Balance is the ability to maintain one’s center of mass without falling over. Whether standing, walking, running, riding a bicycle, on a gymnast’s balance beam, etc., this is done using one’s:
- Vestibulo-ocular reflex (i.e., inner ears and vision)
- Proprioception (i.e., the sense of one’s own physical orientation)
- Physical coordination
All of these involve brain activity that is at least in part data-processed by the cerebellum. Thus, balance is achieved using a variety of abilities, and it’s those abilities that degrade with SCA3 as the cerebellum degenerates. Balance is not something that degrades on its own or that is somehow separable as a distinct ability.
In other words, balance is a high-level ability that degrades if one’s low-level constituent abilities degrade.
What are the proportions?
For me with SCA3, and this is as usual totally nonscientific, the degradation I perceive to experience is about so:
|Ocular system (vision)||50%|
A common example of testing one’s proprioception is the finger-to-nose-with-eyes-closed test performed by neurologists on patients and police officers on suspected intoxicated people. With ataxia, it’s primarily poor coordination that makes finding your nose inaccurate with eyes open. With eyes closed, you miss your nose because your fingers (via your cerebellum) aren’t sure where it is.
Being able to walk in the dark (or rather, without visual cues) relies on proprioception. If you’re not impaired, your brain computes your joint and limb configurations without needing to see them or your surroundings. With ataxia, walking in the dark becomes difficult and then impossible.
Here are some other examples of my impaired proprioception:
I must be looking at my feet to safely walk down stairs. I cannot look elsewhere and calculate where my feet are, nor can I be carrying anything that blocks the view of my feet. To know where my feet are, I must be looking at them.
I must constantly touch things to give my brain extra information about where I am and where the world is. (Older people do this, too, as proprioception deteriorates with age.) For example, if I walk into a room and by a table, I will touch the table. If I walk through a door jamb, I will touch the door jamb. I regularly touch walls. And so on. True, sometimes I am in difficulty and put my weight into it, and my situation will only get worse over time, but for the most part, what I need most is that touch to help optimize my balance. If I don’t use that touch, then maintaining my balance becomes arduous; sometimes my legs shake.
I use a walking stick, not because it holds me up (though it is more and more as time passes) but because it provides information to my proprioception system. Consistent with the above, it’s essentially a way to touch the floor or ground, like touching everything else. Eventually, I will graduate from using a walking stick to using a four-wheeled walker, when I need more physical support to stay upright.
An interesting example of proprioception is being in the shower with my eyes closed. As long as the shower water is hitting me, my brain can compute enough about where everything is for me to keep my balance, even with my eyes closed and touching nothing with my hands. If I turn off the water, then I must touch something in the shower stall in order to keep my balance.
Hanging in the balance
My mother, from whom I inherited the genetic defect, wrote about “not having any point of reference as to what is up or down or sideways.” I therefore assumed that somehow my balance itself—i.e., my sense of balance—was what I was going to lose. In fact, many (countless?) people have described ataxia as impairing one’s balance, leading me to believe that my sense of balance would be affected and that that would in turn mess up my gait. Wrong.
Based on how most people express themselves, I’ve been confused and surprised for over eleven years to find that it’s my coordination slowly degrading that’s resulting in a sloppy and therefore impaired ability to keep my center of mass balanced as I walk. My impaired ability to balance is the side-effect of this sloppy coordination, not the other way around.
Put another way: I walk funny, and that causes me to have poor balance; I do not walk funny because I have poor balance. In fact, this helps explain why it’s easier to ride a bicycle than it is to walk. The coordination required to ride a bicycle is much simpler than the coordination required to walk.
This also explains why those with later stage ataxia aren’t falling out of their wheelchairs. I’m guessing here, but if your actual sense of balance via your vestibular system is messed up, you might fall over even when sitting in a chair. I expect my sense of balance to stay intact, even as my incoordination prevents me from being able to stand and walk.