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CHROMOSOMAL
DISORDERS
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Chromosome abnormality
Chromosome abnormality
◦ A chromosome anomaly, abnormality, aberration, or mutation is a missing, extra, or
irregular portion of chromosomal DNA. It can be from an atypical number of
chromosomes or a structural abnormality in one or more chromosomes.
◦ A karyotype refers to a full set of chromosomes from an individual which can be
compared to a "normal" karyotype for the species via genetic testing.
◦ A chromosome anomaly may be detected or confirmed in this manner. Chromosome
anomalies usually occur when there is an error in celldivision following meiosis or mitosis.
◦ There are many types of chromosome anomalies. They can be organized into two
basic groups, numerical and structural anomalies
Numerical disorders
◦ This is called aneuploidy (an abnormal number of chromosomes), and occurs when an
individual is missing either a chromosome from a pair (monosomy) or has more than
two chromosomes of a pair (trisomy, tetrasomy, etc.).
◦ In humans an example of a condition caused by a numerical anomaly is Down
Syndrome, also known as Trisomy 21 (an individual with Down Syndrome has three
copies of chromosome 21, rather than two). Trisomy has been determined to be a
function of maternal age.
◦ An example of monosomy is Turner Syndrome, where the individual is born with only
one sex chromosome, an X.
Structural abnormalities
◦ Deletions: A portion of the chromosome is missing or deleted. Known disorders in
humans include Wolf-Hirschhorn syndrome, which is caused by partial deletion of the
short arm of chromosome 4; and Jacobsen syndrome, also called the terminal 11q
deletion disorder.
◦ Duplications: A portion of the chromosome is duplicated, resulting in extra genetic
material. Known human disorders include Charcot-Marie-Tooth disease type 1A which
may be caused by duplication of the gene encoding peripheral myelin protein
22 (PMP22) on chromosome 17.
Structural abnormalities
◦ Translocations: A portion of one chromosome is transferred to another chromosome.
There are two main types of translocations:
◦ Reciprocal translocation: Segments from two different chromosomes have been exchanged.
◦ Robertsonian translocation: An entire chromosome has attached to another at the centromere
- in humans these only occur with chromosomes 13, 14, 15, 21 and 22.
◦ Inversions: A portion of the chromosome has broken off, turned upside down and
reattached, therefore the genetic material is inverted.
◦ Insertions: A portion of one chromosome has been deleted from its normal place and
inserted into another chromosome.
Structural abnormalities
◦ Rings: A portion of a chromosome has broken off and formed a circle or ring. This can
happen with or without loss of genetic material.
◦ Isochromosome: Formed by the mirror image copy of a chromosome segment
including the centromere.
◦ Chromosome instability syndromes are a group of disorders characterized by
chromosomal instability and breakage. They often lead to an increased tendency to
develop certain types of malignancies.
Inheritance
◦ Most chromosome abnormalities occur as an accident in the egg or sperm, and
therefore the anomaly is present in every cell of the body. Some anomalies, however,
can happen after conception, resulting in Mosaicism (where some cells have the
anomaly and some do not).
◦ Chromosome anomalies can be inherited from a parent or be "de novo". This is why
chromosome studies are often performed on parents when a child is found to have an
anomaly.
◦ If the parents do not possess the abnormality it was not initially inherited; however it
may be transmitted to subsequent generations.
Cri du chat
Cri du chat
◦ Cri du chat syndrome, also known as chromosome 5p deletion syndrome, 5p- (said
minus) syndrome or Lejeune’s syndrome, is a rare genetic disorder due to a missing part
(deletion) of chromosome 5.
◦ Its name is a French term (cat-cry or call of the cat) referring to the characteristic catlike cry of affected children. It was first described by Jérôme Lejeune in 1963.
◦ The condition affects an estimated 1 in 50,000 live births, strikes all ethnicities, and is
more common in females by a 4:3 ratio.
Signs and symptoms
◦ The syndrome gets its name from the characteristic cry of affected infants, which is
similar to that of a meowing kitten, due to problems with the larynx and nervous system.
About 1/3 of children lose the cry by age 2. Other symptoms of cri du chat syndrome
may include:
Signs and symptoms
◦ feeding problems because of difficulty swallowing and sucking;
◦ low birth weight and poor growth;
◦ severe cognitive, speech, and motor delays;
◦ behavioral problems such as hyperactivity, aggression, tantrums, and repetitive
movements;
◦ unusual facial features which may change over time;
Signs and symptoms
◦ excessive drooling;
◦ small head and jaw;
◦ wide eyes;
◦ skin tags in front of eyes.
Genetics
◦ Cri du chat syndrome is due to a partial deletion
of chromosome number 5, also called "5p monosomy".
of
the
short
arm
◦ Approximately 90% of cases result from a sporadic, or randomly occurring, de
novo deletion. The remaining 10-15% are due to unequal segregation of a
parental balanced translocation where the 5p monosomy is often accompanied by a
trisomic portion of the genome.
◦ These individuals may have more severe disease than those with isolated monosomy of
5p. A recent study suggests this may not be the case where a trisomy of chromosome
4q is involved.
Genetics
◦ Most cases involve total loss of the most distant 10-20% of the material on the short arm.
Fewer than 10% of cases have other rare cytogenetic aberrations (e.g., interstitial
deletions,mosaicisms, rings and de novo translocations). The deleted chromosome 5 is
paternal in origin in about 80% of de novo cases.
◦ Loss of a small region in band 5p15.2 (cri du chat critical region) correlates with all the
clinical features of the syndrome with the exception of the catlike cry, which maps to
band 5p15.3 (catlike critical region). The results suggest that 2 noncontiguous critical
regions contain genes involved in this condition's etiology. Two genes in these
regions, Semaphorine F (SEMA5A) and delta catenin (CTNND2), are potentially
involved in cerebral development.
◦ The deletion of the telomerase reverse transcriptase (hTERT) gene localized in 5p15.33
may contribute to the phenotypic changes in cri du chat syndrome as well.
Diagnosis and management
◦ Diagnosis is based on the distinctive cry and accompanying physical problems. Seeing
as these symptoms are quite easily observable, affected children are typically
diagnosed by a doctor or nurse at birth.
◦ Genetic counseling and genetic testing may be offered to families with individuals who
have cri du chat syndrome. Prenatally the deletion of the cri du chat related region in
the p arm of chromosome 5 can be detected from amniotic fluid or chorionic villi
samples with BACs-on-Beads technology.G-banded karyotype of a carrier is also useful.
