21trisomia

21trisomia, commonly referred to as trisomy 21 or Down syndrome, is a chromosomal condition defined by the presence of an extra copy of chromosome 21 in some or all cells. Most cases involve full trisomy 21, caused by nondisjunction during meiosis, producing three copies of chromosome 21 in all cells. Less common forms include mosaic trisomy 21, where only a portion of cells carry the extra chromosome, and Robertsonian translocation, in which a fragment of chromosome 21 attaches to another chromosome and can be inherited.

Prevalence and causes: Trisomy 21 is the most common chromosomal condition diagnosed in newborns. In many populations the risk is about 1 in 700 live births, and it increases with maternal age. Many conceptions with trisomy 21 do not result in live birth and end in miscarriage or stillbirth.

Clinical features and health considerations: Intellectual disability ranges from mild to moderate, with broad variation in individual abilities. Typical features include hypotonia in infancy and characteristic facial features. About half or more of individuals have congenital heart defects, most commonly atrioventricular septal defects. Other associated health issues include thyroid disorders, hearing and vision problems, obstructive sleep apnea, and an increased lifetime risk of leukemia. Adults with Down syndrome have a higher risk of early-onset Alzheimer disease. Atlantoaxial instability is a potential neuromuscular concern.

Diagnosis and screening: Prenatal screening and diagnostic tests can detect trisomy 21, including noninvasive prenatal testing (NIPT) and ultrasound markers; diagnostic procedures such as chorionic villus sampling or amniocentesis determine fetal karyotype. Postnatal diagnosis is confirmed by karyotyping or chromosomal analysis.

Management and prognosis: Care is multidisciplinary, focusing on early intervention, education, medical monitoring, and treatment of associated conditions. Life expectancy and quality of life have improved substantially with advances in medical care and social support, though outcomes vary by health care access and individual needs.