Timothy C. Hain, MD DizzinessTumor index Page last modified: May 25, 2016
|Acoustic Neuroma (swelling of 8th nerve, just under Facial nerve)|
Acoustic neuromas, also known as "acoustics" or vestibular schwannomas, are non-malignant tumors of the 8th cranial nerve. Most commonly they arise from the covering cells (Schwann cells) of the inferior vestibular nerve (Komatsuzaki and Tsunoda, 2001; Krais, 2007). They can also arise within the labyrinth (Neff et al, 2003).
Acoustics comprise about 6 percent of all intracranial tumors (Anderson et al, 2000), about 30% of brainstem tumors, and about 85% of tumors in the region of the cerebellopontine angle -- another 10% are meningiomas. The 6% number is probably much too high as meningiomas and pituitary tumors are underreported.
Only about 10 acoustic tumors are newly diagnosed each year per million persons (Evans et al, 2005; Carlson et al, 2015), corresponding to between 2000 and 3000 new cases each year in the US. The prevalence -- the number of people with an acoustic is of course much higher.
Another way of looking at this is that an average person has a risk of about 1/1000 of developing an acoustic neuroma in their lifetime (Evans et al, 2005). In the entire country of Denmark, the annual incidence was estimated to be 7.8 patients operated/year (Tos et al, 1992). As technology has improved, more small tumors have been diagnosed, resulting in a similar estimate of about 10 tumors/million/year. White persons have the highest incidence (1.61 per 100K), and blacks the lowest (0.43/100K), according to Carlson et al (2016) who analyzed a national tumor registry. Of course this statistic actually reports the number of acoustics diagnosed, not the number that might be found through a prospective study.
In patients with hearing asymmetry, it is believed that only about 1 in 1000 has acoustic neuroma (source: NIH), although some report prevalences as high as 1-2.5% (Stewart et al, 1975; Baker et al. 2003). The higher prevalence figures do not correspond to our clinical experience in our practice in Chicago, or the findings of others. Lin et al (2005) suggested that only 2/10,000 persons have acoustic neuromas, based on an imaging database of 46414 people. In our practice in Chicago, where we see roughly 800 new patients/year with dizziness or hearing loss, we typically diagnose ourselves only about one new acoustic/year. As we have a symptomatic population that presumably would be more likely to have acoustics than individuals with no hearing or dizziness symptoms, this implies that the prevalence in the general population is probably much lower than 1%. Of course, this does not really answer the question as to the number of persons with hearing asymmetry that have AN.
Because the prevalence of acoustic neuroma is low, and because the cost of MRI scans used in screening is very high, cost-analysis suggests that routine MRI scanning for asymmetrical hearing loss is not cost effective (Pan et al, 2015).
Acoustic neuromas occur largely in adults -- they are very uncommon in children. Only 39 cases in children had been reported in the literature as of 2001 (Pothula et al, 2001).
Acoustic neuroma occurs in two forms: a sporadic form and a form associated with an inherited syndrome called neurofibromatosis type II (NF2). About 95 percent of all cases are sporadic.
NF2 is rare; there are only several thousand affected individuals in the entire United States, corresponding to about 1 in 40,000 individuals (see MRI image below). Roughly 5% of patients with acoustic neuroma have type II neurofibromatosis. A recent study suggested that the mechanism of hearing loss in NF2 is elevated intralabyrinthine protein rather than compression of the 8th nerve (Asthaqiri et al, 2012). While we doubt that this is always the case, we have also invoked this cause in patients with tiny tumors such as intralabyrinthine schwannomas, driven by MRI findings. More information about NF2 can be found at http://ghr.nlm.nih.gov/gene=nf2.
There is no credible evidence that radiation from cellular phones causes acoustic neuroma (Muscat et al, 2002).
Diagnosis of an acoustic usually requires either a physician with otologic expertise who can integrate together the entire picture, or an MRI with gadolinium. Because acoustic neuromas are very rare, and MRI's are very expensive, in our opinion -- all patients with a substantial risk of having an acoustic should be evaluated by a medical doctor with otologic expertise. In other words, patients with unexplained stable asymmetrical hearing loss should generally all be evaluated by a physician with otologic expertise. The following text describes how this integration process can be done.
While hearing loss is common in acoustic neuroma (i.e. it is sensitive), there are myriads of other causes of hearing loss (i.e. hearing loss is very nonspecific). Roughly 20% of adults have hearing impairment of 25 dB or more (Haggard et al, 1981). Because of the high sensitivity but low specificity, routine use of a very expensive diagnostic test such as a gad-MRI of the IAC's in all persons with asymmetrical hearing is not always justified. In other words, mistakes are justifiable on the basis of social cost/benefit ratio. While some clinicians "scan" all patients with asymmetrical hearing - -this is a very expensive way to "find" an acoustic.
Hearing loss is the most frequent symptom of acoustic neuroma, occurring in more than 95 percent of patients. About 90 percent present with a one-sided, slowly progressive hearing impairment. An example is shown below.
Clinicians often attempt to estimate the risk of an acoustic neuroma by looking at the pattern of hearing loss (see here for more about patterns). A high-frequency sensorineural pattern is the most common type, occurring in approximately two-thirds of patients. In the remaining third the next most common observation is hearing loss at low frequency (which would be more typical of Meniere's disease). Even less commonly, some have the "cookie bite" pattern (suggestive of congenital hearing loss or a noise notch).
