Lecture 2
General characterization of chronosome diseases. Clinical peculiarities of the main forms of chronosome diseases.
Down syndrome
Down syndrome (the most common term in US English) , Down’s syndrome (standard in British English), or trisomy 21 is a chromosomal disorder caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British doctor who described the syndrome in 1866. The disorder was identified as a chromosome 21 trisomy by Jérôme Lejeune in 1959. The condition is characterized by a combination of major and minor differences in structure. Often Down syndrome is associated with some impairment of cognitive ability and physical growth as well as facial appearance. Down syndrome in a baby can be identified with amniocentesis during pregnancy or at birth.
Individuals with Down syndrome tend to have a lower than average cognitive ability, often ranging from mild to moderate developmental disabilities. A small number have severe to profound mental disability. The incidence of Down syndrome is estimated at 1 per 800 to 1,000 births, although these statistics are heavily influenced by older mothers. Other factors may also play a role.
Many of the common physical features of Down syndrome may also appear in people with a standard set of chromosomes, including microgenia (an abnormally small chin), an unusually round face, macroglossia (protruding or oversized tongue), an almond shape to the eyes caused by an epicanthic fold of the eyelid, upslanting palpebral fissures (the separation between the upper and lower eyelids), shorter limbs, a single transverse palmar crease (a single instead of a double crease across one or both palms, also called the Simian crease), poor muscle tone, a larger thaormal space between the big and second toes. Health concerns for individuals with Down syndrome include a higher risk for congenital heart defects, gastroesophageal reflux disease, recurrent ear infections, obstructive sleep apnea, and thyroid dysfunctions.
Early childhood intervention, screening for common problems, medical treatment where indicated, a conducive family environment, and vocational training can improve the overall development of children with Down syndrome. Although some of the physical genetic limitations of Down syndrome cannot be overcome, education and proper care will improve quality of life.
Characteristics
Individuals with Down syndrome may have some or all of the following physical characteristics: microgenia (abnormally small chin), oblique eye fissures with epicanthic skin folds on the inner corner of the eyes (formerly known as a mongoloid fold), muscle hypotonia (poor muscle tone), a flat nasal bridge, a single palmar fold, a protruding tongue (due to small oral cavity, and an enlarged tongue near the tonsils) or macroglossia, a short neck, white spots on the iris known as Brushfield spots,excessive joint laxity including atlanto-axial instability, congenital heart defects, excessive space between large toe and second toe, a single flexion furrow of the fifth finger, and a higher number of ulnar loop dermatoglyphs. Most individuals with Down syndrome have mental retardation in the mild (IQ 50–70) to moderate (IQ 35–50) range,with individuals having Mosaic Down syndrome typically 10–30 points higher. In addition, individuals with Down syndrome can have serious abnormalities affecting any body system. They also may have a broad head and a very round face.
The medical consequences of the extra genetic material in Down syndrome are highly variable and may affect the function of any organ system or bodily process. The health aspects of Down syndrome encompass anticipating and preventing effects of the condition, recognizing complications of the disorder, managing individual symptoms, and assisting the individual and his/her family in coping and thriving with any related disability or illnesses.
Down syndrome can result from several different genetic mechanisms. This results in a wide variability in individual symptoms due to complex gene and environment interactions. Prior to birth, it is not possible to predict the symptoms that an individual with Down syndrome will develop. Some problems are present at birth, such as certain heart malformations. Others become apparent over time, such as epilepsy.
The most common manifestations of Down syndrome are the characteristic facial features, cognitive impairment, congenital heart disease (typically a ventricular septal defect), hearing deficits (maybe due to sensory-neural factors, or chronic serous otitis media, also known as Glue-ear), short stature, thyroid disorders, and Alzheimer’s disease. Other less common serious illnesses include leukemia, immune deficiencies, and epilepsy.
However, health benefits of Down syndrome include greatly reduced incidence of many common malignancies except leukemia and testicular cancer — although it is, as yet, unclear whether the reduced incidence of various fatal cancers among people with Down syndrome is as a direct result of tumor-suppressor genes on chromosome 21 (such as Ets2), because of reduced exposure to environmental factors that contribute to cancer risk, or some other as-yet unspecified factor. In addition to a reduced risk of most kinds of cancer, people with Down syndrome also have a much lower risk of hardening of the arteries and diabetic retinopathy.
Cognitive development
Cognitive development in children with Down syndrome is quite variable. It is not currently possible at birth to predict the capabilities of any individual reliably, nor are the number or appearance of physical features predictive of future ability. The identification of the best methods of teaching each particular child ideally begins soon after birth through early intervention programs. Since children with Down syndrome have a wide range of abilities, success at school can vary greatly, which underlines the importance of evaluating children individually. The cognitive problems that are found among children with Down syndrome can also be found among typical children. Therefore, parents can use general programs that are offered through the schools or other means.
Language skills show a difference between understanding speech and expressing speech, and commonly individuals with Down syndrome have a speech delay, requiring speech therapy to improve expressive language. Fine motor skills are delayed and often lag behind gross motor skills and can interfere with cognitive development. Effects of the disorder on the development of gross motor skills are quite variable. Some children will begin walking at around 2 years of age, while others will not walk until age 4. Physical therapy, and/or participation in a program of adapted physical education (APE), may promote enhanced development of gross motor skills in Down syndrome children.
