Acoustic Neuroma NY: Long Island Neurosurgeon
Acoustic neuroma is a benign primary intracranial tumor, or brain tumor, of the myelin forming cells called "Schwann cells" (Schwannoma) of the 8th cranial nerve --- also known as the acoustic nerve, (or more properly the vestibulocochlear nerve).
The term "acoustic neuroma" is actually a misnomer since the tumor never arises from the acoustic division of the vestibulocochlear nerve (cranial nerve VIII) and the tumor is not a neuroma but a schwannoma.
Approximately 3000 cases are diagnosed each year in the United States with an incidence of about 1 in 100,000. Incidence peaks in the fifth and sixth decades and both sexes are affected equally.
Indicated treatments for acoustic neuroma include neurosurgery - surgical removal and radiotherapy.
Because these neuromata grow so slowly, a physician may opt for conservative treatment beginning with an observation period. In such a case, the tumor is monitored by annual MRI to monitor growth. Records suggest that about 45% of acoustic neuromata do not grow detectably over the 3-5 years of observation. In rare cases, acoustical neuromata have been known to shrink spontaneously. Often people with acoustic neruromata die of other causes before the neuroma becomes life-threatening. (This is especially true of elderly people possessing a small neuroma.)
Since the growth rate of an acoustic neuroma rarely accelerates, annual observation is essential.
Removal of acoustic neuromas may be performed using several approaches. Each approach has its advantages and disadvantages. Microsurgery for acoustic neuroma is the only technique that removes the tumor. Radiation treatment (discussed in another section) does not remove the tumor, but has the potential to slow or stop its growth. Surgery is the only treatment that will definitively treat balance symptoms associated with tumor growth, as the vestibular nerves are removed at surgery.
Choice of surgical approach is based on the patient's age, medical condition, size of tumor, and preoperative hearing thresholds and speech discrimination, as well as other tests such as electronystamography, imaging, and auditory brainstem response testing. The patient's and surgeon's preferences also play a significant role.
During removal of the tumor, the tumor along with the superior and inferior vestibular nerves are removed. This results in an acute loss of vestibular input to the brain from the operated side. However, vestibular function improves rapidly due to compensation by the other ear and other balance mechanisms.
Surgery carries risk to the facial nerve which is monitored with facial nerve monitoring during the procedure. Best results (normal or near normal facial function) are more likely with small acoustic neuromas.
Acoustic neuroma surgery is highly technically demanding, and patients are advised to seek out surgical teams with extensive experience.
Radiation therapy is done in a variety of ways, but mainly by two methods: gamma knife radiosurgery or fractionated stereotactic radiotherapy. In the gamma knife approach, 201 beams of gamma radiation are focused on the tumor in a single session. The damage to the tumor at the convergence point may cause it to stop growing but usually does not cause it to shrink in the long term. It may cause short-term shrinkage due to necrosis in the tumor. The damage may be to the tumor cells and/or to the tumor vasculature.
It is not clear what percentage of tumors are controlled by this method for long periods. In earlier times when higher radiation doses were used, the failure rate was about 12% (which then required surgery). Most surgeons feel that these tumors are much more difficult to remove after radiation treatment. Radiation does not remove the tumor, and when irradiated tumors are surgically removed, it is often found that they have growing tumor cells in them.
Fractionated stereotactic therapy involves a beam of ionizing radiation focused on the tumor from a moving gantry. The beam is wider and less accurate than that of the gamma knife. The total dose is also much higher than that used in gamma knife radiosurgery, but the fractionation of the dose (done on many different days) spares normal tissue. This method has not been done on as many patients as gamma knife procedures and there have not been as many years of follow-up study. This means that the tumor control by this method is not yet established, and the incidence of secondary effects of the radiation are not yet known.
Earliest symptoms of acoustic neuromas include hearing loss/deafness, disturbed sense of balance and altered gait, vertigo with associated nausea and vomiting, and pressure in the ear, all of which can be attributed to the disruption of normal vestibulocochlear nerve function. Additionally more than 80% of patients have reported tinnitus (most often a unilateral high-pitched ringing, sometimes a machinery-like roaring or hissing sound, like a steam kettle).
Large tumors that compress the adjacent brainstem may affect other local cranial nerves. Involvement of the 7th cranial nerve (facial nerve) may lead to facial weakness, sensory impairment, and impairment of glandular secretions; involvement of the 5th cranial nerve (trigeminal nerve) may lead to loss of taste and loss of sensation in the involved side's face and mouth. Involvement of the 9th and 10th cranial nerves are uncommon, but may lead to altered gag or swallowing reflexes.
Even larger tumors may lead to increased intracranial pressure, with its associated symptoms such as headache, vomiting, and altered consciousness.
Contrast-enhanced CT will detect almost all acoustic neuromas > 2.0 cm in diameter and project further than 1.5 cm into the cerebellopontine angle. Those tumors that are smaller may be detected by MRI with gadolinium enhancement. Audiology and vestibular tests should be concurrently evaluated using the Weber's and Rinne's test to assess for sensorineural versus conduction hearing loss.