Uhthoff’s Phenomenon: Heat and Multiple Sclerosis


If you’ve ever noticed that your multiple sclerosis (MS) symptoms get worse when you’re overheated, there’s a term for this—Uhthoff’s phenomenon. It’s a unique sign in MS characterized by a temporary worsening of one or more of your MS symptoms when your body’s core temperature is elevated, even by a small amount, like one-half of a degree. This sensitivity to heat is quite common, affecting an estimated 60 percent to 80 percent of MS patients.



Uhthoff’s phenomenon (also known as Uhthoff’s syndrome, Uhthoff’s symptom, and Uhthoff’s sign) was first described by Wilhelm Uhthoff, a German ophthalmologist, in 1890. He noticed that people with optic neuritis, a common symptom of MS in which your optic nerve is swollen, had a temporary worsening of their vision when they exercised.

Initially, both Uhthoff and other specialists believed that the exertion involved in exercising was the cause of this phenomenon, but researchers later discovered that the real culprit was the resulting increase in body temperature—not exercise itself.

This observation led to the hot bath test, which doctors used to help diagnose people with MS before the advent of modern imaging tests. Exactly like it sounds, the hot bath test involved placing a patient suspected of having MS in a hot bath to see if his or her symptoms worsened with heat.


In MS, myelin—the protective, fatty covering around nerve fibers—is damaged or destroyed by your immune system. Myelin is what allows your nerves to communicate with each other effectively and quickly, so when it’s damaged, your nerve cells can’t properly transmit messages. Depending on which nerves are affected, these impaired signaling pathways cause a variety of MS symptoms, like blurry vision, numbness and tingling, muscle weakness, and thinking problems.

Though scientists don’t know exactly what causes Uhthoff’s phenomenon, they believe that heat acutely worsens these already damaged nerve cells pathways, which then triggers your current MS symptoms. For example, you may notice that your fatigue gets worse when you’re overheated. When your body temperature returns to normal, though, your fatigue lessens and goes back to baseline.

Any source of heat can trigger Uhthoff’s phenomenon, such as:

  • Hot and humid weather
  • Direct sunlight
  • Using a hairdryer
  • Taking a hot (or even warm) shower or bath
  • Sitting in a sauna or hot tub
  • A fever from an infection
  • Exercise
  • Hormonal fluctuations that occur with menstruation and menopause, which can increase body temperature

Keep in Mind

No permanent neurological damage is done by Uhthoff’s phenomenon. Any MS symptoms that get worse with heat are reversible once your body temperature goes back to normal.


Your best bet for preventing Uhthoff’s phenomenon from occurring is to identify what your unique triggers are and do your best to avoid them.

You can also use cooling strategies to help keep your body temperature steady, especially when you know you’re going to be in a situation that makes you hot. These may potentially allow you to still enjoy some of your triggers without getting overheated.


3 Women Share Their Experiences Managing MS in the Heat

Some examples of cooling strategies that may be helpful include:

  • Drink cold water throughout the day, especially during the hot summer months.
  • Carry a portable fan in your purse or backpack in case you get into a situation where you’re too hot.
  • Apply a cold washcloth to your wrists or neck when you feel like your body is getting warm.
  • Wear a hat and stay in the shade on warm, sunny days, or stay in an air-conditioned area.
  • Wear loose, breathable clothing, like cotton.
  • Sit by an open window or fan.
  • Suck on ice cubes or a popsicle.
  • Mist your face and clothes with water periodically.
  • Take cool showers or baths.
  • Try a cooling pillow for sleeping.

Staying Active With Uhthoff’s Phenomenon

Since exercise raises your internal temperature and can be a trigger for Uhthoff’s phenomenon, you may be thinking that you need to avoid it. Thankfully, this isn’t the case.

First of all, if you’ve never experienced Uhthoff’s phenomenon and you’re worried that exercising might trigger it for the first time, know that there are plenty of people with MS who never deal with heat intolerance and you may just be one of them.

And even if exercising does trigger Uhthoff’s phenomenon, whether now or in the future, there are ways you can cope.

Exercise is too important to your overall health and well-being to skip it for fear of overheating, and research even shows that it can help reduce MS symptoms.

One approach is to wear cooling garments like a cooling scarf, vest, or headwrap when you exercise. Try to do your workout in a cool environment too; for example, in an air-conditioned room or next to a fan.


Another method you can try is cooling your body down before you work out. A 2019 review of studies on MS patients pre-cooling before exercise found that not only does this help prevent Uhthoff’s phenomenon from occurring, it also improves MS patients’ ability to exercise.

Some of the methods used to pre-cool in the studies included:

  • Wearing cooling garments for 30 to 60 minutes before exercising
  • Immersing the lower body in 62-degree F water for 30 minutes prior to exercise

These methods lowered the participants’ core temperatures within 30 minutes to one hour.

Could It Be a Relapse?

It’s totally normal to wonder if your sudden leg numbness, fatigue, or other MS symptom is from the heat or from a new MS lesion in your central nervous system. Truthfully, until you’re more experienced with MS, it can be tricky to differentiate between an MS relapse and Uhthoff’s phenomenon.

One easy way to distinguish between an MS relapse and Uhthoff’s phenomenon is to see if your symptoms go away when the heat trigger is removed, like after cooling down from a hot shower or when your fever returns to normal. While you may not feel better instantaneously once you’re cooled (it can take a few hours, depending on the symptom), your neurological symptoms should return to baseline if heat is the culprit. With an MS relapse, the symptoms will persist.

It’s a good idea to contact your neurologist or your MS nurse when you experience Uhthoff’s phenomenon, especially if you’re unsure about what’s going on. An MS relapse may require treatment, like steroids, whereas Uhthoff’s phenomenon doesn’t require any treatment beyond removal of the trigger and reassurance.

A Word From Get Meds Info

Managing your MS symptoms is a delicate task, and the fact that temperature may affect them can complicate things. Do your best to stay cool and remember that if your MS symptoms do flare up because you are overheated, they’ll go away soon after the source of heat is removed. It may help to remind yourself, too, that though it can be alarming and uncomfortable, Uhthoff’s phenomenon doesn’t cause any damage and it doesn’t mean that your MS is worsening.

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