Space adaptation syndrome compared to BPPV (vertigo)

Question

Has there ever been any discussion comparing the typical SAS experience and benign paroxysmal positional vertigo (when tiny calcium particles clump up in canals of the inner ear). Having recently experienced BPPV I spent more than 24 hours unable to rise out of my bed without getting violently ill. I'm imaging that would rate as a 1 on the Garn Scale.

"Oral History 2 Transcript" (PDF). Johnson Space Center Oral History Project. NASA. May 13, 1999. pp. 13–35. Retrieved April 22, 2011. [Dr. Robert Stevenson:] Jake Garn was sick, was pretty sick. I don't know whether we should tell stories like that. But anyway, Jake Garn, he has made a mark in the Astronaut Corps because he represents the maximum level of space sickness that anyone can ever attain, and so the mark of being totally sick and totally incompetent is one Garn. Most guys will get maybe to a tenth Garn, if that high. And within the Astronaut Corps, he forever will be remembered by that'

Answer 1

Yes. They have little in common, beyond both resulting in a mismatch between vestibular data input to the brain and what’s actually going on, resulting in symptoms that impair performance.

In SAS, our best theory is that the shift from 1G to microgravity environment essentially results in a dropoff from baseline vestibular input that the brain seeks to rectify by dialing up the gain.

Think about it this way: On earth, every time you tilt your head up, down, etc you are leveraging it around a 1G vector that your otolith organ senses as linear acceleration, and so provides position information about your orientation in space to your brain. Similarly your canals will detect angular accelerations, but let’s stick to the linear accel for now.

I put you in microgravity, and now you get a fundamental mismatch in sensory inputs when you tilt your head - your eyeballs and proprioceptive feedback from your neck muscles tell your brain you’ve reoriented your head, but without any shift in otolith organ input it expects (because you now lack the constant 1G vector to leverage around). This results in an uneasy sensation, with nausea arising in most periodically.

It can be accentuated by stacking a visual input that also seems at odds with the remaining data - crew new to station will report symptoms upon entering a module where other crew members are oriented horizontally or inverted from their POV.

The initial response of the brain to not getting the input it expects is as I said, to essentially dial up the gain to amplify any signal that’s there. After this doesn’t pay off, after about 5 days the brain decides to ignore the null signal and just run off of visual and proprioceptive input. Which is great for working in space......

....But is horrible for coming home. Because after 6 months of getting no vestibular input and having cranked the gain up, when we bring crewmembers home they now have to go through a (usually much worse) mirror adaptation period to re-learn to incorporate those sensory inputs. This is a significant operational concern for crew performance should any emergency procedures be needed post-landing, and with a return to water landings on the horizon we expect it’ll be even more provocative than the levels of deconditioning we’ve gotten used to with Soyuz land recoveries.