Down syndrome
Down syndrome or sometimes Down's syndrome, also known as Trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is usually associated with developmental delays, mild to moderate intellectual disability, and characteristic physical features.
The parents of the affected individual are usually genetically normal. The incidence of the syndrome increases with the age of the mother, from less than 0.1% for 20-year-old mothers to 3% for those of age 45. It is believed to occur by chance, with no known behavioral activity or environmental factor that changes the probability. Three different genetic forms have been identified. The most common, trisomy 21, involves an extra copy of chromosome 21 in all cells. The extra chromosome is provided at conception as the egg and sperm combine. Translocation Down syndrome involves attachment of extra chromosome 21 material. In 1–2% of cases, the additional chromosome is added in the embryo stage and only affects some of the cells in the body; this is known as mosaic Down syndrome.
Down syndrome can be identified during pregnancy by prenatal screening, followed by diagnostic testing, or after birth by direct observation and genetic testing. Since the introduction of screening, Down syndrome pregnancies are often aborted.
There is no cure for Down syndrome. Education and proper care have been shown to provide better quality of life. Some children with Down syndrome are educated in typical school classes, while others require more specialized education. Some individuals with Down syndrome graduate from high school, and a few attend post-secondary education. In adulthood, about 20% in the United States do some paid work, with many requiring a sheltered work environment. Caregiver 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. Regular screening for health issues common in Down syndrome is recommended throughout the person's life.
Down syndrome is the most common chromosomal abnormality, occurring in about 1 in 1,000 babies born worldwide, and one in 700 in the US. In 2015, there were 5.4 million people with Down syndrome globally, of whom 27,000 died, down from 43,000 deaths in 1990. The syndrome is named after British physician John Langdon Down, who dedicated his medical practice to the cause. Some aspects were described earlier by French psychiatrist Jean-Étienne Dominique Esquirol in 1838 and French physician Édouard Séguin in 1844. The genetic cause was discovered in 1959.
Signs and symptoms
Those with Down syndrome nearly always have physical and intellectual disabilities. As adults, their mental abilities are typically similar to those of an eight- or nine-year-old. At the same time, their emotional and social awareness is very high. They can have poor immune function and generally reach developmental milestones at a later age. They have an increased risk of a number of health concerns, such as congenital heart disease, epilepsy, leukemia, and thyroid diseases.| Characteristics | Percentage | Characteristics | Percentage |
| Mental impairment | 99% | Abnormal teeth | 60% |
| Stunted growth | 90% | Slanted eyes | 60% |
| Umbilical hernia | 90% | Shortened hands | 60% |
| Increased skin on back of neck | 80% | Short neck | 60% |
| Low muscle tone | 80% | Obstructive sleep apnea | 60% |
| Narrow roof of mouth | 76% | Bent fifth finger tip | 57% |
| Flat head | 75% | Brushfield spots in the iris | 56% |
| Flexible ligaments | 75% | Single transverse palmar crease | 53% |
| Proportionally large tongue | 75% | Protruding tongue | 47% |
| Abnormal outer ears | 70% | Congenital heart disease | 40% |
| Flattened nose | 68% | Strabismus | ≈35% |
| Separation of first and second toes | 68% | Undescended testicles | 20% |
Physical
People with Down syndrome may have these physical characteristics: a small chin, epicanthic folds, low muscle tone, a flat nasal bridge, and a protruding tongue. A protruding tongue is caused by low tone and weak facial muscles, and often corrected with myofunctional exercises. Some characteristic airway features can lead to obstructive sleep apnea in around half of those with Down syndrome. Other common features include: excessive joint flexibility, extra space between big toe and second toe, a single crease of the palm, and short fingers.Instability of the atlantoaxial joint occurs in about 1–2%. Atlantoaxial instability may cause myelopathy due to cervical spinal cord compression later in life, this often manifests as new onset weakness, problems with co-ordination, bowel or bladder incontinence, and gait dysfunction. Serial imaging cannot reliably predict future cervical cord compression, but changes can be seen on neurological exam. The condition is surgically corrected with spine surgery.
