Medical References
General Information on Down Syndrome
Incidence of Down Syndrome
Down syndrome affects people of all ages, races and
economic levels. It is the most frequently occurring
chromosomal abnormality, occurring once in approximately
every 800 to 1,000 live births. Over 350,000 people in the
United States alone have Down syndrome.
When was Down syndrome discovered?
For centuries, people with Down syndrome have been alluded
to in art, literature and science. It wasn't until the late 19th
century, however, that John Langdon Down, an English
physician, published an accurate description of a person with
Down syndrome. It was this scholarly work, published in 1866,
which earned Down the recognition as the "father" of the
syndrome. Although other people had previously recognized
the characteristics of the syndrome, it was Down who
described the condition as a distinct and separate entity.
Throughout the 20th century, advances in medicine and
science enabled researchers to investigate the characteristics
of people with Down syndrome. In 1959, the French physician,
Jerome Lejeune, identified Down syndrome as a
chromosomal anomaly. Instead of the usual 46 chromosomes
present in each cell, Lejeune observed 47 in the cells of
individuals with Down syndrome. It was later determined that
an extra partial or complete 21st chromosome results in the
characteristics associated with Down syndrome.
Why is Down syndrome referred to
as a "genetic
condition"?
The human body is made of cells; all cells contain a center,
called a nucleus, in which genetic material is stored. This
genetic material, known as genes, carries the codes
responsible for all our inherited characteristics.
Genes are grouped along rod-like structures called
chromosomes. Normally, the nucleus of each cell contains 23
pairs of chromosomes, half of which are inherited from each
parent.
In Down syndrome, however, the cells usually contain not 46,
but 47 chromosomes; with the extra chromosome being a
number 21. This excess genetic material, in the form of
additional genes along the 21st chromosome, results in Down
syndrome.
Because 95 percent of all cases of Down syndrome occur
because there are three copies of the 21st chromosome, it is
referred to as "trisomy 21."
Chromosomes may be studied by examining blood or tissue
cells. Individual chromosomes are identified, stained and
numbered from largest to smallest. The visual display of the
chromosomes is known as a karyotype.
What causes Down syndrome?
Down syndrome is usually caused by an error in cell division
called non-disjunction. However, two other types of
chromosomal abnormalities, mosaicism and translocation, are
also implicated in Down syndrome – although to a much lesser
extent. Regardless of the type of Down syndrome which a
person may have, all people with Down syndrome have an
extra, critical portion of the number 21 chromosome present in
all, or some, of their cells. This additional genetic material
alters the course of development and causes the
characteristics associated with the syndrome.
Nondisjunction is a faulty cell division which results in an
embryo with three number 21 chromosomes instead of two.
Prior to, or at, conception, a pair of number 21 chromosomes,
in either the sperm or the egg, fail to separate. As the embryo
develops, the extra chromosome is replicated in every cell of
the body. This faulty cell division is responsible for 95 percent
of all cases of Down syndrome.
Why nondisjunction occurs is currently unknown, although it
does seem to be related to advancing maternal age. Many
people are surprised to find out that 80 percent of children
born with Down syndrome are born to women under 35 years
of age. This is because younger women have higher fertility
rates. It does not contradict the fact that the incidence of births
of children with Down syndrome increases with the age of the
mother.
Although nondisjunction can be of paternal origin, this occurs
less frequently. Because this error in cell division is often
present in the egg prior to conception, and women are born
with their complete store of eggs, it has been postulated that
some environmental factors may be implicated in
nondisjunction. However, despite years of research, the cause
(or causes) of nondisjunction, is still unknown. There seems to
be no connection between any type of Down syndrome and
parents' activities before or during pregnancy.
Mosaicism occurs when nondisjunction of the 21st
chromosome takes place in one of the initial cell divisions
after fertilization. When this occurs, there is a mixture of two
types of cells, some containing 46 chromosomes and some
containing 47. Those cells with 47 chromosomes contain an
extra 21st chromosome. Because of the "mosaic" pattern of
the cells, the term mosaicism is used. Mosaicism is rare,
being responsible for only one to two percent of all cases of
Down syndrome.
