General intellectual functioning is
determined by the results of astandarzed test of intelligence, and the
term “significantly sub average” is define as an intelligence quotient of
approximately 70 or below or two standard deviations below the mean for
the particular test. The category of borderline mental retardation
(between one and two standard deviations below the test) was eliminated in
The prevalence of mental retardation at
any one time is estimated to be about 1 percent of the population. The
incidence of mental retardation is difficult to calculate accurately
because of the impossibility of stating when mental retardation is
diagnosed in a person. In many cases, retardation may be latent for a ling
time before the persons limitations are recognized, or because of good
adaptation, the formal diagnosis is not warranted at a particular point in
the person’s life. The high-test incidence is in school age children, with
the peak at ages 10 to 14. Mental retardation is approximately 1 and half
times more common among men than among women. In older populations,
prevalence is less, as those with severe or profound mental retardation
have high mortality rates resulting from the complications of associated
On the basis of current
knowledge, approximately 25 percent of all cases of mental retardation are
known to be caused by biological abnormalities. Chromosomal and metabolic
disorders such as Down’s syndrome, fragile X syndrome, and phenylketonuria
(PKU) are the most common disorders manifesting mental retardation. Mental
retardation associated with these disorder is usually diagnosed at birth
or relatively early in childhood, and the severity is generally moderate
No specific biological causes
can be identified I n the remaining 75 percent of the cases. The level of
intellectual impairment of a person with no known cause is usually mild,
with an I.Q. between 50 and 70. The diagnosis of mild retardation is not
usually made before grade school. In mild mental retardation, a familial
pattern is often seen in parents and siblings.
The low socioeconomic groups are
over represented in the case of mild retardation and the significance of
this is not clear. However psychosocial deprivation such as deprivation in
social, linguistic and intellectual stimulation has been suspected of
contributing to mental retardation without a known biological cause.
Current knowledge suggest that three sets of causative factors are
involved, either singly or in combination, genetic factors, environmental
biological factors ( e.g., malnutrition) and early child-rearing
Important prerequisites for the
overall development of the fetus include the mother’s physical,
psychological and nutritional health during pregnancy. Maternal chronic
illnesses and conditions affecting the normal development of the fetus’s
central nervous system include uncontrolled diabetes, anemia, emphysema,
hypertension and long term use of alcohol and narcotic substances.
Maternal infections during pregnancy, especially viral infections, have
been known to cause fetal damage and mental retardation. The degree of
fetal damage depends on variables such as the type of viral infection, the
gestational age of the fetus, and the severity of the illness. Although
numerous infectious diseases have been reported to affect the fetus’s
central nervous system. The following three infectious disorders have been
definitely identified as high risk conditions for mental retardation
rubella (German measles), cytomegalic inclusion disease and syphilis.
Rubella (German measles) .
Rubella has replaced syphilis as
the major cause of congenital malformations and mental retardation caused
by maternal infection. The children of affected mothers may present a
number of abnormalities, including congenital heart disease, mental
retardation, cataracts, deafness, microcephaly and microphthalmia. Timing
is crucial, as the extent and the frequency of the complications are
inversely related to the duration of the pregnancy at the time of the
maternal infection. When mothers are infected in the first trimester of
pregnancy 10 to 15 percent of the children are affected, but the incidence
rises to almost 50 percent when the infection occurs in the first month of
pregnancy. The situation is often complicated by sub clinical forms of
maternal infection, which often go undetected. Maternal rubella can be
prevented by immunization.
Cytomegalic inclusion disease.
In many cases, cytomegalic
inclusion disease remains dormant in the mother. Some children are
stillborn, and others have jaundice, microcephaly, hepatosplenomegaly and
radiographic findings of intracerbral calcification. Children with mental
retardation from this disease frequently have cerebral calcification,
microphaly, or hydrocephalus. The diagnosis is confirmed by positive
throat and urine cultures of the virus and the recovery of inclusion
bearing cells in the urine.
Syphilis in pregnant women used
to be the main cause of various neuropathological changes in their
offspring, including mental retardation. Today the incidence of syphilitic
complications of pregnancy fluctuates with the incidence of syphilis in
the general population. Some recent alarming statistics from several major
cities in the United States indicate that there is still no room for
Brain damage caused by
toxoplasmosis transmitted from the pregnant mother to the fetus is another
universally recognized but relatively complication of pregnancy that often
results I n mental retardation and a variety of brain malformations.
Damage to the fetus from maternal hepatitis has also been reported.
