The balance parts of the inner ear—the labyrinths–are filled with fluid and surrounded by the bone of the skull. They are designed to sense only tiny movements of fluid caused by moving the head, so they need to be protected from other things that could cause fluid movement. Right next door to the labyrinth is the hearing part of the ear, the cochlea, which also senses fluid movement. In the cochlea this fluid movement is caused by a pressure wave from sound entering the inner ear through a thin membrane. The sound waves go right past the labyrinth and head straight into the cochlea, so only sound is heard and no motion is felt. As long as the bone protects and surrounds the labyrinth, this system works perfectly. Sometimes, though, the bone protecting the labyrinth is cracked or a hole forms, and suddenly a new problem shows up: sound can be felt as spinning.
There are a few different things that can cause this problem. Bacterial ear infections can dissolve bone around the labyrinth. Some diseases of the skull also cause bone in that area to disappear. Surgery to treat ear disease can leave a hole in the bone. A blow to the head severe enough to crack the skull can do this. These causes are very obvious to the person who is affected, but there is one cause that is not so obvious because it comes on without other ear diseases. It is called superior semicircular canal dehiscence or SSCD.
SSCD usually affects one of the two superior/anterior semicircular canals, the spinning sensors that are placed highest and furthest forward in the skull. In every case there is an opening between this canal and the inside of the skull. The inner ear is placed right under the brain, just barely separated from the brain by a thin layer of bone over the superior canal. Some people don’t form this bone fully and have a hole there all their lives, but most do have thin coverage at first. As they age, the pressure of the brain gradually wears down the skull until it is completely gone over the canal. When the bone is very thin, even a small bump can crack it, or it can wear away gradually. This causes very odd symptoms, much more than just feeling vertigo with loud sounds. I’ll go over this in the next post.