People with multiple sclerosis (MS) often experience a temporary worsening of their symptoms in hot weather or when they run a fever. These temporary changes can result from as little as one-quarter to one-half of a degree elevation in core body temperature, as an elevated temperature further impairs the ability of demyelinated nerves to conduct electrical impulses.﻿﻿

3:01 3 Women Share Their Experiences Managing MS in the Heat

Symptoms

Any MS symptom can be much worse in the heat; sometimes, new and unfamiliar symptoms can appear. Common symptoms triggered by heat include:

Numbness in the extremities

Fatigue

Blurred vision

Tremor

Weakness

Cognitive problems

Heat intolerance can also differ 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.

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.

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. ﻿ ﻿

Verywell / Emily Roberts

Causes

MS can result in demyelinating plaques in the brain, optic nerves, and spinal cord. This slows the ability of the affected areas to function, and heat further slows down nerve impulse transmission in these regions.

Typically heat worsens symptoms that the person has previously had before. So if they had an MS flare where they had numbness which later improved, the heat may bring out those symptoms again since the myelin is not fully repaired and the heat is a stressor.

A worsening of symptoms as a result of becoming overheated (with the most common one being blurred vision) is a phenomenon known as Uhthoff's sign.﻿﻿

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.

These sources 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 this is different from a true relapse. In the case of a pseudo-exacerbation, when the body’s temperature returns to normal, MS symptoms disappear. Fortunately, no damage—such as inflammation, loss of myelin, or new lesions—occurs during such an episode.

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

Did You Know? Before there were more sophisticated screening tests for MS, 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.

Treatment

Worsening of symptoms in the heat is temporary, and they resolve when the body cools down. Simple cooling techniques usually do the trick.

Here's what you can do to cool off:

Stay in an air-conditioned environment during periods of extreme heat and humidity.

Use cooling products such as vests, neck wraps, or a wet bandana during exercise or outdoor activity.

Wear lightweight, loose, breathable clothing.

Drink cold drinks or eat popsicles.

Stay hydrated with electrolyte-containing sports drinks.

If you exercise outside, pick cooler times of the day to do so (early morning or evening are usually best).

Use an oscillating fan or air conditioning during indoor exercise.

Take a cool bath or shower to help reduce core body temperature following activity or exposure to a hot environment. ﻿ ﻿

A Word From Verywell

While heat intolerance can be extremely debilitating, the good news is that there are simple ways to manage it. Those who are unable to function well at even slightly elevated temperatures may want to consider moving to a cooler geographic location. For most, though, simple strategies are enough to ease heat-related symptoms.