There is fusion across the midline of the grey matter at one location. The thalami are normal and there is a normal corpus callosum. The sulcation of the grey matter of the grey matter is less than normal indicating abnormal neuronal migration, i.e pachygyria or polymicogyria. There is also an anomaly of a cortical vein.
Abnormal neuronal migration of the grey matter includes pachygyria, polymicrogyria, cortical heterotopia, band heterotopia, subependymal heterotopia, schizencephaly and holprosencephaly.
Middle Interhemispheric holo- prosencephaly or syntelencephaly is an unusual form of semilobar holo-prosencephaly.
In this form, the inter-hemispheric fissure is formed frontally and occipitally, but the hemispheres are fused in the frontal and/or parietal regions.
Usually the corpus callosum is absent only in the body.
They may have failure of midline cleavage of the thalamus and also may have dorsal cysts.
In this case, the form is so mild that the corpus callosum is intact and only a small portion of brain is fused across the midline.
Holoprosencephaly has an equal gender incidence. It occurs in syndromes such as trisomy 13, trisomy 18 and Edward’s.
There are teratogenic factors such as maternal diabetes that have been implicated.Whereas in more severe forms of holoprosencphaly, there is hypotelorism, in this form facial abnormalities are mild or absent.
The prognosis varies in holoprosencephaly, but is better in this form.
This anomaly occurs as a result of failure of differentiation and cleavage in the prosencephalon early in the first trimester of gestation.