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Radiologic Findings: T1-weighted images (Figs. 1 and 2) show a 4 cm x 3 cm right cerebellopontine angle (CPA) mass with decreased signal intensity. It extends into the porous acousticus and maintains acute angles with the petrous bone. A moderate amount of mass effect involves the pons and cerebellum. A T2-weighted image (Fig. 3) demonstrates heterogeneously increased signal intensity in the mass. Following Gadolinium administration, T1-weighted images (Figs. 4 and 5) show intense homogeneous enhancement of the mass.
Diagnosis:Vestibular Schwannoma.
Discussion:Vestibular schwannomas are the most common tumors of the CPA, accounting for 80-90% of all tumors in this location. They most commonly arise from superior vestibular branch of the 8th nerve, slightly more commonly than inferior vestibular nerves. The 5th and 7th nerves are the next most common nerves of origin of schwannomas.
Schwannomas arise from perineural Schwann cells, which are responsible for myelination of peripheral nerves. Although "neuroma" tends to be used interchangeably with "schwannoma", by strict pathologic definition, neuromas refer to a post-traumatic proliferation of nerve cells rather than a true neoplasm.
Arachnoid cysts co-exist in 7-10% of cases of vestibular schwannoma. Bilateral vestibular schwannomas are the primary diagnostic criterion for type 2 neurofibromatosis.
The differential diagnosis includes a meningioma, which maintains obtuse angles with the petrous bone and exhibits the dural tail sign. An epidermoid, an aneurysm, and a metastasis is also noted in the differential.
References:
1. Willing, Atlas of Neuroradiology, W. B. Saunders, Philadelphia, PA, 1995, pgs. 153-155.
2. Grossman and Yousem, Neuroradiology: The Requisites, Mosby Year Book, St. Louis, MO,
1994, pgs. 73-74.
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