Summer, vacations in the sun, a hot bath: for many people with multiple sclerosis, these seemingly harmless situations can trigger a temporary worsening of symptoms. This is the Uhthoff phenomenon, well known to patients and neurologists. This phenomenon can affect cognitive functions as much as physical capacities. Understanding this mechanism and knowing how to adapt is essential to get through the hot periods serenely.

What is the Uhthoff phenomenon?

The Uhthoff phenomenon, named after the German ophthalmologist who first described it in 1890, refers to the temporary worsening of MS symptoms triggered by an increase in body temperature. Wilhelm Uhthoff observed that some of his patients experienced deteriorating vision after physical exertion. It is now known that this phenomenon is not limited to vision and can affect all MS symptoms, including cognitive functions.

The mechanism involved

To understand the Uhthoff phenomenon, we need to go back to the basics of MS. The disease causes demyelination of nerve fibers: the myelin sheath, which normally insulates and accelerates the transmission of nerve signals, is damaged. Now, the transmission of the nerve impulse in demyelinated fibers is very sensitive to temperature. Even a slight increase in body temperature (around 0.5°C) may be enough to block or slow down the transmission in these weakened fibers.

The result is a temporary worsening of symptoms: vision may blur, muscle weakness may intensify, fatigue may worsen, and cognitive functions may deteriorate. These symptoms usually disappear when the body temperature returns to normal, without leaving any permanent sequelae. It is not a relapse but a transient and reversible phenomenon.

Impact of heat on cognitive functions

The Uhthoff phenomenon is often associated with visible physical symptoms: vision disturbances, leg weakness, worsening of walking. But cognitive functions can be just as affected, sometimes more subtly but nonetheless significantly.

Cognitive symptoms related to heat

• Mental fog: Feeling of thinking through cotton, difficulty having clear ideas
• Cognitive slowdown: Information processing slower than usual
• Concentration troubles: Difficulty maintaining attention, increased distractibility
• Major cognitive fatigue: Rapid mental exhaustion with the slightest intellectual effort
• Memory difficulties: More frequent forgetfulness, difficulty retaining information
• Language disturbances: Difficulty finding words

These symptoms can be particularly troublesome in everyday and professional life. An important meeting on a hot day, an exam to take in summer, complex tasks to perform in hot weather: so many situations where the Uhthoff phenomenon can complicate the lives of people with MS.

The summers are my nightmare. As soon as the temperature exceeds 25°C, I feel my brain go into slow mode. I make more mistakes, forget things, am much slower. At first, it scared me, I thought it was a relapse. Now I know it's the effect of the heat and that it will pass.

Nathalie, 48 years old, MS for 15 years

Strategies to protect yourself from heat

The good news is that by knowing the Uhthoff phenomenon, measures can be taken to prevent it or limit its effects. Several strategies allow you to get through the hot periods more serenely.

Avoid heat sources

• Air conditioning: Essential during heat waves for sensitive people
• Avoid hot hours: Stay indoors between 12 p.m. and 4 p.m. in summer
• Warm baths and showers: Avoid hot baths and saunas
• Moderate physical activity: Avoid intense efforts that raise body temperature
• Light clothing: Prefer loose and light-colored clothing made from natural fibers

Actively cool down

Cooling techniques can help lower body temperature or keep it stable despite ambient heat. Cooling vests, cold towels on the neck, misting sprays are useful tools. Regularly drinking fresh (not iced) water also helps regulate temperature.

Adapt cognitive stimulation to the heat

When the Uhthoff phenomenon affects your cognitive capacities, it is important to adapt your brain training. This is not the time to force or impose demanding exercises. The goal is to maintain light cognitive activity without adding stress to an already struggling brain.

Favor EDITH during hot periods

During heat waves or days when you feel the effects of Uhthoff, favor EDITH with its stress-free approach. Choose the easy level if you feel particularly affected. Short sessions of 10-15 minutes are preferable to long sessions that may fatigue you further.

Postpone JOE to cooler periods

JOE's timed exercises require vivacity and speed that can be compromised by the Uhthoff phenomenon. It is preferable to reserve them for times when you feel well, for example early in the morning before the heat rises, or after you have cooled down effectively.

Don't feel guilty

If despite your precautions you feel too affected by the heat to do your cognitive exercises, don't feel guilty. It is sometimes preferable to take a break for a few days during a heat wave rather than force and generate frustration. Your brain will catch up when the temperatures are milder.

Last summer, during the heat wave, I tried to continue my JOE exercises as usual. It was a mistake: my scores were catastrophic, I felt useless, it completely demotivated me. This year, I adapted: EDITH easy level when it's hot, and JOE when it feels better. It's much more enjoyable and it keeps my spirits up.

Anne, 41 years old, MS for 9 years

Other factors that raise body temperature

Summer and heat waves are not the only situations that can trigger the Uhthoff phenomenon. Other factors raise body temperature and can have the same effects.

• Fever: Even a mild infection with a fever can temporarily worsen symptoms
• Intense physical effort: Exercise raises body temperature, especially in hot environments
• Hot bath or sauna: Absolutely to be avoided for sensitive people
• Intense stress: Stress can slightly raise body temperature
• Diet: Heavy and spicy meals can generate heat