I used to spar kickboxing a few years back. I was in the ring and got clipped on the side of the head with an overhand right. It knocked me silly and caused what I can describe as "temporary nystagmus." I imagine the raised body temperature due to physical exertion, coupled with my brain smashing into the side of my skull (inflammation/heat response) caused this temporary symptom to arise. A hot shower doesn't exacerbate my symptoms, but overall heightened temperature does the trick in some PWMS.
However, the shot didn't knock me down. I finished the round, albeit with eyes crossed. For the rest of the night, I sat out. That was the last time I sparred.
I carefully drove back home, thinking I was having a new MS attack. At home, I drank a blender-full of ice water. I got momentary brain freeze and my vision corrected. I did this for a few more times until I couldn't drink anymore. Then, I went in under very cold water in the shower. I never had problems with spasticity, so the cold water was only very cold and wasn't locking up my muscles. I did this a few more times and with plenty of rest (no physical exertion for a week or so) and eventually my vision came back to normal. Thinking back, it always feels like I dodged a bullet.