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An Overview of Heat Intolerance and MS


Updated May 30, 2014

For many of us with multiple sclerosis (MS), summer fills us with anxiety. Hot weather means MS symptoms and, well, suffering. As most people are going outside to enjoy the hot weather, we are drawing the blinds and resigning ourselves to cleaning out closets and reading novels. Even vacations are a challenge, as each year we look for places further from the equator or “adventures” that take place in air-conditioned coolness.

Heat intolerance in MS shows up as a "pseudoexacerbation" -- the experience of having symptoms appear or worsen due to heat exposure. This is different than a true relapse. In the case of a pseudoexacerbation, when the body’s temperature returns to normal, these symptoms disappear. No damage, such as inflammation, demyelination or new lesions, has been done during these pseudoexacerbations.

Note: In this article, I am referring to heat from increased activity or external sources, such as hot weather, hot baths and showers, saunas and hot tubs. I am not talking about fever, which is a separate topic.

What Does It Feel Like?

Heat intolerance is felt as increased symptoms, such as:
  • Decreased cognitive function
  • Numbness in the extremities
  • Fatigue
  • Blurred vision, known as Uhthoff’s sign
  • Tremor
  • Weakness

Really, any MS symptoms can be much worse in the heat. Sometimes, symptoms appear that we might not have felt before, which is the result of a lesion in a corresponding area of the brain or spinal cord that was slight enough that it did not cause a relapse or symptoms dramatic enough to notice. For instance, Uhthoff’s sign refers to blurred vision as a result of heat exposure. This is caused by a lesion on the optic nerve, known as optic neuritis. However, many people that experience Uhthoff’s sign never had classic symptoms of optic neuritis and may be unaware that they ever had it until the appearance of Uhthoff’s sign.

Heat intolerance differs for people in terms of:

  • Threshold: Some people can be just fine taking a brisk walk in 90-degree weather, as long as they avoid the sun and drink cold beverages. Others start feeling symptoms at much lower temperatures and with much less activity.
  • Severity and type of symptoms: Again, depending on the person, symptoms can range from annoying (such as tingling in the feet) or debilitating (such as crushing fatigue or severe weakness).
  • Length of time to resolve symptoms: While all symptoms that are results of heat intolerance should resolve once body temperature returns to normal, this takes longer for some people.

What Causes Heat Intolerance?

As people with MS, we have plaques on our nerves where demyelination has taken place. This slows the ability of the nerves to function, and heat further slows down nerve impulse transmission in demylinated regions. Even a very slight increase of as little as one-quarter to one-half a degree in the body’s core temperature is enough to cause symptoms of heat intolerance.

How Common Is Heat Intolerance?

While I don’t have an exact statistic on this, I can say that most of us are sensitive to the heat. In fact, 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 watched to see if neurologic symptoms appeared or got worse, which would earn them a diagnosis of MS. (This practice was stopped when MRIs came on the scene, but is still practiced in some countries where MRIs are not readily accessible.)

How Severe Can It Get?

For some people, heat intolerance can be debilitating enough that they are unable to function well at even slightly elevated temperatures and must consider moving to a cooler geographic location. Deaths have even been reported among people with MS who were sunbathing or relaxing in hot tubs and presumably lost the ability to get out of the heat.

Additional Points:

The people that are the most sensitive to the heat and develop symptoms the quickest are also the most responsive to cooling down and their symptoms disappear quickly.

There are some people who are more sensitive to cold than to heat, and their symptoms - especially spasticity - worsen in cold temperatures. Some of us unlucky people with MS are sensitive to both cold and heat, usually with different symptoms appearing under different temperature extremes.

My Experience

It started out innocently enough -- a short walk to a coffee shop with my husband and twins. OK, so maybe it was a little hotter outside. And maybe we took a route with a little less shade. Still, I was completely unprepared for the effect the heat had on me: My feet and lower legs felt so heavy, it began to feel like I was walking through knee-deep mud. I felt my old friend, the MS hug (which I call “the grippies”) tightening around my ribs and making it difficult to breathe. The stroller I was pushing seemed to have square wheels and be made out of lead. Even my hearing started fading in and out, like a bad cell phone connection.

We finally got to the coffee shop, which was blasting its air conditioning. Within just a few minutes of being inside the cool environment and sipping an iced coffee, things started to come back into focus. A few minutes later, I was absolutely energized and gleeful (think long-haired dog being let out on a cold day, sniffing at the air and wagging all over).

When it was time to leave, my husband offered to go get the car to pick me up, but we settled on walking with "911" set on speed dial and his finger on the cell phone button. Interestingly, I felt just fine the whole way home. I guess I had sufficiently cooled down so that the heat did not effect my core temperature.

Think I’ll check the weather a little more carefully next time… I also upgraded my espresso maker so that I can make my own coffee treats to enjoy at home while looking out the window at the shimmering heat during the summer months.


David W.E. Smith. Taming the DOG DAYS - caring for heat intolerance symptoms. Inside MS. Summer, 2000.

Berger JR, Sheremata WA. Persistent neurological deficit precipitated by hot bath test in multiple sclerosis. JAMA. 1983 Apr 1;249(13):1751-3.

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