Some research has shown that individuals with mosaic Down
syndrome are less affected than those with trisomy 21;
however, broad generalizations are not possible due to the
wide range of abilities that people with Down syndrome
possess.
Translocation is a different type of chromosomal problem and
occurs in only three to four percent of people with Down
syndrome. Translocation occurs when part of the number 21
chromosome breaks off during cell division and attaches to
another chromosome. While the total number of chromosomes
in the cells remains 46, the presence of an extra part of the
number 21 chromosome causes the features of Down
syndrome. As with nondisjunction trisomy 21, translocation
occurs either prior to or at conception.
Unlike nondisjunction, maternal age is not linked to the risk of
translocation. Most cases are sporadic, chance events.
However, in about one-third of cases, one parent is a carrier of
a translocated chromosome. For this reason, the risk of
recurrence for translocation is higher than that of
nondisjunction. Genetic counseling can be sought to
determine the origin of the translocation.
What is the chance of having a
second child with Down
syndrome?
In general, it is estimated that the risk of having a second child
with trisomy 21 or mosaic Down syndrome is about 1 in 100.
The risk is higher if one parent is a carrier of a translocated
cell.
Are any prenatal tests available
to detect Down
syndrome?
Yes. There are two types of procedures available to pregnant
women: screening tests and diagnostic tests. Screening tests
estimate the risk of the fetus having Down syndrome;
diagnostic tests tell whether or not the fetus actually has the
condition.
The most commonly used screening tests are the Triple
Screen and the Alpha-fetoprotein Plus. These tests measure
quantities of various substances in the blood
(alpha-fetoprotein, human chorionic gonadotropin and
unconjugated estriol) and together with the woman's age,
estimate her risk of having a child with Down syndrome. These
screening tests are typically offered between fifteen and twenty
weeks of gestation.
Screening tests are of limited value and are often performed in
conjunction with a detailed sonogram. These tests are only
able to accurately detect about sixty percent of fetuses with
Down syndrome. Many women who undergo these tests will
be given false-positive readings, and some women will be
given false-negative readings.
The procedures available for prenatal diagnosis of Down
syndrome are chorionic villus sampling (CVS), amniocentesis
and percutaneous umbilical blood sampling (PUBS). Each one
of these procedures carries a small risk of miscarriage as
tissue is extracted from the placenta or the umbilical cord to
examine the fetus' chromosomes. The procedures are about
98 to 99 percent accurate in the detection of Down syndrome.
Amniocentesis is usually performed between 12 and 20 weeks
of gestation, CVS between eight and 12 weeks and PUBS
after 20 weeks.
How is Down syndrome diagnosed
in a newborn?
The diagnosis of Down syndrome is usually suspected after
birth as a result of the baby's appearance. It is a particularly
difficult time, coupled with the natural stresses of childbirth.
Although there is no easy way to be informed, most families
agree that having the baby present, being together and being
told as soon as possible is the best way to proceed.
There are many physical characteristics which form the basis
for suspecting an infant has Down syndrome. Many of these
characteristics are found, to some extent, in the general
population of individuals who do not have Down syndrome.
Hence, if Down syndrome is suspected, a karyotype will be
performed to ascertain the diagnosis. Some infants with Down
syndrome have only a few of these traits, while others have
many. Among the most common traits are:
Muscle hypotonia, low muscle tone
Flat facial profile, a somewhat depressed nasal bridge
and a small nose
Oblique palpebral fissures, an upward slant to the eyes
Dysplastic ear, an abnormal shape of the ear
A single deep crease across the center of the palm
Hyperflexibility, an excessive ability to extend the joints
Dysplastic middle phalanx of the fifth finger, fifth finger
has one flexion furrow instead of two
Epicanthal folds, small skin folds on the inner corner of
the eyes
Excessive space between large and second toe
Enlargement of tongue in relationship to size of mouth
Do babies with Down syndrome have medical
problems?
