When we hear that our child's ear hurts, it's usually an infection of the middle ear, or in medical terms, otitis media. There is another kind of earache, due to an infection of the ear canal (otitis externa, or swimmer's ear) which we'll talk about below.
Normally the eardrum is airtight and watertight. To keep pressure inside and outside the eardrum equal, we have natural "drainage" tubes (the Eustachian tubes) that connect the middle ears (the space behind the eardrum) to the back of the nose and throat. Our ears always produce small amounts of fluid; this normally drains down the Eustachian tubes, and is usually in such a small amount that we don't even notice it in the throat. The tubes have one-way valves which allow air to escape from the middle ears to the throat; yawning, chewing, or swallowing helps open the valves in the other direction so that air (and sometimes fluid) can go into the middle ears.
One time we all notice pressure changes is when flying. Airliners keep some air pressure in their cabins when cruising, but not as much as the pressure on the ground. When you climb in a plane, the air pressure around you drops; air escapes easily from your middle ears through the valves, and you don't notice very much happening to you. When you descend, on the other hand, the cabin air pressure goes up, and air has a tough time going back into the middle ear. Until it does, your ears will pop and sometimes hurt. Yawning and swallowing will open up the valves and cut down on the popping, and decongestants can help too, especially if you have a cold.
When we get a cold, the mucous membranes in the nose and throat swell up and produce much more fluid than usual. Often the mucous membranes in the tubes will produce fluid too, but they usually swell up also. The swelling blocks the tubes and allows fluid to accumulate in the middle ear. This fluid blocks high-pitched sounds by absorbing the sound energy. Worse, since bacteria can (and usually do) enter the middle ear through the tubes, if the fluid can't be kept out bacteria will start growing in it, and this inflames the eardrum and increases the pressure inside the ear even more. The pain comes both from the pressure increase pushing on the drum and from the inflamed drum itself. When we examine children to see if they have a middle ear infection, we always look to see if the eardrum is moving, using a rubber bulb attached to our otoscope (the light we use to look at ears) -- we can see the eardrum move as we blow air into the ear with the rubber bulb, and we do not treat with antibiotics unless we see that the eardrum is not moving.
The smaller your child is, the smaller the Eustachian tubes, and the more likely they will be blocked with a cold. Adults sometimes get middle ear infections too, but they have larger tubes so it's much less likely that a cold will block off the tubes enough to let an infection start.
There are two approaches to helping someone with an ear infection. The first is antibiotic treatment; this kills the bacteria in the middle ear, allowing the inflammation to subside. Since most colds are viral rather than bacterial, antibiotics will not help with the cold itself, or even get rid of the fluid completely. They will help get rid of the bacteria in the fluid, though; and some doctors give antibiotics when they see clear fluid in the ear to help keep bacteria from growing in it. However, you can also have a middle ear infection due to a virus, and if that's the problem, then antibiotics will not help at all -- in fact, using antibiotics may result in a bacterial ear infection with a bacteria that's resistant to the antibiotic you took.
Any bacteria that is found in the nose, mouth, and throat can cause an ear infection. Two of the most common bacteria seen in middle ears are Hemophilus influenzae ("Hib" or "H. flu" -- but it doesn't have a lot to do with the flu) and Streptococcus pneumoniae or "pneumococcus" (which does often cause pneumonia -- and other things as well). We have vaccines against both of these bacteria, and they help prevent some ear infections -- in the 20 years or so since we started giving HiB vaccine, we have seen fewer and fewer HiB-related ear infections, and the pneumococcus vaccine seems so far to be cutting down on pneumococcal ear infections as well.
The second approach uses decongestants to reduce the mucous membrane inflammation. This should open up the tubes so the fluid can drain more easily. Unfortunately, studies have shown that decongestants aren't as good as opening up blocked tubes as we previously thought they were.
