The boy rolled over at three to four months, sat without support at seven months and crawled at seven to eight months. He began to walk at 16 months but still did not talk. The mother first became concerned about lack of speech when the child was 13 months old. The remainder of the medical history, the review of systems and the family history were noncontributory.

At 16 months, the child's height and weight were less than the 5th percentile; head circumference was at the 25th percentile. The child was pleasant, alert, active and cooperative. No vocalization of any kind was noted during the examination. Physical findings included a slightly prominent forehead with a depressed wide nasal bridge and a flat nose. The midface appeared depressed; the child's face closely resembled his mother's. The only other abnormal finding was a small left testis. The boy could stand for a few seconds without support and was able to take a few steps. His muscle tone was mildly low but within the reference range.

The physician's impressions included the following: midface hypoplasia, small stature, rule out hearing loss, speech and language delays and global developmental delay, hypoplastic left testicle, incoordinated swallowing, rule out genetic syndrome related to hypoplastic facial features, developmental delay, small stature and familial resemblance.

In the pediatric genetics dysmorphology clinic, the above dysmorphic features were confirmed. He also had distinctive blepharophimosis, ptosis, epicanthal folds, altered palmar creases and hyperextensibility of the fingers and knees. Ohdo blepharophimosis syndrome was diagnosed, based on a London Dysmorphology Database search. Chromosomal status was 46,XY.

At 17 months' chronologic age, his developmental quotient was 61, with most delay occurring in speech, which was at the five-month level. Motor skills were at the 11- to 12-month level. He was evaluated by subspecialists who addressed his various problems. Initial hearing assessment revealed moderate hearing loss. The child was referred to an early intervention program.

Subsequent follow-up at 52 months of age revealed that the boy still had difficulty with feedings and was not yet toilet trained. His cognitive skills were at approximately a 27-month level, and genetic follow-up confirmed Ohdo blepharophimosis syndrome. The mother had subsequently given birth to a second child with the same syndrome.


The physician must have a high index of suspicion to consider the diagnosis of mental retardation in any child. Some helpful clues include delayed speech, dysmorphic features (minor anomalies), hypotonia generally or of the extremities, general inability to do things for self and, not least, expressed concern by the parents.

The first and most important step in the diagnosis of mental retardation is to obtain a comprehensive patient and family history. Previous gynecologic and obstetric history may reveal infertility or fetal loss. Assessment of maternal health status during pregnancy with the involved child should include questions regarding use of tobacco, alcohol and drugs (prescribed and illicit); lifestyle or other risks for sexually transmitted diseases; weight gain or loss; signs of infection; serious illness or injury; and surgery or hospitalization.

To establish a knowledgeable baseline history of the child, the physician should obtain information regarding length of pregnancy, premature onset of labor or rupture of the membranes, duration and course of labor, type of delivery and any complications. Apgar scores at one and (especially) five minutes should be reviewed, and birth weight, length and head circumference measurements obtained and plotted on appropriate growth charts. The parents should be asked about any illnesses, feeding or sleeping difficulties in the newborn period and problems with sucking or swallowing, as well as the baby's general disposition. Extremes in infant temperament are often the first clue to an atypical course in child development.

The systems review of the child should be complete, with special attention to growth problems, history of seizures, lethargy and episodic vomiting. A developmental screen should be used at all well-child visits to obtain information about the timing of the child's developmental milestones, any concerns by parents or caregivers and comparison of the child's developmental rate and pattern with those of siblings. Specific questions about the child's current developmental abilities should be asked at each visit.

The Revised Denver Prescreening Developmental Questionnaire10 is a useful screening tool that parents can readily complete to help determine the need for further evaluation with the time-honored Denver Developmental Screening Test.11,12 Another practical and reliable tool with which to monitor development in infants is the Kansas Infant Development Screen.13 The findings can be recorded and plotted just as with somatic growth charts and shared with parents. Other developmental screening tests are also available.

Delays in speech development are common and may become more obvious when contrasted with the speech development of a sibling. Inquiry should be made regarding concerns about hearing and vision. One cannot overemphasize the importance of addressing concerns voiced by a parent about a child's development, behavior and learning, because these expressed concerns accurately target the majority of children with developmental problems.

Information should be obtained about the family unit, parents' occupations and educational achievements, educational and developmental status of siblings, role of the patient in the family, discipline of the children and identity of the child's caregiver when the parents are not home. Family history of fetal loss, mental retardation, severe learning problems, congenital abnormalities and unexplained childhood deaths, as well as other serious illnesses in first- and second-degree family members, should be elicited.

