A 67-year-old woman comes in with a complaint of hearing loss. She states that over the past few years she has had increasing difficulty understanding speech in noisy environments, particularly in restaurants and cocktail parties. Two years ago, she had similar hearing complaints and obtained a hearing test and hearing aids. However, she finds that the hearing aids are of no great benefit and do not amplify sounds enough to help her understand spoken words. Examination of her ears shows completely normal results. Cranial nerve examination shows a decreased sensation in the left mid-face area. The patient's audiogram is on the right (Fig. 1).
Diagnosis: Vestibular Schwannoma (Acoustic Neuroma)
Difficulty in understanding speech in noisy environments is a common complaint among patients with high-frequency sensorineural hearing loss. The most ambient noise is in the low- to mid-frequencies. People generally rely on high-frequency hearing to understand speech in a noisy environment. For those with high-frequency hearing loss, it is, therefore, natural to have word discrimination difficulties in noisy environments. This patient's audiogram shows a 92 percent word discrimination on the left side despite the significant high-frequency loss. A retrospective look at her previous audiogram from two years prior shows that she also had some asymmetric high-frequency hearing loss with 100 percent word discrimination bilaterally (Fig. 2).
Deciding when further workup for asymmetric high-frequency hearing loss should be undertaken is a difficult task. Setting the bar too high and looking for a very significant asymmetry before obtaining further studies to evaluate for retrocochlear pathologies risk missing tumors at an earlier stage. Setting the bar too low and obtaining imaging for the smallest degree of asymmetry will lead to possibly unnecessary MRIs and considerable medical expenses. A generally accepted criterion for further workup for asymmetric hearing loss is an asymmetry of 15 dB at three adjacent frequencies. Considering this patient's audiogram two years ago, it shows that there is definitely an asymmetry in the four highest frequencies. The patient's word discrimination ability was normal. However, pure-tone testing results clearly showed a difference between the patient's two ears. While the audiologist had recommended medical clearance prior to obtaining hearing aids, the otolaryngologist may not have paid as much attention to this asymmetry and medically cleared the patient for hearing aids.
A second otolaryngologist saw the patient two years later and recognized that the asymmetry had not been worked up properly. After performing a thorough clinical examination, he became suspicious of the presence of a tumor that is causing the facial sensation abnormalities and ordered an MRI, which in fact showed a tumor (Fig. 3). The decreased sensation on the left side mid-face is an early sign of a large tumor in the cerebellopontine angle. The patient's tumor, commonly known as an acoustic neuroma, is technically a vestibular nerve tumor that formed due to the overproduction of Schwann cells, which cover the peripheral nerve fibers. Therefore, an acoustic neuroma is neither acoustic nor a neuroma, and is more correctly termed as vestibular schwannoma.
The most common symptoms of vestibular schwannoma include asymmetric hearing loss, unilateral tinnitus, sudden hearing loss, facial hypesthesia (numbness), and dizziness or imbalance. When a tumor becomes very large, it can result in headaches and other discomfort due to an increased intracranial pressure. Facial paralysis is generally not a sign of vestibular schwannoma. Patients presenting with facial paralysis and a tumor in the cerebellopontine angle are more likely to be suffering from a facial nerve schwannoma instead of a vestibular one.
The treatment of vestibular schwannoma has substantially evolved over the past three decades. Thirty years ago, nearly all patients would have a resection of their tumor. As more data became available, it has been recognized that small tumors in older patients generally do not grow, and some tumors in the younger population may not grow. It would be very uncommon that a large tumor would not grow. In addition, the use of lower dose stereotactic radiation protocols has significantly improved the morbidity and side effects associated with this treatment of vestibular schwannoma. Prior to the use of low-dose techniques, the GammaKnife and CyberKnife techniques were employed but commonly caused damage to the surrounding cranial nerves. Furthermore, prior treatments also caused damage to the arachnoid granulations, which function to recycle the cerebrospinal fluid, resulting in high incidences of hydrocephalus. Over time, low radiation protocols were tried and found to be just as successful in controlling tumor growth.
Generally, 90-95 percent of patients undergoing stereotactic radiation will have tumor control, which means the tumor will either shrink or not grow any further. For patients with vestibular schwannoma to qualify for stereotactic radiation, their tumor must be within the size limit of 3 cm. This measurement is obtained by determining the extracanalicular portion of the tumor (the portion in the angle), which is outside the internal auditory canal. This size limit is observed because tumors swell after the initial radiation treatment. A large tumor that is already compressing the brainstem and the fourth ventricle, through which the cerebrospinal fluid flows between the brain and the spine occurs, may lead to a blockage of the fourth ventricle and consequently acute hydrocephalus (increased fluid pressure in the brain). Therefore, large tumors generally need to be treated with surgical resection rather than stereotactic radiation. After the initial edema, large tumors commonly shrink. At one year post-radiation, the degree of tumor control and size can be assessed, and an MRI can be obtained to serve as the new baseline for the tumor.
Radiation therapy has two main effects. First is to kill actively growing cells in a tumor. In vestibular schwannoma, however, only one to two percent of the cells are actually growing and the rest of the cells are dormant. The radiation primarily destroys actively growing cells to stop tumor growth. The second effect of radiation, which takes approximately one to two years to occur, is the destruction of the blood vessels within and around the tumor. These blood vessels feed the tumor cells with oxygen and nutrients, which are essential to their survival. Destroying these blood vessels leads to infarction and volume loss of the tumor. These infarctions sometimes lead to shrinkage or cyst formation within the tumor. Long-term observation is necessary to evaluate for regrowth, complications, and other issues in these patients.
BONUS VIDEOS: VISUAL DIAGNOSIS
Read this month's Clinical Consultation case, then watch the accompanying videos from Hamid R. Djalilian, MD, to review the patient's imaging for yourself.
- Video 1. Axial post-gadolinium T1 MRI showing the hyperintensity (bright white) of the tumor and compression of the middle cerebral peduncle.
- Video 2. Coronal post-gadolinium T1 MRI demonstrating the bright white enhancing tumor and its relationship to the pons.
- Video 3. Axial CISS MRI showing the relationship between the trigeminal nerve and the tumor.
- Video 4. Sagittal post-gadolinium T1 MRI demonstrating the bright white tumor and its relationship with the cerebellum.
- Video 5. Axial T2 showing the anterior inferior cerebellar artery relationship with the tumor.
- Video 6. Coronal T2 FL2D hemo sequence showing the vasculature within the tumor parenchyma (dark strings inside the tumor).
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