Normal human cells contain 23 pairs of chromosomes. In each pair, one chromosome comes from the mother and the other from the father. Sometimes, there is abnormal cell division involving the chromosome that results inDown syndrome. This abnormal cell division results in an extra partial or full chromosome 21, which is responsible for all the features associated withDown syndrome. Down syndrome is usually notinherited. Abnormal cell division in the early stages offetal developmentcauses Down syndrome.
There are no known external or behavioral factors that cause Down syndrome.
There are three possible genetic variations that can cause Down syndrome
- Trisomy 21:Trisomy 21 is the most common and is seen in 95% of the cases. The person is born with three copies of chromosome 21. This abnormality occurs during cell division during the development of the sperm or egg.
- Mosaic Down syndrome:This is rare. The person is born with only some abnormal cells with trisomy 21, whereas the rest are normal. Hence, the condition is called “mosaic” due to the mosaic pattern of normal and abnormal cells. This usually occurs due to abnormal cell division afterfertilizationof the sperm and egg.
- Translocation Down syndrome:This form of Down syndrome occurs when a portion of chromosome 21 gets attached to another chromosome (translocation). This can occur before or duringconception. Children with this condition are born with the usual two copies of chromosome 21 but have additional genetic material from chromosome 21 attached to another chromosome.
Risk factors:Certain factors increase the risk of having a baby with Down syndrome. High-risk parents are usually advised to get genetic counseling
- Advancing maternal age:A woman's risk of having a child with Down syndrome increases after 35 years of age because older eggs have a high risk of abnormal cells and chromosomal division. However, most children with Down syndrome are usually born to women under 35 years of age because more births are observed among younger women.
- Being carriers of genetic translocation for Down syndrome:The child can inherit genetic translocation for Down syndrome from either parent.
- Having one child with Down syndrome:Having one child with Down syndrome can increase the risk of having another child with Down syndrome.
What is Down syndrome?
Down syndrome is agenetic disordercaused by abnormal cell division resulting in chromosomal abnormality. Abnormalities include an extra full or partial copy of chromosome 21. This genetic abnormality leads to developmental and physical changes that are characteristic of Down syndrome.
The severity of Down syndrome varies. Some cases may be mild, whereas others have a severe mental disability and developmental retardation. They may also have systemic medical abnormalities, includingheartand gastrointestinal (bowel) disorders. The lifespan of patients with Down syndrome has increased to more than 60 years, depending on the severity of health problems because of advancements in medical treatment options. Treatment requires a multidisciplinary approach involving doctors from several medical specialties, healthcare providers, and special education teachers. Lifelong treatment is required to manage their medical and intellectual problems.
What are the signs and symptoms of Down syndrome?
唐氏综合症患者have characteristic facial features including
- Small head
- Flattened face
- Short neck
- Small ears
- Upward slanting eyelids
- Protruding tongue
- Broad, short hands with a single crease in the palm
- Short fingers
- Small hands andfeet
- Short stature
- 可怜的肌肉
- Excessive joint flexibility
- Brushfield's spots (small white spots on theiris, the colored portion of the eyes)
Intellectual disabilities
- Cognitive impairment
- Delayed speech and language
- Poor memory
- Learning disabilities
Complications of Down syndrome
Complications of Down syndrome
- 心defects
- Gastrointestinal (GI) defects
- Decreased vision
- Dental problems
- Immune system disorders
- Seizures
- Sleep apnea(a type ofsleepdisorder in whichbreathingrepeatedly stops and starts)
- Obesity
- Recurrentear infections
- Decreasedhearing
- Spinal disorders
- Bloodcancer
- Dementiaand Alzheimer’s disease
How is Down syndrome diagnosed?
Screening tests and diagnostic tests are usually recommended for allpregnantwomen, irrespective of age.
Screening tests duringpregnancy:They indicate the risk of carrying a baby with Down syndrome.
First-trimester tests
- Blood tests:Abnormal levels ofpregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (HCG) levels in the blood indicate a problem with the baby.
- Nuchal translucency test:Anultrasoundis done to measure a particular area of the back of the baby's neck. In the presence of abnormalities, there is increased fluid collection in the neck.
Second-trimester tests
- The quad screenis the measurement of four pregnancy-associated substances, namely HCG, alpha-fetoprotein, inhibin A and estriol in the blood.
Diagnostic tests during pregnancy:They can confirm the diagnosis of Down syndrome.
- Chorionic villus sampling(CVS):Cells are taken from the placenta and analyzed between 10 and 13 weeks of pregnancy.
- Amniocentesis:Amniotic fluidsurrounds the fetus in the uterus. The fluid is aspirated from the mother's uterus using a needle. The cells in the amniotic fluid are analyzed for chromosomal abnormalities. This is usually performed after 15 weeks of pregnancy.
Diagnosis in newborns:A preliminary diagnosis is based on the baby's appearance. The diagnosis can be confirmed by analyzing the chromosomes in the cells from a blood sample.
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