Non-neoplastic Cystic Lesions of the Central Nervous System, Part 2: Idiopathic and Acquired Cysts SA–CME REVIEW When they are large, glioependy- mal cysts may compress the adjacent brain parenchyma or obstruct the ventricles. Symptoms and signs may include headache, dizziness, motor deficit, seizures, hemifacial spasms, and coma. 6 There is no gender pre- dilection. Most cysts present during the first and third decades of life. 6 Glioependymal cysts are typically hypodense on CT and isointense to CSF on all MRI sequences. There is typically no enhancement in the wall (Figure 2). Robles, et al, suggested lack of adjacent bone thinning may differentiate these cysts from arachnoid cysts. 6 Defini- tive diagnosis can only be made on histopathology. The cysts are lined with a thin inner-ependymal layer, an intermediate glial layer, and an outer connective tissue layer. Multiple names have been used to describe these cysts in the literature. They include neuroglial, ependy- mal, supratentorial, epithelial, and neuroepithelial cysts. 6,7 Owing to the lesions’ unique wall structure, glioependymal cyst remains the preferred name. 8 Choroidal Fissure Cyst Choroidal fissure cysts are common, small (<30mm) intracra- nial cysts centered in the choroidal fissure along the mesial temporal lobe. The choroidal fissure is a cleſt that contains the choroid plexus of the inferior horn of the lateral ventricle and extends from the hippocampal fissure in the anterior temporal lobe to the atrium of the lateral ventricle at the level of the foramen of Monro. 9 The separation between the tem- poral horn and the choroidal fissure is formed by the tela choroidea, epi- thelially derived from the roof plate of the telencephalon. Formation of an ependymal diverticulum, which subsequently becomes isolated from the ventricular system and its vascu- lar pia mater, results in a choroidal fissure cyst. 10 Choroidal fissure cysts tend to be oval, with their long axes parallel to the choroidal fissure in the antero- posterior plane. 10 These cysts rarely enlarge; however, when they do, headaches, epilepsy, tremor, pares- thesia, hemiparesis, gait disturbance, vertigo, or hearing loss may result. 10 On imaging, choroidal fissure cysts are isodense to CSF on CT and iso- intense to CSF on all MRI sequenc- es (Figure 3). There should be no Figure 5. Porencephalic cyst. A middle-aged adult with prior hemorrhage. Follow up MRI exam show cystic encephalomalacia in the left periventricular whiter. Axial FLAIR (A) shows CSF intensity cyst that communicates with the left lateral ventricle. Notice the high signal intensity in the wall (arrow) representing gliosis. Figure 6. Porencephalic cyst. An older adult with dizziness. Axial FLAIR shows a large, intra-axial CSF intensity cyst in the left deep cerebral whiter communicating with the left lateral ventricle. Note the mass effect and lack of significant gliosis in the wall. Applied Radiology 11 September / October 2022