When there is a ‘mixed’ hearing loss, how are the varying degrees of sensorii-neural and conductive components measured?
I know the different sorts can be diagnosed with the Rinne and Weber tests, but are they always accurate? Are there other tests employed to accurately measure the type and degree of loss?
Is a sensori-neural type deafness always permanent? Can it ‘get better’ of it’s own accord? Or, in other words, can you be diagnosed with a SN hearing loss at one point that on later examination is not evident, with the SN loss replaced by a conductive loss?
Considering that I just finished the day testing kids with all sorts of losses (conductive, sensorineural, and mixed), answering this question is going to feel like a refresher course.
My apologies for the length of my answer, but it was the only way I could do justice to kambuckta’s questions.
Let’s start with how a hearing loss is determined the first place. I’ll use a fictional ten-year-old patient named Jimmy to help illustrate examples.
Since Jimmy is ten years old, he will likely be cooperative enough for me to test his hearing using insert earphones. These are earphones with foam inserts that fit snugly into the ear canal. They have various advantadges over the standard headphones, the specifics of which need not concern us here.
I measure his air conduction thresholds in the 250 to 8000 Hertz range using pure tones. In this context, “air conduction” means that the signal has to travel through the ear canal and the middle ear en route to the inner ear in order for Jimmy to perceive it. I obtain thresholds of 60 dB HL across the entire frequency range in both ears.
Now, at this point, I do not know if this hearing loss is sensorineural, mixed, or conductive in origin. All I know for sure is that there is a hearing loss in both ears.
The way to determine the type of hearing loss is with bone conduction testing. While the Rinne & Weber tests are probably still used by various physicians, I myself have never used them, and I’ve never seen an audiologist rely on those particular tests. For an audiologist, “bone conduction testing” refers exclusively to assessing a patient’s bone conduction thresholds by placing an oscillator somewhere on the patient’s head…the most common location is the mastoid process (that little bump behind your ear). The signals in bone conduction testing travel more or less directly to the inner ear, skipping past the ear canal and the middle ear space. By comparing the bone conduction thresholds and the air conduction thresholds, it can be determined if there is anything about the ear canal or the middle ear that is contributing to the hearing loss. If the bone conduction thresholds are better than the air conduction thresholds, then a conductive hearing loss is said to exist. If they are the same as the air conduction thresholds, then a sensorineural hearing loss is said to exist.
For kids, the normal range of hearing is typically considered to be 15 dB and lower. At least, that’s the standard where I work. For adults, the upper range of normal hearing sensitivity can be as high as 30, although some places will use 25 dB as the standard.
So, let’s say Jimmy’s bone conduction thresholds are 5 dB across the board in both ears, which is 55 dB better than his air conduction thresholds. This means that he has an air-bone gap of 55 dB, and definitely has a bilateral conductive hearing loss. Since his bone conduction thresholds are 5 dB across the board, I know that if it wasn’t for the conductive loss, he would have perfectly normal hearing.
If his bone conduction thresholds were anywhere from 50 to 60 dB, then the hearing loss would be considered sensorineural in origin.
So how is a hearing loss of mixed origin is determined? A mixed hearing loss is one that entails both a sensorineural hearing loss and a conductive hearing loss.
So, let’s say Jimmy has a mixed hearing loss instead. With air conduction thresholds at 60 dB, I find that his bone conduction thresholds are 40 dB across the board. This means that there is an air-bone gap of 20 dB. A conductive hearing loss is definitely present; however, even if no conductive hearing loss was present, he would still have a hearing loss…air conduction thresholds would be no better than 40 dB once the conductive component is removed. This is a mixed hearing loss: a conductive hearing loss overlaying an existing sensorineural hearing loss.
When a conductive hearing loss is present, masking must be used in order to isolate the response from a particular ear. The signal from a bone conduction oscillator is assumed to reach both inner ears, and whichever one has the better sensitivity will be the one to respond. Masking typically involves the introduction of frequency-specific narrow-band noise…sort of sounds like wind rushing in your ear. The details of masking defy quick summary, but I hope it suffices to say that it is essential in determining which ear has a conductive loss. If both ears have a conductive loss, lots of masking will be involved.
Kids under about the age of three are commonly tested in the soundfield instead of using headphones, using a procedure called visual reinforcement audiometry (VRA). Similar principles apply here…some kids may tolerate insert earphones, and I’ll use them while testing kids using VRA, and that can get me good ear-specific results.
Quite accurate, especially if you’re dealing with a cooperative patient. Occasionally, though, I’ll run across a faker, but there are ways of catching fakers, and most audiologists worth their salt won’t be fooled by a faker.
There are various other tests that can be useful in determining the presence and/or type of dysfunction in the ear. I’ll mention a couple of them here:
Tympanometry is a test that I always administer to every kid, regardless. I’ll use a somewhat simplified explanation of tympanometry here: tympanometry essentially measures the mobility of the eardrum and middle ear system by introducing changes in air pressure within the ear canal. It is not painful, and can be done rather quickly. In a matter of a few seconds, tympanometry can give me a wealth of information about the middle ear status of a patient. If a conductive component is present, the tympanometry will typically show results that are consistent with middle ear dysfunction. Although the sensitivity of tympanometry to middle ear dysfunction is very, very good, it isn’t 100%…just earlier this week I saw a patient that had a conductive hearing loss in both ears, and yet the tympanometry was normal.
