FINAL DIAGNOSIS: INTRAVENTRICULAR SCHWANNOMA (WHO GRADE I).
Schwannomas account for up to 8% of intracranial neoplasms (14). The vast majority of cases arise from the cranial nerves, with the 8th nerve being most commonly involved (14). Rare cases of schwannomas may develop as primarily meningeal, intracerebral or intraventricular tumors (14). In a review of the literature, we found that only few cases of intraventricular Schwann cell tumors have been reported so far (1,4-9,12,13). These tumors were generally located in the lateral or fourth ventricles. To our knowledge, the tumor of our patient is the first reported schwannoma of the third ventricle.
The origin of intraventricular schwannomas is unknown. However, similar to the hypotheses about the histogenesis of spinal intramedullary (3) and intracerebral schwannomas (11), which are slightly more common than intraventricular schwannomas, different possibilities may be considered. Schwannomas may arise from the sympathetic nerve plexus surrounding blood vessels. These vascular nerves can be found around medullary vessels, in the choroid plexus and in the meninges (10). An alternative hypothesis is the origin from ectopic neural crest-derived cells, which had been displaced during embryogenesis. This hypothesis may point to a link between intraventricular schwannomas and the so-called neurocristopathies, which comprise a spectrum of dysgenetic and neoplastic disorders associated with alterations in the migration, growth and differentiation of neural crest tissue during embryogenesis (2). Prominent examples include the neurofibromatoses, Hirschsprung's disease, and Waardenburg syndrome (2). However, our patient did not demonstrate any clinical signs or symptoms indicative of these diseases.
Neuroradiologically, intraventricular schwannomas cannot be differentiated with certainty from other, more common intraventricular neoplasms such as ependymal and choroid plexus tumors. Histologically, the diagnosis is straight forward, with the main differential diagnoses being pilocytic astrocytoma and fibroblastic meningioma. However, the abundance of reticulin fibers and the lack of GFAP immunoreactivity rules out a pilocytic astrocytoma, while the strong expression of protein S-100 and absence of immunoreactivity for epithelial membrane antigen excludes a fibroblastic meningioma.
In summary, we have reported on a benign schwannoma in the posterior part of the third ventricle. The origin of this tumor is unclear. Schwann cell tumors should be included as a rare differential diagnosis in patients with intraventricular tumors.
Contributed by A. Martina Messing-Jünger, MD, Markus J. Riemenschneider, MD, Guido Reifenberger, MD, PhD