Individuals with Down syndrome differ considerably in their language and communication skills. It is routine to screen for middle ear problems and hearing loss; low gain hearing aids or other amplification devices can be useful for language learning. Early communication intervention fosters linguistic skills. Language assessments can help profile strengths and weaknesses; for example, it is common for receptive language skills to exceed expressive skills. Individualized speech therapy can target specific speech errors, increase speech intelligibility, and in some cases encourage advanced language and literacy. Augmentative and alternative communication (AAC) methods, such as pointing, body language, objects, or graphics are often used to aid communication. Relatively little research has focused on the effectiveness of communications intervention strategies.
In education, mainstreaming of children with Down syndrome is becoming less controversial in many countries. For example, there is a presumption of mainstream in many parts of the
Some European countries such as
Fertility
Fertility amongst both males and females is reduced; males are usually unable to father children, while females demonstrate significantly lower rates of conception relative to unaffected individuals.[citatioeeded] Approximately half of the offspring of someone with Down syndrome also have the syndrome themselves. There have been only three recorded instances of males with Down syndrome fathering children.
Genetics
Down syndrome is a chromosomal abnormality characterized by the presence of an extra copy of genetic material on the 21st chromosome, either in whole (trisomy 21) or part (such as due to translocations). The effects of the extra copy vary greatly among people, depending on the extent of the extra copy, genetic history, and pure chance. Down syndrome occurs in all human populations, and analogous effects have been found in other species such as chimpanzees and mice. Recently, researchers have created transgenic mice with most of human chromosome 21 (in addition to the normal mouse chromosomes). The extra chromosomal material can come about in several distinct ways. A typical human karyotype is designated as 46,XX or 46,XY, indicating 46 chromosomes with an XX arrangement typical of females and 46 chromosomes with an XY arrangement typical of males.
Trisomy 21
Trisomy 21 (47,XX,+21) 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 21; the gamete thus has 24 chromosomes. When combined with a normal gamete from the other parent, the embryo now has 47 chromosomes, with three copies of chromosome 21. Trisomy 21 is the cause of approximately 95% of observed Down syndromes, with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete.
Mosaicism
Trisomy 21 is usually caused by nondisjunction in the gametes prior to conception, and all cells in the body are affected. However, when some of the cells in the body are normal and other cells have trisomy 21, it is called mosaic Down syndrome (46,XX/47,XX,+21). This can occur in one of two ways: a nondisjunction event during an early cell division in a normal embryo leads to a fraction of the cells with trisomy 21; or a Down syndrome embryo undergoes nondisjunction and some of the cells in the embryo revert to the normal chromosomal arrangement. There is considerable variability in the fraction of trisomy 21, both as a whole and among tissues. This is the cause of 1–2% of the observed Down syndromes.
Robertsonian translocation
The extra chromosome 21 material that causes Down syndrome may be due to a Robertsonian translocation in the karyotype of one of the parents. In this case, the long arm of chromosome 21 is attached to another chromosome, often chromosome 14 (45,XX, t(14;21q)) or itself (called an isochromosome, 45,XX, t(21q;21q)). A person with such a translocation is phenotypically normal. During reproduction, normal disjunctions leading to gametes have a significant chance of creating a gamete with an extra chromosome 21, producing a child with Down syndrome. Translocation Down syndrome is often referred to as familial Down syndrome. It is the cause of 2–3% of observed cases of Down syndrome. It does not show the maternal age effect, and is just as likely to have come from fathers as mothers.
Duplication of a portion of chromosome 21
Rarely, a region of chromosome 21 will undergo a duplication event. This will lead to extra copies of some, but not all, of the genes on chromosome 21 (46,XX, dup(21q)). If the duplicated region has genes that are responsible for Down syndrome physical and mental characteristics, such individuals will show those characteristics. This cause is very rare and no rate estimates are available.
Screening
Pregnant women can be screened for various complications during pregnancy. Many standard prenatal screens can discover Down syndrome. Genetic counseling along with genetic testing, such as amniocentesis, chorionic villus sampling (CVS), or percutaneous umbilical cord blood sampling (PUBS) are usually offered to families who may have an increased chance of having a child with Down syndrome, or where normal prenatal exams indicate possible problems. ACOG guidelines recommend that non-invasive screening and invasive testing be offered to all women, regardless of their age, and most likely all physicians currently follow these guidelines. However, some insurance plans will only reimburse invasive testing if a woman is >34 years old or if she has received a high-risk score from a non-invasive screening test.
Amniocentesis and CVS are considered invasive procedures, in that they involve inserting instruments into the uterus, and therefore carry a small risk of causing fetal injury or miscarriage. The risks of miscarriage for CVS and amniocentesis are often quoted as 1% and 0.5% respectively. There are several commoon-invasive screens that can indicate a fetus with Down syndrome. These are normally performed in the late first trimester or early second trimester. Due to the nature of screens, each has a significant chance of a false positive, suggesting a fetus with Down syndrome when, in fact, the fetus does not have this genetic abnormality. Screen positives must be verified before a Down syndrome diagnosis is made.