◦ Children may be treated by speech, sound, and occupational therapists. Cardiac
abnormalities often require surgical correction.
Down syndrome
◦ Down syndrome (DS) or Down's syndrome, also known as trisomy 21, is a genetic
disorder caused by the presence of all or part of a third copy ofchromosome 21.
◦ It is typically associated with physical growth delays, characteristic facial features and
mild to moderate intellectual disability.
◦ The average IQ of a young adult with Down syndrome is around 50, similar to the
mental age of an 8 or 9 year old child.[3]
Down syndrome
◦ Down syndrome can be identified during pregnancy by prenatal screening or after
birth by direct observation and genetic testing. Since the introduction of screening,
pregnancies with the diagnosis are often terminated.[4][5] Regular screening for health
problems common in Down syndrome is recommended throughout the person's life.
Down syndrome
◦ Education and proper care has been shown to improve quality of life.
◦ Some children with Down syndrome are educated in regular school classes while
others require more specialized education.
◦ Some children with Down syndrome graduate from high school and in adulthood some
individuals work in the community.
◦ The degree of independence possible for an affected individual varies, and some
require a more sheltered work environment.
◦ Support in financial and legal matters is often needed.
◦ Life expectancy is around 50 to 60 years in the developed world with proper health
care.
Down syndrome
◦ Down syndrome is the most common chromosome abnormality in humans occurring in
about 1 per 1000 babies born each year.
◦ It is named afterJohn Langdon Down, the British doctor who fully described the
syndrome in 1866.
◦ Some aspects of the condition were described earlier by Jean-Étienne Dominique
Esquirol in 1838 and Édouard Séguin in 1844.[11] The genetic cause of Down syndrome,
an extra copy of chromosome 21, was identified by Dr. Jérôme Lejeune in 1959.[10]
Signs and symptoms
◦ Those with Down syndrome nearly always have physical and mental disabilities.
◦ As adults their mental abilities are typically similar to that of an 8 or 9 year old.
◦ They also typically have poor immune function and developmental
milestones generally are reached at a later age.
◦ There is an increased risk of a number of other health problems including: congenital
heart disease, leukemia, thyroid disorders, and mental illness, among others.
Signs and symptoms
Characteristics
Percentage
Characteristics
Percentage
Stunted growth
90%[14]
Flattened nose
68%[13]
Mental impairment
99%[15]
Abnormal teeth
60%[16]
Increased skin back of neck
80%[10]
Bent fifth finger tip
57%[13]
Signs and symptoms
Separation of 1st and 2nd toes
68%[16]
Umbilical hernia
90%[17]
Flexible ligaments
75%[13]
Short neck
60%[16]
Low muscle tone
80%[18]
Shortened hands
60%[16]
Flat head
75%[13]
Congenital heart disease
40%[16]
Signs and symptoms
Undescended testicles
20%[19]
Single transverse palmar crease
53%[13]
Eyelid fold
60%[16]
Protruding tongue
47%[16]
Shortened arms and legs
70%
Epicanthic fold
59%[13]
Narrow roof of mouth
76%[16]
Strabismus
~35%[2]
Abnormal outer ears
70%[10]
Brushfield spots in the iris
56%[13]
Physical
◦ They may have some or all of the following physical characteristics: abnormally small
chin, slanted eyes, poor muscle tone, a flat nasal bridge, a single crease of the palm, a
protruding tongue due to small mouth, and an enlarged tongue.
◦ Other common features include: a flat and wide face,there is a short neck, excessive
joint flexibility, extra space between big toe and second toe, an abnormal patterns on
the fingertops and short fingers.
◦ Instability of the atlanto-axial joint occurs in approximately 20% and may lead to spinal
cord injury in 1-2%.
◦ Around half of those with DS haveobstructive sleep apnea.[10] Hip dislocations occur
without trauma in an up to a third.
Physical
◦ Growth in height is slower resulting in adults tending to have short stature—the average
height for men is 154 cm (5 feet 1 inch) and for women is 142 cm (4 feet
8 inches).[20] Individuals with DS are at increased risk for obesity as they age.[10] There
are growth charts specifically for those with Down syndrome.[10]
Neurological
◦ Most individuals with Down syndrome have mild (IQ: 50–70) or moderate intellectual
disability (IQ: 35–50) with some cases having severe (IQ: 20–35) difficulties.
◦ As they age they typically perform less well compared to their same aged peers.
◦ Some after 30 years of age may lose their ability to speak.
◦ Those with mosaic Down syndrome typically have scores 10–30 points higher. This
syndrome causes about a third of cases of intellectual disability.
Neurological
◦ Fine motor skills[24] and large scale motor skills are often delayed which can interfere with
cognitive development. Effects of the condition gross motor skills is variable. Some children
will begin walking at around 2 years of age, while others will not walk until age four.
◦ Commonly individuals with Down syndrome have some difficulty speaking with better
language understanding.]
◦ 10 to 45% have either stuttering or rapid and irregular speech making them difficult to
understand.
◦ They typically do fairly well with social skills.
◦ Behavior problems are not geerally as great as an issue as in other syndromes associated
with intellectual disability.[
◦ In children with Down syndrome mental illness occurs in nearly 30% with autism occurring in
5-10%.
◦ While generally happy, symptoms of depression and anxiety may develop in early
adulthood.
Neurological
◦ Commonly individuals with Down syndrome have some difficulty speaking with better
language understanding.10 to 45% have either stuttering or rapid and irregular
speech making them difficult to understand.
◦ They typically do fairly well with social skills.
◦ Behavior problems are not generally as great as an issue as in other syndromes
associated with intellectual disability.
◦ In children with Down syndrome mental illness occurs in nearly 30%
with autism occurring in 5-10%.[9] While generally happy, symptoms
of depression and anxiety may develop in early adulthood.[3]
Neurological
◦ Children and adults with DS are at increased risk of epileptic seizures which occur in 510% of children and up to 50% of adults.
◦ This includes an increased risk of a specific type of seizure called infantile spasms.
◦ Many (15%) who live past 40s develop dementia of the Alzheimer's disease type.[Of
those who reach 60 years, 50-70% have the disease.[3]
Senses
Brushfield spots, visible in the irises of a baby with Down
Syndrome.
Senses
◦ Hearing and vision disorders occur in more than half of people with DS.
◦ Vision problems occur in 38 to 80%. Between 20 and 50% have strabismus, in which the
two eyes do not move in tandem.
◦ Refractive errors requiring glasses or contacts are also common.
