Schwannomas

Schwannomas ("neuromas") are benign tumors arising from the cranial nerve sheaths, which are formed of Schwann cells. Ninety percent occur in the cerebellopontine angle,24,39 and they are usually solitary. The so-called acoustic schwannoma is by far the most common tumor of this type; bilateral acoustic schwannomas are the hallmark of the central form of neurofibromatosis 2 (NF2).5,28 The trigeminal nerve is the next most common site of origin.24 Generally, schwannomas are firm, encapsulated tumors that may contain cysts. As the tumor grows, it may become lobulated, increase in vascularity,29 or develop arachnoid adhesions that may result in arachnoid cysts. Clinical presentation depends on the size and site of origin of the tumor. Acoustic schwannomas initially present with tinnitus and progressive neurosensory hearing loss. Tumor enlargement into the cerebellopontine angle can produce ataxia or compression of the brainstem or exiting cranial nerves.

Cross-sectional imaging demonstrates that schwannomas are extra-axial, well-circumscribed tumors eroding the involved canal with variable contrast enhancement. Angio-graphically, schwannomas are generally less vascular than meningiomas, and the most suggestive angiographic finding is the presence of multiple small puddles of contrast agent that persist into the venous phase.1 Most schwannomas are not sufficiently vascular to warrant preoperative embolization, but this has been shown to be efficacious in the more vascular schwan-nomas,1,4 because it reduces tumor blood supply and eases surgical resection, as with meningiomas.

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