Down syndrome is a chromosomal disorder that affects more than 350,000 U.S. citizens. Down syndrome ranks second to fragile X syndrome as the most frequent genetic cause of mental retardation. Although recognized by Edouard Seguin, a French physician and educator, as early as 1846, the first written description of the disorder was published in 1866 by John Langdon Down, a British physician from whom the syndrome derives its name. Jerome LeJeune, a French geneticist, and his colleagues established the genetic basis of Down syndrome in 1959.
Down syndrome is estimated to occur in 1 in 1,000 live births. The syndrome can be diagnosed prenatally through chromosomal analysis derived from either chorionic villus sampling or amniocentesis. Boys outnumber girls 1.3 to 1.0, and the disorder occurs in all racial and ethnic groups. Down syndrome is predominantly (95% of cases) due to trisomy of chromosome 21, involving a nondisjunction or failure of the two chromosomes of pair 21 to separate during meiosis prior to ovulation. Five percent of Down syndrome cases result from translocation in which part of chromosome 21 attaches to another chromosome or from mosaicism due to an error in cell division soon after conception. Increased maternal age is related to both nondisjunction and mosaicism. Down syndrome is incurable. The only current preventive strategy is termination of pregnancy, an option that appears to be increasingly utilized.
Physical And Medical Aspects
Affected individuals share to varying degrees a set of physical characteristics. The most common features of Down syndrome include a flattened face with a recessed bridge of the nose; upward slanting eyes with epicanthal folds; small ears and mouth; large tongue; short, broad hands and feet; stubby fingers; broad neck; stocky appearance; and loose skin folds at the nape of the neck. Down syndrome has a large number of associated medical problems including congenital gastrointestinal and cardiac abnormalities, eye problems, low muscle tone, and mild to moderate conductive hearing loss secondary to chronic middle ear infections. Infants and young children with Down syndrome are also at greater risk for acute leukemia than the general population.
High rates of hypothyroidism have also been linked to Down syndrome, using normative values from the general population. Lower thyroid functioning values for people with Down syndrome may reflect the susceptibility of adults with Down syndrome to experience “premature aging” and not thyroid disorder. Despite increased truncal obesity among people with Down syndrome, traditional risk factors for type II diabetes and cardiac disease, including hypertension, elevated lipids, and elevated glucose, are less than those reported by the general U.S. population.
Children with Down syndrome are delayed in reaching early developmental milestones compared with typically developing children. In particular, gross motor development is attained at later ages in children with Down syndrome. Independent sitting is usually attained at 1 year of age, while independent walking is not achieved until 2 years of age. Early language development, especially language production, is also slow to progress.
People with Down syndrome typically function within the mild to moderate range of mental retardation on standardized intelligence tests, although some function in the borderline range and a few have severe mental retardation. The intellectual impairments of people with Down syndrome are generally attributed to a reduction in the size of the cortex and cerebellum relative to matched controls and an immaturity of brain development evident in neurons and their synaptic connections. People with Down syndrome show greater deficits in verbo-linguistic skills relative to visuospatial skills. Delayed language acquisition may be related to the overall slowed intellectual development revealed in longitudinal studies of infants and young children with Down syndrome. People with Down syndrome mosaicism typically have IQs that are 12 to 15 points higher than people with trisomy 21. Adaptive behavior is generally commensurate with intellectual ability.
Children with Down syndrome show more maladaptive behaviors than typically developing children, but have a lower risk for psychopathology than other children with intellectual disabilities. Commonly occurring problems include oppositional behavior, stubbornness, and inattention. Research suggests that maladaptive behaviors are most prevalent during adolescence. Adults with Down syndrome show fewer externalizing problems but may have increased internalizing problems such as depression. However, stubbornness often remains characteristic of people with Down syndrome across ages. Interestingly, 7% to 8% of people with Down syndrome may meet criteria for autism.
Children with Down syndrome express fewer positive social signals, show delayed responsiveness, and generate less predictable responses in mother-child interactions than typically developing children. Without clear and frequent signals from their children, mothers of children with Down syndrome may be more likely than mothers of typically developing children to adopt a controlling and directive interactive style during naturalistic play.