Currently, acquired immune
deficiency syndrome (AIDS) has become an important public health issue and
extensive research is being conducted to study its effects on fetuses and
newborn infants. The pregnancy of a woman who has a confirmed case of AIDS
results in fetal death, stillbirth, spontaneous abortion, or death of the
baby within a few years, although some survive longer. Neurological
symptoms in AIDS infected children include development delay and
retrogression. The brain can be invaded by the AIDS virus and by
The role of other maternal
infections during pregnancy such as influenza, common cold viruses,
pneumonia, and urinary tract infections in causing mental retardation is
being investigated, and the results are not yet conclusive.
Complications of pregnancy .
Toxemia of pregnancy and
uncontrolled maternal diabetes present hazards to the fetus and sometimes
result in mental retardation. Maternal malnutrition during pregnancy often
results in prematurely and other obstetrical complications. Vaginal
hemorrhage, placenta previa, premature separation of the placenta and
prolapsed of the cord may damage the fetal brain by causing anoxia.
sematogenic effect of
pharmacological agents administered during pregnancy was widely publicized
after the thalidomide tragedy (the drug produced a high percentage of
deformed babies when given to pregnant women.) So far, with the exception
of metabolites used in cancer chemotherapy, no usual doses are known to
damage the fetus’s central nervous system, but caution and restraint in
prescribing drugs to pregnant women are certainly indicated. The use of
lithium during pregnancy was recently implicated in some congenital
malformations, especially of the cardiovascular system ( e.g., Epstein's
Many infants have been exposed to
alcohol and drugs during the mother’s pregnancy. Those born with fetal
alcohol syndrome may be retarded and have physical stigmata, such as
midfacial hypoplasia, short palpebral fissures, cardiac defects and
possibly microcephaly. Those born addicted to cocaine and heroin may be
developmentally delayed, with the long term prognosis yet unknown.
Abnormalities in autosomal
chromosomes are associated with mental retardation, although aberrations
in sex chromosomes are not always associated with mental retardation (
such as 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.
Down’s syndrome was first
described by the English physician Langdon Down in 1866 and was based on
the physical characteristics associated with subnormal mental functioning.
Since then, Down’s syndrome has remained the most investigated and the
most discussed syndrome in mental retardation. The 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 past 100 years, the cause of Down’s
syndrome is still unknown. There is agreement on very few predisposing
factors in chromosomal disorders among them, the increased age of the
mother, possibly the increased age of the father and X-ray radiation. The
problem of cause is complicated even further by the recent recognition of
three types of chromosomal aberrations in Down’s syndrome.
1) Patients with trisomy 21 (three of
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 no disjunction during meiosis, occurring
for yet unknown reasons, is held responsible for this disorder.
2) No disjunction occurring after
fertilization in any cell division results in mosaicism, a condition in
which both normal and trisomic cells are found in various tissues.
3) In translocation there is a fusion of
two chromosomes, mostly 21 and 15, resulting in a total of 46 chromosomes,
despite the presence of an extra chromosome 21.The disorder, unlikely
trisomy 21 is about 1 in 100births.but,when translation is present, the
risk is about one in three. These facts assume special importance in
Amniocentesis, in which a small
amount of amniotic fluid is removed from the amniotic activity
Tran abdominally between the 14th and the 16th weeks
gestation, has been useful in diagnosing various infant abnormalities,
especially Down’s syndrome Amniotic fluid cells, mostly fetal in origin,
are cultured for cytogenetic and biochemical studies. Many serious
hereditary disorders can be predicted with this method of prevention.
Amniocentesis is recommended for all pregnant women over the age of 35.
Fortunately, most chromosomal anomalies occur only in a family.
chronic villus sampling CVS is
a new screening technique to determine fetal abnormalities. It is done at
8 to10 weeks of gestation, which is 6 weeks earlier than amniocentesis.
Results are available in a short time (hours or days) and if the result is
abnormal, the decision to terminate the pregnancy can be made within the
first trimester. There is a miscarriage risk of between 2 and 5 percent as
a result of the procedure.
