Laboratory Studies
Routine lab studies are generally not required.
Imaging Studies
See the list below:
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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.
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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.
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If suspicion is high and MRI is contraindicated, air-contrast cisternography has high sensitivity and can detect relatively small intracanalicular tumors.
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.
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).
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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This table shows the distribution of presenting symptoms, ie, the symptom that brought the patient to a physician and that constituted the patient's chief ailment.
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A small acoustic neuroma within the internal auditory canal is easily observed on postgadolinium MRI.
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These large bilateral acoustic neuromas are easily observed on MRI. This patient has neurofibromatosis II. Both tumors were eventually removed, leading to anacusis. Facial nerve function remained entirely normal bilaterally.
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The nerves of the internal auditory canal as observed in a cadaveric dissection are shown. The posterior wall of the internal auditory canal has been removed. F indicates the facial nerve. S is the superior vestibular nerve. VIII indicates the statoacoustic nerve as it leaves the brain stem, and P indicates the posterior ampullary nerve. The hollow arrow points to the posterior lip of the boney porous acusticus, and the solid arrow indicates the position of the vestibule. C indicates the cochlear aqueduct.
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The bone that must be removed for a middle cranial fossa approach is indicated in yellow. The tumor is in orange.
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The bone that must be removed for a translabyrinthine approach is indicated in yellow. The tumor is in orange.
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The bone that must be removed for a posterior fossa approach is indicated in yellow. The tumor is in orange.