A sudden hearing loss occurs in about 25 percent of patients with acoustic neuroma. However, because acoustic neuroma is a rare condition, sudden hearing loss attributable to an acoustic tumor occurs in only 1-5 percent of patients with sudden hearing loss as there are many more common causes (Daniels et al, 2000). Even a sudden hearing loss with complete recovery can be caused by an acoustic(Nageris and Popovtzer, 2003).
Asymmetrical hearing is sensitive but extremely nonspecific for acoustic neuroma. The lack of specificity and rarity of acoustic neuromas compared to the myriad of other causes of asymmetrical hearing makes the "cost" of scanning every person with asymmetrical hearing to find an acoustic in 1/1000 persons (or even less) extremely high. The lack of specificity has been commented upon by Margolis and Saly (2008). This conclusion needs to be tempered by other clinical information - -someone with a progressive asymmetrical sensorineural hearing reduction would (in our opinion) be far more likely to have an acoustic than someone with a static or improving asymmetry. Nevertheless, if the chance of finding an acoustic in someone with asymmetrical hearing is between 1/1000 and 1/10,000 and the cost of an MRI is roughly $2000, then it costs between 2 million and 20 million $ in MRI studies to diagnose every acoustic neuroma. Pan et al (2015) reported that the estimated screening cost per patient for those who benefitted from surgery or radiation was $147,030 while the US compensation for unilateral hearing loss is $44,888.
Specificity is another consideration. In this regard, some find that hearing is completely normal in as many as 11% of patients (Morrison and Sterkers, 1996). In our opinion, this percentage is high, but nevertheless certainly acoustic neuroma's can be found in persons with symmetrical hearing.
Tinnitus is very common in acoustic neuroma, is usually unilateral and confined to the affected ear. Looking at things the other way, if you have tinnitus, it is very very unlikely that you have an acoustic neuroma, because these tumors are far rarer than other mechanisms of ear damage.
In spite of the usual origin of acoustics in the inferior vestibular nerve (Komatsuzaki and Tsunoda, 2001; Krais et al, 2007), vertigo (spinning) prior to surgery is not common, occurring in only about 20 percent of persons with acoustic neuroma. As the inferior vestibular nerve innervates the posterior semicircular canal and saccule, one might expect VEMP's, which test saccule function, to be uniformly abnormal in acoustic neuroma's, and in fact they are quite sensitive. Similarly, one might expect ipsilateral BPPV to be rare. This question has not been addressed. One might also expect abnormalities in OAE's as auditory efferents enter the cochlear area via the inferior division of the nerve. Again, this question has not been addressed.
Vertigo is more common with smaller tumors. Unsteadiness is much more prevalent than vertigo, and approximately 70 percent of patients with large tumors have this symptom. Cerebellar symptoms (i.e. poor coordination of the arms) are unusual.
Hyperventilation induced nystagmus is a little known physical sign that may be far more specific for acoustic neuroma. Evaluation of HVIN requires more sophisticated equipment than is available in most offices. It also requires the examiner to be familiar with this sign -- and it is somewhat obscure.
It has recently been pointed out that some forms of acoustic neuromas have increased intralabyrinthine protein. One would expect that this would create timing differences between the ears without spontaneous nystagmus, and result in head-shaking nystagmus, without spontaneous or hyperventilation induced nystagmus.
Facial sensory disturbances occurs only in large tumors (about 50 percent of those greater than 2 cm in size). The facial sensory disturbance may respond to carbamazepine or oxcarbamazine medication for neuralgia. Facial weakness is uncommon. Facial twitching, also known as facial synkinesis or hemifacial spasm, occurs in about 10 percent of patients. Headache prior to surgery occurs in roughly 40 percent of those with large tumors.
As mentioned above -- acoustics can be diagnosed either by a medical doctor with otologic expertise who can integrate together the entire picture, or with an MRI scan with gadolinium of the brain. Because MRI's are so expensive, the most cost efficient method of diagnosis (at this writing - -in 2015), is to have all patients with unexplained asymmetrical hearing loss be evaluated by a medical doctor with otologic expertise.
|Typical audiogram for patient with an acoustic neuroma.|
Conventional audiometry is the most useful diagnostic test for acoustic neuroma. The most common abnormality is an asymmetrical high-frequency sensorineural hearing loss (see figure above). No more than 1 out of 20 patients with large tumors have symmetry within 15 dB at 4000 Hz. However, recall that only about 1 in 1000 patients with hearing asymmetry have acoustics. It has been estimated that 5 percent of persons with sensorineural hearing loss have acoustics (Daniels et al, 2000), but this estimate is probably wrong as it would imply a much higher prevalence of acoustic neuromas than are commonly accepted. Speech reception (SRT) is normal in many patients with small tumors. Excellent speech discrimination is found in about 50% of patients with small tumors, and one third of patients with large tumors still have near-normal (> 80%) speech discrimination. Because hearing asymmetry is mainly due to other conditions than acoustic neuromas, other pieces of information need to be integrated to make the clinical diagnosis of acoustic neuroma. Usually this integration process is done by a medical doctor with otologic expertise, and not by allied health persons such as audiologists.
|Audiogram of patient with large acoustic neuroma on left side, but (nearly) symmetrical hearing. This example shows that symmetrical hearing testing does not always exclude the diagnosis of an acoustic neuroma. See comment in text.|
Symmetrical hearing impairment or even normal hearing does not exclude an acoustic, but it is very rare. The author of this article has encountered several patients with symmetrical hearing but a large acoustic on one side. Above is an example of a man who had a large acoustic on the left side -- to be very sure one must do an MRI, and to be as sure as possible, one must do a high-field MRI of the IAC with gadolinium. Because this is terribly expensive (to screen everyone with hearing impairment of any type), mistakes are sometimes made. While this is really a decision for health care economists, it seems to us that occasional errors are overall permissible, when one considers what is best for the population at large.
