Down syndrome or Trisomy 21 refers to the presence of three instead of two 21st chromosomes in each of the body’s cells. This results in cognitive impairment, slightly different appearance, and risk for certain health issues or birth defects. Mosaicism is a term used to describe a condition where people have different kinds of cells in the body. Mosaic Down syndrome can thus be understood as a condition where people have some cells with the Trisomy 21 arrangement, and some cells that only posses two #21 chromosomes.
Mosaic Down Syndrome remains a rare variant of Trisomy 21, and it’s usually tested for in a blood sample. Doctors look for evidence of any cells with the normal arrangement to show whether this condition is present. Number of normal cells is expressed in a percentage value based on the Trisomy cells. A measurement of 80 out of 100 trisomy cells would be called 80% mosaicism. Percentages may vary depending on the part of the body tested.
From a logical standpoint, it would make sense to think that lower percentages of mosaicism would always correspond to less expression of Down syndrome. This isn’t necessarily the case, and it doesn’t seem that percentage will always predict the degree to which people are affected. In general it can be said that many people with Mosaic Down Syndrome have fewer symptoms, but this isn’t always the case.
Though a medically interesting variant, this type of Down syndrome is sometimes just as likely as total Trisomy 21 to have the following symptoms:
- Impaired cognition, which may be slight to major
- Characteristic appearance of smaller hands, short neck, tilted eyes, small mouth, flattened nose, and shorter stature
- Great risk of congenital heart defects (about 50% of kids with Down syndrome)
- Higher risk for leukemia and thyroid dysfunction
- More likelihood of vision and hearing problems
Some huge differences exist between Mosaic Down syndrome and the standard syndrome expression. First, most Down cases are diagnosed prior to birth or at birth. Mosaicism may not be diagnosed until children are between 1 and 4. It’s also the case that Mosaic Down syndrome is much rarer than standard Trisomy 21. An estimated 3% of Downs’s cases show mosaicism.
The child with Trisomy 21 or Mosaic Down Syndrome does have many difficult things to overcome. These children may need more medical intervention than a child without a third #21 chromosome and they also benefit from early learning opportunities. Despite the deficits accompanying this condition, many of these kids can do well, be mainstreamed in school environments to a certain point, and hold jobs as adults while they live independently or semi-independently in group living settings.