◦ Cataracts (opacity of the lens) occur in 15%, and may be present at birth.
◦ Keratoconus (thin, cone-shaped corneas), and glaucoma (increased eye pressures)
are also more common.
◦ Brushfield spots (small white or grayish/brown spots on the periphery of the iris) are
present in 38 to 85%.
Senses
◦ Hearing problems are found in 38-78% of children with Down syndrome compared to
2.5% of normal children.
◦ Diagnosis and aggressive treatment of chronic ear disease (e.g.otitis media) in
children with Down syndrome can bring many of the children up to normal hearing
levels.
◦ Age related hearing loss of the sensorineural type occurs at a much earlier age
affected 10-70%.[3]
Senses
◦ Otitis media with effusion is the most common cause of hearing loss in children with
Down's occurring in 50-70%.
◦ Ear infections often start at birth and continue throughout the children's life.[28]The ear
infections are mainly due to poor eustachian tube function.
◦ However, excessive wax can also cause obstruction of the outer ear canal and
hearing problems. Middle ear problems account for 83% of hearing loss in children with
Down syndrome.
◦ The degree of hearing loss varies but even a mild degree can have major
consequences for speech understanding, language learning, and academics[2] if not
detected in time and corrected.
◦ It is important to rule out hearing loss as a contributing factor in social and mental
deterioration.
Heart
◦ The rate of congenital heart disease in newborns with Down syndrome is around 40%.
◦ An atrioventricular septal defect also known as endocardial cushion defect is the most
common form with up to 40% affected. This is closely followed by ventricular septal
defect that affects approximately 35%
◦ Mitral valve problems become common as people age, even in those without heart
problems at birth.
◦ Other problems that may occur include: tetralogy of Fallot and patent ductus
arteriosus.
◦ People with Down syndrome have a lower risk of hardening of the arteries.
Cancer
◦ Although the general incidence of cancer amongst individuals with Down syndrome is
the same as in the general population,there is a reduced risk of solid cancers and an
increased risk of leukemia and testicular cancer. Solid cancers are believed to be less
common due to the tumor suppressor genes present on chromosome 21.
Cancer
◦ Cancers of the blood are 10 to 15 times more common in children with DS.
◦ In particular, acute lymphoblastic leukemia is 20 times more common and
the megakaryoblastic form of acute myelogenous leukemia is 500 times more
common.
◦ Transient myeloproliferative disease, a disorder of blood cell production that does not
occur outside of Down syndrome, affects 3-10% of infants.The disorder is typically not
serious but occasionally can be.
◦ It resolves most times without treatment; however, in those who have had it there is a
20 to 30 percent risk of developing acute lymphoblastic leukemia at a latter time
Endocrine
◦ Problems of the thyroid gland occur in 20-50%.
◦ Low thyroid is the most common, occurring in almost half of those with DS. Thyroid
problems can be due to a poorly or non functioning thyroid at birth (known
as congenital hypothyroidism) which occurs in 1%or can develop latter due to an
attack on the thyroid by the immune system resulting in Graves disease or autoimmune
hypothyroidism.
◦ Type 1 diabetes mellitus is also more common.
Gastrointestinal
◦ Constipation occurs in nearly half of people with DS and may result in changes in
behavior.
◦ One potential cause is Hirschsprung's disease, which is due to a lack of nerve cells
controlling thecolon, which occurs in 2 to 15%.
◦ Other frequent congenital problems include: duodenal atresia, pyloric stenosis, Meckel
diverticulum and imperforate anus.
◦ Celiac disease affects about 7-20% and gastroesophageal reflux disease is also more
common
Fertility
◦ Males with Down syndrome usually do not father children, while females have lower
rates of fertility relative those who are unaffected.
◦ Fertility is estimated to be present in 30-50% of women and they often have difficulties
with miscarriages, premature births, and labor.
◦ Menopause typically occurs at an earlier age.[3] The poor fertility in men is thought to
be due to problems with sperm development; however, it may also be related to not
being sexually active.
◦ As of 2006 there have been three recorded instances of males with DS fathering
children and 26 cases of women having children.Without assisted reproductive
technologies, approximately half of the pregnancies of someone with Down syndrome
will also have the syndrome.
Genetics
◦ Down syndrome is caused by having three copies of the genes on chromosome 21,
rather than the usual two.
◦ The parents of the affected individual are typically genetically normal.[ Those who
have one child with Down syndrome have about a 1% risk of having a second child
with the syndrome, if both parents are found to have normal karyotypes.
Genetics
◦ The extra chromosome content can arise through several different mechanisms. The
most common cause (approximately 92-95% of cases) is a complete extra copy of
chromosome 21, resulting in trisomy 21.
◦ In 1 to 2.5%, some of the cells in the body are normal and others have trisomy 21,
known as mosaic Down syndrome.[41][45] The other common mechanisms that can give
rise to Down syndrome include Robertsonian translocation, isochromosomes, ring
chromosomee, which contain additional material from chromosome 21. These findings
occurs in approximately 2.5% of cases.
◦ Isochromosomes result when the two long arms separate together on one
chromosome.
Karyotype
Karyotype
◦ Trisomy 21 (also known by the karyotype 47,XX,+21 for males and 47,XY,+21 for
females) is caused by a failure of the chromosomes to separate during egg or sperm
development.
◦ As a result, a sperm or egg cell is produced with an extra copy of chromosome 21; this
cell thus has 24 chromosomes.
◦ When combined with a normal cell from the other parent, the embryo and baby has
47 chromosomes, with three copies of chromosome 21. Trisomy 21 is the cause of
approximately 92 to 95% of cases of Down syndrome, with 88% of cases resulting from
non separation of the chromosomes in the mother and 8% coming from non separation
in the father.
Translocation
◦ The extra chromosome 21 material may also occur due to a Robertsonian translocation in 24% cases.This may be a new mutation or previously present in one of the parents.
◦ In this case, the long arm of chromosome 21 is attached to another chromosome,
often chromosome 14 known as 45XY,t(14q21q) in males. The risk of this type of Down
syndrome is not related to the mothers age.The parent with such a translocation is usually
normal physically and mentally
◦ however, during production of egg or sperm cells there is a higher chance of creating
reproductive cells with extra chromosome 21 material.This results in a 15% chance of having
a child with DS when the mother is affected and a less than 5% risk if the father is affected.[
◦ Additionally some children may inherit this translocation while not have DS but are
subsequently at higher risk of having children with DS themselves.
