Summertime With MS: How Heat Affects Multiple Sclerosis

A Closer Look at Pseudoexacerbation

Close-up of a woman wiping sweat from her brow
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Summer can fill a person with multiple sclerosis (MS) with anxiety. Hot weather means more symptoms and, well, suffering. While most people are heading 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.

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. It is our intolerance to these things that can cause "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.

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 it presents itself.

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. The people who 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.

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 demyelinated 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.

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.

How Common Is Heat Intolerance?

While there aren't any exact statistics on this, most of us who suffer from 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 used 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.


Smith DWE. Taming the DOG DAYS - caring for heat intolerance symptoms. Inside MS; 2000.

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

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