Mental retardation is the
overriding feature of Down’s syndrome. The majority of patients belong to
the moderately and severely retarded groups, with only a minority having
an I.Q. above 50. Mental development seems to progress normally from birth
to 6 months of age. I.Q. scores gradually decrease from near normal at 1
year of age to about 30 at older ages. This decline in intelligence may be
real or apparent. It could be that infantile tests do not reveal the full
extent of the defect, which may become manifest when more sophisticated
tests are used in early childhood. According to many sources, patients
with Don’s syndrome are placid, cheerful, and cooperative, which
facilitates their adjustment at home. The picture, however, seems to
change in adolescents, especially those in institutions, who may develop
various emotional difficulties, behavior disorders and (rarely) psychotic
The diagnosis of Down’s 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 fissues, abundant neck skin, a small
flattened skull, high cheekbones, and a protruding tongue. The hands are
broad and thick, with a single palmer transversal crease and the little
fingers are short and curved inward. Moro’s reflex is weak or absent. More
than 100 signs or stigmata are described in Down’s syndrome, but rarely
are all found in person.
Life expectancy used to be about
12 years. With the advent of antibiotics, however, few young patients
succumb to infections, but most of them do not live beyond the age of 40
when they already have many signs of senescence, which are similar to
those of Alzheimer’s disease. Despite numerous therapeutic
recommendations, no treatment has proved effective.
Cate-cry ( cri-du-chat) syndrome.
Children with cat-cry syndrome
are missing part of the fifth chromosome. They are severely retarded and
show many stigmata often associated with chromosomal aberrations, such as
microcephaly, low-set ears, oblique palpebral fissues, hypertelorism and
micrognathis. That gave the syndrome is name gradually changes and
disappears with increasing age.
Other syndrome of autosomal
aberrations associated with mental retardation are much less prevalent
than is Down’s syndrome.
Fragile X syndrome.
Fragile X, a recently discovred
syndrome, is a known genetic cause of retardation. Second only to Down’s
syndrome, it occurs in 0.5 to 1 in every 1,000 male births. The fragile
site on the X chromosome is expressed when cells are cultured in a
folate-poor medium. The typical phenotype includes a large head and ears,
a long and narrow face, short stature and postpubertal macroorchidism.
The child’s intellectual level ranges from low average to severely
retarded. Many children have ranges from low average to severely retarded.
Many children have symptoms of attention deficit hyperactivity disorder
and specific developmental disorders. Female carriers are usually less
impaired than males with fragil X, but female carriers can manifest the
typical physical characteristics and can be mildly retarded.
In 1966 Dr. Andreas Rett from
Vienna reported on 22 girls with a serious neurological disability
characterized by a deterioration of hand skills and stereotyped hand
movements. Autisticlike symptoms, ataxia, facial grimacing, teeth grinding
and loss of speech were noted. There was progressive spasticity,
scoliostis, difficulty with gait (if walking had occurred) and onset of
seizures. To date the longest survival age has been 34 years, but the
complete description of the spectrum is still being established.
Phenylketonuria (PKU) was first described by Asbjorn Folling in 1934 as
the paradigmatic inborn error of metabolism. PKU is transmitted as a
simple recessive autosomal Mendelian trait and occurs in approximately 1
in every 10,000 to 15,000 live births. To the parents who have already had
a child with PKU is one in every four to five successive pregnancies.
Although the disease is reported predominantly in people of north European
origin, a few cases have been described in blacks, Yemenite Jews and
Asians. The frequency among institutionalized defectives is about 1
The basic metabolic difect 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 due to a
deficiency of an enzyme, dihydroperdine reductase, and the other to 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.
The majority of patients with
PKU are severely retarded, but some are reported to have borderline or
normal intelligence. Eczema, vomiting and convulsions are present in about
a third of all cases. Although the clinical picture varies, typical PKU
children are hyperactive and exhibit erratic unpredictable behavior, which
makes them difficult to manage. They frequently have temper tantrums and
often display bizarre movements of their bodies and upper extremities and
twisting hand mannerism and their behavior sometimes resembles that of
autistic or schizophrenic children. Verbal and nonverbal communication is
usually severely impaired or nonexistent. Their coordination is poor and
they have many perceptual difficulties.
This disease was previously
diagnosed on the basis of a urine test. Phenylypruvice acid in the urine
reacts with ferric chloride solution to yield a vivd green color. However,
this phenylpyruic acid in urine before the baby is 5 or 6 weeks old, and
it may give positive responses with other aminoacidurias. Currently, a
more reliable screening test that is widely used is the Guthrie inhibition
assay, which uses a bacteriological procedure to detect blood
Early diagnosis is important as
a low phenylalanine diet, in use since 1955, significantly improves both
behavior and development progress. The best result seem to be obtained
with early diagnosis and the start of dietary treatment before the child
is 6 months of age.
