Uhthoff's Phenomenon: Understanding Overheating

Written by Ann Pietrangelo | Published on March 24, 2014
Medically Reviewed by George T. Krucik, MD, MBA on March 24, 2014

Often, one of the first signs of MS is blurred vision when overheating, also called Uhthoff's phenomenon.

What Is Uhthoff’s Phenomenon?

Some people with multiple sclerosis (MS) can feel their symptoms worsen when they become overheated. When elevated body temperature impairs vision, it’s called Uhthoff’s phenomenon. Uhthoff’s occurs because of damage to the optic nerve, which interferes with the transmission of signals between the eye and the brain. Symptoms include blurry or reduced vision.

Uhthoff’s phenomenon is often one of the first signs of MS. Sometimes called “Uhthoff’s syndrome” or “Uhthoff’s sign,” the condition is named for William Uhthoff (1853-1927), a German professor of ophthalmology.

How MS Works

MS is a chronic disease in which the immune system attacks the protective covering that surrounds nerve cells (myelin) in the central nervous system (CNS). Inflammation damages the nerve cells and causes scar tissue (lesions) to form. This damage inhibits the ability of the CNS to send signals to the rest of the body. Symptoms of MS vary, depending on the location of the lesions. Symptoms include vision problems, bladder dysfunction, and trouble with balance and coordination.

How Heat Affects MS

A 2011 Swedish study reported that between 60 and 80 percent of people with MS experience heat sensitivity. Demyelinated fibers in the CNS are hyper sensitive to a rise in body temperature, making it even more difficult for the body to receive internal signals. This can result in worsening of any MS-related symptoms, the most common of which is blurred vision.

The good news is that heat sensitivity and its related worsening of MS symptoms is only temporary (pseudo-exacerbation). It doesn’t mean you have new lesions, that you’re experiencing lasting neurological damage, or that your MS is progressing more rapidly.

What to Do When Uhthoff’s Strikes

Don’t panic. Uhthoff’s is only temporary. If your vision is seriously impaired, ask someone to assist you.

The solution is to cool your body temperature down. If you’re outdoors on a hot or humid day, find a shady area to rest. If you can, get indoors and take advantage of air conditioning or a fan. If you’re at home, take a cool bath or shower. Drink a cool beverage or use ice cubes to cool your body.

Once you’re cooled down, symptoms generally improve fairly quickly.

When to Seek Medical Attention

Medical intervention is not necessary for symptoms of Uhthoff’s. If you’ve experienced it before, you’ll know what to expect. However, it’s easy to confuse heat-related symptoms with other medical emergencies. 

Seek medical attention if:

  • you’ve never experienced heat-related symptoms of MS before
  • you’re not certain that your symptoms are Uhthoff’s or MS-related
  • you have accompanying symptoms unrelated to MS
  • cooling down doesn’t resolve symptoms

How to Avoid Uhthoff’s

The key is to avoid overheating in the first place. Try to stay indoors and use an air conditioner or fan during times of extreme heat and humidity, especially when exercising. Avoid hot showers, baths, and saunas.

Outdoors, try to stay in the shade and avoid heavy exercise. Use cooling products such as cooling vests, neck wraps, and portable fans. Enjoy icy beverages. Avoid hot tubs and swimming pools that are heated above 85 degrees Fahrenheit.

MS and Uhthoff’s don’t always go together. If you’re not sensitive to the heat, there’s no need to avoid activities you enjoy.

Is a Colder Climate the Answer?

While heat and humidity can trigger a wide range of MS-related symptoms, cold weather can too. Involuntary muscle spasms or stiffness (spasticity) can be triggered by high humidity or by cold temperatures.

If you suffer from heat or cold-related symptoms of MS, it’s best to avoid exposure to temperature extremes. If you’re thinking of relocating for health reasons, it’s a good idea to spend some time in a different climate first, to see if it makes a difference.

Did You Know?

Fever can also cause Uhthoff’s. The remedy is to treat the fever and its underlying cause.

MS-related heat sensitivity doesn’t cause new symptoms, but it can aggravate existing symptoms. In addition to vision problems, these include:

  • fatigue
  • numbness
  • weakness
  • bladder problems

Odd, but true: Before the arrival of magnetic resonance imaging (MRI) and other modern medical tests, diagnosing MS was very difficult. When suspected of having MS, patients were immersed in a hot bath. If neurological symptoms like weakness and blurred vision worsened, the patient was assumed to have MS.

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