Down syndrome

موضوع: Down syndrome الثلاثاء أغسطس 10, 2010 4:20 pm

r trisomy G, is a chromosomal disorder caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866. The disorder was identified as a chromosome 21 trisomy by Jérôme Lejeune in 1959. The condition is characterized by a combination of major and minor differences in structure. Often Down syndrome is associated with some impairment of cognitive ability and physical growth, and a particular set of facial characteristics. Down syndrome in a fetus can be identified with amniocentesis during pregnancy, or in a baby at birth.

Individuals with Down syndrome tend to have a lower than average cognitive ability, often ranging from mild to moderate developmental disabilities. A small number have severe to profound mental disability. The average IQ of children with Down syndrome is around 50, compared to normal children with an IQ of 100.[3] The incidence of Down syndrome is estimated at 1 per 800 to 1,000 births, although it is statistically much more common with older mothers. Other factors may also play a role.

Many of the common physical features of Down syndrome may also appear in people with a standard set of chromosomes, including microgenia (an abnormally small chin),[4] an unusually round face, macroglossia[5] (protruding or oversized tongue), an almond shape to the eyes caused by an epicanthic fold of the eyelid, upslanting palpebral fissures (the separation between the upper and lower eyelids), shorter limbs, a single transverse palmar crease (a single instead of a double crease across one or both palms, also called the Simian crease), poor muscle tone, and a larger than normal space between the big and second toes. Health concerns for individuals with Down syndrome include a higher risk for congenital heart defects, gastroesophageal reflux disease, recurrent ear infections, obstructive sleep apnea, and thyroid dysfunctions.

Early childhood intervention, screening for common problems, medical treatment where indicated, a conducive family environment, and vocational training can improve the overall development of children with Down syndrome. Although some of the physical genetic limitations of Down syndrome cannot be overcome, education and proper care will improve

Signs and symptoms

Down syndrome can result from several different genetic mechanisms. This results in a wide variability in individual signs and symptoms due to complex gene and environment interactions. Prior to birth, it is not possible to predict the symptoms that an individual with Down syndrome will develop.

Individuals with Down syndrome may have some or all of the following physical characteristics: microgenia (abnormally small chin),[4] oblique eye fissures with epicanthic skin folds on the inner corner of the eyes (formerly known as a mongoloid fold),[5] muscle hypotonia (poor muscle tone), a flat nasal bridge, a single palmar fold, a protruding tongue (due to small oral cavity, and an enlarged tongue near the tonsils) or macroglossia,[5] a short neck, white spots on the iris known as Brushfield spots,[7] excessive joint laxity including atlanto-axial instability, excessive space between large toe and second toe, a single flexion furrow of the fifth finger, and a higher number of ulnar loop dermatoglyphs. Most individuals with Down syndrome have mental retardation in the mild (IQ 50–70) to moderate (IQ 35–50) range,[8] with individuals having Mosaic Down syndrome typically 10–30 points higher.[9] They also may have a broad head and a very round face.

Language skills show a difference between understanding speech and expressing speech, and commonly individuals with Down syndrome have a speech delay.[10] Fine motor skills are delayed[11] and often lag behind gross motor skills and can interfere with cognitive development. Effects of the disorder on the development of gross motor skills are quite variable. Some children will begin walking at around 2 years of age, while others will not walk until age 4. Physical therapy, and/or participation in a program of adapted physical education (APE), may promote enhanced development of gross motor skills in Down syndrome children.[12]

Growth parameters such as height, weight, and head circumference are smaller in children with DS than with individuals of the same age. Adults with DS tend to have short stature — the average height for men is 5 feet 1 inch (157 cm) and for women is four feet 9 inches (144 cm).[13] Individuals with DS are also at increased risk for obesity as they ag

Complications

Individuals with Down syndrome have a higher risk for many conditions. The medical consequences of the extra genetic material in Down syndrome are highly variable and may affect the function of any organ system or bodily process. Some problems are present at birth, such as certain heart malformations. Others become apparent over time, such as epilepsy.

