Holoprosencephaly (alobar)

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Holoprosencephaly, with a birth prevalence of about 1 in 10 000, is characterized by a spectrum of cerebral abnormalities resulting from incomplete cleavage of the forebrain. There are three types according to the degree of forebrain cleavage. The alobar type is characterized by a monoventricular cavity and fusion of the thalami. In the semilobar type there is partial segmentation of the ventricles and cerebral hemispheres posteriorly with incomplete fusion of the thalami. In lobar holoprosencephaly, there is normal separation of the ventricles and thalami but absence of the septum pellucidum. The first two types are often accompanied by facial abnormalities.
Alobar and semilobar types are often accompanied by facial abnormalities. Those types of holoprosencephaly are strongly associated with chromosomal defects (at least 50% of cases). Trisomy 13 (Patau syndrome) is the most common aneuploidy. Other chromosomal defects include triploidy, trisomy 18 and other rare chromosomal conditions. The prognosis of alobar holoprosencephaly is very poor. Even in the cases without chromosomal defects the condition is lethal or associated with profuse neurological disability.