I found the distinction between conductive and sensorineural hearing loss confusing, so I wanted to write about it. Please note that I’m not an audiologist, so this explanation is just based on my own understanding!
First, here’s a diagram of the ear:
When we hear a sound, the sound goes in through the outer ear (the pinna), travels through the ear canal, and vibrates the ear drum. The vibration of the ear drum causes the ossicles to move (these are three tiny bones in the middle ear), which then causes fluid in the cochlea (the inner ear) to move. The moving fluid in the cochlea causes tiny hair cells in the cochlea to bend, and the location of the hair cells that bend indicate the frequency of the sound (how high or low pitched it is). The information from the hair cells then travels to the brain.
A conductive hearing loss occurs when someone has a hearing loss that stems from a problem in the outer or middle ear. Conversely, a sensorineural hearing loss occurs when someone has a hearing that stems from a problem with the hair cells of the cochlea or with the nerve that travels from the cochlea up to the brain.
The vast majority of diagnosed hearing losses are sensorineural, especially in adults. Sensorineural hearing loss can occur when hair cells are damaged due to medication, infection, and especially from exposure to really loud noise (don’t turn your headphone volume up too high! If people can hear your music when you’re wearing headphones, it’s too loud!). In babies diagnosed with sensorineural hearing loss, the cause might be genetic.
Conductive hearing losses are less common, although they do occur pretty frequently in children. A conductive hearing loss means that sound is having trouble reaching the cochlea – this could be due to a malformation of the ear canal, too much ear wax, or, most commonly in children, fluid in the middle ear and/or an ear infection.
Many conductive hearing losses can be treated or will go away on their own – for example, ear wax can be removed and ear infections can be treated. Conversely, a sensorineural hearing loss can’t be cured – once hair cells are gone, they can’t be grown back! (at least not yet. scientists are walking on this!). Hearing aids and cochlear implants don’t fix sensorineural hearing loss – in the case of hearing aids they amplify sounds to stimulate the remaining hair cells, and in the case of cochlear implants, they bypass the hair cells completely to stimulate the nerve that transmits sound information to the brain.
So how do audiologists determine whether a hearing loss is conductive or sensorineural? I think this is tricky, especially with babies! One thing T’s audiologist does is compare his audiograms measured with air conduction with his audiograms measured with bone conduction. Sounds played by air conduction go through the full ear chain – from the outer ear to the middle ear and then to the inner ear. With bone conduction, the audiologist puts a tiny oscillator on T’s mastoid bone, and the vibrations cause a sound at a particular frequency to be played – but this sound bypasses the outer and middle ears and goes right to the inner ear.
By comparing the air conduction and bone conduction audiograms, the audiologist can get an indication of if there’s something wrong with the outer and middle ears. If both the air conduction and bone conduction audiograms show a hearing loss, and the losses are similar, this indicates a sensorineural hearing loss. On the other hand, if the bone conduction and air conduction results are very different from each other, this may indicate a conductive hearing loss. For example, if the bone conduction audiogram results show normal hearing thresholds, this indicates that the inner ear is normal. If, however, the air conduction audiogram shows abnormal thresholds where the bone conduction results are normal, this indicates that although the inner ear is normal, there is a problem with sound getting to the inner ear – that is, there might be a problem with the outer and middle ears – a conductive hearing loss.
I think this is particularly difficult to get a handle on for babies and young children, because they are too young to be able to tell you if they are hearing stuff differently in one ear compared to the other, the way an adult would be able to if they had ear wax build up or fluid in one ear. And, I think conductive hearing losses in particular can fluctuate a lot, especially for babies and children who go to daycare and school and may frequently have colds or ear infections, so that variability just makes this even trickier to nail down!