I have no personal experience with this malady, and I hadn't heard that a tub could make it worse before. Would you say that this is the normal response, or does this happen with only some folks who have MS?
Heat/Temperature Sensitivity in MS
From The MS Information Sourcebook, produced by the National MS Society.
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. Myelin is the protective sheath that surrounds and protects nerve fibers. The destruction of myelin causes the formation of plaques-abnormal areas-on the nerves that slow nerve impulses and produce the symptoms of MS.
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.
Symptoms Are Generally Rapidly Reversed
It is important to remember that heat generally produces only temporary worsening of symptoms and does not cause more actual tissue damage (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.
I hope this helps . . . .