Parents of children with Down syndrome report experiencing less stress and feeling more rewarded by their children than do parents of children with other intellectual disabilities. Similarly, siblings of children with Down syndrome experience fewer adjustment problems than siblings of children with other intellectual disabilities. Reasons offered for this Down syndrome advantage include mothers being “older and wiser,” parents having a better understanding of their child’s condition than parents of children with less researched syndromes, greater sociability and cheerfulness of children with Down syndrome compared to children with other learning difficulties, greater public knowledge and recognition of Down syndrome, and the perceived youthfulness of people with Down syndrome due to their immature-appearing physical features and “babyfaceness.”
People with Down syndrome have a greater early mortality rate as compared to people of similar ages from the general population and to people with comparable levels of mental retardation due to other causes. Congenital heart disease contributes greatly to the increased early mortality rate, but so do respiratory tract infections, leukemia, and congenital gastrointestinal tract anomalies. Although improved medical management of respiratory infection and congenital heart disease in young children with Down syndrome has significantly increased the life expectancy, people with Down syndrome have lower life expectancies than the general population. The average life expectancy for people with Down syndrome is approximately 55 years.
Interestingly, genes linked to the development of dementia of the Alzheimer type are also located on chromosome 21. The triplication of chromosome 21 may result in an increased risk for developing Alzheimer’s dementia. Seventy-five percent of adults with Down syndrome over 60 years of age show neurological abnormalities and behavioral signs of Alzheimer’s disease. These symptoms frequently include seizures, personality change, focal neurological signs, apathy, and loss of conversational skills. The allele for the gene apolipoprotein E (ApoE) may affect the risk for Alzheimer’s dementia. Down syndrome people with the ApoE e4 allele have been found to have a heightened risk for developing Alzheimer’s type dementia, whereas the ApoE e2 allele may serve as a preventive factor. Recent studies suggest that Aricept (donepezil hydrochloride), a drug targeting deficiencies in cholinergic neurotransmission, may improve cognitive functioning, and specifically language performance, in nondemented people with Down syndrome and slow deterioration in cognitive functioning and adaptive behavior among people with Down syndrome and Alzheimer’s type dementia.
High rates of epilepsy (8% to 13%) have been noted among people with Down syndrome. It is estimated that 40% of cases begin before 1 year of age and another 40% appear in the second to third decades of life. Early onset epilepsy often involves infantile spasms and tonicclonic seizures with myoclonus, whereas epilepsy in adults typically involves partial simple or complex seizures and may be associated with Alzheimer’s disease. Prevalence of epilepsy is thought to increase with age and is estimated to be as high as 46% among adults with Down syndrome who are over 50 years old.
Interventions And Community Living
Infants and young children with Down syndrome typically receive early intervention designed to help them reach their fullest developmental potential. Although it remains unclear whether early interventions lead to increased intelligence, improvements in independence, community functioning, quality of life, language, and motor skills have been shown. Reading, writing, and arithmetic are commonly combined with daily living and social skill training for school-aged children and adolescents with Down syndrome. Successful transitions from school to work and family to community living hinge on obtaining the necessary skills for self-care, maintaining strong interpersonal relationships, and developing appropriate work habits and skills.
Lifelong programs aimed at monitoring nutrition, preventing obesity, and maintaining bone density are associated with improved health outcomes. These programs commonly include food selection, behavioral interventions, physical activities, and social activities. Programs targeting exercise and health education have been found to increase positive attitudes toward exercise, increase life satisfaction, and slightly reduce risk for depression. Cognitive and affective (e.g., too boring and too difficult) and pragmatic obstacles (e.g., lack of transportation and high costs) often hinder people with Down syndrome from engaging in fitness programs.
Adults with Down syndrome typically live in family homes, community group homes, or in independent or semi-independent living situations. Many adults with Down syndrome maintain paid employment, often with some support (e.g., supervision, training, or transportation). Increased longevity requires parents to adopt a long-term perspective in planning for their child with Down syndrome.
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- National Down Syndrome Congress, http://www.ndsc center.org
- National Down Syndrome Society, http://www.ndss.org
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