Children with Down syndrome are at increased risk for certain
health problems. Congenital heart defects, increased
susceptibility to infection, respiratory problems, obstructed
digestive tracts and childhood leukemia occur with greater
frequency among children who have Down syndrome.
However, advances in medicine have rendered most of these
health problems treatable and the majority of people born with
Down syndrome today have a life expectancy of approximately
55 years.
Adults with Down syndrome are at increased risk for
Alzheimer's disease. Whereas approximately six percent of
the general population will develop the disease, the figure is
about 25 percent for people with Down syndrome. Many
individuals with Down syndrome have the changes in the brain
associated with Alzheimer's, but do not necessarily develop
the clinical disorder.
How does Down syndrome affect a
person's
development?
Most people with Down syndrome have some level of mental
retardation; however, the level usually falls into the mild to
moderate range and is not indicative of the many strengths
and talents that each individual possesses. Children with
Down syndrome learn to sit, walk, talk, play, toilet train and do
most other activities – only somewhat later than their peers
without Down syndrome. Because speech is often delayed,
careful attention should be paid to the child's hearing, as
retention of fluid in the inner ear is a very common cause of
hearing and speech difficulties.
Early intervention services, which begin shortly after birth, help
children with Down syndrome develop to their full potential.
Quality educational programs, along with a stimulating home
environment and good medical care enable people with Down
syndrome to become contributing members of their families
and communities.
People with Down syndrome are highly responsive to their
physical and social environment. Those who receive good
medical care and are included in the activities of the
community can be expected to adapt successfully – to attend
school, make friends, find work, participate in decisions which
affect them and make a positive contribution to society.
People with Down syndrome have the same emotions and
needs as their peers and deserve the same opportunities.
What does the future hold for people
with Down
syndrome?
People with Down syndrome are people first. They may have
some of the characteristics generally associated with this
condition, but they are overwhelmingly unique and must be
treated as individuals. Over the past few decades, beginning
with Section 504 of The Rehabilitation Act of 1973, continuing
with The Education for All Handicapped Children Act of 1975
and culminating with the passage of the Americans with
Disabilities Act in 1991, people with Down syndrome have
been granted equal protections under federal law.
Ensuring equal treatment and access to services is a struggle
that every family of a child with Down syndrome faces. Daily,
these individuals strive to accomplish the same goals as
everyone else: self-fulfillment, pride in their achievements,
inclusion in the activities of the community and the challenge of
reaching their full potential.
Daily, people with Down syndrome venture out into the
community: to schools, jobs and leisure activities. Some live
with family, some with friends and some independently. They
form ongoing interpersonal relationships and some may marry.
Women with Down syndrome are fertile and can have children.
The opportunities available to people with Down syndrome
today have never been greater. However, it is only through the
collective efforts of parents, professionals and concerned
citizens that acceptance is becoming widespread. It is the goal
of the National Down Syndrome Society to ensure that all
people with Down syndrome are provided the opportunity to
achieve their full potential in all aspects of community life.
Questions & Answers
Why should we be concerned about Down syndrome?
Chromosomal abnormalities are a widespread medical
problem, with Down syndrome being the most common
genetic condition. One in every 800 to 1,000 children is born
with Down syndrome.
More than 50 percent of miscarriages are caused by a
chromosomal abnormality. As many as 25 percent of all
miscarriages are caused by a trisomy, which is the presence
of three copies of a particular chromosome, rather than the
normal two.
The most common form of Down syndrome is called Trisomy
21, because it involves an extra copy of the 21st chromosome.
Twenty to 25 percent of children conceived with Down
syndrome survive past birth.
What impact does Down syndrome
have on society?
Individuals with Down syndrome are becoming increasingly
integrated into society and community organizations, such as
school, health care systems, work forces and social and
recreational activities. Individuals with Down syndrome
possess varying degrees of mental retardation, from very mild
to severe. Most people with Down syndrome have IQs in the
mild to moderate range of mental retardation.
