Children whose seizures have some of these features are less likely to have the syndrome of Childhood Absence Epilepsy. 1  An absence seizure with mild clonic components involves mild, often subtle twitching of the eyelids, the corners of the mouth, and sometimes the arms.
1 Trisomy 21 â More than 90% of Down syndrome cases are caused by trisomy 21. 2  An extra chromosome (chromosome 21) originates in the development of either the sperm or the egg. 3  When the egg and the sperm unite to form the fertilized egg, three (rather than two) chromosomes 21 are present.
These cell division abnormalities result in extra genetic material from chromosome 21, which is responsible for the characteristic features and developmental problems of Down syndrome. Any one of three genetic variations can cause Down syndrome: 1  Trisomy 21.2  About 95 percent of the time, Down syndrome is caused by trisomy 21 â the child has three copies of chromosome 21 (instead of the usual two copies) in all cells. Mosaic Down syndrome. 2  In this rare form of Down syndrome, children have some cells with an extra copy of chromosome 21. 3  This mosaic of normal and abnormal cells is caused by abnormal cell division after fertilization. 4  Translocation Down syndrome.