Heat sensitivity in MS

Dr Ollie Jay from the University of Sydney provides an update on his research into why heat sensitivity occurs in MS and strategies to help people manage it. Dr Jay’s research is supported by MS Research Australia.

Heat sensitivity is a major issue for 90% of Australians living with MS. It is characterised by a temporary but reversible worsening of MS signs and symptoms. One of the most commonly exacerbated symptoms experienced in the heat is fatigue, which often limits a person’s ability to complete physical exercise and even perform day to day tasks, especially in summer.

Our research team at the Thermal Ergonomics Laboratory at the University of Sydney, in association with international collaborators, conducts research to develop a better understanding of the physiology of heat sensitivity in MS, and identify the most effective strategies that people can use to help prevent heat-related fatigue during physical activity.

What do we know about heat sensitivity with MS?

It is unclear exactly what causes heat sensitivity (also known as Uhtoff’s phenomenon). It is thought that even a 0.2 to 0.5°C increase in body temperature can lead to a worsening of MS symptoms. However, the exact increase in temperature needed or even whether an increase in body temperature is actually needed to induce Uhtoff’s phenomenon still needs to be confirmed. Our work has recently shown that people with MS exercising in 30°C temperatures stop because of exhaustion when their core temperatures have increased by as little as 0.2°C or as much as 1.0°C. Our American colleagues have also shown that at 40°C people with MS display an increase in postural sway (the movement of the body during standing – an indicator of disability status), and that this occurs without the core body temperature increasing at all. Their results suggest that even increases in skin temperature, without core temperature increasing, may worsen symptoms.

We have also found that people with MS may also not be able to control their body temperature as efficiently as people without MS, as they only start sweating at a higher body temperature than people without MS. Additionally, the ability of people with MS to sense the temperature of the environment may also be altered. For example, we have found that the capacity to distinguish between different cold temperatures is blunted with MS, whereas the ability to sense warm temperatures remains unaltered.

Ways to prevent heat sensitivity when indoors

When indoors during hot weather, one of the primary methods to cope with heat sensitivity is the use of air conditioning (AC). Australians with MS run their AC approximately 15 times more than a regular household. However, despite the fact that some people with MS can get medical energy rebates, it’s use can still prove costly and is vulnerable to potential power outages during extreme heat events.

One alternative is using a fan. Fans move air across the skin instead of chilling the air, and they use far less electricity. The same amount of heat loss can be achieved with an electric or battery operated fan as with AC because air flow also accelerates the evaporation of moisture on the skin, which further cools the body. Our research team has recently demonstrated that in a simulated heat wave, using a fan can deliver a cooling effect on the body equivalent to a reduction in air temperature of at least 6-7°C in healthy young men. This benefit is further enhanced if the skin is wetted with a damp cloth while using a fan.

Research team at the Thermal Ergonomics Laboratory at the University of Sydney

Research team at the Thermal Ergonomics Laboratory at the University of Sydney

Ways to prevent heat sensitivity when physically active and/or outdoors

In everyday life, remaining indoors in an air-conditioned environment is highly restrictive and unrealistic. Many jobs require physical activity or outdoor exposure and it is important to try and incorporate physical exercise in your lifestyle. Physical exercise is well known to have profound health benefits including positive impacts on psychological wellbeing. Unfortunately, these essential activities put people with MS at a much greater risk of heat related problems, especially in summer.

To date, it has been assumed that heat sensitivity is due to an increase in core body temperature and nearly all recommendations or strategies for people with MS planning to be physically active in the heat focus on reducing core temperature. These strategies include immersing the lower half of the body in 20°C water for 30 minutes prior to exercise. This has been shown to reduce core temperature by 0.5°C and enable people with MS to exercise in a warm environment with fewer side effects.

Another recommendation is to wear a cooling vest, (vests with ice or cold gel packs built in). These have been shown to improve movement and relieve fatigue. However, as these vests tend to be heavy, they can result in higher physical exertion and therefore increased internal heat production. Additionally, once the ice packs have melted the vests tend to be heavy and can instead act as an insulating barrier trapping the body heat in.

It has also been suggested that cooling the neck or the head can be beneficial, but more evidence is needed for this method. However, emerging evidence from studies in our lab indicate that reducing the core temperature may not be necessary to reduce the effects of heat sensitivity in people with MS. Drinking 250 ml of cold (1.5°C) water every 15 minutes was found to increase the capacity of people with relapsing-remitting MS to continue exercising in a warm (30°C) environment by about 30%. This increased exercise capacity was observed despite no reductions in core or skin temperature with cold-water compared to drinking body temperature (37°C) water. We think that stimulating cold sensitive receptors in the mouth and throat may blunt the development of MS related fatigue, despite the increase in core body temperature.

Given that the majority of these temperature sensors along the gastrointestinal tract are located in the mouth and tongue, it seems likely that a similar reduction in heat sensitivity could be achieved by simply swilling cold water in the mouth, without swallowing. This approach would be particularly beneficial for people with MS who may prefer to limit how much they drink due to bladder symptoms (although staying hydrated is still really important).

Through our ongoing research we hope to help people with MS manage heat sensitivity through evidence-based practical methods.

Top tips on how to reduce heat sensitivity without air conditioning

Dr Ollie Jay

Dr Ollie Jay is a Senior Lecturer in Thermoregulatory Physiology, and Director of the Thermal Ergonomics Laboratory, in the Faculty of Health Sciences at the University of Sydney, and Lead Researcher of the Charles Perkins Centre (CPC) Research Node on Climate Adaptation and Health. @ollie_jay13 @ThermalErgLab
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Heat sensitivity in MS