Due to advances in medical technology, individuals with Down
syndrome are living longer than ever before. In 1910, children
with Down syndrome were expected to survive to age nine.
With the discovery of antibiotics, the average survival age
increased to 19 or 20. Now, with recent advancements in
clinical treatment, as many as 80 percent of adults with Down
syndrome reach age 55, and many live even longer.
In the United States, approximately 350,000 families are
affected by Down syndrome. Approximately 5,000 children
with Down syndrome are born each year. As the mortality rate
associated with Down syndrome is decreasing, the
prevalence of individuals with Down syndrome in our society
will increase. Some experts project that the number of people
with Down syndrome will double in the next 10 years. More and
more Americans will interact with individuals with this genetic
condition, increasing the need for widespread public
education and acceptance.
Is Down syndrome transmitted from
the mother or
father?
The additional genetic material which causes Down syndrome
can originate from either the father or the mother.
Approximately five percent of the cases have been traced to
the father.
Who has the highest risk of having
a child with Down
syndrome?
Down syndrome affects people of all races and economic
levels. Women age 35 and older have a significantly increased
risk of having a child with Down syndrome. A 35-year-old
woman has a one in 400 chance of conceiving a child with
Down syndrome and this chance increases gradually to one in
110 by age 40. At age 45 the incidence becomes
approximately one in 35.
Since many couples are postponing parenting until later in life,
the incidence of Down syndrome conceptions is expected to
increase. Therefore, genetic counseling for parents is
becoming increasingly important. Still, many physicians are not
fully informed about advising their patients about the of
incidence of Down syndrome, advancements in diagnosis and
the protocols for care and treatment of babies born with Down
syndrome.
Why is it important to raise children
with Down
syndrome at home?
A greater understanding of Down syndrome and
advancements in treatment of Down syndrome-related health
problems have allowed people with Down syndrome to enjoy
fuller and more active lives.
Children raised at home and included in all aspects of
community life can best reach their potential and function in
society with a greater degree of independence. Parental love,
nurturing and support, as well as early intervention programs,
educational opportunities and community involvement, have a
direct relationship to the degree that a person with Down
syndrome is able to achieve his/her potential.
Why are medical researchers following
Down syndrome
work so closely?
Down syndrome is a developmental disorder. As researchers
learn more about the molecular genetics and other aspects of
Down syndrome, they also obtain valuable information about
human development and can advance the study of many
biological processes.
In addition, individuals with Down syndrome have a higher
incidence of certain medical problems and the study of Down
syndrome may yield important breakthroughs in those areas.
Research in Down syndrome provides a way for looking at
many important problems:
Heart disease: Up to 50 percent of individuals with
Down syndrome are born with congenital heart defects.
The majority of heart defects in children with Down
syndrome can now be surgically corrected with resulting
long-term health improvements. However, scientists
continue to search for the cause of this problem and
look for means of prevention.
Alzheimer's disease: Estimates vary, but it is
reasonable to conclude that 25 percent or more of
individuals with Down syndrome over the age of 35 will
develop the clinical signs and symptoms of
Alzheimer's-type dementia.
Leukemia: Individuals with Down syndrome have a 15 to
20 times greater risk of developing leukemia. The
majority of cases are categorized as acute
megakaryoblastic leukemia, which tends to occur in the
first three years of life, and for which there is a high cure
rate. A transient form of leukemia is also seen in
newborns with Down syndrome, disappearing
spontaneously during the first two to three months of life.
Why hasn't Down syndrome received
much attention in
the past?
Even though Dr. Jerome Lejeune discovered in 1959 that it
was an extra 21st chromosome that caused Down syndrome,
it is only in the last few years that a focus has been placed on
the study of the 21st chromosome.
In May 2000, researchers completed sequencing the
approximately 225 genes on the 21st chromosome - only the
second chromosome to be fully sequenced at this time.
Although these findings will not have an immediate impact on
the Down syndrome community, they will open the door to
valuable research on this small set of genes.