Common screening procedures for Down syndrome are given in Table 1.
|
Screen
|
When performed (weeks gestation) |
Detection rate |
False positive rate |
Description |
|
Quad screen |
15–20 |
81% |
5% |
This test measures the maternal serum alpha feto protein (a fetal liver protein), estriol (a pregnancy hormone), human chorionic gonadotropin (hCG, a pregnancy hormone), and inhibin-Alpha (INHA). |
|
Nuchal translucency/free beta/PAPPA screen (aka “1st Trimester Combined Test”) |
10–13.5 |
85% |
5% |
Uses ultrasound to measure Nuchal Translucency in addition to the freeBeta hCG and PAPPA (pregnancy-associated plasma protein A). NIH has confirmed that this first trimester test is more accurate than second trimester screening methods. Performing an NT ultrasound requires considerable skill; a Combined test may be less accurate if there is operator error, resulting in a lower-than-advertised sensitivity and higher false-positive rate, possibly in the 5-10% range. |
|
Integrated Test |
10-13.5 and 15–20 |
95% |
5% |
The Integrated test uses measurements from both the 1st Trimester Combined test and the 2nd trimester Quad test to yield a more accurate screening result. Because all of these tests are dependent on accurate calculation of the gestational age of the fetus, the real-world false-positive rate is >5% and maybe be closer to 7.5%. |
Even with the best non-invasive screens, the detection rate is 90%–95% and the rate of false positive is 2%–5%. Inaccuracies can be caused by undetected multiple fetuses (very rare with the ultrasound tests), incorrect date of pregnancy, or normal variation in the proteins.
Confirmation of screen positive is normally accomplished with amniocentesis or chorionic villus sampling (CVS). Amniocentesis is an invasive procedure and involves taking amniotic fluid from the amniotic sac and identifying fetal cells. The lab work can take several weeks but will detect over 99.8% of all numerical chromosomal problems with a very low false positive rate.
Plastic surgery
Plastic surgery has sometimes been advocated and performed on children with Down syndrome, based on the assumption that surgery can reduce the facial features associated with Down syndrome, therefore decreasing social stigma, and leading to a better quality of life. Plastic surgery on children with Down syndrome is uncommon, and continues to be controversial. Researchers have found that for facial reconstruction, “…although most patients reported improvements in their child’s speech and appearance, independent raters could not readily discern improvement….” For partial glossectomy (tongue reduction), one researcher found that 1 out of 3 patients “achieved oral competence,” with 2 out of 3 showing speech improvement. Len Leshin, physician and author of the ds-health website, has stated, “Despite being in use for over twenty years, there is still not a lot of solid evidence in favor of the use of plastic surgery in children with Down syndrome.” The National Down Syndrome Society has issued a “Position Statement on Cosmetic Surgery for Children with Down Syndrome” which states that “The goal of inclusion and acceptance is mutual respect based on who we are as individuals, not how we look.”
Alternative treatment
The Institutes for the Achievement of Human Potential is a non-profit organization which treats children who have, as the IAHP terms it, “some form of brain injury,” including children with Down syndrome. The approach of “Psychomotor Patterning” is not proven, and is considered alternative medicine.
Prognosis
These factors can contribute to a shorter life expectancy for people with Down syndrome. One study, carried out in the
Epidemiology
The incidence of Down syndrome is estimated at one per 800 to one per 1000 births. In 2006, the Centers for Disease Control and Prevention estimated the rate as one per 733 live births in the
Maternal age influences the chances of conceiving a baby with Down syndrome. At maternal age 20 to 24, the probability is one in 1562; at age 35 to 39 the probability is one in 214, and above age 45 the probability is one in 19. Although the probability increases with maternal age, 80% of children with Down syndrome are born to women under the age of 35, reflecting the overall fertility of that age group. Recent data also suggest that paternal age, especially beyond 42, also increases the risk of Down Syndrome manifesting in pregnancies in older mothers.
Current research (as of 2008) has shown that Down syndrome is due to a random event during the formation of sex cells or pregnancy. There has beeo evidence that it is due to parental behavior (other than age) or environmental factors.
Edwards syndrome
Trisomy 18 (T18) (also known as Trisomy E or Edwards Syndrome) is a genetic disorder caused by the presence of all or part of an extra 18th chromosome. It is named after John H. Edwards, who first described the syndrome in 1960. It is the second most common autosomal trisomy, after Down Syndrome, that carries to term.
Trisomy 18 is caused by the presence of three—as opposed to two—copies of chromosome
Prognosis
The survival rate of Edwards Syndrome is very low. About 95% die in utero. Of liveborn infants, only 50% live to 2 months, and only 5–10% will survive their first year of life. Major causes of death include apnea and heart abnormalities. It is impossible to predict the exact prognosis of an Edwards Syndrome child during pregnancy or the neonatal period. The median life span is five to fifteen days. One percent of children born with this syndrome live to age ten, typically in cases of the less severe mosaic Edwards syndrome.
Incidence/prevalence
The rate of occurrence for Edwards Syndrome is approximately one in 3,000 (for conception) and approximately one in 6,000 (for live births), as 50% of those diagnosed prenatally with the condition will not survive the prenatal period. Although women in their 20s and early 30s may conceive Edwards Syndrome babies, there is an increased risk of conceiving a child with Edwards Syndrome as a woman’s age increases, with the average age for this disorder being 32½.
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 part (such as due to translocations). The additional chromosome usually occurs before conception. The effects of the extra copy vary greatly among people, depending on the extent of the extra copy, genetic history, and chance. Edwards syndrome occurs in all human populations, but is more prevalent in females.
A healthy egg or sperm cell contains individual chromosomes — one to contribute to each of the 23 pairs of chromosomes needed to form a normal cell with typical human karyotype of 46 chromosomes. Numerical errors arise at either of the two meiotic divisions and cause the failure of segregation of a chromosome into the daughter cells (nondisjunction). This results in an extra chromosome making the haploid number 24 rather than 23. Fertilization of these eggs or sperm that contain an extra chromosome results in trisomy, or three copies of a chromosome rather than two.
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 now has 47 chromosomes, with three copies of chromosome 18.
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 people have two copies of chromosome 18, plus extra material from chromosome 18 attached to another chromosome. With a translocation, the person has a partial trisomy for chromosome 18 and the abnormalities are often less than for the typical Edwards syndrome.
