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Many people with MS experience a temporary worsening of their symptoms when the weather is very hot or humid or they run a fever, sunbathe, get overheated from exercise, or take very hot showers or baths. For example, some people notice that their vision becomes blurred when they get overheated—a phenomenon known as Uhthoff's sign. These temporary changes can result from even a very slight elevation in core body temperature (one-quarter to one-half of a degree) because an elevated temperature further impairs the ability of a demyelinated nerve to conduct electrical impulses.
The ‘Hot Bath’ Test Used for Years to Diagnose MS

For many years, the “Hot Bath” test was used to diagnose MS. A person suspected of having MS was immersed in a hot tub of water, and the appearance of neurologic symptoms or their worsening was taken as evidence that the person had MS.
Heat-Related Symptoms are Temporary

It is important to remember that heat generally produces only temporary worsening of symptoms and does not cause more disease activity (demyelination or damage to the nerves themselves). The symptoms are generally rapidly reversed when the source of increased temperature is removed. People with MS who are planning to move to a very warm climate should try to visit first; plans may have to be changed if the weather seriously worsens MS symptoms.
Strategies for Easing the Effects of Heat

* Stay in an air-conditioned environment during periods of extreme heat and humidity. [If an air conditioner is needed to help minimize the symptoms of MS, the cost of this equipment may be tax deductible if the physician has written a prescription for it.]
* Use cooling products (vests, neck wraps, bandanas, etc.) during exercise or outdoor activity, or pre- and post-cool.
* Wear lightweight, loose, “breathe-able” clothing.
* Icy drinks such as “slurpees” or popsicles can provide temporary relief.
* Use an oscillating fan during indoor exercise.
* Exercise in a cool pool (<85 degrees).

Cold Can Also Be a Problem

Some people with MS notice that symptoms, particularly spasticity, become worse in cold weather. It is generally recommended that people with MS who are sensitive to temperature try to avoid extremes of either hot or cold, and that people who are considering a move to a "better" climate try to visit first to see if the climate change is, indeed, beneficial.