Researchers continue to look for the genes related to the
development of intelligence and the physical characteristics
associated with Down syndrome. Once identified, it is hoped
that the biochemical process which causes Down syndrome
can be decoded, leading to the development of an intervention
and cure. See the Research section of this Web site for more
information about sequencing the 21st chromosome.
What is the National Down Syndrome
Society doing to
further research?
NDSS is currently sponsoring three scientists through the
NDSS Science Scholar Award Program initiated in 1983.
Under this grant program, each researcher receives $35,000 a
year for two years to carry out scientific research into the
causes and/or amelioration of Down syndrome. To date, more
than 20 scholars have been supported by this program.
NDSS also sponsors a series of annual symposia focusing on
Down syndrome research. This sharing of information,
technology and the collective knowledge of the world's
foremost researchers is expected to produce an acceleration
of advancements in a variety of areas. The proceedings of
these scientific symposia are published annually and are
available through NDSS.
Myths and Truths
Myth: Down syndrome is a rare genetic disorder.
Truth: Down syndrome is the most commonly occurring
genetic condition. One in every 800 to 1,000 live births is a
child with Down syndrome, representing approximately 5,000
births per year in the United States alone. Today, Down
syndrome affects more than 350,000 people in the United
States.
Myth: Most children with Down syndrome are born to
older parents.
Truth: Eighty percent of children born with Down syndrome
are born to women younger than 35-years-old. However, the
incidence of births of children with Down syndrome increases
with the age of the mother.
Myth: People with Down syndrome are severely
retarded.
Truth: Most people with Down syndrome have IQs that fall in
the mild to moderate range of retardation. Children with Down
syndrome are definitely educable and educators and
researchers are still discovering the full educational potential of
people with Down syndrome.
Myth: Most people with Down syndrome are
institutionalized.
Truth: Today people with Down syndrome live at home with
their families and are active participants in the educational,
vocational, social and recreational activities of the community.
They are integrated into the regular education system, and
take part in sports, camping, music, art programs and all the
other activities of their communities. In addition, they are
socializing with people with and without disabilities, and as
adults are obtaining employment and living in group homes
and other independent housing arrangements.
Myth: Parents will not find community support
in
bringing up their child with Down syndrome.
Truth: In almost every community of the United States there
are parent support groups and other community organizations
directly involved in providing services to families of individuals
with Down syndrome.
Myth: Children with Down syndrome must be placed
in
segregated special education programs.
Truth: Children with Down syndrome have been included in
regular academic classrooms in schools across the country. In
some instances they are integrated into specific courses, while
in other situations students are fully included in the regular
classroom for all subjects. The degree of mainstreaming is
based in the abilities of the individual; but the trend is for full
inclusion in the social and educational life of the community.
Myth: Adults with Down syndrome are unemployable.
Truth: Businesses are seeking young adults with Down
syndrome for a variety of positions. They are being employed
in small and medium sized offices: by banks, corporations,
nursing homes, hotels and restaurants. They work in the music
and entertainment industry, in clerical positions and in the
computer industry. People with Down syndrome bring to their
jobs enthusiasm, reliability and dedication.
Myth: People with Down syndrome are always happy.
Truth: People with Down syndrome have feelings just like
everyone else in the population. They respond to positive
expressions of friendship and they are hurt and upset by
inconsiderate behavior.
Myth: Adults with Down syndrome are unable to form
close interpersonal relationships leading to marriage.
Truth: People with Down syndrome date, socialize and form
ongoing relationships. Some are beginning to marry. Women
with Down syndrome can and do have children, but there is a
50 percent chance that their child will have Down syndrome.
Men with Down syndrome are believed to be sterile, with only
one documented instance of a male with Down syndrome who
has fathered a child.
Myth: Down syndrome can never be cured.
Truth: Research on Down syndrome is making great strides
in identifying the genes on chromosome 21 that cause the
characteristics of Down syndrome. Scientists now feel
strongly that it will be possible to improve, correct or prevent
many of the problems associated with Down syndrome in the
future.