When abnormal with a progressively worsening pattern, audiometry usually leads to further testing such as ABR (auditory brainstem response, also known as BAER for brainstem auditory evoked response) and gadolinium enhanced MRI (magnetic resonance imaging) which establishes the diagnosis. ABR testing is less sensitive than MRI (false negative rate about 33%), but it is considerably less expensive. A new technique called "summated ABR", essentially several ABRs compared over time, may provide better sensitivity. In our clinical context in Chicago, this technique is rarely relevant as the more sensitive test for acoustics (MRI) is so common. Nevertheless, this method might have utility in contexts where MRI's are difficult to access.
A characteristic finding on ABR in a person with an acoustic neuroma would be a wave I with nothing after it -- no waves 3 or 5 (10-20% of cases). A wave I-III interval delay is common, and a wave V delay occurs in 40-60% of cases. ABR's have high false-positive as well as false negative rates. As many as 1/3 of patients with small tumors (on MRI) have normal ABR.
Vestibular testing in acoustic neuromas
At this writing, we have ENG testing, Rotatory chair testing, VHIT testing, VEMP testing, and Posturography to choose from when we are attempting to diagnose dizzy patients. The general rule is that tests that distinguish between sides (i.e. Caloric, VEMP, perhaps VHIT), do better than those that test both sides at the same time (rotatory chair, posturography).
Electroencephalography, (ENG testing) is frequently abnormal in persons with acoustic neuromas and about 60 percent of all tumors are associated with unilateral loss of clerics (Hulshof et al, 1989). We have diagnosed a few acoustics by noting that ENG function is lost but hearing is relatively preserved. As in the more common condition of Meniere's disease, hearing loss precedes vestibular impairment, the opposite pattern is suggestive of a tumor (especially an intralabyrinthine schwannoma).
Nevertheless, ENG is not a sufficient test by itself for acoustic neuroma because it is not specific, and also because there are far more other causes for caloric loss than acoustic neuromas. Rotatory chair testing is less sensitive than caloric testing. VHIT testing is probably also less sensitive than caloric testing but it depends on the tumor size.
Posturography is insensitive to acoustic neuroma.
Acoustic reflex decay is also insensitive to acoustic neuroma (about 36%) and this test is rarely used for this purpose.
Otoacoustic emissions are also considered a poor test for acoustic neuroma, although one would think that they would do as well as hearing testing. As OAE's are generated by the cochlea, and acoustics involve the 8th nerve, which is further in, the lack of sensitivity may be due to this anatomical specificity.
As mentioned above, VEMP testing would be expected to be sensitive to acoustic neuroma's. Chiarovano et al (2015) reported that cervical VEMPs were abnormal in 65% of patients with acoustic neuroma. This is similar performance to ENG testing.
|MRI scan of brain (coronal) showing an acoustic neuroma (the white spot on the left side of the picture).||MRI scan of brain (axial with contrast) showing a largely intracanalicular acoustic neuroma on right side of brain (left side of scan).||Another intracanalicular acoustic on right side.||Large 3cm acoustic indenting brainstem on L side.||A coronal view of another acoustic, again indenting brainstem on L side.|
|Another large acoustic indenting brainstem on the L side (contrast).|
Although it is more costly compared to audiometry or ABR, the optimal test for excluding an acoustic neuroma is a gadolinium enhanced T1 MRI (see picture above). Parenthetically, it is puzzling that an old technology like MRI should keep increasing in cost every year.
On MRI, acoustic neuromas are frequently uniformly enhanced and dense. The best protocol MRI protocol is generally a T1 with contrast (gadolinium) of the IAC, in a closed MRI with the highest resolution available (3T is the best right now -- in 2015). It is a mistake to get an "open MRI" if you are seriously suspecting an acoustic neuroma.
One looks for lighting up of the tumor (enhancement) on the images with dye. A fast spin-echo T2 variant of MRI is very sensitive to acoustics, and in some clinical settings, can be done fairly inexpensively (but we don't know of anyone who does this in Chicago). If a person has a pacemaker or other metallic device, then they may have to have a CT scan with contrast instead of an MRI.
|CT with contrast of acoustic neuroma||MRI with contrast of same acoustic neuroma|
CT scans with IV contrast are poor tests for diagnosis of acoustic neuromas, as they have a high false negative rate (about 37%). Once again, one looks for enhancement (lighting up of the tumor with contrast, and no lighting up without contrast). As CT scans show bone, another way to diagnose them is to see a process that expands the internal auditory meatus (canal) -- IAC. In persons with metal in vital places -- such as a pacemaker, sometimes CT scan is the only test available. As persons with pacemakers often have marginal kidney function, one must be careful here not to damage their kidneys with the CT scan dye. All in all, CT's are a much less satisfactory situation than when one has the option of using an MRI.
