The dorsal cyst of holoprosencephaly is a large cerebrospinal fluid cavity present in holoprosencephaly that occupies the area above the dorsocaudal aspect of the diencephalon. This communicates directly with the prosencephalic, telencephalic, or diencephalic ventricle. This cavity usually abuts the cranial vault in the midline parieto-occipital area and lies directly on the cerebellum because of tentorial dysplasia. With lobar holoprosencephaly, this cyst may occur on one side.
The wall of the cyst is a thin transparent membrane consisting of ependymal epithelium and leptomeninges, in which, occasionally, a layer of cerebral tissue or heterotopic glial rest is seen.
Although the term dorsal cyst has been traditionally reserved for holoprosencephaly, histologically identical cavities are also found in other dorsal midline anomalies. These include
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