A complete physical examination can begin with a review of growth curves since birth, if these are available. The head circumference should continue to be plotted. The examination should be thorough, with special attention to physical findings that are compatible with any risk factors obtained from the history.

The child should be examined closely for dysmorphic features or minor abnormalities, such as unusual eyebrow pattern, eyes that are widely or closely spaced, low-set ears or abnormal palmar crease patterns. Minor abnormalities are defined as defects that have unusual morphologic features without serious medical implications or untoward cosmetic appearance.8 Most minor abnormalities involve the face, ears, hands or feet, and are readily recognized even on cursory examination.14 The presence of three or more minor abnormalities in newborns is correlated with a 90 percent frequency of coexistent major abnormalities,15 suggesting close association with morphogenesis in utero. Thus, minor abnormalities may provide clues to developmental problems of possible prenatal origin.

Evaluation of the head, face, eyes, ears and mouth must include general assessment of visual acuity and hearing. Examination of the chest, heart, spine, abdomen, genitalia, extremities, muscles and neurologic reflexes can reveal abnormalities that may be associated with retardation. Table 2 highlights five common syndromes or problems associated with retardation.

DiagnosisIncidenceEtiology, including inheritanceClinical manifestations and early recognitionAssociated conditionsDiagnostic evaluation*PrognosisSpecial considerations
Down syndrome1 in 600 to 800 birthsResults from extra copy of chromosome 21, usually a sporadic event; 2% of cases may be inherited from a balanced translocation carrier parentHypotonia; flat facial profile; upslanting palpebral fissures; small ears; in-curving fifth fingers; single transverse palmar creasesSlow growth; congenital heart defect; thyroid dysfunction; developmental delay, especially speechChromosome analysis in all patients; chromosome analysis of parents if translocation is found; pediatric cardiology evaluation with echocardiogram by 6 weeks of ageCognitive limitations, with most in mild to moderate MR range; decreased life expectancy can be associated with congenital heart defect, especially if not recognized in early infancyExcept in cases where parent has a translocation, risk for recurrence is 1%
Fetal alcohol syndrome0.05 to 3 in 1,000 children diagnosed annually in United StatesAlcohol consumption by mother during pregnancyDiagnosis can be made at birth, based on history, baby's facial features (medial epicanthal folds, wide nasal bridge, small upturned nose, long philtrum, narrow or wide upper lip), low birth measurementsMay include retardation, behavior problems, ADHD, seizures, autismGood history and physical examination imperative; history of maternal drinking, pre- and postnatal growth retardation, dysmorphic facial features, CNS involvement; no laboratory tests availableVaries; growth may improve during adolescence and facial features may soften, but behaviors may cause serious problemsMany of these children are adopted; FAS and fetal alcohol effects (usually developmental and behavioral problems) are totally preventable
Fragile X syndrome1 in 2,000 to 3,000 male live births; females may also be affectedAbnormality in FMR-1 gene located on X chromosome; inherited in X-linked manner so males are more severely affectedMacrocephaly; large ears; enlarged testicles after puberty; hyperextensible fingersAutism/autistic- like behaviors; developmental delay, especially speech; clumsiness; mitral valve prolapseDNA testing for fragile X mutation (chromosome testing for fragile X misses up to 7% of cases); mothers of affected boys are obligate carriers of the geneNormal life expectancy; mild to profound MRFemales usually less severely affected than males; up to 50% of females with mutation have MR or educational difficulties; risk for recurrence is 50%
[ corrected] Velocardiofacial syndrome1 in 700 live birthsDeletion of chromosome 22; usually de novo but may be inherited in an autosomal dominant mannerCleft palate; congenital heart defect; speech delay; elongated face with almond-shaped eyes; wide nose with hypoplastic alae nasi; small ears; slender, hyperextensible fingersLearning disabilities ± mild MR; psychiatric disorder in 10%High-resolution chromosome analysis with chromosome painting (FISH) to detect chromosome 22 deletion; parents should also be testedNormal life expectancy unless severe heart defect (e.g., truncus arteriosus, interrupted aortic arch) is presentRisk for recurrence as high as 50%, depending on family history
Unknown cause of MR30 to 50% of all cases of MRVariable; diagnosis may evolve over time, so repeated evaluations may be helpfulNonspecific cluster of minor malformations; delayed milestones, especially language developmentBehavioral phenotype may also aid diagnosis as course evolvesCytogenetic studies; brain imaging; metabolic studiesWill vary considerably based on etiology (if it can be established) and/or severityDiagnostic techniques that may aid in diagnosis are constantly being refined

Evaluation and Referrals

Findings from the history and physical examination of the child will help determine which diagnostic tests and referrals are appropriate for further assessment. The physician needs to explain to the parents what these findings are and the reasons for further evaluation. The laboratory and radiographic assessment of individual children should be based on clinical presentation (Table 3).16

The rightsholder did not grant rights to reproduce this item in electronic media. For the missing item, see the original print version of this publication.