A comparison between two scans of the same acoustic, the CT on the left and MRI on the right is shown above. Clearly the MRI is much more informative.
While MRI's are the most sensitive test to acoustics, they also can make errors. False negative errors mainly occur in persons with very small tumors, or very bad scans (such as a scan done in a low-field unit, such as an open MRI, without contrast). False positive errors are very rare but also possible (House et a, 2008).
Acoustic neuromas range in size up to 4 cm. The smallest, the intracanalicular acoustic, is measured in millimeters. A "small" acoustic is less than 1.5 cm (above left). A "moderate" acoustic is 1.5-3 cm, and a "large" acoustic is 3 cm or greater.
Tumors are staged by a combination of their location and size. An "intracanalicular" is small and in the IAC (internal auditory canal). A "cisternal" tumor has extended outside the IAC. A "compressive" tumor is touching the cerebellum or brainstem, and a "hydrocephalus" tumor is obstructing CSF drainage pathways in the IV'th ventricle. Acoustic neuromas can extend from the nerve into the inner ear, which can make their removal more difficult (Falcioni et al, 2003). Intralabyrinthine schwannomas as well as intracochlear schwannoma's exist (Kennedy et al, 2004). The acoustic on the right upper panel has a small intracochlear extension.
|Axial image of patient with NF2 showing acoustic neuromas on both sides (Image courtesy of Dr. Richard Wiet).||Higher image showing multiple meningiomas in same patient with NF2.|
Rarely, acoustic neuromas are inherited. Acoustic neuroma caused by type-II neurofibromatosis should be suspected in young patients and those with a family history of neural tumors. The figure above shows an example of such a person. It is common in this disease to become deaf due to bilateral acoustic neuromas. Genetic testing for NF1 and NF2 is available commercially, for example, from Athena Diagnostics.
There are several other tumors that can occur in the same region of the brain, the cerebello-pontine angle or CPA, as acoustic neuromas. Of all lesions in the CPA, acoustic neuromas account for 70-90 percent. Meningiomas are second most common (10 percent), followed by epidermoids, and then lipomas. Occasionally tumors in other locations, such as the lung, can metastasize to the CPA. Metastatic tumors usually grow rapidly -- hearing goes down quickly, and often the facial nerve is involved with a Bells palsy, over a few weeks time.
As CT and MRI scans become more commonly used, there are more acoustics being discovered accidentally -- serendipitously. For example, a person who has a migraine headache, might get an MRI which reveals an acoustic neuroma. Or someone who experienced an auto accident, might get an MRI which reveals an acoustic neuroma.
This situation has considerable potential for trouble. It is not very likely that a previously silent acoustic neuroma, will suddenly manifest itself at the same time as another problem - -such as a minor head injury or migraine. Rather, it is commonly the case that the person will have one of the common causes of dizziness, and just happen to also have a silent acoustic.
If someone in such a situation proceeds with surgery or radiation treatment, it is certain that the surgery or radiation will create a vestibular imbalance as well deafness. This will eventually occur anyway, but treatment of a tumor that is not growing will speed up the process. For this reason, extreme caution is suggested -- in our opinion, except for very large tumors, it is best to have objective evidence -- i.e. progression of hearing loss or a enlargement on MRI -- that the tumor is growing before embarking on surgery or radiation treatment. Should someone be suggesting an invasive procedure, a second opinion is well worth obtaining.
Nikolopoulos and O'Donoghue recently reviewed 111 articles on acoustic neuroma treatment and stated that "well-designed comparisons between treatment methods do not exist, and therefore claims by clinicians favoring a particular treatment are unfounded" (2002). Similarly, studies of long-term quality of life show no significant difference (Robinett et al, 2014). We do not think that clinical wisdom can be completely discounted, but certainly the present situation seems to be that acoustic neuroma treatment is an art and no single option is clearly superior.
According to Carlson et al (2015), about half of all patients are operated as soon as they are diagnosed, about 24% are radiated, and about 29% were observed. They also stated that in the US, there is a trend toward more observation and less surgery/radiation, mainly because tumors are being diagnosed earlier, when their size is smaller.
Over time, more acoustic neuromas are being diagnosed because there are far more MRI scans being done. There is a trend towards having smaller acoustics, and also towards having less aggressive acoustic neuromas over time. In other words, the playing field is changing and at present, we have a lot less aggressive tumors than in the past, due to sampling bias.
There are three distinct management options:
Medical Management of acoustic neuroma:
In practices with great experience, most acoustics less than 20 mm (i.e. 2 cm) in size are managed medically (Caye-Thomasen, 2016) -- i.e. with "watchful waiting". This to us appears ot be a good strategy as well.
Between 29 and 50% of all acoustic neuromas are currently treated with medical management (Caye-Thomasen, 2016; Carlson et al, 2015; Stangerup and Caye-Thomasen, 2012). Medical management consists of periodic monitoring of the patient's neurological status, use of hearing aids when appropriate, and periodic imaging studies (such as MRI's). It is an appropriate method of management in some patients (Hoistad et al, 2001).
There is no medication known to have a substantial effect on the growth of acoustic neuroma tumors. The tumors often grow very slowly (or not at all), less than 1 mm/year, and one may elect to follow a tumor with serial audiometry and/or MRI scans (Shin et al, 2000). In individuals of advanced age, a serious threat to life or bodily function from tumor growth may be judged unlikely in the remainder of a patient's expected lifespan, and for this reason, medical management may be elected (Perry et al, 2001). Once a tumor is diagnosed, a repeat scan is obtained at 6 months and then at yearly intervals (Perry et al, 2001).