Features and characteristics
Infants 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, mental retardation, developmental delays, growth deficiency, feeding difficulties, breathing difficulties, and arthrogryposis (a muscle disorder that causes multiple joint contractures at birth).
Some physical malformations associated with Edwards syndrome include: a 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, underdeveloped thumbs and or nails, absent radius, webbing of the second and third toes, clubfoot or Rocker bottom feet, and undescended testicles in males.
In utero, the most common characteristic is cardiac anomalies, followed by central nervous system anomalies such as head shape abnormalities. The most common head shape anomaly is the presence of choroid plexus cysts, which is a pocket of fluid on the brain that is not problematic in itself but may be a marker for Trisomy 18. Sometimes excess amniotic fluid or polyhydramnios is exhibited.
Klinefelter’s Syndrome
Klinefelter’s Syndrome, genetic disease affecting
Both men and women normally have 23 pairs of chromosomes. One of these pairs is the sex chromosome. A female normally inherits an X chromosome from each parent so that her chromosomal complement is XX. A male inherits an X chromosome from his mother and a Y chromosome from his father so that his chromosomal complement is XY. It is the presence of the Y chromosome that determines maleness. A male with Klinefelter’s syndrome inherits an extra X chromosome, giving him an abnormal chromosomal complement of XXY. In some cases, more than one extra X chromosome is inherited. The cause of Klinefelter’s syndrome is unknown, although it occurs slightly more often in boys born to older mothers.
In most cases, a boy with Klinefelter’s syndrome has a normal physical appearance until he reaches puberty. Diagnosis of the disorder may be delayed until physical symptoms develop, or until the adult male is tested for infertility. Diagnosis of the disorder is made by performing a chromosomal analysis in which body cells are studied in the laboratory to identify any chromosomal irregularities.
There is no treatment for Klinefelter’s syndrome, although regular injections of the male sex hormone testosterone may increase muscle size and strength, stimulate the growth of facial and body hair, and produce a normal sex drive in some cases. Enlarged breasts may be reduced surgically. Reversing infertility associated with Klinefelter’s syndrome may not be possible. Some men with the disorder may produce a small number of sperm, and they may benefit from modern fertility techniques in which a single sperm is injected into an egg to achieve fertilization
, 47, XXY or XXY syndrome is a condition in which males have an extra X sex chromosome. While females have an XX chromosomal makeup, and males an XY, affected individuals have at least two X chromosomes and at least one Y chromosome. Klinefelter’s syndrome is the most common sex chromosome disorder and the second most common condition caused by the presence of extra chromosomes. The condition exists in roughly 1 out of every 1000 males. One in every 500 males have an extra x chromosome but do not have the syndrome.
The principal effects are development of small testicles and reduced fertility. A variety of other physical and behavioral differences and problems are common, though severity varies and many boys and men with the condition have few detectable symptoms. Named after Dr. Harry Klinefelter, an endocrinologist at Massachusetts General Hospital, Boston, Massachusetts, who first described it in 1942[4]. Because of the extra chromosome, individuals with the condition are usually referred to as “XXY Males”, or “47, XXY Males”.
Signs and symptoms
Affected males are almost always effectively infertile although advanced reproductive assistance is sometimes possible. Some degree of language learning impairment may be present, and neuropsychological testing often reveals deficits in executive functions. 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, but only about 10% of XXY males’ gynecomastia is noticeable enough to require surgery.
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 hypogonadism, patients 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 often also have “microorchidism” (i.e. small testicles).
The more severe end of the spectrum of symptom expression is also associated with an increased risk of germ cell tumors, breast cancer, and osteoporosis, risks shared to varying degrees with females. Additionally, medical literature shows some individual case studies of Klinefelter’s syndrome coexisting with other disorders, such as pulmonary disease, varicose veins, diabetes mellitus, and rheumatoid arthritis, but possible correlations between Klinefelter’s and these other conditions are not well characterized or understood.
In contrast to these potentially increased risks, it is currently thought that rare X-linked recessive conditions occur even less frequently in XXY males than iormal 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.
There are many variances within the XXY population, just as in the most common 46,XY population. While it is possible to characterise 47,XXY males with certain body types, that in itself should not be the method of identification as to whether or not someone has 47,XXY. The only reliable method of identification is karyotype testing.
Diagnosis
A karyotype is used to confirm the diagnosis. In this procedure, a small blood sample is drawn. White blood cells are then separated from the sample, mixed with tissue culture medium, incubated, and checked for chromosomal abnormalities, such as an extra X chromosome.
Diagnosis can also be made prenatally via chorionic villus sampling or amniocentesis, tests in which fetal tissue is extracted and the fetal DNA is examined for genetic abnormalities. A 2002 literature review of elective abortion rates found that approximately 50% of pregnancies in the
Cause
The extra X chromosome is retained because of a nondisjunction event during meiosis (sex cell division). The XXY chromosome arrangement is one of the most common genetic variations from the XY karyotype, occurring in about
In mammals with more than one X chromosome, the genes on all but one X chromosome are not expressed; this is known as X inactivation. This happens in XXY males as well as normal XX females.However, in XXY males, a few genes located in the pseudoautosomal regions of their X chromosomes, have corresponding genes on their Y chromosome and are capable of being expressed.These triploid genes in XXY males may be responsible for symptoms associated with Klinefelter’s syndrome.
The first published report of a man with a 47,XXY karyotype was by Patricia A. Jacobs and Dr. J.A. Strong at
Treatment
The genetic variation is irreversible. Testosterone treatment is an option for some individuals who desire a more masculine appearance and identity. 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.
