Holoprosencephaly is a structural abnormality of the brain resulting from a failed or incomplete division of the forebrain in the third or fourth week of gestation (it is incompletely divided into the right and left hemispheres, also affecting the deep brain structures and the olfactory and optic bulbs and tracts).
Severe forms are first identified during prenatal ultrasound, intermediate forms are most frequently diagnosed during the neonatal period, when abnormal facial findings and/or neurological presentation require further evaluation. Less severe forms, with normal or only mildly dysmorphic facies and mild brain abnormalities, cannot be diagnosed until the first year of life when neuroimaging studies obtained during evaluation for developmental delay and/or growth retardation reveal holoprosencephaly.
