What I should have known about Otitis media but didn't

2012 March 12
by Daniel Lakeland

Otitis media is the most common cause of "earache", it's a condition in which the middle ear becomes inflamed, often from an infection. Following the inflammation you have a lot of pain in the ears, and you may have pus build up inside the ear, or a variety of unpleasant things.

In the US, this was a much more common and painful condition when I was a child than for my children (I'm told) and the main reasons are the existence of the Pneumococcal vaccine (PCV) and the significant reduction in secondhand smoke exposure for children.

My understanding of this condition has always been along the lines of the following statement (from wikipedia article on Otitis media):

"When the middle ear becomes acutely infected, pressure builds up behind the eardrum (tympanic membrane), frequently causing intense pain."

This statement makes it seem like the inner ear is filled with a high pressure and it causes your eardrum to bulge outward and then sometimes rupture. But further along in the Wikipedia article they correct this misconception:

"At an anatomic level, the typical progression of acute otitis media occurs as follows: the tissues surrounding the Eustachian tube swell due to an upper respiratory infectionallergies, or dysfunction of the tubes. The Eustachian tube remains blocked most of the time. The air present in the middle ear is slowly absorbed into the surrounding tissues. A strong negative pressure creates a vacuum in the middle ear, and eventually the vacuum reaches a point where fluid from the surrounding tissues accumulates in the middle ear."

This interpretation is consistent with other sites that I've read as well.

So the "pressure" in the middle ear is actually a reduced pressure vs the surrounding air. This reduced pressure causes the flow of fluids from the surrounding tissue into the middle ear, and then the fluid provides a medium for bacterial infection.

Since this is a blog about modeling, let's examine a simple mechanical model for what is going on.

Imagine a tube, open at both ends, with a membrane separating the left and right halves. Pressure outside the tube is the atmospheric pressure. Now let's call the left half of the tube your middle ear, and the right half your outer ear. The outer ear, under most conditions, communicates with very low impedance with the atmosphere (unless you have perhaps a very serious earwax buildup), so the pressure on the right side of the membrane is 1 atm +- whatever the barometer and your altitude are doing today.

On the other hand, the left half of the pipe is thinner, and more flexible, it's your Eustachian tube and it goes from your middle ear down into your throat. Suppose that we inflame the eustachian tube so that the walls become thicker and the inner diameter of the tube becomes smaller. Now the flow of air into the space behind the membrane is impeded. With enough inflammation, and enough fluid droplets in the tube, we can stop the flow of air entirely. As soon as we do this, the air trapped in the middle ear ceases to be resupplied and processes going on in the middle ear which absorb the air cause the pressure to decrease. As the pressure in the middle ear decreases, the eustachian tube's pressure decreases and the external atmospheric pressure tends to clamp this tube further shut. In other words, we have a feedback mechanism. The opposite case, of high pressure inside the tube, would tend to cause stretching of the tube, and widening of the diameter. It is possible for this high pressure blockage to occur, and it's called a reverse block by divers because it occurs when ascending to the surface, but the reverse block is a less stable process, it requires that the tube be blocked by a relatively mechanically strong blockage. The vacuum condition on the other hand would seem to feed back on itself, so that a small blockage can become worse and worse.

Also, once we have reduced pressure inside the ear, the flow of any fluid into that space is energetically favorable (fluids flow from high pressure to low pressure) so in this case liquid is extracted from the tissue surrounding your middle ear and fills the middle ear cavity. This liquid contains proteins and things that form a broth that bacteria can grow in.

So when your ear hurts like crazy, it's most likely because the external pressure in the atmosphere is higher than the pressure inside your ear, and your ear drum is being forced deep into your ear. If you know this, and you detect the blockage early enough, gentle Valsalva maneuvers such as the ones that SCUBA divers routinely use to "clear their ears" while diving might be able to help avoid the feedback loop and a lot of pain and suffering. Of course, you might do some damage if you over pressurize so you really must be gentle, and as every SCUBA diver knows, it's better to equalize early and often.

Another thing that we can remember is that inflammation of the eustachian tube is the root of all this evil. Anti-inflammatory drugs such as ibuprofen, and membrane shrinking drugs like pseudoephedrine used in combination with things like saline gargling to flush out irritating mucus from your throat might help you get air back into your middle ear. Also taking these drugs are not just treating the symptoms (pain) but also will prevent or reduce the fluid buildup that leads to a bacterial infection broth.

Anyway, when I was feeling under water yesterday and could barely communicate with my wife I finally looked up some info on all of this, and the resulting theory helped me adjust my method of treatment so that by this morning I was feeling much better. I hope it helps some of you and I wish this was more commonly understood, as it might have led to a lot less suffering through earaches when I was a child.

 

3 Responses leave one →
  1. jean permalink
    March 13, 2012

    What is the Valsalva maneuver?

    • Daniel Lakeland
      March 13, 2012

      The Valsalva maneuver is the action that divers do to clear their ears when they descend. You hold your nose closed and gently blow outwards, the increased pressure forces air into the middle ear. It has to be done carefully or you can seriously overpressurize your ears.

Trackbacks and Pingbacks

  1. The molecular pump in the inner ear? | Models Of Reality

Leave a Reply

Note: You can use basic XHTML in your comments. Your email address will never be published.

Subscribe to this comment feed via RSS