Congenital heart disease

The incidence of congenital heart disease in children with Down syndrome is up to 50%.[15] A ventricular septal defect is the most common form.

Malignancies

Hematologic malignancies such as leukemia are more common in children with DS. In particular, the risk for acute lymphoblastic leukemia is at least 10 times more common in DS and for the megakaryoblastic form of acute myelogenous leukemia is at least 50 times more common in DS. Transient leukemia is a form of leukemia which is rare in individuals without DS but affects up to 20 percent of newborns with DS. This form of leukemia is typically benign and resolves on its own over several months, though it can lead to other serious illnesses.[16] In contrast to hematologic malignancies, solid tumor malignancies are less common in DS, possibly due to increased numbers of tumor suppressor genes contained in the extra genetic material.[17]

Thyroid disorders

Individuals with DS are at increased risk for dysfunction of the thyroid gland, an organ which helps control metabolism. Low thyroid (hypothyroidism) is most common, occurring in almost a third of those with DS. This can be due to absence of the thyroid at birth (congenital hypothyroidism) or due to attack on the thyroid by the immune system.[18] Reproduction is also affected by DS.

Gastrointestinal

Down syndrome increases the risk of Hirschsprung's disease, in which the nerve cells that control the function of parts of the colon are not present.[19] This results in severe constipation. Other congenital anomalies occurring more frequently in DS include duodenal atresia, annular pancreas, and imperforate anus. Gastroesophageal reflux disease and celiac disease are also more common among people with DS.[20]

Infertility

There is infertility amongst both males and females with Down syndrome; males are usually unable to father children, while females demonstrate significantly lower rates of conception relative to unaffected individuals.[citation needed] Women with DS are less fertile and often have difficulties with miscarriage, premature birth, and difficult labor. Without preimplantation genetic diagnosis, approximately half of the offspring of someone with Down syndrome also have the syndrome themselves.[21] Men with DS are almost uniformly infertile, exhibiting defects in spermatogenesis.[22] There have been only three recorded instances of males with Down syndrome fathering children.[23][24]

Neurology

Children and adults with DS are at increased risk for developing epilepsy.[25][26] The risk for Alzheimer's disease is increased in individuals with DS, with 10-25% of individuals with DS showing signs of AD before age 50, up to 50% with clinical symptoms in the sixth decade, and up to 75% in the 7th decade. This sharp increase in the incidence and prevalence of dementia may be one of the factors driving the decreased life expectancy of persons with Down Syndrome.

Ophthalmology and otolaryngology

Eye disorders are more common in people with DS. Almost half have strabismus, in which the two eyes do not move in tandem. Refractive errors requiring glasses or contacts are also common. Cataracts (opacity of the lens) and glaucoma (increased eye pressures) are also more common in DS.[27] Brushfield spots (small white or grayish/brown spots on the periphery of the iris) may be present.

Other complications

In the past, prior to current treatment, there was a 38-78% incidence of hearing loss in children with Down syndrome. Fortunately, with aggressive, meticulous and compulsive diagnosis and treatment of chronic ear disease (e.g. otitis media, also known as Glue-ear) in children with Down syndrome, approximately 98% of the children have normal hearing levels. [28]

Instability of the atlanto-axial joint occurs in ~15% of people with DS, probably due to ligamental laxity. It may lead to the neurologic symptoms of spinal cord compression.[29] Periodic screening, with cervical x-rays, is recommended to identify this abnormality.

Other serious illnesses include immune deficiencies.

موضوع: رد: Down syndrome الخميس أغسطس 12, 2010 1:45 pm

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موضوع: رد: Down syndrome الجمعة أغسطس 13, 2010 8:11 am

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موضوع: رد: Down syndrome الثلاثاء أغسطس 24, 2010 10:25 am

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