Down syndrome also known as trisomy 21, is considered to be one of the most common diseases due to chromosomal abnormality in humans. In this diseases there is an extra copy of chromosome 21. Common characteristics of the disease include delayed physical growth, typical facial patterns and varying degree of intellectual disability.
Several persons with Down syndrome complete high school education, even some of them are eligible for further education. In the US about 20% of the total Down syndrome affected persons do paid work although some of them require sheltered working environment. Currently with proper care and support life expectancy of a person with Down syndrome is about 50 to 60 years.
Complications
Common complications with Down syndrome are
1) Defect in the heart
2) Cancer: blood cancer and cancer of the testes are quite common.
3) Infectious diseases: infectious diseases quite common due to malfunctioning of the immune system
4) Disturbed Sleep: This occurs due to sudden stoppage of breathing due to obstruction of the airways.
5) Obesity
6) Skin diseases like psoriasis
Symptoms
In every person having Down syndrome there are intellectual and developmental delays; The presenting symptoms of Down syndrome can be discussed under following headings
Physical symptoms:
Commonly associated facial features of Down syndrome include
1) Flattening of all the facial features
2) Small sized head (microcephaly)
3) Short neck
4) Protrusion of tongue
5) Typical upward slanting of the eyes (not consistent with the ethnic background of the child)
6) Unusual shape of the ears or abnormally small ears
7) Reduced muscle tone
8) Broadening and shortening of hands with characteristic single crease  on the palm and short fingers
9) Abnormal flexibility
10) Appearance of small white spots on the iris the colored part of the eyes (known as Brushfield spots)
11) Abnormally short height: Usually infants with Down syndrome are of average size but the rate of growth is slower and the developmental milestones up to 2 years (like sitting, crawling etc) are reached with delay.
Nerve related symptoms
1) Intellectual disability: mild to moderate degree of intellectual disability is common (IQ between 20 and 35).
2) Linguistic Feature: They have better understanding of language than speech; some of the affected person might loose the ability to speak after 30 years. Rapid and irregular speech are common problems.
3) Epileptic seizure is common in Down syndrome
4) Dementia due to (memory loss) Alzheimer’s disease is common.
Hearing and vision related symptoms   
1) Hearing problems: affect about 50 to 90% of the children having Down syndrome. Ear infections are common in these children due to collection of wax in the outer ear and due to poor functioning of the Eustachian tube.
2) Visual problems: it affects about 38 to 80% of the people. Inability to focus both the eyes at the same point and the shape of the cornea became conical and thinner.
Other Symptoms
1) Hormonal problems: Malfunctioning of the thyroid gland and Diabetes mellitus (type 1) are more common in these people.
2) Reproductive problems: Both male and female are rarely fertile. Affected females are more prone to early occurrence of menopause.
3) Gastrointestinal symptoms: about half of the people withDown syndrome suffer from constipation due to poor development of nerve cells at the colon.
Risk factors
Common risk factors of Down syndrome include
1. Maternal age: the chance of having a baby with Down syndrome increases as the mother gets older and the older eggs are more prone to faulty cell division.
2. History of having a baby with Down syndrome: chance of having the second baby with Down syndrome is 1 out of 100 (1%).
3. Carriers of genetic translocation: carrier state in either of the parents increases the chance of babies with Down syndrome.
Treatment
Intervention at an early stage makes a lot of difference in the future life of the affected child. This helps to develop the motor skill, social skills, language etc of the child.The affected child usually requires care from the specialist to manage and prevent the complications.
Prevention
There is no measure available to avoid Down syndrome; although at risk persons like those having Down syndrome, past history of having a baby with Down syndrome, etc. should consult a genetic counselor before getting pregnant.