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Causes for Mental Retardation
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Mental Retardation
Associated With Sociocultural Deprivation
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Adverse sociocultural conditions,
particularly those involving a deprivation of normal stimulation, may play a primary role
in the etiology of mental retardation.
Two subtype of mental retardation can be mental retardation associated
with extreme sensory and social deprivation, such as prolonged isolation during the
development years and cultural-familial
retardation, in which the child is not subjected to extreme isolation but rather
suffers from an inferior quality of interaction with his cultural environment and with
other people.
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Cultural-familial mental retardation |
| Children who fall in this category are
usually mild mental retardates; they make up the majority of persons labeled as mentally
retarded and show no identifiable brain pathology. They are usually not diagnosed as
mentally retarded until they enter school and have serious difficulties in their studies. A
number of studies point out that most of these children come from
poverty-stricken, unstable, and often disrupted family backgrounds characterized by a lack
of intellectual stimulation, an inferior quality of interaction with others, and general
environmental deprivation.
Since a child’s current level of intellectual functioning is based largely on
previous learning - and since schoolwork requires complex skills such as being able to
control one's attention, follow instructions, and recognize the meaning of a considerable
range of words - the child is at a disadvantage from the beginning if his environment has
deprived him of the opportunity to learn requisite background skills and be motivated
toward further learning. Thus with each succeeding year these children tend to fall
farther behind in school performance and relative ratings on intelligence tests, unless
remedial measures are undertaken. Many of these children do reveal a history of
prematurity, inadequate diets, a little or no medical care.
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MENTAL
RETARDATION ASSOCIATED WITH ORGANIC CAUSES |
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Genetic-Chromosomal
Factors |
Mental retardation tends to run in families. This is
particularly of mild retardation,
which presumably is heavily influenced by the many genetic factors responsible for
variations in intelligence. Genetic factors play a much clearer role in the etiology of
relatively rare types of mental retardation such as Down's Syndrome. Here, specific
genetic defects are responsible for metabolic alterations that adversely affect
development of the brain. Genetic defects leading to metabolic alterations may, of course,
involve many other developmental anomalies besides mental retardation. In general, mental
retardation associated with known genetic-chromosomal defects is moderate to severe in
degree.
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| Infections and
Toxic Agents |
| Mental retardation may be associated with a wide range
of conditions due to infection. The fetus of a mother with certain virus diseases, such as
German Measles, may suffer brain damage, as may the fetus of a mother with Syphilis. And,
as in the case of viral encephalitis, brain damage may result from infections occurring
after birth. A number of toxic agents, such as carbon monoxide and lead, may also cause
brain damage during fetal development or after birth. In some instances, immunological
agents, such as antitetanus serum or typhoid vaccine, may lead to brain damage. Similarly
certain drugs taken by the mother during pregnancy may lead to congenital malformations,
or an overdose of drugs administered to the infant may result in toxicity and brain
damage. In rare cases, brain damage results from incompatibility in blood types between
mother and fetus – Rh or ABO system incompatibility. Fortunately, early diagnosis and
blood transfusions can now minimize the effects of this disorder.
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| Prematurity
and Trauma ( Physical Injury) |
| Follow up studies of children born prematurely (weighing
less than 1500 grams at birth) have revealed a high incidence of neurological disorders,
including mental retardation. In fact, very small premature babies are about ten times
more likely to be mentally retarded than normal infants. Physical injury at birth can
also result in retardation. Although normally the fetus is well protected by its fluid
filled bag, and its skull appears designed to resist delivery stresses, accidents do
happen during delivery, as well as after birth.
Difficulties in labor due to malposition of the fetus or other complications may
irreparably damage the infant’s brain. Bleeding within the brain is probably the most
common result of birth trauma. Anoxia – lack of sufficient oxygen to the brain
stemming from delayed breathing or other causes – is another type of birth trauma
that may damage the brain.
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| Ionizing Radiation |
| In recent years a good deal of scientific
attention has been focused on the damaging effects of ionizing radiation on sex cells and
other bodily cells and tissues. Radiation may act directly on the fertilized ovum or may
produce gene mutations in the sex cells of either or both parents, which, in turn, may
lead to defective offspring. Sources of harmful radiation were once limited primarily
to high-energy X rays used for diagnosis and therapy, but the list has grown to include
nuclear weapons testing and other radioactive materials to which people may be exposed.
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| Malnutrition and Other Biological Factors |
| Deficiencies in proteins and other essential nutrients
during early development can result in irreversible physical and mental damage. Protein
deficiencies in the mother’s diet, as well as in the baby’s diet after the
birth, have been pinpointed as particularly potent causes of lowered intelligence. A
limited number of cases of mental retardation are also associated with other biological
agents, such as brain tumors that either damage the brain tissue directly or lead to
increased cranial pressure and concomitant brain damage. In some instances of mental
retardation – particularly of severe
and profound types
– the causes are uncertain or unknown, although extensive brain pathology is evident.
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