This procedure has its own risks. Even when the tumor is not growing on MRI, there is a risk of losing useful hearing in this situation, making the individual no longer a candidate for hearing preservation type surgery. Somewhere between 10 and 43% of patients followed for about 2 years lose "useful" hearing (Warrick et al, 1999; Shin et al, 2000; Lin et al, 2005).
On the other hand, no matter what treatment procedure is used, surgical or not, in the long term, retention of "serviceable" hearing is very unlikely. The so-called "hearing preservation surgery" rarely preserves useful hearing, and hearing also tends to deteriorate fairly rapidly with time, with or without a tumor still being there. Because of this observation, some surgeons simply recommend taking out the entire 8th nerve when most convenient as this approach makes tumor recurrence very unlikely. In our opinion, this is should be a judgment call -- but not an unreasonable idea. A reasonable estimate is that over a year, about 75% of tumors will have visible enlargement, averaging 1.5 mm, and about 25% will not. Some variants grow much faster than others. In recent years, far more acoustics are being detected that are not growing rapidly, and the 1.5 mm/year statistic is likely much too high.
Wait/See patients are not necessarily doomed to go death -- Stangerup et al (2010) stated that "Most vestibular schwannoma patients with 100% speech discrimination at diagnosis maintain good hearing even after many years of observation. "
In persons with neurofibromatosis, hearing is likely to remain stable in unoperated ears for about 1-2 years (Masuda et al, 2004).
Gamma Knife: for acoustic neuroma
According to Carlson et al (2015), about 29% of acoustics are initially treated with radiation. When the risk of surgery is high because of other medical problems, or where the patient simply refuses surgery, the "gamma knife" procedure may be used. This is a method of irradiating the tumor, invented by Lars Leksell in 1971. This procedure avoids surgery with its attendant risks. In the past, this option was usually recommended only for higher risk surgical cases because of the possibilities of late radiation complications, and the need for ongoing MRI monitoring of the results of the procedure. Billing for this procedure is amazingly high, so this is not something that be generally used without health insurance.
As acoustic neuromas recently seem to have become more benign, one might argue that radiation is being overused, and that there should be more "medical management".
Acoustic neuroma's are generally not "killed" by radiation. They are damaged, they are scarred, but it is just not the nature of radiation to completely obliterate tumors. (Jacob et al, 2015) For this reason, periodic MRI scans are prudent post gamma knife surgery. Generally speaking, this is once/year and then less often if there is no change.
Muzevic et al (2014), in a Cochrane review, stated that "There is no high quality evidence in the literature from RCTs to determine whether stereotactic radiotherapy is better than microsurgical resection or observation alone for patients with a vestibular schwannoma."
The author of this review does not favor high-dose gamma knife because of the possibility of radiation complications at 2 years and beyond. However, low dose gamma knife is looking much better and there are certainly many times when it is the best option. Lower doses of radiation (e.g. 13 Gy) are presently advised because of the much lower risk of facial weakness and numbness (Wackym et al, 2004).
An interesting consequence of the lower-dose radiation protocol is that patients are now seen who do not have complete loss of hearing or vestibular function after radiation. In some cases this can be very annoying as it may result in nerve irritability symptoms such as hyperventilation induced nystagmus. This is probably a consequence of using a treatment methodology that works more slowly than surgery. It seems likely that this complication is more common in persons who have small tumors.
Hyperventilation induced nystagmus for this situation beats towards the lesion (unlike vibration induced nystagmus which beats away from the lesion). It is often very powerful. In persons with this sign, one can either wait for it to go away (this may take several years), try a medication that reduces nerve irritability, or reconsider surgical treatment.
Supplemental material on the site DVD: Video of hyperventilation induced nystagmus in patient with left sided acoustic neuroma
Stereotactic radiotherapy of acoustic neuroma
Radiation other than gamma rays can also be used to treat acoustic neuroma. They include linear accelerator (LINAC) and Cyberknife. These other modalities are similar to gamma knife in overall features. Long term hearing preservation is very rare in persons with stereotactic radiotherapy (6.7% according to Lin et al, 2005). In other words, although the "goal" is to preserve hearing, practically this is unrealistic. We see no particular reason to seek out stereotactic radiotherapy rather than gamma knife. The chance of recurrent tumor using current dose regimens is roughly 5-10%. Tumor growth is rare in patients who remain stable 6-7 years post therapy.
Issues in radiotherapy are recurrence (5-10%), hearing loss (eventually 93%), risk of radiating large (>2 cm) tumors due to swelling of the tumor in the first year, risk for malignancy (Tanbouzi et al, 2011; Markou et al, 2011), hydrocephalus (rare), ruptured IAC aneurysm (rare), and accelerated vertebrobasilar atherosclerosis (e.g. Jackler, 2007).
For larger tumors, cystic lesions, and neuromas with brainstem compression, according to the neurosurgeons, microsurgical resection in experienced neurosurgical centers is the preferred option (Unger et al, 2010). These are generally larger than 20 mm, but there is some "wiggle rooom" depending on where the tumor is located and the size of the individuals posterior fossa.
|Very large acoustic neuroma (coronal view, the tumor is the large white blob). Source: Mayo Clinic Neuroscience Update.|
Surgical Treatment of acoustic neuroma
About half of all acoustic neuromas are presently treated with immediate surgery (Carlson et al, 2015). This number appears to us to be unreasonably high, as recent data suggests that acoustics grow more slowly than was previously appreciated, and also it is difficult to see the advantage of doing immediate surgery compared to a waiting process.
