Ependymal cysts within or adjacent to the brain and spinal cord most probably arise during the second trimester and may be silent for a long period to occasionally become symptomatic through their space-occupying properties at any age, from the fetal period to adulthood. This entity presents as cysts of ependymal epithelial origin, probably arising by budding and separating from the embryonic central canal or early ventricular system and presenting as a space-occupying process. They must be distinguished from neoplastic cysts, arachnoidal cysts, neurocysticercosis, and enterogenous cysts by their neuroradiological and neuropathological features. A subgroup arises as part of multisystemic disorders, such as orofaciodigital syndromes and Aicardi syndrome. The outcome of treatment of solitary cysts has vastly benefited from the application of endoscopic neurosurgery. The improvement in neurosurgical techniques underlines the need for refined differential diagnosis, separating this group from malignant cystic tumors such as cystic gliomas. In this article, the author, whose special interest is in developmental and genetic disorders, reviews the present knowledge of ependymal cysts and also addresses a related entity, the choroid plexus cyst.
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• Ependymal cysts arise from focal collections of ependymal cells, inside or outside and adjacent to the brain and spinal cord.
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• Ependymal cysts may become space occupying and may require neurosurgical intervention, particularly if they obstruct CSF flow within the ventricular system.
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• Ependymal cysts can arise as part of congenital malformation syndromes such as Aicardi syndrome or orofaciodigital syndromes.
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• Most ependymal cysts appear prominently in the second trimester by fetal ultrasound or MRI but “disappear” later in gestation or in early infancy because they rupture into the surrounding CSF and do not reform.
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• Choroid plexus cysts arise from choroid plexus in any of the ventricles and are loculated cysts with the secretory epithelium facing outward into the ventricular cavity or inward to the cyst lumen. They may occur sporadically as isolated, multiple, and may be associated with other brain malformations and genetic disorders including trisomy-18.
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• Subependymal cysts due to small germinal matrix hemorrhages are a major differential diagnosis.
Historical note and terminology
Different types of nonmalignant space-occupying cysts are associated with the central nervous system. The first case reports of ependymal cysts date from the thirties cited by Friede and Yasargil (26). Gradual improvement in pathological techniques, using electron microscopy of the cyst wall, and antibody staining have refined the diagnostic criteria. Because of the rarity of the diagnosis, most of the literature has accrued from case reports, mentioned in this article. Clinical interest in ependymal cysts centers on 2 different entities:
(1) Solitary ependymal cysts arising as space-occupying cysts with clear fluid. In the transverse axis these may present anywhere between the ventricular system and the leptomeningeal spaces surrounding the brain and spinal cord. Growth in volume may prompt symptoms at any age. Proper radiological and neuropathological diagnosis of this essentially benign process is required to exclude other space-occupying lesions such as cystic gliomas and arachnoid cysts. Treatment results have improved through better differential diagnosis and refined neurosurgical interventions in the last decades.
(2) Multiple ependymal cysts arising as part of a congenital disorder, some of which may carry a genetic risk for recurrence.
The group of true intracranial and intraspinal epithelial cysts includes ependymal (also known as glioependymal cysts) and enterogenous cysts. Ependymal cysts develop as heterotopic ependyma, with the inner layer recognizable as ependyma. They contain an outer layer of neuroglial tissue (astrocytes). However, ependymal cells themselves may also be positive for astroglial immunocytochemical markers. A layer of connective tissue on the outside may be present due to vascularization. These cysts may arise in any place within or surrounding the central nervous system. They may be the only structural abnormality, or they may form part of a compound cerebral and extracerebral malformation syndrome.
Ependymal cysts may be space occupying, either by displacement of adjacent structures or by obstructing CSF circulation. Clinical signs are due to these effects or to associated malformations (callosal dysgenesis, neuronal heterotopia, cortical dysplasia) or associated malformation syndromes (orofaciodigital syndromes, especially types I and II, and Aicardi syndrome).
Antibody reactivity of ependymal cyst specimens allows their differentiation from cysts of other origins (17; 48; 51). Ependymal cysts are immunoreactive for glial fibrillary acidic protein (GFAP) and S-100, both glial markers expressed by normal immature ependymal epithelium. Cytokeratin staining is positive in cysts that develop from non-neural epithelia, such as colloidal and enteric cysts, but the latter display negative results for glial markers (40). Inoue and colleagues studied a variety of epithelial cysts and found S-100β positive and GFAP negative in 2 neurenteric cysts and a case of colloidal cyst (37). Therefore, S-100β protein is less specific for ependymal cysts than GFAP.
A single case report mentions the finding of positive staining for aquaporins (types 1 and 4) in the cyst wall in a case of an intramedullary cyst (64), the significance of which remains unclear.