Variations
The 48, XXYY (male) syndrome occurs
Males with Klinefelter syndrome may have a mosaic 47,XXY/46,XY constitutional karyotype and varying degrees of spermatogenic failure. Mosaicism 47,XXY/46,XX with clinical features suggestive of Klinefelter syndrome is very rare. Thus far, only about 10 cases have been described in literature.
Trisomy 18 Edwards’ Syndrome
Introduction
Trisomy 18 syndrome (also known as Edwards’ syndrome, after Dr. John Edwards) is a rare chromosomal disorder in which there are three copies of chromosome 18 (trisomy) rather than the usual two. It is characterized by specific dysmorphic features and organ malformations. While the majority of Trisomy 18 cases are “full” trisomy (where there are three copies of chromosome
Features and Characteristic
Symptoms and findings may be extremely variable from case to case. However, in many affected infants, the following may be found:
Growth deficiency
Feeding difficulties
Breathing difficulties
Developmental delays
Mental Retardation
Undescended testicles in males
Prominent back portion of the head
Small head (microcephaly)
Low-set, malformed ears
Abnormally small jaw (micrognathia)
Small mouth
Cleft lip/palate
Upturned nose
Narrow eyelid folds (palpebral fissures)
Widely-spaced eyes (ocular hypertelorism)
Dropping of the upper eyelids (ptosis)
Overlapped, flexed fingers
Underdeveloped or absent thumbs
Underdeveloped nails
Webbing of the second and third toes
Clubfeet
Small pelvis with limited movements of the hips
Short breastbone
Kidney malformations
Structural heart defects at birth (i.e., ventricular septal defect, atrial septal defect, patent ductus arteriosus)
Diagnosis
Intrauterine diagnosis is possible with an amniocentesis and chromosome studies. Chromosome studies may be indicated when the mother’s uterus appears unusually large during pregnancy, there is feeble fetal activity, there is an excess of fluid in the fetal sac, a small placenta is noted, and there is a single umbilical artery.
For those cases who are not diagnosed prenatally, the first sign of Trisomy 18 may be seen at birth when the baby appears thin and frail, he or she fails to thrive, has a weak cry, and is small for his or her gestational age. Various other features (as mentioned above) may be indicative of the syndrome as well. Chromosome studies, however, are performed to verify the diagnosis.
Treatment
There is no cure for Trisomy 18, therefore, treatment is based on managing symptoms. For example, many babies with Trisomy 18 have feeding problems that involve breathing, sucking, and swallowing difficulties. Others may have clefts, reflux, or problems with aspiration. For these children, a dysphagia clinic or feeding specialist (i.e., an occupational therapist) may be able to help improve feeding skills. In other cases, a G-tube may be necessary.
Some children with heart problems have difficulty gaining weight. For babies with this problem, parents may work with a nutritionist to devise ways to increase the baby’s caloric intake.
Many children with Trisomy 18 suffer from irritability due to constipation. To help with this, special formula may be needed to form a softer stool or a stool softener medication may be needed. Parents should always seek the advice of their child’s doctor before trying any medication to treat constipation or other health concerns.
Problems with muscle tone and other nervous system abnormalities are common in children with Trisomy 18. Motor skills are often affected and can lead to other problems, such as scoliosis. Physical and occupational therapy should be provided to improve fine and gross motor skills.
Some children have hearing and vision impairments. Hearing aides and glasses should be considered for such children.
Turner syndrome
Turner syndrome or Ullrich-Turner syndrome (also known as “Gonadal dysgenesis”:550) encompasses several conditions, of which monosomy X (deletion of an entire X chromosome) is most common. It is a chromosomal disorder in which all or part of one of the sex chromosomes is absent (unaffected humans have 46 chromosomes, of which 2 are sex chromosomes). Typical females have 2 X chromosomes, but in Turner syndrome, one of those sex chromosomes is missing or has other abnormalities. In some cases, the missing chromosome is present in some cells but not others, a condition referred to as mosaicism.
Occurring in 1 out of every 2500 girls, the syndrome manifests itself in a number of ways. There are characteristic physical abnormalities, such as short stature, swelling, broad chest, low hairline, low-set ears, and webbed 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 are also frequently present, including congenital heart disease, hypothyroidism (reduced hormone secretion by the thyroid), diabetes, vision problems, hearing concerns, and many other autoimmune diseases. Finally, a specific pattern of cognitive deficits is often observed, with particular difficulties in visuospatial, mathematic, and memory areas.
Symptoms
The arrows point to some of the classical features of Turner’s syndrome: (A) short webbed neck; (B) cubitus valgus; (C) lymphedema.
Common symptoms of Turner syndrome include:
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)
Amenorrhea, or the absence of a menstrual period
Increased weight, obesity
Shield shaped thorax of heart
Shortened metacarpal IV (of hand)
Small fingernails
Characteristic facial features
Webbed neck from cystic hygroma in infancy
Coarctation of the aorta
Poor breast development
Horseshoe kidney
Visual impairments sclera, cornea, glaucoma, etc.
Ear infections and hearing loss
Other symptoms may include a small lower jaw (micrognathia), cubitus valgus (turned-out elbows), 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, and no two individuals will share the same symptoms.
Risk factors
Risk factors for Turner syndrome are not well known. Nondisjunctions increase with maternal age, such as for Down syndrome, but that effect is not clear for Turner syndrome. It is also unknown if there is a genetic predisposition present that causes the abnormality, though most researchers and doctors treating Turners women agree that this is highly unlikely. There is currently no known cause for Turner syndrome, though there are several theories surrounding the subject. The only solid fact that is known today, is that during conception part or all of the X chromosome is not transferred to the fetus.