Dietary treatment, however is
not without risk. Phenylalanine is an essential amino acid and its
omission from the diet may lead to such severe complications as anemia,
hypoglycemia, edema and even death. Dietary treatment of PKU should be
continued indefinitely. Children who receive a diagnosis before the age of
3 months and are placed on an optimal dietary regimen may have normal
intelligence. For untreated older children and adolescents with PKU, a low
phenylalanine diet does not influence the level of mental retardation.
However, the diet does decrease their irritability and abnormal EEG
changes and does increase their social responsiveness and attention span.
The parents of PKU children and
some of these children’s normal siblings are heterozygous carriers. The
disease can be detected by a phenylalanine tolerance test, which may be
important in the genetic counseling of these people.
Maple syrup urine disease (Menkes’
The clinical symptoms of Menkes’
disease appear during the first week of life. The infant deteriorates
rapidly and has decrebrate rigidity, seizures, respiratory irregularity
and hypoglycemia. If untreated most patients die in the first months of
life and the survivors are severely retarded. Some variants have been
reported with transient ataxia and only mild retardation.
Treatment follows the general
principles established for PKU and consists of a diet very low in the
three involved amino acids leucine, isoleucine, and valine.
Other enzyme deficiency disorders:
Several enzyme deficiency disorders associated with mental retardation
have been identified, and still more disease are being added as new
discoveries are made. Some of these include Hartnup disease, galactosemia
and glycogen storage disease.
Acquired Childhood Diseases
Occasionally, a child’s
developmental status changes dramatically as a result of a specific
disease or physical trauma. In retrospect, it is sometimes difficult to
ascertrain the full normally of the child’s developmental progress before
the insult, but the adverse effects on the child’s development or skills
undoubtedly have a new origin.
The most serious infections
affecting cerebral integrity are encephalitis and meningitis. Measles
encephalitis has been virtually eliminated by the universal use of measles
vaccine and the incidences of other bacterial infections of the central
nervous system have been markedly reduced with antibacterial agents. Most
episodes of encephalitis are caused by household accidents such as falls
from tables, from open windows and on stairways. Child abuse is also a
cause of head injury.
Other issues .
Brain damage from cardiac arrest during
anesthesia is rare. One cause of complete or partial decortication is
asphyxia associated with near drowning. Chronic exposure to lead is a
well-established cause of compromised intelligence and learning skills.
Intracranial tumors of various types and origins in themselves or from the
effects of surgery and chemotherapy can also adversely affect brain
Environmental and Sociocultural Factors
It is well known that mild
retardation is significantly more prevalent among persons of culturally
deprived, low socioeconomic groups and that many of the family members or
relatives are affected with similar degrees of mental retardation. No
biological causes have been identified in these cases.
In any case it is clear that
children in poor, sociocultrally deprived families are subjected to
potentially pathogenic and developmentally adverse conditions. The
prenatal environment is compromised by poor medical care and poor maternal
nutrition. Teenage pregnancies are frequent and are associated with
obstetrical complications, prematurely and low birth weight. Poor
postnatal medical care, malnutrition, exposure to such toxic substances as
lead and physical traumata are frequent. Family instability, frequent
moves and multiple but inadequate caretakers are common. Further more the
mothers in such families are often poorly educated and ill equipped to
give the child appropriate stimulation.
Another unresolved issue is the
influence of severe parental mental illness. It has been hypothesized that
such illness adversely affects the child’s care and stimulation and other
aspects of the environment, thus putting the child at a developmental
risk. Children of parents with mood disorder and schizophrenia are known
to be at risk for these and related disorders. Recent studies also suggest
a high prevalence of motor and other developmental delays among these
children but not necessarily frank retardation.
Personality and Behavioral Patterns
The most common misconception
among professionals and the general public has been the belief that
mentally retarded persons are a behaviorally homogeneous group. In fact,
retarded persons display more personality styles and behaviors than do the
non retarded. For example, a mildly retarded person who is living
independently with some supervision and who is partially self-supporting
has more in common with a non retarded coworker than with a profoundly
retarded person who is totally dependent on others, care. Another
misconception is the belief that the maladaptive behaviors of retarded
persons are the result of mental retardation and organicity, rather than
All behaviors and personality
patterns exhibited by retarded persons ar seen also in non retarded
persons. However, certain behavioral patterns may be expected to be more
frequently associated with mental retardation because of retarded persons
cognitive and other deficits and life experiences. Egocentricity and
concreteness of thinking are often seen in retarded persons and are
related to cognitive deficits, particularly difficulties in concept
formation and abstract thinking.
abnormalities usually cannot be readily linked to behavioral patterns,
especially in mildly or moderately impaired persons. Such neurological
abnormalities are more common in profoundly retarded persons. They may be
associated with motor hyperactivity and short attention span. Contrary to
another misconception, aggressive behavior is not organically based and is
not an especially common behavioral feature of retarded persons.
