How Heat Affects Multiple Sclerosis

Heat Can Trigger MS Symptoms
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Summer can fill a person with multiple sclerosis (MS) with anxiety, as hot weather often means more symptoms.

So while most people are heading outside to enjoy the hot weather, people with MS are often drawing the blinds, drinking cold drinks, and turning their fans on. Even vacations are a challenge, as each year people with MS often seek out places further from the equator or “adventures” that take place in air-conditioned coolness.


Increased activity, hot weather, hot baths and showers, saunas and hot tubs, are all sources of heat that can cause issues for someone living with MS. They can trigger a phenomenon known as an MS "pseudoexacerbation," which is the experience of having symptoms appear or worsen due to heat exposure.

It's important to understand that a pseudoexacerbation is different from a true relapse. In the case of a pseudoexacerbation, when the body’s temperature returns to normal, MS symptoms disappear. You can also be reassured that no damage, such as inflammation, loss of myelin, or new lesions, has occurred during a pseudoexacerbation.

What Heat Intolerance Feels Like

Common symptoms worsened with heat (although every person with MS is different) include: 

  • Numbness in the extremities
  • Fatigue
  • Blurred vision
  • Tremor
  • Weakness
  • Thinking problems

The truth is that any MS symptom 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 example, a common target of the central nervous system for myelin attack in MS is the optic nerve. When demyelination occurs in the optic nerve, a person may develop blurry vision and/or eye pain. That being said, sometimes a person's experiences no symptoms or very subtle symptoms of optic neuritis. However, with heat exposure, their vision can become blurry. While uncomfortable, their vision restores when they cool down. 

How Heat Intolerance Can Present

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, to debilitating, such as crushing fatigue or severe weakness.
  • Length of time to resolve symptoms: While all symptoms that result from heat intolerance should resolve once body temperature returns to normal, this takes longer for some people.


As people with MS, we can have demyelinating plaques in our brain, optic nerves, and spinal cords. This slows the ability of the affected areas to function, and heat further slows down nerve impulse transmission in these regions. In fact, even a very slight increase (as little as one-quarter to one-half a degree) in the body’s core temperature is enough to cause symptoms of heat intolerance.

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


While there aren't any exact statistics on this, most of us have MS 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. Now that we have more advanced screening options, like MRIs, this practice isn't needed.

A Word From Verywell

While heat intolerance can be extremely debilitating, the good news is that there are ways to manage it with cooling vests, hand-held mini fans, regularly drinking cold water, wearing loose-fitted, light clothes, and avoiding direct sunlight.

That being said, for some people, heat intolerance can be disabling enough that they are unable to function well at even slightly elevated temperatures. In this instance, moving to a cooler geographic location may be a sensible idea.

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Article Sources
  • Birnbaum, M.D. George. (2013). Multiple Sclerosis: Clinician’s Guide to Diagnosis and Treatment, 2nd Edition. New York, New York. Oxford University Press.

  • Multiple Sclerosis Society. (2013). Fact Sheet: Hot and Cold - The Effects of Temperature on MS.