Incidence
Approximately 98% of all fetuses with Turner syndrome result in miscarriage.[citation needed] Turner syndrome accounts for about 10% of the total number of spontaneous abortions in the
History
The syndrome is named after Henry Turner, an
Diagnosis
Turner syndrome may be diagnosed by amniocentesis during pregnancy. Sometimes, fetuses with Turner syndrome are identified by abnormal ultrasound findings (i.e. heart defect, kidney abnormality, cystic hygroma, ascites). Although the recurrence risk is not increased, genetic counseling is often recommended for families who have had a pregnancy or child with Turner syndrome.
A test, called a karyotype or a chromosome analysis, analyzes the chromosomal composition of the individual. This is the test of choice to diagnose Turner syndrome.
Prognosis
While most of the physical findings in Turner syndrome are harmless, there can be significant medical problems associated with the syndrome.
Cardiovascular
Price et al. (1986 study of 156 female patients with Turner syndrome) showed a significantly greater number of deaths from diseases of the circulatory system than expected, half of them due to congenital heart disease—mostly preductal coarctation of the aorta. When patients with congenital heart disease were omitted from the sample of the study, the mortality from circulatory disorders was not significantly increased.
Cardiovascular malformations are a serious concern as it is the most common cause of death in adults with Turner syndrome. It takes an important part in the 3-fold increase in overall mortality and the reduced life expectancy (up to 13 years) associated with Turner syndrome.
Cause
According to Sybert, 1998 the data is inadequate to allow conclusions about phenotype-karyotype correlations in regard to cardiovascular malformations in Turner syndrome because the number of individuals studied within the less common karyotype groups is too small. Other studies also suggest the presence of hidden mosaicisms that are not diagnosed on usual karyotypic analyses in some patients with 45,X karyotype.
In conclusion, the associations between karyotype and phenotypic characteristics, including cardiovascular malformations, remain questionable.
Prevalence of cardiovascular malformations
The prevalence of cardiovascular malformations among patients with Turner syndrome ranges from 17% (Landin-Wilhelmsen et al., 2001) to 45% (Dawson-Falk et al., 1992).
The variations found in the different studies are mainly attributable to variations in non-invasive methods used for screening and the types of lesions that they can characterize (Ho et al., 2004). However Sybert, 1998 suggests that it could be simply attributable to the small number of subjects in most studies.
Different karyotypes may have differing prevalence of cardiovascular malformations. Two studies found a prevalence of cardiovascular malformations of 30% and 38% in a group of pure 45,X monosomy. But considering other karyotype groups, they reported a prevalence of 24.3% and 11% in patients with mosaic X monosomy , and a prevalence of 11% in patients with X chromosomal structural abnormalities.
The higher prevalence in the group of pure 45,X monosomy is primarily due to a significant difference in the prevalence of aortic valve abnormalities and aortic coarctation, the two most common cardiovascular malformations.
Congenital heart disease
The most commonly observed are congenital obstructive lesions of the left side of the heart, leading to reduced flow on this side of the heart. This includes bicuspid aortic valve and coarctation of the aorta. Sybert, 1998 found that more than 50% of the cardiovascular malformations observed in her study of individuals with Turner syndrome were bicuspid aortic valves or coarctation of the aorta, alone or in combination.
Other congenital cardiovascular malformations such partial anomalous venous drainage and aortic stenosis or aortic regurgitation are also more common in Turner syndrome than in the general population. Hypoplastic left heart syndrome represents the most severe reduction in left-sided structures.
Bicuspid aortic valve. Up to 15% of adults with Turner syndrome have bicuspid aortic valves, meaning that there are only two, instead of three, parts to the valves in the main blood vessel leading from the heart. Since bicuspid valves are capable of regulating blood flow properly, this condition may go undetected without regular screening. However, bicuspid valves are more likely to deteriorate and later fail. Calcification also occurs in the valves, which may lead to a progressive valvular dysfunction as evidenced by aortic stenosis or regurgitation.
With a prevalence from 12.5% to 17.5% (Dawson-Falk et al., 1992), bicuspid aortic valve is the most common congenital malformation affecting the heart in this syndrome. It is usually isolated but it may be seen in combination with other anomalies, particularly coarctation of the aorta.
Coarctation of the aorta. Between 5% and 10% of those born with Turner syndrome have coarctation of the aorta, a congenital narrowing of the descending aorta, usually just distal to the origin of the left subclavian artery and opposite to the duct (and so termed “juxtaductal”). Estimates of the prevalence of this malformation in patients with Turner syndrome ranges from 6.9%[8] to 12.5% (Dawson-Falk et al., 1992). A coarctation of the aorta in a female is suggestive of Turner syndrome, and suggests the need for further tests, such as a karyotype.
Partial anomalous venous drainage. This abnormality is a relatively rare congenital heart disease in the general population. The prevalence of this abnormality also is low (around 2.9%) in Turner syndrome. However, its relative risk is
In the management of a patient with Turner syndrome it is essential to keep in mind that these left-sided cardiovascular malformations in Turner syndrome result in an increased susceptibility to bacterial endocarditis. Therefore prophylactic antibiotics should be considered when procedures with high risk endocarditis are performed, such as dental cleaning.
Turner syndrome is often associated with persistent hypertension, sometimes in childhood. In the majority of Turner syndrome patients with hypertension, there is no specific cause. In the remainder, it is usually associated with cardiovascular or kidney abnormalities, including coarctation of the aorta.
