FINAL DIAGNOSIS: INTRASELLAR CELLULAR SCHWANNOMA
The differential diagnosis of intrasellar lesions is extensive. Pituitary adenomas account for the majority of primary intrasellar neoplasms. Other hormonally inactive tumors are less common in this location but may clinically mimic non-secreting pituitary adenomas due to mass effects and compression of neighbouring structures. Hypopituitarism and visual field defects are typically found (4). The most common non-pituitary intrasellar neoplasms are craniopharyngeomas, germ cell tumors, meningeomas, chordomas, and pituicytomas (4, 14). Schwannomas of the sellar region are extremely rare and usually not included in the differential diagnosis of intra- and parasellar lesions. Schwannomas of the central nervous system most commonly arise from the sensory roots of spinal nerves and account for 30% of primary spinal tumors (15). Intracranial presentation is less frequent and accounts for 8 - 10% of all primary intracranial tumors. Intracranial schwannomas occur predominantly in association with the vestibular portion of the VIII cranial nerve in the cerebellopontine angle (14). There are only a few reported cases of intrasellar schwannomas, the majority arise in the parasellar region from the nerve sheath of adjacent cranial nerves (7, 12). True intrasellar schwannomas without involvement of cranial nerves have only been reported in four cases to date (5, 9, 16). While a nerve of origin is frequently identified in peripheral schwannomas, the origin of true intrasellar tumors is under debate. Hypothesized origins of intrasellar schwannomas include Schwann cells in autonomic vasomotor nerve plexi (10), in the walls of the cavernous sinus (2), or ectopic Schwann cells in the wall of the sella turcica (13). Other authors propose transformed mesenchymal or pial cells as potential origins for intrasellar schwannoma (1, 6).
Macroscopically, intrasellar schwannomas have a brownish mucoid appearance with frequent dense dural attachment and high vascularisation which makes complete resection difficult (11). Histologically the intrasellar schwannoma in this report displayed features of a cellular schwannoma consisting of spindle-shaped cells of uniformly high cellularity with no evidence of hypocellular areas.
The histopathologic differential diagnosis of intrasellar cellular Schwannoma includes other spindle-shaped tumors that are more common in this area, i.e. fibrous meningioma, astrocytoma of the posterior pituitary gland (pituicytoma), and melanocytoma (3, 4, 8). These can be distinguished by immunohistochemical and ultrastructural analysis. While meningiomas do not display Verocay bodies observed in cellular schwannoma as in our case, they do show nuclear pseudoinclusions and an immunohistochemical positivity for epithelial membrane antigen. Schwannomas show a strong diffuse cytoplasmic expression of S100 protein that is also typically observed in melanocytomas. In contrast to schwannomas, melanocytomas display infrequent pigmentation and frequent expression of HMB45. A pituicytoma can be distinguished by strong expression of glial fibrillary-acidic protein, a condition rarely displayed by other intrasellar tumors.
In summary, schwannomas are uncommon intrasellar neoplasms which can clinically and radiologically mimic non-secreting pituitary adenomas. Vascularisation may pose a challenge to the neurosurgeon regarding radical removal of the tumor. Histopathologically, these spindle cell tumors must be distinguished from fibrous meningioma, melanocytoma, and pituicytoma.
Contributed by Christian Bernreuther, Jörg Flitsch, Dieter K. Lüdecke, Christian Hagel