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I. Intensive and convalescent care

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  1. Definition

    1. In the mid-1800s several physicians described groups of patients who had intellectual disability, short stature, and specific facial characteristics including upslanting palpebral fissures, epicanthal folds, flat nasal bridge, and protruding tongue.

    2. Subsequently, J. Langdon Down, after whom Down syndrome (DS) was named, emphasized that affected individuals could be distinguished from the heterogeneous group of all those with intellectual disabilities based on their unique physical features.

    3. Unlike other genetic conditions, individuals with DS are typically diagnosed in the newborn period based on these distinctive physical characteristics.

    4. Clinical syndrome is phenotype of individuals with Trisomy 21.

  2. Incidence

    1. DS, or trisomy 21, is the most common genetic cause of intellectual disability, occurring in one in 691 live births.

    2. DS occurs in all ethnic groups, socioeconomic levels, and geographic regions.

    3. Once the medical complications at birth are appropriately treated, individuals with DS can live well into their 60s and beyond with productive lives if appropriate educational, medical, and societal resources are provided.

  3. Pathophysiology

    1. DS is described as the presence of an additional (third) copy of the genetic material located on the 21st chromosome.

    2. The typical human karyotype is composed of 46 chromosomes, which group into 23 pairs. The first 22 pairs of chromosomes are known as the autosomes. The last pair, composed of the X chromosome and/or the Y chromosome, is known as the sex chromosomes. Females will have two X chromosomes, while males will have one X chromosome and one Y chromosome.

    3. DS can occur in one of three ways.

      1. Nondisjunction

        • DS due to nondisjunction, also known as standard trisomy 21, occurs when like chromosomes fail to separate during meiotic division. This results in the egg or sperm cell contributing two copies of chromosome 21, producing a child with three complete copies of the 21st chromosome.

        • Standard trisomy 21 is the most common form of DS, accounting for 95% of all known cases.

        • The recurrence risk for parents of a child with standard trisomy 21 is 1% or one in 100, until the maternal age–related risk for having a child with a chromosomal variation exceeds 1% (>38 y/o).

        • The karyotype for an individual with standard trisomy 21 will be 47,XY,+21 in males or 47,XX,+21 in females.

      2. Robertsonian translocation

        • DS due to a Robertsonian translocation occurs when an individual inherits a complete copy of the 21st chromosome from each parent, as well as, a copy of the long arm of the 21st chromosome fused to the long arm of another acrocentric chromosome (chromosome 13, 14, 15, 21, or 22).

        • DS due to a Robertsonian translocation accounts for 3% to 4% of all known cases.

        • The recurrence risk for parents of a child with a Robertsonian translocation can vary between 0.5% and 100% depending on which parent is the carrier of the balanced translocation, as well as which acrocentric chromosome was involved. Thus, it is important that parental chromosome karyotype analysis be performed prior to providing a recurrence risk.

        • The karyotype for ...

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