Mental development in Down syndrome mosaicism.

Serial assessments of mental status of 30 subjects with Down syndrome mosaicism were compared with those of 30 subjects with trisomy 21 Down syndrome. The samples were matched for age, sex, and parental socioeconomic background. The mean IQ level of the mosaic Down syndrome group was significantly higher than that of the trisomy 21 group. Many subjects with mosaicism showed better verbal abilities, and some also demonstrated normal visual-perceptual skills in paper- and-pencil tasks. Children with Down syndrome with IQs over 60 at 5 years of age and with relatively normal speech can be viewed as a suspected case of mosaicism. Chromosomal analysis is mandatory to confirm or disprove this diagnosis.