Heat does cause a "pseudo exacerbation" and many multiple sclerosis patients are sensitive to heat.
These changes are called "pseudo exacerbations" because the changes that occur are reflect changes in physiology and not damage caused by the immune system.
The myelin that covers nerves is there to make electrical impulses travel very quickly. These electrical impulses travel most efficiently at normal body temperature. As you increase the temperature, the impulses slow down. In normal myelin this is not a problem, but in areas of the nervous system damaged by MS, the impulse will slow enough to bring out symptoms. There is no damage to the nervous system.
When this happens you need to cool down to improve the symptoms.
If you are heat sensitive and have MS, you need to protect yourself from getting overheated.
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