Female, 35 years old, with headache and hydrocephalus. A midline cystic lesion is evident centered on the midbrain and measuring up to 29 mm AP, 19 mm CC, and 17 mm TV. Inferiorly, the lesion contains multiple septations or cystic loculations, and this portion of the lesion appears to be intraparenchymal within the midbrain tegmentum. Superiorly, the dominant portion of the cyst projects into the posterior aspect of the third ventricle, elevating and stretching the midbrain tectum, and likely obstructing outflow through the cerebral aqueduct. No clear evidence of any solid or enhancing tissue is seen within the lesion.The lateral and third ventricles are enlarged with evidence of mild transependymal CSF flow. The fourth ventricle is within normal limits. Incidental note is made of descent of the cerebellar tonsils approximately 10 mm below the level of the foramen magnum.
A cystic lesion is evident centered on the midbrain which projects into the third ventricle and likely effaces the cerebral aqueduct. Consequently, hydrocephalus involving the lateral and third ventricles is seen. The lesion appears to be at least partly intraparenchymal within the midbrain tegmentum, and this portion of the lesion demonstrates multiple septations or loculations. The differential diagnosis would include a neuroglial cyst, with arachnoid cyst a somewhat less likely consideration given location and imaging features. A cystic neoplasm is felt to be unlikely due to lack of solid or enhancing tissue.Inferior herniation of the cerebellar tonsils is seen which technically meets criteria for a Chiari I malformation. However, this finding should be assessed after correction of hydrocephalus as the appearance may change.