You might have permanent and even severe damage to one of your inner ears, yet still get back to feeling normal with good balance. This ability to recover is called compensation. Being able to do this well is easier for humans than for some other animals. For example, rabbits can’t do it well at all. They become permanently crippled from a damaged ear. For humans, what explains this almost magical power of recovery?
Thoughts have changed on this after more than a century of study. At first compensation was seen to be almost magical—the brain was believed to rewire itself in some way. Over time, the situation has become more nuanced. Now we understand that, while the vestibular system does rebalance itself, other senses have a major role in making up for the lost sense of balance.
When one ear has a vestibular injury suddenly, it is felt as intense spinning, and the environment can be seen to turn or shift almost continuously. Turning the head can cause blurring of vision, like a bad video. Head movements can cause vomiting and you might feel nauseated all the time. You may stagger or drift to one side while walking. Over a few days, however, a lot of this dwindles away. The environment can stop spinning in a few days, and the internal spinning feeling gradually tapers. Drifting when walking can gradually disappear over a few weeks. How does this happen?
When you first have a vestibular injury, one ear senses normal movement while the other senses nothing because injuries usually shut off all or most function. The brain determines whether it is turning by comparing the movement sensation of the two ears. If one is “on” while the other is “off” because it is injured, it makes you feel as if you are turning, and makes your eyes move (nystagmus). Seeing the room spin is very disturbing, and your brain will attempt to get this to stop by reducing the input from the good ear, so it better matches the bad one. Then it will slowly adjust this until an optimum stage is reached giving you more visual stability.
To do this, your brain must change how it uses vision. Your eyes have a few different ways of tracking moving things. You can follow a tennis ball with just your eyes or track a deer moving in the woods. This form of visual tracking is called smooth pursuit. You can also have normal nystagmus when you are tracking a series of things moving steadily across your field of vision, like fenceposts or trees as you drive by. This is optokinetic nystagmus (OKN). Both allow you to see clearly even though the eyes or the objects you are viewing are in motion.
When you have a vestibular injury, this clearness of vision is lost because the eyes are having nystagmus even though what you are watching is still. Your brain notices the problem—your vision is blurred by it—and uses smooth pursuit and OKN to attempt to follow the spinning objects caused by your nystagmus. This slows down the nystagmus, and over the course of a few days you will be able to counteract and cancel the nystagmus entirely. We can be certain that this is what happens, because if we take vision away by putting you in a dark room, the nystagmus will immediately start up again. That is why trying to focus when you have constant vertigo can help lower the speed of spinning.
There are other ways vision can help you compensate, and these are necessary for long-term recovery. We’ll discuss this in the next post.
Ending Vertigo Today 