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There is no known single cause of schizophrenia. Many diseases, such as
heart disease, result from an interplay of genetic, behavioral and other
factors, and this may be the case for schizophrenia as well.
Scientists do not yet understand all of the factors necessary to produce
schizophrenia, but all the tools of modern biomedical research are being
used to search for genes, critical moments in brain development, and other
factors that may lead to the illness.
Can It Be Inherited?
It has long been known that schizophrenia runs in families. People who have
a close relative with schizophrenia are more likely to develop the disorder
than are people who have no relatives with the illness. For example, a
monozygotic (identical) twin of a person with schizophrenia has the highest
risk -- 40 to 50 percent -- of developing the illness. A child whose parent
has schizophrenia has about a 10 percent chance. By comparison, the risk of
schizophrenia in the general population is about 1 percent.
Scientists are studying genetic factors in schizophrenia.
It appears likely
that multiple genes are involved in creating a predisposition to develop the
disorder. In addition, factors such as prenatal difficulties like
intrauterine starvation or viral infections, perinatal complications, and
various nonspecific stressors, seem to influence the development of
schizophrenia. However, it is not yet understood how the genetic
predisposition is transmitted, and it cannot yet be accurately predicted
whether a given person will or will not develop the disorder.
Several regions of the human genome are being investigated
to identify genes
that may confer susceptibility for schizophrenia. The strongest evidence to
date leads to chromosomes 13 and 6 but remains unconfirmed. Identification
of specific genes involved in the development of schizophrenia will provide
important clues into what goes wrong in the brain to produce and sustain the
illness and will guide the development of new and better treatments. To
learn more about the genetic basis for schizophrenia, the NIMH has
established a Schizophrenia Genetics Initiative that is gathering data from
a large number of families of people with the illness.
Is It Caused by a Chemical Defect in the Brain?
Basic knowledge about brain chemistry and its link to schizophrenia is
expanding rapidly. Neurotransmitters, substances that allow communication
between nerve cells, have long been thought to be involved in the
development of schizophrenia. It is likely, although not yet certain, that
the disorder is associated with some imbalance of the complex, interrelated
chemical systems of the brain, perhaps involving the neurotransmitters
dopamine and glutamate. This area of research is promising.
Is It Caused by a Physical Abnormality in the Brain?
There have been dramatic advances in neuroimaging technology that permit
scientists to study brain structure and function in living individuals. Many
studies of people with schizophrenia have found abnormalities in brain
structure (for example, enlargement of the fluid-filled cavities, called the
ventricles, in the interior of the brain, and decreased size of certain
brain regions) or function (for example, decreased metabolic activity in
certain brain regions).
It should be emphasized that these abnormalities are quite
subtle and are
not characteristic of all people with schizophrenia, nor do they occur only
in individuals with this illness. Microscopic studies of brain tissue after
death have also shown small changes in distribution or number of brain cells
in people with schizophrenia. It appears that many (but probably not all) of
these changes are present before an individual becomes ill, and
schizophrenia may be, in part, a disorder in development of the brain.
Developmental neurobiologists funded by the National Institute
of Mental
Health (NIMH) have found that schizophrenia may be a developmental disorder
resulting when neurons form inappropriate connections during fetal
development. These errors may lie dormant until puberty, when changes in the
brain that occur normally during this critical stage of maturation interact
adversely with the faulty connections. This research has spurred efforts to
identify prenatal factors that may have some bearing on the apparent
developmental abnormality.
In other studies, investigators using brain-imaging techniques
have found
evidence of early biochemical changes that may precede the onset of disease
symptoms, prompting examination of the neural circuits that are most likely
to be involved in producing those symptoms. Meanwhile, scientists working at
the molecular level are exploring the genetic basis for abnormalities in
brain development and in the neurotransmitter systems regulating brain
function.
Reprinted with permission:
National Institute of Mental Health
Date Published: February, 1999
Date Reviewed: September, 1999
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