This fraction will likely decline over the next few decades as watchful waiting as well as use of gamma-knife and other radition based treatments grow. The figure above shows a large acoustic neuroma in which surgical management would generally be preferred. In most instances surgical removal of a large tumor is the preferred option because it prevents potentially fatal complications of tumor growth (although this would be very unusual). Surgery may theoretically enable "preservation" of hearing, although it is very rare that hearing is actually serviceable after surgery. Usually the surgery is done at an academic center by a team of surgeons including a neurotologist (a specialized otolaryngologist) and a neurosurgeon. There are several operative approaches.
Common surgical approaches to acoustic neuroma
(Image from Jackler R, Atlas of Neurotology and Skull Base Surgery (Mosby 1996, First edition, with permission).
The translabyrinthine approach makes no attempt to preserve hearing, and does not open the skull. This approach is generally a good one in terms of little collateral damage (Springborg et al, 2012), but of course, would not be appropriate for someone with substantial hearing. .
The middle fossa approach (not shown above), can theoretically preserve hearing, but like the retrosigmoid, the approach is intrinsically more dangerous as it involves retraction of the brain, and also more prone to complications.
Compared to radiosurgery, the primary advantage of conventional surgery is avoidance of late complications associated with radiation of neighboring structures. This is a reasonable consideration which, in our opinion, makes conventional surgery advantageous when the patient is a reasonable surgical risk.
Each of these surgical approaches has advantages and disadvantages that must be considered in selecting an optimal approach. Surgical treatment where the brain is exposed is nearly always performed by a team of surgeons, usually including a neurotologist and a neurosurgeon. Most patients are admitted to the hospital a day before the operation. After surgery, they spend a night in a monitored unit. Most are discharged from the hospital within 4-6 days after surgery, and return to work is usually possible in 6 weeks. MRI's are usually obtained at 1 and 5 years to detect residual or recurrent tumor. Total or near total (95%) removal of the tumor is advised (Sanna et al, 2002). Careful follow up with MRI is advised to detect recurrence. Nodular or progressive enhancement in the internal auditory canal may represent regrowth (Brors et al, 2003).
Tumors that extend into the labyrinth itself (i.e. "intracochlear schwannomas") may not be removed by the "retrolab" approach to tumors, and may recur. There are also many significant complications that can result from surgery that should be considered (see below).
With respect to hearing preservation surgery, while of course this is certainly desirable, unfortunately the chance of hearing being preserved after acoustic neuroma surgery is slight. Nedzelski and colleagues recently reported that only about 16% of patients had "serviceable" hearing in the follow up interval after "hearing preservation surgery". (Lin et al, 2005). While hearing may be "preserved" immediately after surgery, it usually deteriorates in most within a few years. Peng and Wilkinson (2016), on the other hand, stated that " microsurgery, including the middle fossa approach, may provide excellent hearing outcomes, particularly when a small tumour has begun to cause hearing loss." There is a "catch-22" problem here. Small tumors are doing very little damage, so one might want to just let them grow. On the other hand, a surgeon might argue that it is too late to save hearing once the damage is done. We ourselves favor the conclusion of the Nedzelski group -- that hearing preservation is possible but unlikely after surgery on the 8th nerve.
At the author's clinical practice in Chicago Illinois, it is suggested that prospective operative candidates primarily consider safety and the probability of complications when considering surgery or gamma knife. If one has serviceable hearing, and there is no other danger of waiting (such as needing a bigger operation), one might reasonably simply wait until hearing becomes unserviceable (i.e. useless) before proceeding with surgery or radiation. Here the procedure would be periodic hearing tests, and less frequent MRI scans. The frequency of testing is mainly determined by the rate of change in the measures, but about every 6 months for hearing testing and about once/year for MRI scanning is usually appropriate. If there is no change after the first 6 months, it is reasonable to move the rechecks out farther -- to once/yearly, or sometimes once every few years.
Medical management, surgery and gamma-knife methods of managing acoustic tumors all seem reasonable at this writing, with the choice depending on individual factors. Medical management is almost always the best strategy for small tumors. Surgical management has the advantage that it "gets it over with quickly", but normally dizziness is much worse shortly after tumor removal (El-Kashlan et al, 1998). Gamma knife is less acutely stressful, but it can delay resolution of dizziness.
In most instances, acoustic neuroma surgery results in complete loss of vestibular function on the operated side. Gamma knife treatment also is associated with substantial decline in vestibular function (Wackym, 2004). Patients frequently experience vertigo and imbalance post-surgery (Levo et al, 2004; Tufarelli et al, 2007). Vestibular rehabilitation may speed recovery from this deficit. Unless there already is complete loss of vestibular function prior to surgery or radiation (as documented on ENG), we think it is best that the patient who is planning to have acoustic neuroma surgery visit a vestibular physical therapist to make sure that there is a "good fit" and to learn the basic procedures, and for the individual to begin a weekly program of PT for 1-2 months following discharge. It is important that the otologic surgeon who performs the operation be involved with the therapy as in some situations (i.e. patients with CSF leak), therapy should be delayed.
