Some other things that can cause heat intolerance: myasthenia gravis (neuromuscular, not related at all to ON or demyelination) and dysautonomia (flushing, sweating, cardio symptoms often come with this one). Doesn't sound like your case, but just thought I'd throw that out there. My neuro didn't seem to be phased either when I mentioned I needed an hour to recover from a warm shower.
Thank you for your answer, JJ. I didn't think of the difference of some heat intolerance causing flushing etc. I never get that. I have searched high and low on the web and don't seem to come up with anything which is a good explanation for what I get other than the Uhthoff's. Maybe it's a duck then. LOL.
Off the top of my head, the two common causes of heat intolerance are medication side effect and thyroid conditions but keep in mind what's different with MS Uhthoffs compared to the other causes, is that with MS, internal or external exposure to increased temperatures causing the temporary return of symptoms of prior neurological damage and doesn't usually include the other typical signs eg excessive sweating, flushing red and or hot skin, rash, dehydration signs etc that usually accompany the other conditions that are associated with heat intolerance.
hmmmmmm i think at the top of 'your' particular list would actually be the MS connected, Optic Neuritis (ON) and the temporary worsening of your visual and your other previously experienced neurological symptoms with heat exposure, is pretty classic Uhthoffs that's typically associated with MS.
Unfortunately, you're probably going to continue to experience pseudo relapses when ever your temperature rises (internal or external) because there is nerve damage and temperature is known to negatively effect ON and your other issues...
A wise owl once said, if it waddles like a duck, quacks like a duck, behaves like a duck etc.....it's a duck!
Cheers..........JJ