Aortic dilation, dissection, and rupture
Two studies have suggested aortic dilatation in Turner syndrome, typically involving the root of the ascending aorta and occasionally extending through the aortic arch to the descending aorta, or at the site of previous coarctation of the aorta repair.
Allen et al., 1986 who evaluated 28 girls with Turner syndrome, found a significantly greater mean aortic root diameter in patients with Turner syndrome than in the control group (matched for body surface area). Nonetheless, the aortic root diameter found in Turner syndrome patients were still well within the limits.
This has been confirmed by the study of Dawson-Falk et al., 1992 who evaluated 40 patients with Turner syndrome. They presented basically the same findings: a greater mean aortic root diameter, which nevertheless remains within the normal range for body surface area.
Sybert, 1998 points out that it remains unproven that aortic root diameters that are relatively large for body surface area but still well withiormal limits imply a risk for progressive dilatation.
Prevalence of aortic abnormalities
The prevalence of aortic root dilatation ranges from 8.8% to 42% in patients with Turner syndrome. Even if not every aortic root dilatatioecessarily goes on to an aortic dissection (circumferential or transverse tear of the intima), complications such as dissection, aortic rupture resulting in death may occur. The natural history of aortic root dilatation is still unknown, but it is a fact that it is linked to aortic dissection and rupture, which has a high mortality rate.
Aortic dissection affects 1% to 2% of patients with Turner syndrome. As a result any aortic root dilatation should be seriously taken into account as it could become a fatal aortic dissection. Routine surveillance is highly recommended.
Risk factors for aortic rupture
It is well established that cardiovascular malformations (typically bicuspid aortic valve, coarctation of the aorta and some other left-sided cardiac malformations) and hypertension predispose to aortic dilatation and dissection in the general population. At the same time it has been shown that these risk factors are common in Turner syndrome. Indeed these same risk factors are found in more than 90% of patients with Turner syndrome who develop aortic dilatation. Only a small number of patients (around 10%) have no apparent predisposing risk factors. It is important to note that the risk of hypertension is increased 3-fold in patients with Turner syndrome. Because of its relation to aortic dissection blood pressure needs to be regularly monitored and hypertension should be treated aggressively with an aim to keep blood pressure below 140/80 mmHg. It has to be noted that as with the other cardiovascular malformations, complications of aortic dilatation is commonly associated with 45,X karyotype.
Pathogenesis of aortic dissection and rupture
The exact role that all these risk factors play in the process leading to such fatal complications is still quite unclear. Aortic root dilatation is thought to be due to a mesenchymal defect as pathological evidence of cystic medial necrosis has been found by several studies. The association between a similar defect and aortic dilatation is well established in such conditions such as Marfan Syndrome. Also, abnormalities in other mesenchymal tissues (bone matrix and lymphatic vessels) suggests a similar primary mesenchymal defect in patients with Turner syndrome. However there is no evidence to suggest that patients with Turner syndrome have a significantly higher risk of aortic dilatation and dissection in absence of predisposing factors. So the risk of aortic dissection in Turner syndrome appears to be a consequence of structural cardiovascular malformations and hemodynamic risk factors rather than a reflection of an inherent abnormality in connective tissue (Sybert, 1998). The natural history of aortic root dilatation is unknown, but because of its lethal potential, this aortic abnormality needs to be carefully followed.
Pregnancy
As more women with Turner syndrome complete pregnancy thanks to the new modern techniques to treat infertility, it has to be noted that pregnancy may be a risk of cardiovascular complications for the mother. Indeed several studies had suggested an increased risk for aortic dissection in pregnancy. Three deaths have even been reported. The influence of estrogen has been examined but remains unclear. It seems that the high risk of aortic dissection during pregnancy in women with Turner syndrome may be due to the increased hemodynamic load rather than the high estrogen rate. Of course these findings are important and need to be remembered while following a pregnant patient with Turner syndrome.
Cardiovascular malformations in Turner syndrome are also very serious, not only because of their high prevalence in that particular population but mainly because of their high lethal potential and their great implication in the increased mortality found in patients with Turner syndrome. Congenital heart disease needs to be explored in every female newly diagnosed with Turner syndrome. As adults are concerned close surveillance of blood pressure is needed to avoid a high risk of fatal complications due to aortic dissection and rupture.
Skeletal
Normal skeletal development is inhibited due to a large variety of factors, mostly hormonal. The average height of a woman with Turner syndrome, in the absence of growth hormone treatment, is 4’7″, about
The fourth metacarpal bone (fourth toe and ring finger) may be unusually short, as may the fifth.
Due to inadequate production of estrogen, many of those with Turner syndrome develop osteoporosis. This can decrease height further, as well as exacerbate the curvature of the spine, possibly leading to scoliosis. It is also associated with an increased risk of bone fractures.
Kidney
Approximately one-third of all women with Turner syndrome have one of three kidney abnormalities:
A single, horseshoe-shaped kidney on one side of the body.
An abnormal urine-collecting system.
Poor blood flow to the kidneys.
Some of these conditions can be corrected surgically. Even with these abnormalities, the kidneys of most women with Turner syndrome functioormally. However, as noted above, kidney problems may be associated with hypertension.
Thyroid
Approximately one-third of all women with Turner syndrome have a thyroid disorder. Usually it is hypothyroidism, specifically Hashimoto’s thyroiditis. If detected, it can be easily treated with thyroid hormone supplements.
Diabetes
Women with Turner syndrome are at a moderately increased risk of developing type 1 diabetes in childhood and a substantially increased risk of developing type 2 diabetes by adult years. The risk of developing type 2 diabetes can be substantially reduced by maintaining a normal weight.
