I have read enough about Uhthoff's phenomenon to conclude that it is involved in my case excluding vision disturbances (Summer of 2013: heat triggered pain running from eyes into head without optical disturbances, probably inflamed optical nerve). I can imagine having a hard time maintaining posture under warm ambient conditions if it slows down nerve conduction.
It is reversible, and I'm sensitive to this trigger almost 100% of the time just like POIS attacks. Getting stiff muscles by heat is difficult for people to grasp since it is completely the reverse compared to healthy individuals. Saying that I might have MS is a step too far though.
https://pmc.ncbi.nlm.nih.gov/articles/PMC5152601/To clarify personal experience:
Heat triggers:
Hot Weather
Hot showers
Exercise
Radiators: Doing dishes, cooking, room heating radiator, Sun
Hot Baths (had an intolerance to it as a kid)
Infrared sauna (worst heat trigger, felt dangerous)
Hot foods/drinks (depends on the amount though)
Symptoms:
Stiffness
Systemic/whole body weakness
Exhaustion
Pain
Posture difficulties
Fasciculations
Decreased Concentration
Actually more:
Breathing dysregulation
Decreased digestion
Symptoms are resolved within 24 hours
Best time of the day: Morning and night.
The paper I posted mentions that Aspirin is used as a cooling treatment. Aspirin works for my POIS...