Piazza and others recently suggested that in the elderly, the following algorithm should be followed: If the acoustic protrudes less than 1 cm into the cerebellopontine angle, an MRI should be repeated in one year. If the growth rate is < 2 mm/year, the patient should be observed. If greater than this, offered surgery. We are a little puzzled about the 2 mm/year growth rate criterion, as one would think that if a tumor went from 1 to 3 mm total, it would still be very small, and one should still wait. Of course, a tumor that grew 10 mm/year, we would think should have something done to stop its growth.
According to Piazza, for tumors that protrude > 1 cm into the CP angle, and patients in good general health should be offered surgery. Patients in poor general health should be offered radiosurgery (Piazza et al, 2003). We think that this strategy is a little too much in favor of surgery, and instead we would usually advise patients to avoid surgery or radition until there was a reasonable expectation of impending damage to structures other than the inner ear.
Stangerup et al (2008) stated that "After comparing the hearing results of hearing preservation surgery and of radiation therapy with those of 'wait and scan' management, it appears that, in vestibular schwannoma patients with a small tumour and normal speech discrimination, the main indication for active treatment should be established tumour growth."
Surgical treatment of acoustic neuroma has a substantial risk. Overall, the risk of death from acoustic neuroma surgery is about 0.5 to 2 percent. Unexpected post-operative complications occur in roughly 20 percent with more complications occurring in elderly and infirm individuals and those with large tumors (Kaylie et al, 2001). Complications, ordered from rare to frequent, are listed.
A review of acoustic neuroma surgery suggested that CSF leak (9.4%) and meningitis (1.5%) are the most common complications (Slattery et al, 2001). El-Kashlan et al (1998) reported that dizziness from unoperated acoustic neuroma's is generally mild as damage progresses slowly and is handled with compensation. On the other hand, immediately after surgical removal, most patients report "severe vertigo and dysequilibrium".
MRI scan from person who had acoustic neuroma removed via retrolab approach, and with refractory dizziness. On the right side of the picture (left side of head), there is a black area where there was damage to the cerebellum, presumably associated with the surgery. Another scan showing cerebellar damage following acoustic neuroma surgery. This type of injury will most likely be accompanied by permanent imbalance due to the combination of a cerebellar injury and loss of inner ear function on the left. Tumor operated in 1961. There is clear cerebellar damage on the left side of the cerebellum (right side of picture). Residual acoustic neuroma as well as cerebellar damage, in this patient who had surgery in the remote past.
In a review of results of 258 patients operated via the translabyrinthine approach, stroke or cerebellar injury occurred in 1.1%. Cerebellar injury can occur due to traction as well as due to injury to branches of the anterior inferior cerebellar artery (Hegarty et al, 2002). Images of cerebellar traction injury are shown above. These patients generally have very prominent oculomotor signs (e.g. nystagmus) and persistent (e.g. lifelong) imbalance.
There can also be traction injuries to the temporal lobe, after middle fossa route surgery.
Death following acoustic neuroma surgery
There is overall a 0.5 percent incidence of death due to acoustic neuroma surgery. Discharge to long-term care (1.2%), to short-term rehabilitation (4.4%) are also possibilities (Barker et al, 2003). Thus, there is roughly a 1.7% chance of death or long term nursing care being required after acoustic neuroma surgery. The odds are better if one chooses a "high volume" hospital and surgeon.
Other complications include CSF leak in 7.8%, meningitis in 1.6%.
Facial weakness of various degrees appeared in most, but severe weakness with House-Brackman scores of V-VI at 1 year occurred in 6% (Mass et al, 1998). Wiet and others have recently reported results in 500 cases (Wiet et al, 2001). Overall success at retaining useful hearing was 27%, with considerably better results obtained when operating via the middle-fossa approach.
In the literature, surgeons often use the term "serviceable" hearing (e.g. Lin et al, 2005), meaning literally that hearing is of some service, or "better than nothing". Note that they are not talking here about "restoring" hearing to normal -- what is dead is dead. Also, realistically, surgery (or radiation) is a destructive procedure. A procedure that removes some nerve (with tumor) is unlikely to improve hearing. The best one might hope for would be a small reduction in hearing (after surgery), followed by stability.
With that in mind, of course, hearing generally gets worse after acoustic neuroma surgery. Keep in mind also that audiograms are somewhat subjective, there is such a thing as a placebo response, and reported results may sometimes mainly be wishful thinking.
Turning to the data:
Hearing in the operated ear often deteriorates over time to a greater extent than the unoperated ear, even without recurrent tumor (Chee et al. 2003). The percent of persons with "serviceable hearing" may deteriorate by 25% in the late post-operative period. This has been variously attributed to scarring, fibrosis, or microhemmorages during operation.
Over the long term, very few patients retain serviceable hearing (Lin et al, 2005). Some authors have suggested that given the modest hearing that is salvaged in the very few patients who are candidates for hearing sparing surgery, that hearing preservation as an objective of acoustic neuroma surgery is not worthwhile (Tos et al, 1988). Our view is that it is occasionally worth attempting, but one's expectations should be realistic. Hearing occasionally improves for the middle fossa approach (Stidham and Roberson, 2001). The middle fossa approach has more risk to one's brain and other aspects than other approaches.
One might preserve hearing when hearing is good, and the tumor is small. Then the patient must choose between the risk of sitting tight and surgery (or radiation). In other words, a gamble.
