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The abnormal midline cyst of diencephalic origin was termed "diencephalic cyst" by Brocklehurst in 1973. Since then, several case reports of so-called "diencephalic cyst" have appeared, but its clinical picture, especially its relation to other midline dysraphisms, does not seem to be well understood. Our experience with four cases of so-called "diencephalic cyst" were reported and their relation to holoprosencephaly as well as simple agenesis of corpus callosum was discussed. (1) In all of these cases, there was an abnormal space in the midline which freely communicated with the lateral ventricle. In the first two cases, their clinical pictures as well as the features of contrast studies were typical of alobar holoprosencephaly. In the third case, the absence of the olfactory tract, the incompletely separated cerebrum, and the monoventricle were confirmed at autopsy. In the last case, the ventricle was also incompletely separated. From these observations, such a malformation should better be considered as a variant of holoprosencephaly, and it would not be appropriate to classify this malformation, as Brocklehurst claimed, as "a maldevelopment lying between the group associated with anterior neuropore closure and prosencephalization (anterior encephaloceles and holoprosencephaly) and the group associated with maldevelopment of the hind brain (cerebellar and fourth ventricle encephaloceles and Dandy-Walker cyst of the rhombencephalon)." (2) This malformation is apparently formed before the 40 mm stage of the embryo, since by this time the diencephalic roof has already sunk to the level of the adult form. This maldevelopment should be clearly differenciated from simple agenesis of the corpus callosum which is formed after the 60 mm stage of the embryo. In simple agenesis of the corpus callosum, the deep venous system might take an abnormal course, but is well formed, whereas in holoprosencephaly or so-called "diencephalic cyst", the internal cerebral vein as well as the straight sinus and the inferior sagittal sinus is absent. (3) In all of our cases, there was an abnormal space posterior and inferior to the elevated lateral sinus. Autopsy done in two cases revealed that this abnormal space was a Dandy-Walker cyst in one case, the the dorsal sack of diencephalic origin in the other case. The latter case tells that in the cases of holoprosencephaly the lateral sinus does not necessarily mark the boundary between the cerebrum and the cerebellum. (4) "Diencephalic cyst" often accompanies severe hydrocephalus. However, even with successful surgical treatment of hydrocephalus, the outcome of intelligence in these cases would be grave.
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S Matsumoto
March 1981,
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