◦ In this case it is sometimes known as familial Down syndrome.
Mechanism
◦ In general, extra chromosome 21 DNA leads to an over-expression of certain
genes. This over expression is about 50%.
◦ It is estimated that chromosome 21 contains around 310 genes.]\
◦ Some research has shown that the parts of the chromosome which are of greatest
importance are bands 21q22.1-q22.3.
◦ This area includes genes for amyloid, superoxide dismutase, and likely the ETS-2 proto
oncogene.
◦ Other research has not confirmed these findings.
Mechanism
◦ The dementia which occurs in Down syndrome is due to too much amyloid beta
peptides being produced in the brain.
◦ Senile plaques and neurofibrillary tangles are present in nearly all by 35 years of age
even though dementia may not be present.
◦ Those with DS lack a normal number of lymphocytes and produce
less antibodies which contributes to there increased risk of infection.[10]
Screening
◦ Guidelines recommend that screening for Down syndrome be offered to all pregnant
women, regardless of age. A number of tests can be used, with varying levels of
accuracy. They are usually used in combination to increase their detection rate, while
maintaining a low false positive rate, but are not definitive.[10]
◦ If screening is positive either amniocentesis or chorionic villous sampling is required to
confirm the diagnosis.
◦ Screening in both the first and second trimesters is better than just screening in the first
trimester.
◦ The different screening techniques in use are able to pick up 90 to 95% of cases with a
false positive rate of between 2 and 5%.
Ultrasound
Ultrasound
◦ Enlarged NT and absent nasal bone in a fetus at 11 weeks with Down syndrome
◦ Ultrasound imaging can be used to screen for Down syndrome. Finding that increase
the risk, when seen at 14 to 24 weeks of gestation include: a small or no nasal
bone, large ventricles, nuchal fold thickness, and an abnormal right subclavian
artery among others. The presence or absence of many markers is more accurate.
◦ Increased fetal nuchal translucency (NT) indicates an increased risk of Down syndrome
picking up 75-80% of cases and being falsely positive in 6%.
Blood tests
◦ Several blood markers can be measured that to predict the risk of Down syndrome
during the second trimester.[61] Often two or three or used in combination with two or
three of: α-fetoprotein, unconjugated estriol, total hCG, and free βhCG detecting
about 60-70% of cases.[61] First trimester screening with markers is not as accurate and
thus is not generally recommended by itself.
◦ Testing of the mother's blood for fetal DNA is being studied and appears promising in
the first trimester.
Blood tests
◦ The International Society for Prenatal Diagnosis considers it a reasonable screening
option for those women whose pregnancies are at a high risk for trisomy 21.
◦ Accuracy has been reported at 98.6% in the first trimester of pregnancy.
◦ Confirmatory testing by invasive techniques (amniocentesis, CVS) is still required to
confirm the screening result.
Diagnosis
◦ When screening tests predict a high risk of Down syndrome, a more invasive diagnostic
test (amniocentesis or chorionic villus sampling) is needed to confirm the diagnosis.[56] If
Down syndrome occurs in 1 in 500 pregnancies and the test used has a 5% false
positive rate, this means that of 28 women who test positive on screening only 1 will
have Down syndrome confirmed.
◦ If the screening test has a 2% false positive rate this improves to 1 out of 10 who test
positive on screening having a fetus with DS.
◦ Amniocentesis and chorionic villus sampling are more reliable; however, carry an
increased risk of miscarriage of between 0.5 and 1%.
◦ There is also an increased risk of limb problems in the offspring due to the procedure.
◦ The risk from the procedure is greater the earlier it is preformed and thus amniocentesis
is not recommended before 15 weeks gestational age and chorionic villus sampling
before 10 weeks gestational age]
Abortion rates
◦ About 92% of pregnancies in the United Kingdom and Europe with a diagnosis of Down
syndrome are terminated.
◦ In the United States termination rates are around 67%; however this varies significantly
depending upon the population looked at.
◦ When non pregnant people are asked if they would have a termination if their fetus
tested positive 23-33% said yes, when high risk pregnant women were asked 46-86%
said yes, and when women who screen positive are asked 89-97% say yes.[
After birth
◦ The diagnosis can typically be made based on the appearance of the child at birth.
◦ An analysis of the child's chromosomes is recommended to confirm the diagnosis and
determine if a translocation is present as if it this may help determine the risk of the
child's parents having further children with DS.
◦ Parents generally wish to know the possible diagnosis once it is suspected and do not
wish pity.
Management
◦ Efforts such as early childhood intervention, screening for common problems, medical
treatment where indicated, a good family environment, and work related training can
improve the development of children with Down syndrome.
◦ Education and proper care can improve quality of life.
◦ Typical vaccinations are recommended.
Medications
◦ Efforts to prevent respiratory syncytial virus (RSV) with human monoclonal
antibodies should be considered, especially in those with heart problems.[2] In those
who develop dementia there is no evidence for memantine, donepezil,
rivastigmine, or galantamine.
Edwards syndrome
Edwards syndrome
◦ Edwards syndrome (also known as Trisomy 18 [T18]) is a genetic disorder caused by the
presence of all or part of an extra 18th chromosome. This genetic condition almost
always results from nondisjunction during meiosis. It is named after John Hilton Edwards,
who first described the syndrome in 1960.
◦ It is the second most common autosomal trisomy, after Down syndrome, that carries to
term.
Edwards syndrome
◦ Edwards syndrome occurs in around one in 6,000 live births and around 80 percent of
those affected are female. The majority of fetuses with the syndrome die before birth.
◦ The incidence increases as the mother's age increases. The syndrome has a very low
rate of survival, resulting from heart abnormalities, kidney malformations, and other
internal organ disorders.
Signs and symptoms
◦ Children born with Edwards syndrome may have some or all of the following
characteristics: kidney malformations, structural heart defects at birth (i.e., ventricular
septal defect, atrial septal defect, patent ductus arteriosus), intestines protruding
outside the body (omphalocele), esophageal atresia, intellectual disability,
developmental delays, growth deficiency, feeding difficulties,breathing difficulties,
and arthrogryposis (a muscle disorder that causes multiple joint contractures at birth).
