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OVERVIEW
Trisomy 18 (Edwards Syndrome) is a genetic disorder that occurs when an extra copy of the chromosome is present in the 18th chromosomal pairing. This syndrome was named after John H. Edwards, who first described the syndrome in 1960. This is the second most common genetic disorder in humans after Down Syndrome. The statistics claim that between 1 in 2000 and 1 in 7000 embryos are Trisomy 18. Edwards Syndrome is normally characterized by low set ears, deformed fingers, narrow nose and a receding jaw as well as congenital heart disease. (See below for more characteristics.)
Like Trisomy 21, mental retardation is an inevitable result of this extra chromosome, however the majority of these children die within a very short time after birth. A higher risk of the baby dying whilst still in the uterus is also a factor. Very few Trisomy 18 children live beyond one year.
CAUSE
It is caused by the presence of three - instead of two - chromosomes 18 in a fetus or baby's cells. The additional chromosome usually occurs before conception. A healthy egg or sperm cell contains 23 individual chromosomes - one to contribute to each of the 23 pairs of chromosomes needed to form a normal cell with 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 (non-disjunction). 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.
PRENATAL DIAGNOSIS
Chorionic Villus Sampling (CVS) is performed at 10 to 12 weeks and Amniocentesis is performed at 16 weeks. The Triple Test will detect approximately 50 percent of cases. Routine detailed ultrasound scanning at 18 weeks can detect up to 80 percent of cases. A suspected diagnosis of Edwards Syndrome can only be confirmed by amniocentesis or placental biopsy.
MoonDragon's Obgyn Procedures: CVS (Chorionic Villus Sampling)
PRESENTATION
The extra genetic information from the third chromosome causes the abnormalities characteristic of individuals with Edwards Syndrome. Since every cell in the body contains extra information, the ability to grow and develop appropriately is delayed or impaired. This results in characteristic physical abnormalities such as low birth weight; a small, abnormally shaped head; small jaw; small mouth; low-set ears; and clenched fists with overlapping fingers. A characteristic "rocker bottom" foot is also common. Babies with Edwards syndrome also have heart defects, and other organ malformations such that most systems of the body are affected.
PROGNOSIS (OUTCOME)
The survival rate for Edwards Syndrome is very low. About half die in utero. Of live born infants, only 50% live to 2 months, and only 5 to 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. As major medical interventions are routinely withheld from these children, it is also difficult to determine what the survival rate or prognosis would be for the condition if they were treated with the same aggressiveness as their genetically normal peers.
Psychological and behavioral characteristic: Surviving children with Edwards' syndrome have marked developmental and motor delays. Their verbal communication is severely impaired, and is limited to a few single words at best; but they are mostly aware of their environment and are able to communicate some of their needs non-verbally. They continue to acquire new skills over time and develop some understanding of language. A few can walk with assistance. Overall, their skills in daily living, receptive language and social interaction tend to be higher than their motor and expressive language abilities.
Children with trisomy 18 mosaicism or partial trisomy 18 tend to be less severely handicapped; they are able to walk, and have moderate or mild learning difficulties or normal intelligence.
INCIDENCE / PREVALENCE
The rate of occurrence for Edwards Syndrome is ~ 1:3000 conceptions and 1:6000 live births, as 50% of those diagnosed prenatally with the condition will not survive the prenatal period. Although women in their 20's and 30's may conceive Edwards Syndrome babies, there is an increased risk of conceiving a child with Edwards Syndrome as a woman's age increases. Inheritance patterns are seen as being sporadic. In rare cases, a balanced translocation in a parent can give rise to an unbalanced translocation in their child.
GENETIC VARIATIONS
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.
There are three types of the syndrome:
- Full form (severe) - In this every cell in the body has three chromosome 18's instead of two.
- Mosaic form (less severe) - In this some cells have two chromosome 18's while others have three. The extent and severity of the condition will depend upon the ratio of normal to abnormal cells.
- Partial form - In some cases there may be an extra copy of part of chromosome 18. This is referred to as partial trisomy 18. The effects of this may be milder and would require further medical advice.
Children with all their cells affected do not normally survive beyond infancy. Those affected by the mosaic and partial forms may survive to adulthood. Characteristic features include growth deficiency, low-set and malformed ears, clenched hands, bone abnormalities, hernias, skin mottling, heart defects, feeding and breathing problems in infancy and learning disability.
SYMPTOMS, FEATURES & CHARACTERISTICS
Symptoms and findings may be extremely variable from case to case.
The authors present an 18 months old infant presenting with a plurimalfprmative syndrome suggestive for trisomy 18. The clinical characteristics, imaging studies and results of genetic studies definers this syndrome are presented.
However, in many affected infants, the following may be found:
- Arthrogryposis a muscle disorder that causes multiple joint contractures at birth.
- 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/Cleft 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.
- Absent radius.
- Webbing of the second and third toes.
- Clubfeet or Rocker bottom feet.
- 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).
HELPFUL INFORMATION & SUPPORT
The following sites can be useful for people seeking further information or support for Edwards Syndrome.
Resource Links
The Chromosome 18 Registry & Research Society
Trisomy 18 Support Foundation
Rainbows Down Under - A Trisomy 18 & 13 Resource
www.geneticalliance.org
Support Organization for Trisomy 18, 13 & Related Disorders
Ability - See the Ability Not the Disability
The Chromosome 18 Registry & Research Society
March of Dimes
Washington State Genetics Documents
New York Times Video: Perinatal Hospice Care - Preparing For Birth & Death
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