One would not be likely to preserve hearing when the hearing is good, and the tumor is big. Here, the odds are stacked against one.
This is simple stuff -- balance deteriorates after acoustic surgery because the surgery damages or removes remaining function in the vestibular nerve (Levo et al, 2004; Tufarelli et al, 2007). Tufarelli and associated reported that 10% of 459 patients judged their imbalance as disabling, and 73% felt that they had at least moderate oscillopsia (trouble seeing with head moving).
It has been our experience that over the long term (i.e. 2 years), balance generally returns to near normal, but persons who have other deficits such as visual disturbances (e.g. cataract), sensory loss (e.g. neuropathy), brain damage (e.g. cerebellar or brainstem damage due to the tumor or surgery), or poor adaptation (e.g. due to advanced age) may never obtain complete return of balance.
At a recent meeting at the ANA (acoustic neuroma association), the author of this page participated in a 2-hour session and interviewed 8 patients who had persistent vertigo. As a general rule, patients who had vertigo 2 years following surgery, had had a mixture of a cerebellar injury and total vestibular loss, associated with a difficult surgery (usually for a 4cm + tumor). The MRI scan of two patients are shown above. As a general rule, patients who have lost both inner ear function (from the acoustic), and substantial cerebellar function (from the surgery or acoustic), will have an enduring loss of balance.
Immediately after surgery, most patients have vertigo. The timing and prognosis of this vertigo depends on the reason and associated damage to the cerebellum (if any). A detailed discussion of dizziness, vertigo and imbalance, pre and post surgery can be found in this lecture handout.
Significant headache can occur following acoustic neuroma surgery (reviewed by Driscoll and Beatty, 1997). The incidence is very variable among surgeons and also depends on the choice of approach, but an overview of the literature suggests an incidence of about 20-35%. This compares with an incidence of about 8% after radiosurgery.
Schessel et al (1996) observed and documented adherence of neck muscles to the dura after craniotomy and reported a dramatic decrease in headache in patients who had craniotomy with replacement of the bone flap. Similarly, Harner also noted a drop in headache when cranioplasty with methyl methacrylate was used instead of craniotomy alone (Harner et al, 1995). The mechanism here is thought to be traction on the dura by movement of neck muscles. Many patients with this syndrome note aggravation by coughing or straining.
Schessel et al (1996) suggested that patients having surgery via the retrosigmoid approach had significantly higher frequency of headache than those who had the translabyrinthine approach. Several other groups have found a similar pattern. Schaller and Bauman (2003) noted severe headaches requiring daily medication and accompanied by a feeling of incapacity in 34% of patients at 3 months following retrosigmoid surgery. They found that these headaches were associated with aseptic meningitis and furthermore that they were associated with use of fibrin glue and drilling in the posterior aspect of the internal auditory canal. They suggested that prevention of postoperative headache may be accomplished by replacement of bone flap at the end of surgery, use of duraplastic instead of direct dural closure, and avoidance of the use of fibrin glue or extensive drilling of the posterior aspect of internal auditory canal.
Currently there is little information about incidence of headache using the middle fossa approach, but the few series available suggest a rather low incidence (Driscoll et al, 1997).
Management of post-operative headache utilizes analgesics, muscle relaxants, antidepressants and anticonvulsants, in a way similar to migraine management. Migraine abortive agents, however, and specific prophylactic drugs for migraine are not recommended in most instances. Nevertheless, a recent report found that sumatriptan (a migraine drug) improved headache in 9/10 patients with post-surgical headaches. This probably reflects the fact that migraine is an extremely common health condition that worsens with any type of head pain.
Persistent incisional pain may occur from entrapment of the occipital nerve or from formation of an occipital neuroma. Massage, local heat, and analgesics may help. Occipital nerve blocks (such as is done in the pain clinic) may also be beneficial.
Another mechanism that have been suggested is that bone dust trapped within the intracranial cavity may cause a protracted inflammatory response resulting in chronic headache (Driscoll, 1997). MRI images sometimes show dural enhancement (the membrane covering the brain "lights up") and CT images may show calcification along the brainstem (Schaller and Bauman, 2003). This sort of headache should not respond to blocking of scalp nerves (as is done in the pain clinic) and this procedure may be of some diagnostic use. In these patients, logically treatment might include anti-inflammatory agents and possibly corticosteroids. Narcotic analgesics are occasionally indicated.
No standards exist regarding patient follow-up following complete acoustic neuroma resection. On average though, 3 to 6 scans is common over a follow-up period of about 5 years. (Lee and Isaacson, 2005).
A person with an acoustic neuroma might desire a cochlear implant. Logic would suggest that this is generally a foolish endeavor. Acoustics are nerve tumors, or in electrical terms, disorders of wiring. Cochlear implants stimulate the part of the ear prior to the wiring - -they affect the microphone. Thus, one would expect that a cochlear implant would generally be ineffective in someone whose cochlear nerve has already been damaged by a tumor. In situations where there is one deaf ear, a "BAHA" is the logical choice.
Nevertheless, one might give it a try anyway if an acoustic tumor is present in the only hearing ear or after surgery to remove bilateral acoustic neuromas. Belal (2001) reported that cochlear implantation is possible only if there is an intact cochlear nerve (as shown by a positive response to promontory stimulation), and if the implantation is done at the time of acoustic tumor removal, before the cochlea ossifies.