Signs and symptoms
◦ Some physical malformations associated with Edwards syndrome include small head
(microcephaly) accompanied by a prominent back portion of the head (occiput);
low-set, malformed ears; abnormally small jaw (micrognathia); cleft lip/cleft palate;
upturned nose; narrow eyelid folds (palpebral fissures); widely spaced eyes (ocular
hypertelorism); drooping of the upper eyelids (ptosis); a short breast bone; clenched
hands; choroid plexus cysts; underdeveloped thumbs and or nails, absent
radius, webbing of the second and third toes; clubfoot or Rocker bottom feet; and
inmales, undescended testicles.[3][4]
Genetics
◦ Edwards syndrome is a chromosomal abnormality characterized by the presence of an
extra copy of genetic material on the 18th chromosome, either in whole (trisomy 18) or
in part (such as due to translocations). The additional chromosome usually occurs
before conception. The effects of the extra copy vary greatly, depending on the
extent of the extra copy, genetic history, and chance. Edwards syndrome occurs in all
human populations but is more prevalent in female offspring.[7]
Genetics
◦ A healthy egg and/or sperm cell contains individual chromosomes, each of which
contributes to the 23 pairs of chromosomes needed to form a normal cell with a typical
human karyotype of 46 chromosomes. Numerical errors can arise at either of the
two meiotic divisions and cause the failure of a chromosome to segregate into the
daughter cells (nondisjunction). This results in an extra chromosome, making
the haploid number 24 rather than 23. Fertilization of eggs or insemination by sperm
that contain an extra chromosome results in trisomy, or three copies of a chromosome
rather than two.[8]
Genetics
◦ Trisomy 18 (47,XX,+18) is caused by a meiotic nondisjunction event. With nondisjunction,
a gamete (i.e., a sperm or egg cell) is produced with an extra copy of chromosome 18;
the gamete thus has 24 chromosomes. When combined with a normal gamete from
the other parent, the embryo has 47 chromosomes, with three copies of chromosome
18.
Genetics
◦ A small percentage of cases occur when only some of the body's cells have an extra
copy of chromosome 18, resulting in a mixed population of cells with a differing
number of chromosomes.
◦ Such cases are sometimes called mosaic Edwards syndrome. Very rarely, a piece of
chromosome 18 becomes attached to another chromosome (translocated) before or
after conception.
◦ Affected individuals have two copies of chromosome 18 plus extra material from
chromosome 18 attached to another chromosome. With a translocation, a person has
a partial trisomy for chromosome 18, and the abnormalities are often less severe than
for the typical Edwards syndrome.
Prognosis
◦ In 2008/2009, there were 495 diagnoses of Edwards syndrome (trisomy 18)
in England and Wales, 92% of which were made prenatally. There were 339 abortions,
49 stillbirths/miscarriages/fetal deaths, 72 unknown outcomes, and 35 live births.
◦ Because approximately 3% of cases with unknown outcomes are likely to result in a live
birth, the total number of live births is estimated to be 37 (2008/09 data are provisional).
Major causes of death include apnea and heart abnormalities.
◦ It is impossible to predict an exact prognosis during pregnancy or the neonatal period
Prognosis
◦ Half of infants with this condition do not survive beyond the first week of life.
◦ The median lifespan is 5–15 days.
◦ About 8% of infants survive longer than 1 year.
◦ One percent of children live to age 10, typically in less severe cases of
the mosaic Edwards syndrome.
◦ Parents with surviving children who take part in support groups report that these
children enriched their family and their couple irrespective of the length of their lives.
Epidemiology
◦ Edwards syndrome occurs in approximately 1 in 6,000 live births, but more conceptions
are affected by the syndrome because the majority of those diagnosed with the
condition prenatally will not survive the prenatal period.
◦ Although women in their 20s and early 30s may conceive babies with Edwards
syndrome, the risk of conceiving a child with Edwards syndrome increases with a
woman's age. The average maternal age for conceiving a child with this disorder is
32½.
Patau syndrome
Patau syndrome
◦ Patau syndrome /ˈpætaʊ/ is a syndrome caused by a chromosomal abnormality, in
which some or all of the cells of the body contain extra genetic material
from chromosome 13.
Patau syndrome
◦ This can occur either because each cell contains a full extra copy of chromosome 13
(a disorder known as trisomy 13or trisomy D), or because each cell contains an extra
partial copy of the chromosome (i.e., Robertsonian translocation) or because
of mosaic Patau syndrome. Full trisomy 13 is caused by nondisjunction of chromosomes
during meiosis (the mosaic form is caused by nondisjunction during mitosis).
Patau syndrome
◦ The extra genetic material from chromosome 13 disrupts the normal course of
development, causing multiple and complex organ defects. Like
allnondisjunction conditions (such as Down syndrome and Edwards syndrome), the risk
of this syndrome in the offspring increases with maternal age at pregnancy, with about
31 years being the average.[1] Patau syndrome affects somewhere between 1 in 10,000
and 1 in 21,700 live births.[2
Causes
◦ Patau's syndrome is the result of trisomy 13, meaning each cell in the body has three
copies of chromosome 13 instead of the usual two. A small percentage of cases occur
when only some of the body's cells have an extra copy; such cases are called mosaic
Patau.
Causes
◦ Patau syndrome can also occur when part of chromosome 13 becomes attached to
another chromosome (translocated) before or at conception in aRobertsonian
translocation. Affected people have two copies of chromosome 13, plus extra material
from chromosome 13 attached to another chromosome. With a translocation, the
person has a partial trisomy for chromosome 13 and often the physical signs of the
syndrome differ from the typical Patau syndrome.
Causes
◦ Most cases of Patau syndrome are not inherited, but occur as random events during
the formation of reproductive cells (eggs and sperm). An error in cell division
called non-disjunction can result in reproductive cells with an abnormal number of
chromosomes. For example, an egg or sperm cell may gain an extra copy of the
chromosome. If one of these atypical reproductive cells contributes to the genetic
makeup of a child, the child will have an extra chromosome 13 in each of the body's
cells. Mosaic Patau syndrome is also not inherited. It occurs as a random error during
cell division early in fetal development.
Causes
◦ Patau syndrome due to a translocation can be inherited. An unaffected person can
carry a rearrangement of genetic material between chromosome 13 and another
chromosome. This rearrangement is called a balanced translocation because there is
no extra material from chromosome 13. Although they do not have signs of Patau
syndrome, people who carry this type of balanced translocation are at an increased
risk of having children with the condition.
Manifestations and physical findings
◦ Of those fetuses that do survive to gestation and subsequent birth, common
abnormalities may include:
◦ Nervous system
◦ Intellectual disability and motor disorder
◦ Microcephaly
◦ Holoprosencephaly (failure of the forebrain to divide properly).
