|Artist's depiction of an acoustic neuroma.|
Acoustic neuromas ("acoustics") can be diagnosed either by a medical doctor with otologic expertise 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 2019), 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. Red circles are the right (normal) side. The blue boxes are the left (acoustic) ear.||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|
Conventional audiometry (hearing testing)
Hearing testing is the most useful diagnostic test for acoustic neuroma. The most common abnormality is an asymmetrical high-frequency sensorineural hearing loss (see figure above left). 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 (although other studies suggest 1/100 -- see below). It has also been estimated that 5 percent of persons with asymmetrical sensorineural hearing loss have acoustics (Daniels et al, 2000) -- amounting to 5/100. 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.
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. Hentschel et al (2016) examined 5 studies of ABR testing in acoustic neuroma, and reported that sensitivity ranged from 37% to 91%, and specificity from 57% to 96%.
In our clinical context in Chicago, ABR testing 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). Hentschel et al (2016) examined studies of ENG, calorics and the HVT test, and stated that all resulted in poor diagnostic accuracy.
Electronystagmography, (ENG testing) is frequently abnormal in persons with acoustic neuromas and about 60 percent of all tumors are associated with unilateral loss of calorics (Hulshof et al, 1989). We have diagnosed a few acoustics by noting that ENG function is lost but hearing is relatively preserved. We have also encountered patients with acoustics with no changes in their vestibular function. In other words, sensitivity is only fair. 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, this cannot be relied upon.
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 more 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.
VHIT testing is much quicker than ENG (caloric) testing, and for this reason, we think might be a good screening procedure in persons where an acoustic is suspect, such as someone with progressive asymmetrical hearing. Nevertheless, like everything else, it is not always positive even in large tumors.
|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).||Another small (0.5 cc) intracanalicular tumor. This individual had only a slight high-frequency hearing loss.|
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. Hojjat et al (2016) suggested that MRI imaging of "undifferentiated asymmetrical sensorineural hearing loss" was cost effective, calculating that the "incremental cost effective ratio" was $27,660 to diagnose a tumor, taking into account the costs of scans on normal people. We find this conclusion dubious. We think that MRI is an immensely costly technology, and also that hearing asymmetry is immensely common. We don't think society can afford to detect every single acoustic neuroma.
According to Tolisano et al (2018), the MRI diagnosis rate for patients in a military personel (presumably younger than the general population), with asymmetrical sensorineural hearing loss was 0.85%. An otologic practice reported that in patients where they thought an MRI was reasonable, 1.4% of their patients had a "retrocochlear tumor" (Robinette et al, 2018). So it seems that even in the hands of ear specialists, who presumably know more about what they are doing than non-specialists, the yield of MRI for acoustic tumors is only about 1%.
Thus, roughly 1/100 MRI scans in patients with hearing worse on one side (having a sensorineural pattern) have a tumor. Let see --- $1000/MRI * 100 MRI scans = $100,000 per acoustic neuroma diagnosis. Of course, many patients are referred for MRI scans for less compelling reasons than asymmetrical hearing loss, and MRI's may cost considerably more than $1000, so this figure is probably a bit low. This figure may too high considering that the population being scanned may have less acoustic neuromas than the general population. It does not appear that seeing an expert is all that better than just using asymmetrical hearing loss as one's criterion for MRI. So most of the risk is carried in the hearing test.
Another huge expense in acoustic neuromas is the practice of doing serial imaging. As of 2017, instead of "taking it out", or perhaps using gamma knife, it is conventional to "follow" the patient with yearly MRI scans. Thus, the cost is not only to "diagnose", but also to rescan every year, perhaps for 10 years. Lets see - -$1000/scan, 10 scans. After a while, this adds up.
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 2019). 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.
Radiologists are not always consistent. According to Teh et al (2017), inter-observer difference averages 0.33 mm. As decision making regarding initiating some active treatment usually occurs around 10 mm in size, this isn't bad.
|CT with contrast of acoustic neuroma||MRI with contrast of same acoustic neuroma|
It is now common (as of 2019) to watch acoustics rather than just take them out or radiate them. According to D'Haese et al (2019), tumors that are larger when first diagnosed, grow faster. They said "Growth of the tumor was detected in 56% of patients and mainly observed in the first three years of follow-up. Tumor size remained stable in 34% and decreased in 10% of patients. No baseline information, symptom, or sign was found to be predictive for growth." It is curious that tumors sometimes decreased. One wonders if they really had tumors, as these "diagnoses" were based on MRI appearance, not pathology.
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 schwannomas 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 cerebellopontine angle or the 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.