Why do patients with multiple sclerosis experience worsening of neurological symptoms after heat exposure, that is, after taking a hot bath or after exercise?
It's known as Uhthoff's phenomenon.
Uhthoff's phenomena is due to ion channel modifications, in conjunction with thermoregulatory derangements that occur in MS, transiently altering the conduction properties of demyelinated axons.
This is from uptodate:
Heat sensitivity — Heat sensitivity (Uhthoff phenomenon) is a well-known occurrence in MS; small increases in the body temperature can temporarily worsen current or preexisting signs and symptoms.
This phenomenon is presumably the result of conduction block developing in central pathways as the body temperature increases. Normally, the nerve conduction safety factor decreases with increasing temperature until a point is reached at which conduction block occurs; this point of conduction block is reached at a much lower temperature in demyelinated nerves.
(This was discussed on our study group.)