I occasionally recommend decongestants when flying with a cold. I've found from flying with a cold myself that descending at the end of a flight with a cold is a really miserable experience. Taking a decongestant 1/2 to 1 hour before the plane starts to descend takes much of the edge off the popping and pain. Or at least it does for me -- but, again, controlled don't really show that much advantage in general to using decongestants. There are also some of us (and I've done this myself once in a while) who give decongestants when we see clear fluid behind the eardrums to try and increase drainage, rather than give antibiotics. This approach makes sense physiologically; unfortunately, studies have shown that decongestants don't help clear the fluid out nearly as well as antibiotics do. I will occasionally try decongestants in addition to antibiotics when a child has had fluid for a long time and antibiotics alone don't seem to help, but in those cases, and especially if the child's hearing is being affected, I will also have an ear, nose, and throat surgeon evaluate the child to see if artificial drainage tubes may be helpful.
It's also very important for us to check the ear after treatment (I usually see a child three weeks after starting antibiotics) to make sure
Another kind of earache, usually not as common as otitis media but seen a lot in summer, is otitis externa or "swimmer's ear". This is a bit of a misnomer, since this can happen without swimming, but we'll stick with the usual term for now.
Swimmer's ear is an infection of the ear canal, rather than the eardrum and middle ear. The actual infection is simply a boil on the skin within the canal. This can be in the canal's outer part, whose skin is much like that on the rest of your body, or in the inner "membranous" part which is much thinner skin and sometimes more likely to be infected if it's irritated.
The bacteria that produce a swimmer's ear (and swimmer's ears are almost always bacterial) are the same kind of bacteria that you'd normally find on your skin, with a couple of additions that are peculiar to dark and damp part of your body (like your feet). They are often on the skin surface in the canal normally -- remember that it's pretty much impossible to keep all bacteria off your skin -- but can get below the surface if the skin's irritated. (This is one reason why you should never put a Q-tip, or anything else smaller than your finger, in your ear canals. Q-tips in particular can scratch the canal's skin enough for bacteria to get in and start growing.) If water sits in the canal for very long, it mixes with the ear wax and bacteria start growing in the mixture, and the wet skin is easier than dry skin for bacteria to penetrate. We call it "swimmer's ear" because most often we see it in people who have been swimming a lot recently.
Sometimes it's hard to tell if an earache is due to swimmer's ear or to otitis media -- one reason why I always want to examine a child with a "new" earache. With otitis media, the canal looks fine, but the eardrum looks dull, a little red, or sometimes badly inflamed. With swimmer's ear, on the other hand, the canal may look inflamed, or we may see a boil on the skin inside the canal, but the eardrum itself will look OK. (Of course, we occasionally see an ear with both swimmer's ear and otitis media.) One way to tell the two apart without looking directly into the canal is to wiggle the earlobe gently: with otitis media moving the earlobe doesn't change the pain, whereas with swimmer's ear moving the lobe hurts more. This is not absolutely dependable, though, so we still prefer to look into the canal and the eardrum ourselves.
Since we are dealing with different bacteria, we need to treat swimmer's ear differently from otitis media. Most of the bugs on the skin and in the canal are resistant to the regular oral antibiotics we use for otitis media. In fact, some are resistant to almost every oral antibiotic on the market. Luckily, since these are skin infections, we can use eardrops that sit on the skin in the canal and kill bacteria on the skin; if the skin is inflamed (as it always is in swimmer's ear) enough of the drops will penetrate the inflamed area to kill the bacteria causing the inflammation.
With a swimmer's ear, it's important to keep the canal as dry as possible until the infection is gone. Avoiding swimming or other sources of water in the ears completely is best, but it may not be practical to keep your child out of the pool, especially on a hot summer's day. You may have to keep them out of the water for the first day or so, however. Soft (wax) earplugs will help keep the canals dry if they insist on swimming. Just as important is making sure that as much of the water as possible gets out of the ears after swimming. I like to give one of the daily doses of antibiotic eardrops right after swimming, too: this helps kill bacteria when they are most likely to establish themselves in the canal, and helps protect the canal until it dries out naturally. Never put Q-tips in the canal after swimming (or any other time, but the canal skin is most fragile when it's wet).