trisomy 8, a congenital condition associated with the presence of an extra chromosome 8 within the C group. Those with the condition are slender and of normal height and have a large asymmetric head, prominent forehead, deep-set eyes, low-set prominent ears, and thick lips. There are mild to severe cognitive and motor impairments, often with delayed and poorly articulated speech. Skeletal anomalies and joint limitation, especially permanent flexion of one or more fingers, may occur. There are unusually deep palmar and plantar creases, which are diagnostically significant. Most trisomy 8 individuals are mosaic, with no abnormal or only slight clinical manifestations, or they are only partially trisomic, with part of the extra chromosome 8 missing, and show varying degrees of the clinical symptoms. In general, trisomy 8 is a less severe condition than other trisomies, especially trisomy 13 and trisomy 18, so that mortality is low. Also called trisomy C syndrome.
