[Sonographic diagnosis of alobar holoprosencephaly].

The alobar form of holoprosencephaly is characterised by a huge spherical ventricle localized in the midline. The monoventricle communicates with a large occipital cyst, the "dorsal sac". Both thalami and plexus chorioidei are fused in the midline. The interhemisperic fissure and the falx cerebri are absent as well as the corpus callosum and the septum pellucidum.