◦ Structural eye defects, including microphthalmia, Peters anomaly (a type of eye
abnormality), cataract, iris and/or fundus (coloboma), retinal dysplasia or retinal detachment,
sensory nystagmus, cortical visual loss, and optic nerve hypoplasia
◦ Meningomyelocele (a spinal defect)
◦ Musculoskeletal and cutaneous
◦ Polydactyly (extra digits)
◦ Cyclopia
◦ Proboscis
◦ Low-set ears[3]
◦ Prominent heel
◦ Deformed feet known as rocker-bottom feet
◦ Omphalocele (abdominal defect)
◦ Abnormal palm pattern
◦ Overlapping of fingers over thumb
◦ Cutis aplasia (missing portion of the skin/hair)
◦ Cleft palate
◦ Urogenital
◦ Abnormal genitalia
◦ Kidney defects
◦ Other
◦ Heart defects (ventricular septal defect) (Patent Ductus Arteriosus)
◦ Dextrocardia
◦ Single umbilical artery
Diagnosis
◦ Diagnosis is usually based on clinical findings, although fetal chromosome testing will
show trisomy 13. While many of the physical findings are similar to Edward's syndrome
there are a few unique traits, such as polydactyly.
◦ However, unlike Edward's syndrome and Down syndrome, the quad screen does not
provide a reliable means of screening for this disorder. This is due to the variability of the
results seen in fetuses with Patau.
Treatment
◦ Medical management of children with Trisomy 13 is planned on a case-by-case basis
and depends on the individual circumstances of the patient. Treatment of Patau
syndrome focuses on the particular physical problems with which each child is born.
Many infants have difficulty surviving the first few days or weeks due to severe
neurological problems or complex heart defects. Surgery may be necessary to repair
heart defects or cleft lip and cleft palate.
Treatment
◦ Physical, occupational, and speech therapy will help individuals with Patau syndrome
reach their full developmental potential. Surviving children are described as happy
and parents report that they enrich their lives.
◦ The cited study grouped Edwards syndrome, which is sometimes survivable beyond
toddlerhood, along with Patau, hence the median age of 4 at the time of data
collection
Prognosis
◦ More than 80% of children with Patau syndrome die within the first year of life.
Trisomy 8
◦ Trisomy 8, also known as Warkany syndrome 2, is a human chromosomal
disorder caused by having three copies (trisomy) of chromosome 8. It can appear with
or without mosaicism.
Characteristics
◦ Complete trisomy 8 causes severe effects on the developing fetus and can be a cause
of miscarriage.
◦ Complete trisomy 8 is usually an early lethal condition, whereas trisomy 8 mosaicism is
less severe and individuals with a low proportion of affected cells may exhibit a
comparatively mild range of physical abnormalities and developmental delay.
◦ Individuals with trisomy 8 mosaicism are more likely to survive into childhood and
adulthood, and exhibit a characteristic and recognizable pattern of developmental
abnormalities.
Characteristics
◦ Common findings include retarded psychomotor development, moderate to severe
mental retardation, variable growth patterns which can result in either abnormally short
or tall stature, an expressionless face, and many musculoskeletal, visceral, and eye
abnormalities, as well as other anomalies.[5] A deep plantar furrow is considered to be
pathognomonic of this condition, especially when seen in combination with other
associated features.[6] The type and severity of symptoms are dependent upon the
location and proportion of trisomy 8 cells compared to normal cells.
Other conditions
◦ Trisomy 8 mosaicism affects wide areas of chromosome 8 containing many genes, and
can thus be associated with a range of symptoms.
◦ Mosaic trisomy 8 has been reported in rare cases of Rothmund-Thomson syndrome, a
genetic disorder associated with the DNA helicase RECQL4 on chromosome 8q24.3.
The syndrome is "characterized by skin atrophy, telangiectasia, hyperand hypopigmentation, congenital skeletal abnormalities, short stature, premature
aging, and increased risk of malignant disease".[7]
Other conditions
◦ Some individuals trisomic for chromosome 8 were deficient in production
of coagulation factor VII due to a factor 7 regulation gene (F7R) mapped to 8p23.3p23.1
◦ Trisomy and other rearrangements of chromosome 8 have also been found in tricho–
rhino–phalangeal syndrome
◦ Small regions of chromosome 8 trisomy and monosomy are also created
by recombinant chromosome 8 syndrome (San Luis Valley syndrome), causing
anomalies associated with tetralogy of Fallot, which results from recombination
between a typical chromosome 8 and one carrying a parental paracentric inversion.
◦ Trisomy is also found in some cases of chronic myeloid leukaemia, potentially as a result
of karyotypic instability caused by the bcr:abl fusion gene.
Trisomy 22
◦ Trisomy 22 is a chromosomal disorder in which there are three copies of chromosome
22 rather than two. It is a frequent cause of spontaneous abortion during the first
trimester of pregnancy. Progression to the second trimester and livebirth are rare. This
disorder is found in individuals with an extra copy or a variation of chromosome 22 in
some or all cells of their body. There are many kinds of disorders associated with Trisomy
22:
Emanuel Syndrome
◦ Emanuel Syndrome is named after the genetic contributions made by researcher Dr.
Beverly Emanuel. This condition is assigned to individuals born with an unbalanced
11/22 translocation. That is, when a fragment of chromosome 11 is moved, or
translocated to chromosome 22.
◦ 22q11 Deletion Syndrome [2] is a rare condition which occurs in approximately 1 in 4000
births. This condition is identified when a band in the q11.2 section of the arm of
chromosome 22 is missing or deleted. This condition has several different names, The
22q11.2 Deletion Sydrome, Velocardiofacial syndrome, DiGeorge Syndrome,
Conotruncal Anomaly Face syndrome, Opitz G/BBB Syndrome, Cayler Cardiofacial
Syndrome.The effects of this disorder are different in each individual but similarities exist
such as heart defects, immune system problems, a distinctive facial appearance,
learning challenges, cleft palate, hearing loss, kidney problems, hypocalcemia, and
sometimes psychiatric issues.
22q11 microduplication syndrome
◦ 22q11 microduplication syndrome is the opposite of the 22q11 deletion syndrome, in
this condition, a band of q.11.2 section of chromosome 22 is duplicated. Individuals
carrying this deficiency are relatively “normal” as in they don’t possess any major birth
defects or major medical illnesses.
◦ This microduplication is more common than the deletion; this might be due to the
milder phenotype of the individuals.
