Acoustic Neuroma Workup

Updated: Apr 10, 2018
  • Author: Joe Walter Kutz, Jr, MD, FACS; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Workup

Laboratory Studies

Routine lab studies are generally not required.

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Imaging Studies

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  • The definitive diagnostic test for patients with acoustic tumors is gadolinium-enhanced MRI.

    • Well-performed scanning can demonstrate tumors as small as 1-2 mm in diameter. On the other hand, thin-cut CT scanning can miss tumors as large as 1.5 cm even when intravenous contrast enhancement is used.

    • Gadolinium contrast is critical because nonenhanced MRI can miss small tumors.

    • Fast-spin echo techniques do not require gadolinium enhancement and can be performed very rapidly and relatively inexpensively. However, such highly targeted techniques risk missing other important causes of unilateral sensory hearing loss, including intra-axial tumors, demyelinating disease, and infarcts.

    • MRI is contraindicated in individuals with ferromagnetic implants.

  • Fine-cut CT scanning of the internal auditory canal with contrast can rule out a medium-size or large tumor but cannot be relied upon to detect a tumor smaller than 1-1.5 cm.

  • If suspicion is high and MRI is contraindicated, air-contrast cisternography has high sensitivity and can detect relatively small intracanalicular tumors.

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Diagnostic Procedures

A variety of audiometric tests were developed in the mid-20th century in an attempt to identify patients with increased likelihood of having an acoustic neuroma. That was a worthwhile undertaking when definitive radiographic imaging consisted of some form of either pneumoencephalography or formal arteriography. Such testing is no longer used. Even the auditory brainstem evoked response (ABR) is now infrequently used as a screening test for acoustic neuroma. ABR screening techniques miss 20-35% of acoustic tumors smaller than 1 cm. Moreover, ABR is likely to miss those tumors in patients with excellent hearing, which are the cases most favorable for hearing conservation procedures.

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Histologic Findings

Two histologic types of tissue have been identified in acoustic tumors. Antoni A tissue consists of elongated spindle cells with a palisading pattern. Antoni B tissue, on the other hand, has a loose spongy texture and markedly reduced cellularity. A given acoustic neuroma may contain areas with both Antoni A and Antoni B tissue. Another histologic feature characteristic of schwannomas are rows of palisading nuclei called Verocay bodies. Although the histologic appearance of acoustic tumors is fairly straightforward, they can occasionally be difficult to distinguish from meningiomas. Immunohistochemical staining can distinguish schwannomas from meningiomas in difficult cases. Schwannomas are immunoreactive to S-100 antibody while meningiomas are immunoreactive to epithelial membrane antibody (EMA).

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Staging

As a rule, acoustic neuromas are benign tumors, although rare cases of malignant acoustic neuromas have been reported in the literature. No widely accepted staging system exists for acoustic neuromas.

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