Specific Imaging Findings Holoprosencephaly (HPE) is characterized by lack of separation of the cerebral hemispheres and absence of midline structures. Several forms are identified based on the severity and extent of abnormalities. • Alobar HPE. Complete absence of hemispheric separation and midline structures results in a so-called holosphere. A single monoventricle communicates posteriorly with a dorsal cyst. The hypothalamus is noncleaved to some degree, and the deep gray matter structures are nonseparated in almost all cases. • Semilobar HPE. The occipital and, to some extent, parietal lobes are separated with posteriorly present interhemispheric fissure and falx, as well as the callosal splenium. The trigones and occipital horns of the lateral ventricles are formed, but the frontal horns and septum pellucidum are absent. • Lobar HPE. Lack of separation involves basal frontal regions and, variably, basal ganglia and hypothalamus; thalami are completely separated. Corpus callosum extends more anteriorly, with presence of splenium and at least a part of the body. Rudimentary frontal horns are fused in the midline due to absent septum pellucidum. • Middle interhemispheric HPE (syntelencephaly). Lack of separation involves the perirolandic region with complete separation both anteriorly and posteriorly. Consequently, an intermediate portion of the falx and interhemispheric fissure are absent and the corpus callosum is split into two portions, separated by brain tissue. Septum pellucidum is absent and lateral ventricles are dysmorphic. • Septo-preoptic HPE. Lack of separation involves only a limited zone at the preoptic (subcallosal) and septal (suprachiasmatic-hypothalamic) regions. Partial posterior hypoplasia of the corpus callosum is common.
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