Phelan-McDermid Syndrome
◦ Phelan-McDermid Syndrome / 22q13 Deletion Syndrome is a condition caused by the
deletion of the tip of the q arm on chromosome 22. Most individuals with this disorder
experience cognitive delays as well as low muscle tone and sleeping, eating and
behavioural issues.
Chromosome Ring 22
◦ Chromosome Ring 22 is a rare disorder caused by the break and re-join of both ends of
chromosome 22, forming a ring. The effects on the individual with this disorder are
dependent on the amount of genetic information lost during the break/re-join. Major
characteristics for this disorder are mental retardation, muscle weakness and lack of
coordination
Cat Eye Syndrome
◦ Cat Eye Syndrome / Schmid Fraccaro Syndrome is a condition caused by a partial
trisomy or tetrasomy in chromosome 22. A small extra chromosome is found, made up
of the top half of chromosome 22 and a portion of the q arm at the q11.2 break. This
chromosome can be found three or four times.
◦ This syndrome is referred as “Cat Eye” due to the eye appearance of reported
affected individuals who have coloboma of the iris ; however, this feature is only seen
in about half of the cases.
◦ Mosaic trisomy 22 is a disorder in which an extra chromosome 22 is found only in some
cells of the body. The severity of each case is determined by the number of cells with
this extra copy. Some characteristics of individuals with this condition are cardiac
abnormalities, growth retardation, mental delay, etc..
◦ Complete Trisomy 22[8] is in contrast with Mosaic trisomy 22; this disorder is characterized
by an extra copy of chromosome 22 which is found in each cell of the body of the
affected individual. These cases are very rare, and most of the affected individuals die
before birth or shortly after
Klinefelter syndrome
Klinefelter syndrome
◦ Klinefelter syndrome or Klinefelter's syndrome is the set of symptoms resulting from
additional X genetic material in males. Also known as47,XXY or XXY, Klinefelter
syndrome is a genetic disorder in which there is at least one extra X chromosome to a
standard human male karyotype, for a total of 47 chromosomes rather than the 46
found in genetically typical humans.
◦ While females have an XX chromosomal makeup, and males an XY, individuals with
Klinefelter syndrome have at least two X chromosomes and at least one Y
chromosome.
◦ Because of the extra chromosome, individuals with the condition are usually referred to
as "XXY males", or "47,XXY males".
◦ This chromosome constitution (karyotype) exists in roughly between 1:500 to 1:1000 live
male births but many of these people may not show symptoms. If the physical traits
associated with the syndrome become apparent, they normally appear after the onset
of puberty.
◦ In humans, 47,XXY is the most common sex chromosome aneuploidy in males and the
second most common condition caused by the presence of extra chromosomes.
Other mammals also have the XXY syndrome, including mice.
◦ Principal effects include hypogonadism and sterility. A variety of other physical and
behavioural differences and problems are common, though severity varies and many
XXY boys have few detectable symptoms.
Signs and symptoms
◦ There are many variances within the XXY population, just as within the 46,XY population.
While it is possible to characterise XXY males with certain body types and physical
characteristics, that in itself should not be the method of identification as to whether or
not someone has XXY. The only reliable method of identification is karyotype testing.
The degree to which XXY males are affected, both physically and developmentally,
differs widely from person to person.
Signs and symptoms
◦ Physical
◦ As babies and children, XXY males may have weaker muscles and reduced strength.
As they grow older, they tend to become taller than average. They may have less
muscle control and coordination than other boys their age.
◦ During puberty, the physical traits of the syndrome become more evident; because
these boys do not produce as much testosterone as other boys, they have a less
muscular body, less facial and body hair, and broader hips. As teens, XXY males may
have larger breasts, weaker bones, and a lower energy level than other boys.
Signs and symptoms
◦ As babies and children, XXY males may have weaker muscles and reduced strength.
As they grow older, they tend to become taller than average. They may have less
muscle control and coordination than other boys their age.
◦ During puberty, the physical traits of the syndrome become more evident; because
these boys do not produce as much testosterone as other boys, they have a less
muscular body, less facial and body hair, and broader hips. As teens, XXY males may
have larger breasts, weaker bones, and a lower energy level than other boys.
Signs and symptoms
◦ By adulthood, XXY males look similar to males without the condition, although they are
often taller. In adults, possible characteristics vary widely and include little to no signs of
affectedness, a lanky, youthful build and facial appearance, or a rounded body type
with some degree of gynecomastia (increased breast tissue).
◦ Gynecomastia is present to some extent in about a third of affected individuals, a
slightly higher percentage than in the XY population.
◦ About 10% of XXY males have gynecomastia noticeable enough that they may choose
to have cosmetic surgery.
Signs and symptoms
◦ Affected males are often infertile, or may have reduced fertility. Advanced
reproductive assistance is sometimes possible.
◦ The term hypogonadism in XXY symptoms is often misinterpreted to mean "small
testicles" or "small penis". In fact, it means decreased testicular hormone/endocrine
function. Because of this (primary) hypogonadism, individuals will often have a low
serum testosterone level but high serum follicle-stimulating hormone (FSH)
and luteinizing hormone (LH) levels.
◦ Despite this misunderstanding of the term, however, it is true that XXY men may also
have microorchidism (i.e., small testicles).
Signs and symptoms
◦ XXY males are also more likely than other men to have certain health problems, which
typically affect females, such as autoimmune disorders, breast cancer, venous
thromboembolic disease, and osteoporosis.
◦ In contrast to these potentially increased risks, it is currently thought that rare X-linked
recessive conditions occur less frequently in XXY males than in normal XY males, since
these conditions are transmitted by genes on the X chromosome, and people with two
X chromosomes are typically only carriers rather than affected by these X-linked
recessive conditions.
Cause
◦ The extra X chromosome is retained because of a nondisjunction event during meiosis
I (gametogenesis). Nondisjunction occurs when homologous chromosomes, in this case
the X and Y sex chromosomes, fail to separate, producing a sperm with an X and a Y
chromosome. Fertilizing a normal (X) egg produces an XXY offspring. The XXY
chromosome arrangement is one of the most common genetic variations from the
XY karyotype, occurring in about 1 in 500 live male births.
Cause
◦ Another mechanism for retaining the extra X chromosome is through a nondisjunction
event during meiosis II in the female. Nondisjunction will occur when sister chromatids
on the sex chromosome, in this case an X and an X, fail to separate. (meiosis) An XX
egg is produced which, when fertilized with a Y sperm, yields XXY offspring.
