Acoustic Neuroma

A 29-year-old woman complained of 6 months of tinnitus, hearing loss in the left ear, and leff-sided facial numbness. There was no history of trauma or recognized antecedent infection. Physical examination of the external and middle ears was unremarkable. Vestibular test results and tympanometry results were normal. An audiogram demonstrated nearly total sensorineural hearing loss in the left ear.

Noncontrast MRI at high field showed a 4 × 4.5 × 3-cm mass in the left cerebellar pontine angle (CPA) (A). This mass was displacing the fourth ventricle, pons, and middle cerebellar peduncle medially as well as compressing the left fifth cranial nerve. Gadolinium-enhanced MRI (B) showed the mass to be uniformly, and intensely, enhancing; it extended approximately halfway into the widened left internal auditory canal. This is radiographically diagnostic of an eighth-nerve schwannoma, or acoustic neuroma. The neurovascular bundles of the seventh and eighth cranial nerves are seen laterally in the left internal auditory canal (white arrow) and throughout the contralateral side.

Dr Joel M. Schwartz of Nyack Hospital, Nyack, NY, notes that schwannomas are the most frequently occurring CPA tumors in adults, with the majority presenting in the fourth through seventh decades of life.¹ These neoplasms arise slightly more often in women and tend to be larger and more vascular than the tumors occurring in men. In Figure B, the signal voids in and around the periphery of the mass represent blood vessels (arrows). The acoustic neuromas most commonly arise from the vestibular division of the eighth cranial nerve.²

Contrast-enhanced MRI is the method of choice for evaluation of all cranial nerve schwannomas and particularly acoustic neuromas, but as this case illustrates, large acoustic neuromas can be identified by noncontrast MRI. Because these tumors demonstrate avid enhancement, even those that are quite small are readily apparent with thin-section (3-mm) imaging and contrast administration.

The major radiographic differential diagnosis is a meningioma, which is found less frequently in the CPA. The differentiating feature is extension into the internal auditory canal that occurs in almost all acoustic neuromas but is seen only rarely with meningiomas. Other masses that arise in this region, such as epidermoids, arachnoid cysts, and vascular lesions, are easily differentiated because of their different signal characteristics on MRI scans. Exophytic intra-axial tumors or tumors of the petrous bones also can appear as extra-axial masses in the CPA.

Acoustic neuromas, adds Dr Schwartz, can be found during the work-up for sensorineural hearing loss and tinnitus or as an incidental finding. These tumors are usually unilateral; bilateral acoustic neuromas are characteristic of neurofibromatosis type 2. Surgical resection is the mainstay of treatment. Sterotactic radiosurgery is a viable alternative for high-risk patients or those with bilateral tumors.

Gross total surgical resection of the acoustic neuroma in this patient caused postoperative facial nerve paralysis, which was not unexpected in the setting of such a large tumor. However, before scheduled hypoglossal nerve–facial nerve anastomosis, the facial paralysis began to improve.

REFERENCES:1. Som PM, Bergeron RT, eds. Head and Neck Imaging. 2nd ed. St Louis: mosby Year Book; 1991:1047-1069.
2. Atlas SW, ed. Magnetic Resonance Imaging of the Brain and Spine. New York: Raven Press; 1991:356-362.