Friday, August 6, 2010

On Conductive Hearing Losses

In many cases, conductive hearing losses are caused by earwax or fluid in the ears. This condition is both temporary and easily remedied (with wax removal or pressure-equalization tubes to allow the fluid to drain).

Since Nolan was discovered to have a large conductive component to his hearing loss (the frequencies from 250Hz-1000Hz have a large air-bone gap), I have been trying to figure out what could cause this component. It is not caused by fluid, infection, or any other temporary situation. It has been there since birth, and has not improved with the insertion of PE tubes. In fact, it has only gotten worse or fluctuated over time. In addition, he has had several hearing tests with type A tympanograms (no fluid), which demonstrate a hearing loss with a clear middle ear space.

With some research via Dr. Google, I have found a few causes for air-bone gaps (conductive hearing losses) in children. They include:

  • Ossicular malformations (the middle ear bones are malformed). Nolan's CT scan at the age of six months states: "Bony ossicles are well formed. Middle ear cavities and mastoid air cells are clear." The CT scan rules out any malformation of the middle ear bones.
  • Atresia. This means the external ear canal is not formed and prevents the transmission of sound to the eardrum. Nolan does not have atresia.
  • Congenital cholesteatoma. The MRI and the CT scan did not indicate any cholesteatomas present.
  • Fixed stapes or malleus (middle ear bones). Usually identified on CT scan- Nolan's CT scan was unremarkable.
Of course, Nolan doesn't just have a conductive loss. He has a moderate sensorineural hearing loss from 2000-4000Hz. So, what could cause a congenital hearing loss consisting of both sensorineural and conductive components, which fluctuates over time?

  • Enlarged vestibular aqueducts: there is often a low frequency air-bone gap in children with EVA. Nolan's CT scan report states, however: "Vestibular aqueducts are unremarkable." So it is probably not that.
  • X-Linked Stapes with Perilymphatic Gusher. This occurs only in boys and causes a fixed stapes, which cannot be repaired because a "stapes gusher" will occur and the ear will lose all hearing. This is a mixed hearing loss profile, but is typically severe-profound (Nolan is moderately severe).
  • Otosclerosis. This is rarely encountered in children, and is extremely rare in congenital form. This is also visible on CT scan, and Nolan's CT scan was normal.
I can't really find anything else that would cause Nolan's hearing loss profile. In any case, we either have an idiopathic conductive hearing loss and an idiopathic sensorineural hearing loss, or the two are related and Nolan's CT scan or MRI failed to show the problem.

I plotted his thresholds over time, excluding any tests with middle ear pathology (all scores below are with either type A tympanograms or patent PE tubes). Anyhow, you can see the fluctuation in his air conduction scores over time- something our ENT and audiologist have never really paid attention to:

Left Ear

Right Ear

4 comments:

Apraxia Mom said...

So would an MRI at this point be the next step? A bit more detail oriented than a CT scan.

It's always something....

Joey @ Big Teeth and Clouds said...

It's frustrating that there's so much they can't tell us about our kids ears.

Julia said...

Do the cochlea and the middle ear develop at the same time in utero? I'm just wondering if there was perhaps a common cause for two otherwise unrelated kinds of hearing loss (i.e. not linked by some kind of syndrome).

leah said...

The inner ear development is complete by 5-6 weeks gestational age, and the middle ear doesn't start forming until about 9 weeks GA. They don't form at the same time, which is why two coincidental hearing losses that are unrelated would be very rare. Especially since Nolan's ears were symmetrical until recently- whatever happened has to be related.

Also, he has just as much hearing fluctuation in his purely sensorineural hearing range as he does in his conductive (look at 4000Hz- there is no conductive component here). Something funny is going on- it will be interesting to see what our ENT thinks!