Growth in height is slower, resulting in adults who tend to have short stature—the average height for men is, and for women is. Individuals with Down syndrome are at increased risk for obesity as they age due to increased risk of hypothyroidism, other medical issues and lifestyle. Growth charts have been developed specifically for children with Down syndrome.
Neurological
This syndrome causes about a third of cases of intellectual disability. Many developmental milestones are delayed with the ability to crawl typically occurring around 8–22 months rather than 6–12 months, and the ability to walk independently typically occurring around 1–4 years rather than 9–18 months. Walking is acquired in 50% of children after 24 months.Most individuals with Down syndrome have mild or moderate intellectual disability with some cases having severe difficulties. Those with mosaic Down syndrome typically have IQ scores 10–30 points higher than that. As they age, the gap tends to widen between people with Down syndrome and their same-age peers.
Commonly, individuals with Down syndrome have better language understanding than ability to speak. Babbling typically emerges around 15 months on average. 10–45% of those with Down syndrome have either a stutter or rapid and irregular speech, making it difficult to understand them. After reaching 30 years of age, some may lose their ability to speak.
They typically do fairly well with social skills. Behavior problems are not generally as great 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%. People with Down syndrome experience a wide range of emotions. While people with Down syndrome are generally happy, symptoms of depression and anxiety may develop in early adulthood.
Children and adults with Down syndrome are at increased risk of epileptic seizures, which occur in 5–10% of children and up to 50% of adults. This includes an increased risk of a specific type of seizure called infantile spasms. Many who live 40 years or longer develop Alzheimer's disease. In those who reach 60 years of age, 50–70% have the disease.
Down syndrome regression disorder is a sudden regression with neuropsychiatric symptoms such as catatonia, possibly caused by an autoimmune disease. It primarily appears in teenagers and younger adults.
Senses
Hearing and vision disorders occur in more than half of people with Down syndrome.Ocular findings
, upward slanting palpebral fissures and epicanthal folds are clinical signs at birth suggesting the diagnosis of Down syndrome especially in the Western World. None of these requires treatment.Visually significant congenital cataracts occur more frequently with Down syndrome. Neonates with Down syndrome should be screened for cataract because early recognition and referral reduce the risk of vision loss from amblyopia. Dot-like opacities in the cortex of the lens are present in up to 50% of people with Down syndrome, but may be followed without treatment if they are not visually significant.
Strabismus, nystagmus and nasolacrimal duct obstruction occur more frequently in children with Down syndrome. Screening for these diagnoses should begin within six months of birth. Strabismus is more often acquired than congenital. Early diagnosis and treatment of strabismus reduces the risk of vision loss from amblyopia. In Down syndrome, the presence of epicanthal folds may give the false impression of strabismus, referred to as pseudostrabismus. Nasolacrimal duct obstruction, which causes tearing, is more frequently bilateral and multifactorial than in children without Down syndrome.
Refractive error is more common with Down syndrome, though the rate may not differ until after twelve months of age compared to children without Down syndrome. Early screening is recommended to identify and treat significant refractive error with glasses or contact lenses. Poor accommodation is associated with Down syndrome, which may mean bifocals are indicated.
In keratoconus, the cornea progressively thins and bulges into a cone shape, causing visual blurring or distortion. Keratoconus first presents in the teen years and progresses into the thirties. Down syndrome is a strong risk factor for developing keratoconus, and onset may be occur at a younger age than in those without Down syndrome. Eye rubbing is also a risk factor for developing keratoconus. It is speculated that chronic eye irritation from blepharitis may increase eye rubbing in Down syndrome, contributing to the increased prevalence of keratoconus.
An association between glaucoma and Down syndrome is often cited. Glaucoma in children with Down syndrome is uncommon, with a prevalence of less than 1%. It is currently unclear if the prevalence of glaucoma in those with Down syndrome differs from that in the absence of Down syndrome.
Estimates of prevalence of ocular findings in Down syndrome vary widely depending on the study. Some prevalence estimates follow. Vision problems have been observed in 38–80% of cases. Brushfield spots are present in 38–85% of individuals. Between 20 and 50% have strabismus. Cataracts occur in 15%, and may be present at birth. Keratoconus may occur in as many as 21–30%.