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.
Often, heart defects and other problems are present. It is one of the most common genetic birth defects, affecting approximately 1 in 800 babies. Chromosomes are the structures in cells that contain the genes. Down syndrome is caused by extra genetic material from chromosome 21. Affected persons have two normal copies of number 21 chromosomes, plus extra chromosome number 21 material attached to another chromosome. For that reason, Down syndrome is sometimes referred to as Trisomy 21.