Two cases of congenital hydrocephalus are presented with a large cyst in the region of the incisura tentorii. In one case the cyst was seen to originate from a folium on the superior surface of the cerebellum. Gross and histologic observations in the other case unfortunately must remain incomplete but are quite consistent with the same origin, although it could also have arisen for the medial temporal lobe. Inner neuroglial and outer leptomeningeal laminae are considered most significant microscopic elements of the cyst wall. These cases are considered to represent meningo-encephaloceles of the superior cerebellum, the cyst remaining within the cranial cavity and causing hydrocephalus by extension through the incisura tentorii and compression of the ventricular system about the level of the aqueduct of Sylvius.Such lesions are admittedly rare but have been seen by other investigators, although the origin from the cerebellum has apparently not been previously recognized. It is suggested that these lesions may originate from cystic degeneration of subarachnoid glial heterotopias and that they may be closely related to cerebral arachnoid cysts (intracranial meningoceles). The further suggestion is made that certain cases previously reported as examples of atresia of the foramina of Lusehka and Magendie may represent similar intracranial meningoceles or meningo-encephaloceles originating from the posterior or inferior surfaces of the cerebellum, especially if the anterior wall of the cyst separates the cyst from the fourth ventricle.Although ldquo;congenital cyst of the tentoriumrdquo; is in the standard nomenclature, no evidence has been found to substantiate such a diagnosis, and it is understood that this diagnosis is being removed; in the present cases, although both had cysts in this region, certainly neither originated from the tentorium.
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