Multiple Sclerosis (MS) is a disabling neurological disease. Many people who have MS find that their symptoms become worse when they have a fever or when the weather is very warm or humid.

Myelin is a substance that forms the protective sheath that surrounds the axons of the nerve fibers. When a person has MS, the immune system cells that ordinarily protect the body from bacteria and viruses mistakenly attack myelin in the central nervous system (CNS). The resulting damage can cause a wide range of symptoms.

People can experience a worsening of MS symptoms as a result of a slight elevation in their core temperature. For instance, this might just be a quarter of a degree higher than usual.

One reason for this is that an elevated temperature alters the action of nerves, further impairing the ability of demyelinated nerves by making it harder for them to conduct electrical impulses.

This article looks at MS and the effects of heat and cold in more detail, explaining why MS causes extreme temperature sensitivity. It also discusses the treatment and management of this symptom.

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About 60–80% of people with MS experience temperature sensitivity, also known as Uhthoff’s phenomenon. Increases in body temperature — due to the weather, fever, exercise, or other factors — can cause the neurological symptoms of MS to become worse temporarily.

The symptoms of MS, which higher temperatures can exacerbate, include:

  • muscle weakness
  • difficulty with balance and coordination
  • memory and thinking problems
  • visual disturbances
  • sensations of numbness or pricking
  • fatigue

The temperature sensitivity associated with MS arises largely as a result of temperature-dependent slowing or blocking of neural impulses within the CNS.

This occurs due to changes in the core temperature, or internal temperature, of the body, but changes in the temperature of the skin can also contribute to worsening symptoms.

Researchers have thoroughly documented the effect of temperature sensitivity in MS on autonomic symptoms, such as cardiovascular functioning, and motor symptoms, including fatigue.

People with MS may find that their symptoms worsen in warmer conditions, such as when they are exercising, taking a hot bath or shower, or sunbathing.

The worsening of symptoms, which people sometimes refer to as a pseudo-exacerbation, tends to be temporary. It does not cause more demyelination or damage to the nerves, and the symptoms improve once the person has cooled down.

An example of a symptom that may worsen during exposure to heat is blurring of the vision.

In the past, doctors would use a “hot bath test” to diagnose MS. This involved a person immersing themself in a hot tub of water. The doctor would then base the diagnosis on the appearance or worsening of neurological symptoms.

Although heat sensitivity affects the majority of people with MS, about 20% of people with the disease experience worsening symptoms due to cold temperatures.

For instance, some people with MS have reported that their symptoms — especially spasticity, which is muscle tightening or stiffening — become worse in cold weather.

Experiencing cold temperatures can cause a person with MS to have temporary issues with:

  • tremors
  • tingling
  • numbness
  • mobility
  • balance
  • vision
  • cognition

Experts believe that this occurs because the cold affects the speed of messages traveling along nerves that the disease has already damaged. Researchers have also suggested that cold sensitivity in MS may happen due to MS lesions in the area of the brain that affects body temperature.

People with MS who experience temperature sensitivity can treat and manage the worsening of their symptoms in various ways. These include:

  • wearing light, breathable clothing
  • staying in air-conditioned environments when the weather is very hot or humid
  • exercising in a cool swimming pool with a water temperature lower than 85°F
  • limiting outdoor exercise to the early morning or evening
  • drinking cold fluids and eating popsicles
  • cooling down before and after exercise, such as by sitting in a tub of tepid water
  • using cooling products, such as neck wraps and vests, during exercise

Researchers have shown that drinking cold water may be particularly useful in helping with heat sensitivity. In their study, they asked the 20 participants — 10 of whom had MS — to cycle on a stationary bicycle in a controlled atmosphere of 30°C (86°F) and 30% humidity on three separate occasions. The participants cycled either until exhaustion or for a maximum of 60 minutes.

The team gave the participants a drink of water, either cold or warm, every 15 minutes. The participants who did not have MS all managed to complete 60 minutes of cycling, while only 3 out of 10 people with MS could manage the full hour when they drank warm water. However, 5 of these participants managed to cycle for 60 minutes with the aid of cold water, which also helped the remainder of the MS group cycle for about 30% longer than they had while drinking warm water.

The researchers suggest that temperature sensors in the mouth, abdomen, and digestive tract could be playing a role in signaling to the brain to affect the perception of fatigue, rather than the cold water simply cooling the core temperature of the participants.

People with MS may experience a temporary worsening of symptoms due to very hot or cold temperatures.

This symptom exacerbation happens because the extreme temperatures affect the function of nerves within the CNS.

The worsening symptoms are usually temporary, and extreme temperatures do not cause lasting damage to people with MS.

Severe MS symptoms occur more commonly in the heat than in the cold. About 20% of people with MS experience worsening neurological symptoms in cold weather.

People with MS may be able to manage temperature sensitivity by exercising at cooler times of the day or in a cool pool, wearing light clothing, staying inside an air-conditioned environment, and drinking cold beverages.