We all know drinking alcohol affects the brain, starting with feeling tipsy. If you’ve ever had a lot to drink in one sitting (remember that 21st birthday party?) you might have noticed another effect. Drinking too much too fast can cause vertigo that becomes intense when you lie down. This effect, called the spins, spinners, or bed spins, is caused by a direct effect of alcohol on your inner ear. It’s a mechanical form of vertigo.
The medical term for this is positional alcohol-induced nystagmus or PAN. The inner ear contains spinning sensors shaped like tiny rings. In each ring, there is a sensor, the cupula, a tiny oval flap that bulges to the side when the fluid surrounding it moves. Moving the cupula to one side causes nystagmus, a jerking and drifting movement of the eyes that helps us focus when we move our heads—but that can cause vertigo if it happens when we are not moving.
We don’t have just one cupula; we have six of them in our heads, 3 in each ear. They’re oriented in different directions to sense all 3-dimensional directions of movements. In order to function properly, the cupula and the fluid around it must have the same density, so that the cupula moves only in response to fluid motion, and not in response to gravity.
When you try to mix oil and vinegar for salad dressing, you’ll notice that the vinegar floats on top of the oil. This is because the vinegar is less dense than the oil. A rock dropped in the oil will sink to the bottom of the cup, because it is more dense than the oil.
Now imagine that a cupula were to become denser than the fluid, making it relatively heavy. It would tend to sink down in the direction of gravity. Or if it became less dense, and so was lighter than the fluid around it, it would tend to float up. If all six cupulae were affected, they would all float up, but each of the six canals senses a different direction of movement, so the feeling the person would get from this would depend upon what position their head was in. Lying down on one side is the worst possible position, because the strongest sensor, in the horizontal canal, is perpendicular to the floor in this position and so is most affected.
The effect of alcohol is to make the cupula float, and that sets off the spins. We’ll talk about how this resolves naturally and how to prevent it in the next post.