When someone with MS experiences an increase in temperature, which in turn causes a temporary worsening of one or more symptoms, this is known as “heat sensitivity in MS.” A person’s temperature may increase as a result of exercise or other physical exertion, being in a hot and/or humid environment (which includes taking a hot bath or shower), and fever.1
The most common symptoms reported with heat sensitivity in MS are blurred vision and increased fatigue, weakness, and incoordination. Sometimes speech and cognition are also affected, and this can include a slurring of one’s words and the inability to think quickly or clearly.
As many as 80% of individuals with MS experience heat sensitivity, which was first recognized in the late 1800s by Wilhelm Uhthoff, a German ophthalmologist who specialized in neurologic disorders. He observed that patients with MS and other demyelinating conditions developed blurry vision while exercising. He also found that once the patients cooled down, their vision returned to normal. This manifestation of heat intolerance became known as “Uhthoff’s phenomena” and this term is still sometimes used to describe the worsening of any MS symptom resulting from heat intolerance.1
Multiple sclerosis (MS) is a disease of the central nervous system (CNS), which consists of the brain, optic nerves and spinal cord. With MS, areas of the CNS become inflamed, damaging the protective covering (myelin) that surrounds and insulates the nerves. This damage, known as “demyelination,” disrupts the smooth flow of nerve impulses, which causes the symptoms of MS.
Studies show that nerves with damaged myelin are sensitive to changes in temperature. When a person with MS experiences even a slight increase in core temperature – such as from exercise, fever, or a hot environment – the disrupted flow of nerve impulses along demyelinated nerves temporarily worsens. This further slowing of nerve impulses temporarily exacerbates existing symptoms and/or leads to the temporary appearance of new symptoms.
This temporary worsening of symptoms and/or the appearance of new symptoms, however, does not indicate any additional damage to the nerves or an increase in disease activity. Also referred to as a pseudoexacerbation, the effects of heat sensitivity resolve on their own once the individual cools down and is no longer experiencing an increase in core temperature. No permanent or long-term damage results from heat sensitivity and pseudoexacerbations.
In addition to a hot environment or other causes of heat sensitivity, brain lesions may sometimes be located in an area of the brain that controls the body’s temperature. This can create a dysregulation of the body’s temperature and a temporary increase in temperature, which can also lead to a pseudoexacerbation.
Plan ahead. Scheduling more activities during the cooler times of day when outside, while remaining in a cooler environment inside during the hotter times of day, helps to reduce the risk of feeling the effects of heat sensitivity. Allow time to rest between activities and be realistic with what can comfortably be accomplished.
Be aware of your body temperature. If you are beginning to feel the effects of heat sensitivity from things such as exercise, a warm environment, or a fever, take action to cool down. This can include strategies such as moving to a cooler space, using a cooling garment, sitting and resting, and drinking a cold or frozen (non-alcoholic) drink. Taking a cool shower can also help someone cool down. With a fever, a physician may recommend taking an over-the-counter medication, such as Tylenol® (acetaminophen) or aspirin, to reduce one’s temperature.
Stay hydrated. Especially when exercising, in a warm environment, or experiencing a fever, drinking enough water and other healthy beverages is vital to avoiding dehydration, which can cause a number of dangerous health issues. Dehydration increases the risk of fatigue, heatstroke, low-blood pressure, kidney failure, and other potentially fatal health complications.
Pay attention to weather forecasts. Look to the weather forecasts in your area for both heat and humidity, as the latter can also cause symptoms of heat sensitivity. Knowing when hotter temperatures or a higher heat index are predicted enables you to plan your activities around those times that will be too hot. (The heat index is a figure derived by combining air temperature with humidity to determine how warm the temperature will actually feel.)
Keep cool. Many people accomplish this by wearing cooling apparel as needed. MSAA’s Cooling Distribution Program provides free cooling vests and accessories to individuals with MS who qualify for assistance. Staying in an air-conditioned environment is also recommended whenever the outside temperature and/or heat index is elevated. In addition, many individuals with MS participate in aquatic therapy, where they enjoy the natural resistance and cooling benefits of exercising in water.
In addition to heat sensitivity, some individuals with MS experience a sensitivity to cold temperatures. A study that surveyed 757 people with MS indicated that 58% of participants reported being heat-sensitive only, 29% were sensitive to both heat and cold, while 13% were only sensitive to cold temperatures. All participants stated that hot and cold days were the primary triggers of their heat or cold sensitivity and fatigue was the most common worsening symptom.2
Although people with cold sensitivity experience some of the same symptoms as those with heat sensitivity – including fatigue and mobility issues – those with cold sensitivity tend to experience different symptoms, such as an increase in spasms and spasticity (tight muscles), nerve pain, and depression. Layering of clothes and moving to a warmer environment were common modes of treatment for cold sensitivity in MS.3
1. Frohman T, Davis S, Beh S, et al. Uhthoff’s phenomena in MS – clinical features and pathophysiology. Nat Rev Neurol. 2013 Sep;9(9):535–540 (2013).
2. Christogianni A, et al. Heat and cold sensitivity in multiple sclerosis: A patient-centred perspective on triggers, symptoms, and thermal resilience practices. Multiple Sclerosis and Related Disorders, Volume 67, 104075. November 2022.
3. Wexler M. MS and heat. Multiple Sclerosis News Today. Updated Aug. 22, 2022.
