A 4-years-old male presented with a history of global developmental delay with a delayed achievement of walking without support and speech. There was no significant antenatal history however there was a history of two minutes delayed cry at birth.
The overall incidence of holoprosencephaly varies from 1:10000 live births to 1:250 aborted fetuses.[3,4]
Holoprosencephaly is a rare congenital malformation of the brain due to the failure of bifurcation of the two cerebral hemispheres. Several variants of holoprosencephaly have been identified which are alobar holoprosencephaly, semilobar holoprosencephaly, lobar holoprosencephaly, syntelencephaly , septopreoptic holoprosencephaly,  solitary median maxillary central incisor syndrome  and congenital nasal pyriform aperture stenosis .
Alobar holoprosencephaly: The brain is often small in size, agyric or mildly sulcated with flat pancake or ball-like configuration . There is no falx cerebri, no interhemispheric fissure, no septum pellucidum, fused bilateral basal ganglia , agenesis of corpus callosum . There is a single crescent-shaped monoventricle that communicates dorsally into a large CSF filled dorsal cyst .
Semilobar holoprosencephaly: There is fusion of anterior aspect of the bilateral cerebral hemispheres, rudimentary falx cerebri and IHF, absence of genu and body of corpus callosum, absent septum pellucidum, fused bilateral basal ganglia  and absent frontal horns .
Lobar holoprosencephaly: There is fusion of most anterior and ventral part of the bilateral frontal lobes with rest of the cerebral hemispheres completely separate from each other, dysmorphic frontal horns, absence of falx cerebri and IHF anteriorly, mildly dysplastic genu of corpus callosum, separate basal ganglia  and absent septum pellucidum .
Syntelencephaly is postulated to result from a mutation of zic-2 gene resulting in reduction in mitotic rate and increased apoptosis in the roof plate causing non-formation of the interhemispheric fissure .
Patient usually presents with global developmental delay with spasticity, hypotonia or dystonia. Imaging of brain in syntelencephaly shows incomplete separation of the posterior frontal and parietal lobes . The interhemispheric fissure is present anteriorly and posteriorly but it is absent in the middle . In approximately 85% of cases the sylvian fissures are abnormally oriented and appear to connect with each other across the midline . Grey matter heterotopia and cortical dysplasias are seen in two-thirds of syntelencephaly cases. There is a variable degree of fusion seen in the bilateral thalami in patients of syntelencephaly . There is partial agenesis of the corpus callosum with the formation of genu and splenium but the body is absent . Syntelencephaly is the only brain malformation with such distribution of the corpus callosum. In almost all cases of syntelencephaly, an associated azygos cerebral artery is seen. Many case reports of syntelencephaly have shown its association with cerebellar hypoplasia, chiari malformation, polymicrogyria and cephaloceles . Most patients of syntelencephaly have normal hypothalamus, caudate and basal ganglia .
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