Otoacoustic emissions. Again, I’ll use here the simplified explanation I give to parents at the hospital. The outer hair cells of a normal cochlea (inner ear) will actually generate an extremely faint echo when stimulated by auditory input. Anything more than a mild hearing loss, and this echo will simply not be present.
The echo is called an otoacoustic emissions, or OAEs. If OAEs are present, that is considered indicative of normal cochlear outer hair cell function. If they are absent, that is considered indicative of cochlear outer hair cell dysfunction.
The advantages of OAEs are that it is a fairly quick test to administer, and can be done on a patient of virtually any age. In fact, many infant hearing screening programs use a screening version of the OAE test to determine if a newborn might have a hearing problem. An OAE test is also objective: it does not depend on the cooperative behavioral response of a patient for the results.
The disadvantages of OAEs are that it is extremely easily affected by background noise (so if a kid is crying his little head off, forget about trying the OAE test), it cannot be used to determine the severity of a hearing loss (moderate hearing losses and severe hearing losses will yield the exact same result of absent emissions), and if any type of middle ear disorder is present, it is extremely unlikely that OAEs will be measurable at all, regardless of whether or not the patient has normal hearing.
Absent emissions along with normal tympanometry is very suggestive of a sensorineural hearing loss.
Auditory brainstem response (ABR) evaluation. Think of it as an EEG, but instead of measuring brain activity, an ABR test measures the response of the auditory nerve and brainstem to various types of auditory stimuli. Like OAEs, an ABR test is an objective measure, and can give an indication of whether or not a hearing loss is present. Unlike OAEs, it is not affected by background noise, and it can give specific information on the severity of the hearing loss. With the use of a bone conduction oscillator, it can also be possible to determine if a conductive component is present.
An ABR test can be easily interfered with by muscular activity, so sedation is commonly used when administering this test to children. Where I work, chloral hydrate is the sedative of choice, but only for kids older than 3 months. Under 3 months of age, we do ABRs without sedation. ABR tests are time-consuming and expensive, and are typically done as a last resort when no other means of obtaining reliable and ear-specific information is possible.
There are other tests like acoustic reflex and reflex decay, and there are different flavors of OAEs, but crikey, this post is getting long enough as it is.
I should mention that there is another (and much less sensitive way of distinguishing between conductive and sensorineural hearing losses. The configuration of thresholds on an audiogram can be highly suggestive as to whether or not a hearing loss may be sensorineural or conductive in origin. For instance, any conductive hearing losses, especially those caused by fluid in the middle ear, will have the configuration of a hearing loss that is much greater in the low frequencies than in the high frequencies. It takes some experience and judgment to decide how important the configuration of a hearing loss is in determining the origin (or even the validity) of a hearing loss, and I would never use it as the sole criterion for determining this.
Sensorineural hearing losses are typically permanent, although anything is possible. A few patients, like those with Meniere’s Disease, may have a fluctuating sensorineural hearing loss. Generally speaking, if you have a sensorineural hearing loss, you’re stuck with some degree of hearing loss for the rest of your life.
On the other hand, most conductive hearing losses are medically treatable, and once a conductive hearing loss is resolved, the person will have normal hearing again. Unless they have a mixed hearing loss, in which case the hearing will resolve to wherever the bone conduction thresholds indicate the sensorineural hearing loss to be.
As I mentioned before, there are instances of fluctuating hearing loss. But typically the baseline for a fluctuating hearing loss of sensorineural origin is already at a point that would still be considered a sensorineural hearing loss. I cannot recall ever seeing or hearing about a case where a person had a significant sensorineural hearing loss, and then recovered to the point of normal hearing sensitivity.
When encountering a new diagnosis of hearing loss, I don’t like to rely on a single test to determine the type of hearing loss, especially when dealing with kids. I prefer to use a battery of tests. For instance, just recently I identified a 7-year-old boy with sensorineural hearing loss in both ears. How he made it that far in life without ever having his hearing tested is something I’d like to find out. With this kid, I did both the standard audiometry of air conduction thresholds, bone conduction thresholds, spondee recognition thresholds (that’s another story), word recognition tests, tympanometry, and OAEs. If all the tests are consistent in their agreement with a particular diagnosis, this increases my certainty as to whether or not a kid has a sensorineural hearing loss as opposed to a conventional hearing loss.
I hope this information is helpful. Again, I apologize for the length of the post, but there were a fair number of questions to answer, and I wanted to answer them in the depth that they deserved. Feel free to ask more questions if you need to, either here or at my e-mail address (which can be found in my profile).
Oh, instead of 3 months of age as the cutoff for using sedation in an ABR exam, it’s actually 6 months. I don’t know what my brain was thinking when it popped up with the 3 month figure.
