(Acoustic Neurinoma; 8th Nerve Tumor; Vestibular Schwannoma)
Acoustic neuromas almost always arise from the vestibular division of the 8th cranial nerve and account for about 7% of all intracranial tumors. As the tumor expands, it projects from the internal auditory canal into the cerebellopontine angle, compressing the 7th and 8th cranial nerves. As the tumor continues to enlarge, the cerebellum, brain stem, and nearby cranial nerves (5th and 9th to 12th) can also become compressed.
Bilateral acoustic neuromas are a common feature of neurofibromatosis type 2.
Slowly progressive unilateral sensorineural hearing loss is the hallmark symptom of acoustic neuroma. However, the onset of hearing loss may be abrupt, and the degree of impairment may fluctuate. Other early symptoms include unilateral tinnitus, dizziness and dysequilibrium, headache, sensation of pressure or fullness in the ear, otalgia, trigeminal neuralgia, and numbness or weakness in the face due to involvement of the facial nerve.
Most commonly, an audiogram is the first test done as part of the evaluation to diagnose acoustic neuroma. It usually reveals an asymmetric sensorineural hearing loss and a greater impairment of speech discrimination than would be expected for the degree of hearing loss. Such findings indicate the need for imaging tests, preferably gadolinium-enhanced MRI. However, some tumors are found incidentally when brain imaging is done for another reason.
Other findings include presence of acoustic reflex decay on tympanometry. Auditory brain stem response testing may show the absence of waveforms and/or increased latency of the 5th waveform. Although not usually required in the routine evaluation of a patient with asymmetric sensorineural hearing loss, caloric testing shows marked vestibular hypoactivity (canal paresis) on the affected side.
Small, asymptomatic (ie, discovered incidentally), and nongrowing acoustic neuromas do not require treatment; such tumors may be observed with serial MRI scans and treated if they begin growing or cause symptoms. Whether to use stereotactic radiation therapy (eg, gamma knife or cyberknife radiation therapy) or conventional microsurgery depends on many factors including the amount of residual hearing, tumor size, and patient age and health. Stereotactic radiation therapy tends to be used for patients who are elderly, those with smaller tumors, or those who cannot undergo surgery for medical reasons. Microsurgery can involve a hearing-preservation approach (middle cranial fossa or retrosigmoid approach) or a translabyrinthine approach if there is no useful residual hearing.
Acoustic neuroma is usually unilateral but can be bilateral in neurofibromatosis type 2.
Unilateral hearing loss, sometimes with tinnitus and dizziness, is typical.
Larger and/or symptomatic tumors are treated with stereotactic radiosurgery or conventional microsurgery.
Small or nongrowing tumors can be observed with serial MRI scans.