Alobar Holoprosencephaly.
DISCUSSION
Holoprosencephaly is a spectrum of cerebrofacial anomalies resulting from the complete or partial failure of the diverticulation and cleavage of the primitive forebrain. During the 4th gestational week, the neural tube forms the three primary brain vesicles, namely, prosencephalon, mesencephalon, and rhombencephalon. By the 5th week of intrauterine life, the prosencephalon further divides into the telencephalon and diencephalon. The telencephalon forms the two cerebral hemispheres whereas the diencephalon forms the thalami, the hypothalamus, and the basal ganglia.
The alobar holoprosencephaly is the most severe form and shows undifferentiated holosphere of the cerebral parenchyma with a central monoventricle and fused thalami. The falx, interhemispheric fissure, corpus callosum, optic tracts, olfactory bulbs, and the septum pellucidum are absent. Absence of septum pellucidum may be associated with septooptic dysplasia, holoprosencephaly, corpus callosal agenesis, schizencephaly, Chiari-II malformation, hydranencephaly, porencephaly, and cephaloceles.
Alobar holoprosencephaly can be differentiated from hydrocephalus by the presence of midline echogenic falx, absent septum pellucidum, separated thalami, and distinct lateral ventricles in the latter. Hydranencephaly may also demonstrate absence or deviated falx but the thalami are not fused in this condition. In both hydranencephaly and Dandy Walker malformation, the falx cerebri, interhemispheric fissure, corpus callosum, and 3rd ventricle are present.
The semilobar holoprosencephaly is of intermediate severity with early midline differentiation and sagittal separation. It shows a rudimentary falx, partial interhemispheric fissure, absent septum pellucidum, partial separation of thalami, and large H-shaped monoventricle. The basal ganglia show variable fusion. The facial anomalies are mild, namely, cleft lip, cleft palate, and hypotelorism.
The mildest variety of holoprosencephalies is the lobar holoprosencephaly characterized by near total cleavage of cerebral hemispheres, presence of falx, interhemispheric fissure, and absent septum pellucidum. The frontal horns appear squared off or box like due to the absence of septum pellucidum. The thalami and the basal ganglia are separated. It may be associated with minimal facial dysmorphism like hypotelorism.
There is a fourth variant of holoprosencephaly called the middle hemispheric variant. In this condition, the interhemispheric fissure is formed in the frontal and occipital regions and absent in the parietal region with fusion of the hemispheres