holoprosencephaly /hol′ōpros′ensef″əlē/ [Gk, holos + pro, before, enkephalos, brain] , a congenital defect caused by the failure of the prosencephalon to divide into hemispheres during embryonic development. It is characterized by multiple midline facial defects, including cyclopia in severe cases. It can also be caused by an extra chromosome in the 13–15 or D group, manifested as one of many developmental defects. See also trisomy 13. −holoprosencephalous, holoprosencephalic, adj.
