Identification and Evaluation of Mental Retardation

Author: Donna K. Daily, Grace E. Holmes
Date: Feb 15, 2000

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.)

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.

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.

TABLE 1Developmental Characteristics Related to Level of Mental Retardation(DSM-IV Criteria)Mild retardation Moderate retardation Severe retardation Profound retardation75% to 90% of all cases of ~10% to 25% of all cases of ~10% to 25%of all cases ~10% to 25% of all casesretardation retardation of retardation ofretardationFunction at one half to two Function at one third to one half Function at one fifth to one Function at [LESS THAN] one fifth ofCAthirds of CA (IQ: 50 to 70) of CA (IQ: 35 to 49) thirdof CA (IQ: 20 to 34) (IQ: [LESS THAN] 20)Slow in all areas Noticeable delays, especially in Marked and obvious Marked delays in all areas speech delays; may walk lateMay have no unusual physical May have some unusual physical Littleor no communication Congenital abnormalities often signs signs skills but may have some present understanding of speech and show some responseCan acquire practical skills Can learn simple communication May betaught daily routines Need close supervision and repetitive activitiesUseful reading and math skills Can learn elementary health and May be trained in simple Often need attendant careup to grades 3 to 6 level safety habits self-careCan conform socially Can participate in simple Need direction and May respond to regular physical activities and self-care supervision activity and social stimulationCan acquire vocational skills Can perform tasks in sheltered - Not capable of self-carefor self-maintenance conditionsIntegrated into general society Can travel alone to familiar places - -note: Additional problems with vision, hearing or speech, congenitalabnormalities, seizures, emotional problems or cerebral palsy may bepresent.DSM IV = Diagnostic and Statistical Manual of Mental Disorders, 4th ed.;CA = chronological age; IQ = intelligence quotient.Adapted with permission from Pelegano JP, Healy A. Mental retardation.Part II. Seeing the child within. Fam Pract Recertification 1992;14:63.table 2Common Syndromes Associated with Mental Retardation Etiology, Clinical including manifestations and AssociatedDiagnosis Incidence inheritance early recognition conditions Diagnostic evaluation* Prognosis Special considerationsDown 1 in 600 to Results from Hypotonia; flat facialprofile; Slow growth; Chromosome analysis Cognitivelimitations, with Except in cases wheresyndrome 800 births extra copy of upslantingpalpebral fissures; congenital in all patients; most in mild to moderate parent has a translocation, chromosome 21, small ears; in-curvingfifth heart defect; chromosome analysis MR range; decreased risk for recurrence is 1% usually a sporadic fingers; singletransverse thyroid of parents if translocation life expectancy can be event; 2% of cases palmar creases dysfunction; is found; pediatric associatedwith may be inherited developmental cardiology evaluation congenitalheart defect, from a balanced delay,especially with echocardiogram especially if not translocation carrier speech by 6 weeks of age recognized in early parent infancyFetal alcohol 0.05 to 3 in Alcohol consumption Diagnosiscan be made at birth, May include Good history and physical Varies; growth may Many of these children aresyndrome 1,000 by mother during based onhistory, baby's facial retardation, examinationimperative; improve during adopted; FAS and fetal children pregnancy features (medialepicanthal behavior history of maternal adolescence and alcohol effects (usually diagnosed folds, wide nasal bridge,small problems, drinking, pre- and facialfeatures may developmental and annually in upturned nose, longphiltrum, ADHD, seizures, postnatal growth soften, but behaviors behavioral problems) are United narrow or wide upper lip), autism retardation, dysmorphic may causeserious totally preventable States low birth measurements facial features, CNS problems involvement;no laboratorytests availableFragile X 1 in 2,000 Abnormality in FMR-1 Macrocephaly; largeears; Autism/autistic- DNA testing for fragile X Normal lifeexpectancy; Females usually lesssyndrome to 3,000 gene located on enlargedtesticles after like behaviors; mutation (chromosome mild to profound MR severely affected than male live X chromosome; puberty;hyperextensible developmental testing for fragile X males; up to 50% of births; inherited in fingers delay, especially misses up to 7% of females with mutation females X-linked manner speech; cases); mothers of haveMR or educational may also so males are more clumsiness; affected boys are difficulties; risk for be affected severely affected mitralvalve obligate carriers of the recurrence is 50% prolapse geneDiGeorge 1 in 700 live Deletion of Cleft palate; congenitalheart Learning High-resolution Normal life expectancy Riskfor recurrence as highsyndrome births chromosome 22; defect;speech delay; disabilities chromosome analysis unless severe heart as 50%, depending on usually de novo but elongated facewith almond- [plus-or-minus sign] mild MR; with chromosomedefect(e.g., truncus family history may be inherited in shaped eyes; widenose with psychiatric painting (FISH) to arteriosus, interrupted an autosomal hypoplastic alae nasi;small disorder in 10% detect chromosome aortic arch) is present dominant manner ears; slender,hyperextensible 22 deletion; parents fingers shouldalso be testedUnknown 30 to 50% Variable; diagnosis Nonspecificcluster of minor Behavioral Cytogenetic studies; brain Willvary considerably Diagnostic techniques thatcause of all cases may evolve over malformations; delayed phenotype imaging; metabolic based on etiology may aid in diagnosis areof MR of MR time, so repeated milestones,especially language may also aid studies (if it can be established) constantly being refined evaluations may be development diagnosis as and/or severity helpful course evolves*- See Table 3 for summary of indications for diagnostic and screeningtests.MR = mental retardation; ADHD = attention-deficit/hyperactivitydisorder; CNS = central nervous system; FAS = fetal alcohol syndrome;FISH = fluorescence in situ hybridization.Information from references 4, 5, 7 and 18.Table 3Suggested Indications for Tests When Mental Retardation Is UnexplainedMagnetic resonance imagingof the brainCerebral palsy or motorasymmetryAbnormal head size or shapeCraniofacial malformationLoss or plateau ofdevelopmental skillsMultiple somatic anomaliesNeurocutaneous findingsSeizuresIQ [LESS THAN]50Cytogenetic studiesMicrocephalyMultiple (even minor) somatic anomaliesFamily history of mentalretardationFamily history of fetal lossIQ [LESS THAN]50Skin pigment anomalies (mosaicism)Suspected contiguous gene syndromes (e.g., Prader-Willi, Angelman,Smith-Magenis)Metabolic studiesEpisodic vomiting or lethargyPoor growthSeizuresUnusual body odorsSomatic evidence of storage diseaseLoss or plateau of developmental skillsMovement disorder(choreoathetosis, dystonia, ataxia)Sensory loss (especially retinal abnormality)Acquired cutaneous disordersIQ = intelligence quotient.Adapted with permission from Palmer FB, Capute AJ. Mental retardation.Pediatr Rev 1994;15:476.table 4Internet and Computer Resources on Mental Retardationfor Family PhysiciansSyndrome Diagnosis, Pediatrics in Review, Multimedia Pages CD-ROM, Vol18, No.11-CD, 11/97Family Voices Website: http://www.familyvoices.orgFamily Village Website: http://www.familyvillage.wisc.eduKUMC Website (support groups): Website:

COPYRIGHT 2000 American Academy of Family PhysiciansCOPYRIGHT 2000 Gale Group

© 2006,, All Rights Reserved.