INTRODUCTION

In common with their cell of origin, ependymomas are found throughout the central nervous system, from frontal horns to filum terminale. Intracranially, most occur in the fourth ventricle of children, although supratentorial neoplasms appear in both children and adults. The spinal lesions, especially those of the filum terminale, are usually seen in adults. A rare neoplasm arises as a primary tumor in sacral soft tissues or bone.

The intracranial lesions meet little resistance to intraventricular expansion and can become sizable exophytic masses before the onset of obstructive symptoms. Such neoplasms are lobular, well circumscribed, and often broadly attached to the ventricular floor from which they arise. Secondary adhesions to other ventricular surfaces may also form. Occasional supratentorial lesions arise superficially, seemingly remote from the ventricular system. Characteristically, in the fourth ventricle, there is little invasion of the ventricular wall, and but for their origin from the brain stem, the neoplasms could be excised in toto. The larger ependymomas in any site may be cystic. Calcification may be present.

Intraoperatively, the fourth ventricular ependymoma can resemble a medulloblastoma pendent from the vermis, although it is not usually as soft and necrotic as the medulloblastoma in its typical form or as firm as the cerebellar neoplasm in its desmoplastic variety. In the spinal cord, an ependymoma's discreteness helps differentiate it from the infiltrating astrocytoma. It can, however, be mimicked by the similarly discrete but more vascular and much less common spinal hemangioblastoma. Caudally, the filum terminale can be confused with a nerve root and an ependymoma be mistaken for a schwannoma.

Microscopically, there are two classes of ependymoma: (1) the classic lesion of the brain and spinal cord and (2) the distinctive myxopapillary ependymoma of the filum terminale. The classic ependymoma is a cellular neoplasm formed of cells with small, dark nuclei. Cytoplasmic differentiation produces two basic patterns. The first is the cells' expression of their glial heritage in cell processes packed with filaments. When the ependymal cells are present in low density and these processes are abundant, the lesion can resemble the well-differentiated astrocytoma, and it may be difficult to distinguish the two lesions in small specimens. One helpful differential feature characteristic of ependymomas in such fibrillar areas, as well as in more cellular regions, is the orientation of cell processes to the wall of blood vessels. This produces a perivascular eosinophilic anuclear zone that, together with the perivascular nuclei, is known as a perivascular pseudorosette.

The second distinctive feature of the classic ependymoma is the epithelial differentiation by which cuboidal cells form epithelial surfaces. These include small canals known as true ependymal rosettes, large, flat epithelial surfaces, and crude papillations. In contrast to the choroid plexus papilloma, the epithelial features are never exclusive, and cells with more glial features are found within the interstices between rosettes and canals. As is also true of astrocytomas of the hypothalamus, optic nerve, and cerebellum, clusters of cells resembling oligodendrocytes are occasionally seen. In an uncommon ependymoma variant ("clear cell ependymoma"), such lucent cells are especially prominent.

The structure of the myxopapillary ependymoma is quite different and may reflect the differences in embryology and anatomy between the caudal nervous system, derived from the caudal cell mass, and the principal portion of the nervous system, derived by neurulation. In particular, the unique juxtaposition of dense collagenous tissue and ependyma, found exclusively in the filum terminale, may contribute to the singular morphologic features of the myxopapillary ependymoma. This lesion is extensively vacuolated by microcysts, whose expansion forces the nuclei into intervacuolar interstices. The mucosubstance also swells the walls of the blood vessels to which the neoplastic cells cling as the papillae that give this distinctive lesion its name.

As in the other gliomas, there is a spectrum of histologic anaplasia in the classic ependymoma from the well-differentiated lesion to the markedly cellular neoplasm. Anaplastic lesions are not recognized in the myxopapillary group, although the lesion is capable of wide dissemination though CSF pathways in some instances.

The Kernohan group developed a grading system of four tiers similar to that used for the astrocytomas, but this is not widely used, because it has been difficult to establish precise correlations between histologic grade and biological behaviour due to the over­riding importance of location and extent of excision in the length of postoperative survival. It seems, however, that the more anaplastic lesions behave more aggressively. The term ependymoblastoma has sometimes been used for these more anaplastic lesions, although others reserve this term for a separate, extremely rare, neoplasm of childhood with epithelial surfaces similar to that of the primitive neuroepithelium.

A neoplasm related to the ependymoma is the subependymoma. This discrete lobulated lesion occurs in either the anterior lateral ventricle or the posterior fourth ventricle. In either, it is a reasonably common incidental necropsy finding but a rare surgical entity. Histologically, subependymomas are remarkable for the clustering of uniform nuclei in a highly fibrillar background. Some lesions in the lateral ventricles can be totally excised, but attachment to the medulla generally precludes this possibility in the fourth ventricle.