Apparent atypical callosal dysgenesis: analysis of MR findings in six cases and their relationship to holoprosencephaly.

The MR scans of six pediatric patients with apparent atypical callosal dysgenesis (presence of the dorsal corpus callosum in the absence of a rostral corpus callosum) were critically analyzed and correlated with developmental information in order to assess the anatomic, embryologic, and developmental implications of this unusual anomaly. Four patients had semilobar holoprosencephaly; the dorsal interhemispheric commissure in these four infants resembled a true callosal splenium. All patients in this group had severe developmental delay. The other two patients had complete callosal agenesis with an enlarged hippocampal commissure mimicking a callosal splenium; both were developmentally and neurologically normal. The embryologic implications of the presence of these atypical interhemispheric connections are discussed. Differentiation between semilobar holoprosencephaly and agenesis of the corpus callosum with enlarged hippocampal commissure--two types of apparent atypical callosal dysgenesis--can be made by obtaining coronal, short TR/TE MR images through the frontal lobes. Such differentiation has critical prognostic implications.