Environmental and experiential
influences are probably the main factors responsible for the retarded
person’s behavior. Impersonal, dehumanizing and understaffed custodial
institutions have been the most pathogenic in this respect. The
institutions staff often reward passivity, compliance and lack of
initiative. Some patients, however, thrive on the staff’s negative
attention, which they can elicit through inappropriate behaviors,
including aggression. Overprotection by care takers, especially parents,
is often responsible for the retarded person’s dependency, low frustration
tolerance, sense of inadequacy and low self-esteem.
Vulnerability to Mental Disorders
Negative self-image and low
self-esteem are probably almost universal personality features of retarded
persons, particularly the mildly and moderately retarded. They are well
aware of being different from others, of not meeting their parents and
society’s expectations and of not meeting their parents and society’s
expectations and of progressively falling behind their peers and even
their younger siblings.
Defenses against an intolerable
sense of inadequacy, low self-image and anxiety may often be maladaptive
and pathological and may lead to inappropriate behavior. Some retarded
adolescents and young adults may resort to delinquency and aggressive
The conflict between the
expected self-image and the real self-image may be a source of lifelong
stress and anxiety among mildly retarded persons who are aware of their
handicap. Prolonged dependence on care and support by others prevents most
retarded persons from developing a self-image as a separate person
vulnerability to feeling inadequate and frustrated. Inappropriate
behaviors such as withdrawal, are common. The low self-esteem of mildly
and moderately retarded persons may predispose them to depression.
No reliable data are available
on the incidence of psychiatric disorders in mentally retarded persons,
mainly because of methodological difficulties. However, several reports
indicate a very high risk of psychiatric illness in such persons, with the
incidence ranging from 40 to 75 percent.
Among mildly and moderately
retarded adults, the most frequent diagnoses are adjustment disorders,
mood disorders and psychoses among children, the most frequent diagnose
are mood disorders, anxiety disorders and pervasive developmental
Mental retardation is associated
with several heterogeneous groups of disorders and a multitude of
psychosocial factors. The best treatment of mental retardation is the
preventive medicine model of primary, secondary and tertiary prevention.
Primary prevention refers to
efforts and actions taken to eliminate or reduce the factors and
conditions that lead to the development of the disorders associated with
mental retardation. Such measures include (1) education to increase the
general public’s knowledge and awareness of mental retardation (2)
continuing efforts of health professionals to ensure and upgrade public
health policies, (3) legislation to provide optimal maternal and child
health care and (4) the eradication of the known disorders associated with
central nervous system damage. Family and genetic counseling helps reduce
the incidence of mental retardation in a family with a history of a
genetic disorder with mental retardation. For the children and mothers of
low socioeconomic status, proper prenatal and postnatal medical care and
various supplementary enrichment programs and social service assistance
may help minimize medical and psychosocial complications.
Secondary and Tertiary Prevention
Once a disorder or condition
associated with mental retardation has been identified, the disorder
should be treated so as to shorten the course of the illness ( secondary
prevention) and to minimize the sequelae or consequent handicaps (
Hereditary metabolic and
endocrine disorders, such as PKU and hypothyroidism, can be effectively
treated in an early stage by dietary control or hormone replacement
Mentally retarded children
frequently have emotional and behavioral difficulties requiring
psychiatric treatment. These children’s limited cognitive and social
capabilities require modified psychiatric treatment modalities based on
the children’s level of intelligence. Play therapy and opportunities for
social group interaction often help them express their inner conflicts.
Behavior therapy, especially
positive reinforcement, has proved effective in modifying some maladaptive
behaviors. Occasionally, psychotropic medication can help remove or modify
some target behavioral symptoms, such as hyperactive and impulsive
behavior, anxiety and depression. Lithium or propranolol (Inderal) are
sometimes useful for the management of aggressive behaviors. Naltrexone
(Trexan) has reduced self-injurious behaviors in some retarded persons. In
some institutions insufficient staffing and poor monitoring of medication
result in the unnecessary use or excessively high dosaging of medication .
The parents need continuous
counseling or, if indicated, family therapy. The parents should be allowed
opportunities to express their feelings of guilt, despair, anguish,
recurring denial and anger regarding the child’s disorder and future. The
psychiatrist should be prepared to give the parents all the basic and
current medical information regarding causes, treatment and other
pertinent areas (such as special training and the correction of sensory