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Hearing Loss

Hearing loss occurs in two major forms -- conductive hearing loss and then sensorineural hearing loss, but you also can see mixed forms. Conductive loss is caused by interference with transmission of sound through the external canal, through the tympanic membrane or through the ossicles. Whereas sensorineural problems are caused by damage to the cochlea, the auditory nerve.

Conductive hearing loss: can be either acquired or inherited. Acquired is much more common. Tends to be transient and is often mild to moderate hearing loss. Most often due to things like earwax or sometimes middle ear effusion. If you see more significant loss or a more chronic loss, it’s generally caused by damage to the tympanic membrane and/or the ossicles. So things like chronic perforation, which we mentioned before, cholesteatoma, otosclerosis, or more seriously ossicular chain disruptions, will lead to serious conductive hearing losses.

Sensorineural hearing losses can be acquired or inherited. The acquired ones may be infectious so TORCH infections in newborns, particularly Rubella, can cause sensorineural hearing loss. Bacterial meningitis, especially H. flu can cause hearing loss. And then medications, aminoglycosides, diuretics are two of the most notorious.

A very brief word about speech audiometry. A couple of things that will be tested on, which is more advanced than the pure tone audiometry, one is something that is called the "Speech Reception Threshold" or SRT. That is defined as the lowest sound intensity in decibels at which a child can identify selected words in 50% of the trials. So give them a list of words and they can identify what those words are 50% of the time. In comparison you have the speech discrimination score where you have a percent of words on a standardized list that are correctly identified if presented at a sound level 25-40 decibels above the speech reception threshold. Kind of complicated, pretty unlikely to show up on the test but might be worth kind of noting. One fact worth remembering about these though, is that looking at speech discrimination scores, if you look at these in sensorineural hearing loss, they tend to be lower than conductive loss. So kids with hearing loss due to conductive problems actually probably can pick up speech better than if there is sensorineural hearing loss.

On to tympanometry. In tympanometry what you are trying to assess is two things, essentially. One is mobility of the tympanic membrane and second is indirectly you are trying to measure the pressure, or estimate the pressure, in the middle ear. There are three key things that you want to look at when you are looking at audiometry. And that is, the peak of the curve.

Next one, what you see here is the peak height or the height of the peak is normal so the eardrum can move normally but the problem is that the peak is coming out at the left of this box at negative 200. So what that means is that while there is normal mobility there is negative pressure behind that middle ear, or behind that tympanic membrane within the middle ear and that is consistent with eustachian tube dysfunction and a retracted eardrum.

The third one here is a high peak and this one can be due to a couple of different things. Again, you see this peak falls outside of the box. Moderate levels are generally due to things like thinning of the tympanic membrane. So a child who has had recurrent infections may have thinning of the tympanic membrane and you may see moderate levels. The other thing, though, that can cause a very high peak is ossicular disarticulation and there you may see curves that literally go off the map.

Okay, on to audiologic evaluations. I think the trick here - I’m going to cheat, it’s really not cheating but - if you look at the tympanometry first I think it can suggest a little bit of where the pathology may lie. So here we have a curve that’s shifted to the left, which means negative pressure. The left ear is completely normal. And looking at the legend here, the air conduction on the right is circles, X is left ear and what you see here is essentially the air conduction is shifted down slightly. So this is just a right sided mild eustachian tube dysfunction with a retracted ear drum that’s just not going to function quite as well in terms of hearing. The second one, you see a flat line here. Most likely it’s going to be a middle ear effusion.

Finally, we have one where the tympanic membranes are completely normal on tympanometry. They move very nicely, the pressures are normal. But here you see profound hearing loss bilaterally on both sides and you see both air conduction loss and with the brackets you also see that bone conduction is abnormal and it is much worse at the higher threshold. So this is a fairly classic audiogram for sensorineural hearing loss.