Timothy C. Hain, MD • Page last modified: August 15, 2021
|Acoustic Neuroma (swelling of 8th nerve, just under Facial nerve)|
Acoustic neuromas, also known as "acoustics", acoustic neurinomas, or the currently preferred "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, where they are called "intralabyrinthine schwannomas" (Neff et al, 2003). Sometimes, instead of "neuroma", it is called an "neurinoma". Of course, the name doesn't really matter as everyone understands that both are names for the same thing. A rose is a rose ...
Acoustics comprise about 6% 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. This number is the "incidence". The prevalence -- the number of people with an acoustic -- is of course much higher. According to a recent study from the Mayo clinic, in Rochester Minn (Marinelli et al, 2018), the incidence was 1.5/100,000 during the first decade (i.e. about 1966-1976) and it has increased to 4.2/100,000 in the most recent decade (i.e. 1996-2016). This is likely due to better diagnostic technology -- i.e. MRI scanners. Note however, that this new incidence number corresponds to about 42 cases/million, which is far more than the estimates quoted above. This would correspond then to about 12,000 new cases/year in the USA, given that Minnesota residents are representative of the entire country.
In the author's clinical practice, Chicago Dizziness and Hearing, as of 2019, we have diagnosed or were referred 98 patients with acoustic neuroma patients out of a total of about 20,000 patients with dizziness. Thus acoustic neuroma is not a very common cause of dizziness.
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). Of course, there are more people with acoustics than operated, so this incidence is low. 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 46,414 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, but we also evaluate several patients with known acoustics accounting for the count of about 100 patients/20 years. 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 for society as a whole (Pan et al, 2015). An individual with hearing loss may feel differently however, so there is conflict here between what is good for society and what is good for individual people. Perhaps insurance payment algorithms should be restructured to allow for this difference in interests between individuals and society.
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). Acoustics in persons aged more than 70 tend to grow more slowly and also tend to be smaller (Basu et al, 2019).
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 https://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 physician 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 cVEMP'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 in acoustics. 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 supposedly more common with smaller tumors, although this certainly defies common sense as one would think that more damage would mean more vertigo.
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 arm on the side of the tumor) are extremely 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, but we ourselves have never seen this (perhaps because our cases usually are small tumors). Headache prior to surgery occurs in roughly 40 percent of those with large tumors.
This section is covered under a separate web page on diagnosis of acoustic neuroma using tests.
This section is covered under a separate web page on treatment of acoustic neuroma.