This initial sharing of information with parents is an extremely important step and will probably set the stage for the future physician-family-patient relationship. Ample time should be scheduled to discuss the findings and to allow for questions, which will be numerous. The family should be encouraged to write a list of questions for further communication with the physician. The clinician should clarify the term “developmental delay,” because parents frequently misinterpret this as meaning the child has the ability to catch up.

It is best to seek other opinions as soon as mental retardation is suspected rather than adopting a “wait-and-see” approach. The Individuals with Disabilities Education Act (1997)17 provides for developmental assessment of children older than three years in every school district. For children younger than three, similar infant-toddler assessment and early intervention resources are available, usually through local health departments, school districts or regional assessment centers. (The responsible agency varies in each state.)

Referral may be made to a tertiary-level child development unit that can provide interdisciplinary evaluations (developmental pediatrics, genetics, neurology, ophthalmology) as well as functional assessments (occupational and physical therapy, speech/language pathology, audiology, psychology). Families will usually welcome such a referral and comprehensive evaluation, especially if the mental retardation is unexplained. Evaluations by a nutritionist and a child psychiatrist may also be appropriate for some patients.

The family physician should expect complete information on the findings from this type of team evaluation. The family should expect to be referred back to their local community for ongoing primary care and, in some instances, subspecialty care. Information about early intervention resources in the local community should be shared with the family, and appropriate support services should be identified.

If the child with mental retardation has a head circumference that falls below the 5th percentile (microcephaly) or above the 95th percentile (macrocephaly), a magnetic resonance imaging scan of the brain should be considered. This is usually preferable to computed tomographic scanning because of the enhanced visualization of developmental abnormalities of the cerebral cortex, such as pachygyria, polymicrogyria and schizencephaly. These disorders reflect an abnormality during the first 25 weeks of gestation in the early migration of the neurons into the normally six-layered cortex.

A consultation with a medical geneticist/dysmorphologist is invaluable. This would include a review of a three-generation pedigree and records of pertinent relatives, evaluation for subtle dysmorphic features and assessment for a pattern to the patient's presenting characteristics.

Most mentally retarded patients who visit a genetics office undergo chromosome analysis. While this testing could be done by the referring physician, there are different levels of test quality, and it is usually best performed by a good cytogenetics laboratory associated with a university hospital or children's hospital. This allows for ease in interpretation of the results to the patient's family in the event an abnormality is found. DNA testing for fragile X syndrome should be done instead of cytogenetic testing, which can miss up to 7 percent of those who are affected.18 Metabolic testing in the absence of a history suggestive of metabolic disease is probably of little value.4

Diagnosis may require several periodic visits to a geneticist, because a phenotype may evolve slowly, and new syndromes are constantly being reported. The importance of making a diagnosis in a child with mental retardation cannot be overemphasized. An accurate diagnosis allows for anticipatory guidance for the patient, recurrence risk information and genetic counseling for the parents, and opportunities for the family to become involved in specific support groups. An uncertain diagnosis should be conveyed as such; no diagnosis is preferable to an incorrect one.

Within a given family, the risk of recurrence of mental retardation in future siblings or other relatives of the patient depends on the specific diagnosis. The recurrence risk for mental retardation cannot be given to the family until a diagnosis has been made, although a general discussion with a geneticist may be of benefit. The family physician is a valuable resource in periodically reviewing the recurrence risk for the family.

Practice guidelines for primary care of children with certain conditions (Down syndrome, fragile X syndrome) are also available,1921 as are special somatic growth charts for some syndromes. There are also guidelines for the management of adults with mental retardation who have been deinstitutionalized.22 Table 4 lists Internet resources that may be valuable to the family physician.

Syndrome Diagnosis, Pediatrics in Review, Multimedia Pages CD-ROM, Vol 18, No.11-CD, 11/97
Family Voices Website:
Family Village Website:
KUMC Website (support groups):
KUMC Website:

All physicians who care for children with mental retardation or developmental disabilities should remember that these patients quickly outgrow their childhoods. As they become adolescents and young adults, most of them will need professional intervention to help them become their own advocates in the health care system. Families should be supported as they encourage independent functioning on the part of their adolescent or young-adult son or daughter.23 Issues relating to sexuality, family planning, custodial care, estate planning and guardianship are highly important but are beyond the scope of this article.

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