Cognitive
Turner syndrome does not typically cause mental retardation or impair cognition. However, learning difficulties are common among women with Turner syndrome, particularly a specific difficulty in perceiving spatial relationships, such as Nonverbal Learning Disorder. This may also manifest itself as a difficulty with motor control or with mathematics. While it is non-correctable, in most cases it does not cause difficulty in daily living.
There is also a rare variety of Turner Syndrome, known as “Ring-X Turner Syndrome”, which has an approximate 60% association with mental retardation. This variety accounts for approximately 2 – 4% of all Turner Syndrome cases.
Reproductive
Women with Turner syndrome are almost universally infertile. While some women with Turner syndrome have successfully become pregnant and carried their pregnancies to term, this is very rare and is generally limited to those women whose karyotypes are not 45,X. Even when such pregnancies do occur, there is a higher than average risk of miscarriage or birth defects, including Turner Syndrome or Down Syndrome. Some women with Turner syndrome who are unable to conceive without medical intervention may be able to use IVF or other fertility treatments.
Usually estrogen replacement therapy is used to spur growth of secondary sexual characteristics at the time when puberty should onset. While very few women with Turner Syndrome menstruate spontaneously, estrogen therapy requires a regular shedding of the uterine lining (“withdrawal bleeding”) to prevent its overgrowth. Withdrawal bleeding can be induced monthly, like menstruation, or less often, usually every three months, if the patient desires. Estrogen therapy does not make a woman with nonfunctional ovaries fertile, but it plays an important role in assisted reproduction; the health of the uterus must be maintained with estrogen if an eligible woman with Turner Syndrome wishes to use IVF.
Treatment
As a chromosomal condition, there is no cure for Turner syndrome. However, much can be done to minimize the symptoms. For example:
Growth hormone, either alone or with a low dose of androgen, will increase growth and probably final adult height. Growth hormone is approved by the U.S. Food and Drug Administration for treatment of Turner syndrome and is covered by many insurance plans. There is evidence that this is effective, even in toddlers.
Estrogen replacement therapy has been used since the condition was described in 1938 to promote development of secondary sexual characteristics. Estrogens are crucial for maintaining good bone integrity and tissue health. Women with Turner Syndrome who do not have spontaneous puberty and who are not treated with estrogen are at high risk for osteoporosis.
Modern reproductive technologies have also been used to help women with Turner syndrome become pregnant if they desire. For example, a donor egg can be used to create an embryo, which is carried by the Turner syndrome woman.
Uterine maturity is positively associated with years of estrogen use, history of spontaneous menarche, and negatively associated with the lack of current hormone replacement therapy.
References
Basic:
1. Medical Genetics + Student Consult, 4th Edition. Lynn B. Jorde, John C. Carey, MPH and Michael J. Bamshad, 2010, p. 368. ISBN: 978-032-305-373-0
2. Essential Medical Genetics, 6 edition. Edward S. Tobias, Michael Connor, Malcolm Ferguson Smith. Published by Wiley-Blackwell, 2011, p. 344. ISBN: 978-140-516-974-5
3. Emery’s Elements of Medical Genetics + Student Consult, 14th Edition Peter D Turnpenny, 2012 p. 464. ISBN: 978-070-204-043-6
4. Genes, Chromosomes, and Disease: From Simple Traits, to Complex Traits, to Personalized Medicine. Nicholas Wright Gillham. Published by FT Press Science, 2011. p. 352.
5. Management of Genetic Syndromes. Suzanne B. Cassidy, Judith E. Allanson; 3 edition. Published by Wiley-Blackwell. 2010 p. 984 ISBN: 978-047-019-141-5.
6. Genetic Disorders and the Fetus: Diagnosis, Prevention and Treatment (Milunsky, Genetic Disorders and the Fetus) 6 edition. Aubrey Milunsky and Jeff Milunsky. Published by Wiley-Blackwell, 2010 p. 1184 ISBN: 978-140-519-087-9
7. Molecular Diagnostics: Fundamentals, Methods, & Clinical Applications. 1st edition. Buckingham L, Flaws ML. F.A. Davis. 2007.
8. Passarge E. Color Atlas of Genetics – Thieme, 2007 Р. 497
9. http://intranet.tdmu.edu.ua/data/kafedra/internal/index.php?&path=pediatria2/classes_stud
Additional:
1. Atlas of Inherited Metabolic Diseases 3 edition William L Nyhan Bruce A Barshop, Aida I Al-Aqeel. Published by CRC. Press 2011, p 888. ISBN: 978-144-411-225-2.
2. Inborn Metabolic Diseases: Diagnosis and Treatment Jean-Marie Saudubray, Georges van den Berghe, John H. Walter. 5th ed. Published by Springer, 2012, p. 684. ISBN: 978-364-215-719-6.
3. Chromosome Abnormalities and Genetic Counseling (
4. Mitochondrial Medicine: Mitochondrial Metabolism, Diseases, Diagnosis and Therapy Editored by Anna Gvozdjáková. Published by Soringer, 2008, p. 409 ISBN 978-1-4020-6713-6
5. Mitochondrial DNA, Mitochondria, Disease and Stem Cells (Stem Cell Biology and Regenerative Medicine) Editor Justin C. St. John. Published by Humana Press, 2012 p.199. ISBN: 978-162-703-100-4.
6. Genetic Counseling Practice: Advanced Concepts and Skills. Bonnie S. LeRoy, Patricia M. Veach, Dianne M. Bartels. Published by Wiley-Blackwell, 2010, p. 415. ISBN: 978-047-018-355-7