Treatment
◦ The genetic variation is irreversible. Often individuals that have noticeable breast tissue
or hypogonadism experience depression and/or social anxiety because they are
outside of social norms. This is academically referred to as psychosocial morbidity. At
least one study indicates that planned and timed support should be provided for
young men with Klinefelter syndrome to ameliorate current poor psychosocial
outcomes.
◦ By 2010 over 100 successful pregnancies have been reported using IVF technology with
surgically removed sperm material from males with Klinefelter syndrome.
Prognosis
◦ There is research suggesting Klinefelter syndrome substantially decreases life
expectancy among affected individuals, though the evidence is not definitive. A 1985
publication identified a greater mortality mainly due to diseases of the aortic valve,
development of tumors and possible subarachnoid hemorrhages, reducing life
expectancy by about 5 years. Later studies have reduced this estimated reduction to
an average of 2.1 years. These results are still questioned data, are not absolute, and
will need further testing.
History
◦ The syndrome was named after Harry Klinefelter, who, in 1942, worked with Fuller
Albright at Massachusetts General Hospital in Boston, Massachusetts and first described
it in the same year.[13][29] The account given by Klinefelter came to be known as
Klinefelter syndrome as his name appeared first on the published paper, and
seminiferous tubule dysgenesis was no longer used.
Turner syndrome
◦ Turner syndrome or Ullrich–Turner syndrome (also known as "Gonadal dysgenesis"[1]:550),
45,X, encompasses several conditions in humanfemales, of which monosomy X
(absence of an entire sex chromosome, the Barr body) is most common. It is
a chromosomal abnormality in which all or part of one of the sex chromosomes is
absent or has other abnormalities (unaffected humans have 46 chromosomes, of
which two are sex chromosomes). In some cases, the chromosome is missing in some
cells but not others, a condition referred to as mosaicism or "Turner mosaicism".
Turner syndrome
◦ Occurring in 1 in 2000[3] – 1 in 5000 phenotypic females, the syndrome manifests itself in
a number of ways. There are characteristic physical abnormalities which affect many
but not all people with Turner syndrome, such as short stature, swelling, broad chest,
low hairline, low-set ears, andwebbed necks. Girls with Turner syndrome typically
experience gonadal dysfunction (non-working ovaries), which results
in amenorrhea (absence of menstrual cycle) and sterility. Concurrent health concerns
may also be present, including congenital heart
disease, hypothyroidism (reducedhormone secretion by
the thyroid), diabetes, vision problems, hearing concerns, and many autoimmune
diseases.[6] Finally, a specific pattern of cognitive deficits is often observed, with
particular difficulties in visuospatial, mathematical, and memory areas.
Turner syndrome
Turner syndrome
◦ Short stature
◦ Lymphedema (swelling) of the hands and feet
◦ Broad chest (shield chest) and widely spaced nipples
◦ Low hairline
◦ Low-set ears
◦ Reproductive sterility
◦ Rudimentary ovaries gonadal streak (underdeveloped gonadal structures that later
become fibrosed)
◦ Amenorrhoea, or the absence of a menstrual period
◦ Increased weight, obesity
◦ Shield shaped thorax of heart
◦ Shortened metacarpal IV
◦ Small fingernails
◦ Characteristic facial features
◦ Webbed neck from cystic hygroma in infancy
◦ Aortic valve stenosis
◦ Coarctation of the aorta
◦ Bicuspid aortic valve
◦ Horseshoe kidney
◦ Visual impairments sclera, cornea, glaucoma, etc.
◦ Ear infections and hearing loss
◦ High waist-to-hip ratio (the hips are not much bigger than the waist)
◦ Attention Deficit/Hyperactivity Disorder or ADHD (problems with concentration,
memory, attention with hyperactivity seen mostly in childhood and adolescence)
◦ Nonverbal Learning Disability (problems with math, social skills and spatial relations)
◦ Other features may include a small lower jaw (micrognathia), cubitus valgus,[8] soft
upturned nails, palmar crease, and drooping eyelids. Less common are
pigmented moles, hearing loss, and a high-arch palate (narrow maxilla). Turner
syndrome manifests itself differently in each female affected by the condition,
therefore, no two individuals will share the same features.
Cause
◦ Turner syndrome is caused by the absence of two complete copies of the X
chromosome in some or all the cells. The abnormal cells may have only one X
(monosomy) (45,X) or they may be affected by one of several types of partial
monosomy like a deletion of the short p arm of one X chromosome (46,XdelXp) or the
presence of an isochromosome with two q arms (46XiXq. In mosaic individuals, cells
with X monosomy (45,X) may occur along with cells that are normal (46,XX), cells that
have partial monosomies, or cells that have a Y chromosome (46,XY). The presence of
mosaicism is estimated to be relatively common in affected individuals (67-90%).
Inheritance
◦ In the majority of cases where monosomy occurs, the X chromosome comes from the
mother. This may be due to a nondisjunction in the father. Meiotic errors that lead to
the production of X with p arm deletions or abnormal Y chromosomes are also mostly
found in the father. Isochromosome X or ring chromosome X on the other hand are
formed equally often by both parents. Overall, the functional X chromosome mostly
comes from the mother.
◦ In most cases, Turner syndrome is a sporadic event, and for the parents of an individual
with Turner syndrome the risk of recurrence is not increased for subsequent
pregnancies. Rare exceptions may include the presence of a
balanced translocation of the X chromosome in a parent, or where the mother has XO
mosaicism restricted to her germ cells.
Diagnosis
45,X karyotype, showing an unpaired X at the lower right
Diagnosis
◦ Turner syndrome may be diagnosed by amniocentesis or chorionic villus
sampling during pregnancy.
Prognosis
◦ While most of the physical findings are harmless, there can be significant medical
problems associated with the syndrome.
◦ Prenatal
◦ Despite the excellent postnatal prognosis, 99% of Turner-syndrome conceptions are
thought to end in spontaneous abortion or stillbirth, and as many as 15% of all
spontaneous abortions have the XO karyotype. Among cases that are detected by
routine amniocentesis or chorionic villus sampling, one study found that the prevalence
of Turner syndrome among tested pregnancies was 5.58 and 13.3 times higher
respectively than among live neonates in a similar population.
Epidemiology
◦ Approximately 99 percent of all fetuses with Turner syndrome result in spontaneous
termination during the first trimester. Turner syndrome accounts for about 10 percent of
the total number of spontaneous abortions in the United States. The incidence of Turner
syndrome in live female births is believed to be around 1 in 2000.
References:
◦
http://www.wikipedia.com