Too hot, too cold: the science behind temperature and MS

Lots of people with MS notice symptoms get temporarily worse when they’re too hot or too cold, or both. Professor Filingeri is Associate Professor in Thermal Physiology at the University of Southampton. He studies how temperature affects the human body and mind, including its impact on people with MS. He also teaches health science to trainee nurses and physiotherapists. 

Heat and MS

Thank you for speaking with us, Davide. To start off, can you tell us why so many people with MS are affected by the heat?

There are several theories for why heat affects people with MS. And I think there are a few different things going on.

The classical view is this: when nerve cells get too warm they don’t pass on messages so well. And MS makes them even more vulnerable to heat by stripping off their protective layer, called myelin. 

When messages are disrupted in the brain and spinal cord, people notice their MS symptoms getting worse - until they cool down again. 

This explanation makes sense because we’ve seen in research that signals go slower when the heat is turned up on nerve cells without myelin. Those lab experiments create conditions that are like the inside of our body warming up – like when our ‘core body temperature’ rises.

Temperature and MS symptoms

But I think there could also be something else going on.

People with MS sometimes tell us their symptoms get worse when they feel warm, even when it’s unlikely their core body temperature has risen. I'll give you a practical example. Sometimes fatigue gets worse for someone who’s only spent a short time outside in sunny weather. They can feel the warmth on their skin, but it probably wouldn’t be enough to heat up their core. So the vulnerable nerve cells in the brain and spinal cord aren’t getting warmer. The MS symptom feels worse because of the warmth on the skin. 

There’s research to back this up. For example, we did some experiments in our environmental chamber. That’s basically a big room where we can control the temperature. We gradually increased the heat while people with MS were in the chamber. The room got warm enough to make them notice the heat, but it didn’t raise their core temperature. And these people reported more mental and physical fatigue as they felt hotter. 

In another study, our colleagues in America found that people’s balance got worse when their skin got warmer. In both of these studies, it wasn't hot enough to raise people’s core temperature, just to make them feel warmer through their skin. 

How do we explain all this?

Well, we don’t know for sure. But I think some alternative views come into play. You know, sometimes we think of the body as divided into compartments. You've got the core and then the skin. So if the core doesn’t change, then the MS symptoms shouldn’t change either. But I think it's a bit more nuanced than that. 

For example, we traditionally think of MS affecting nerves in the brain and spinal cord. But perhaps nerves in other parts of the body can be affected too. Some ‘peripheral’ nerves – nerves in the rest of the body – also have myelin around them. And research suggests the myelin there might be affected sometimes. It’s called ‘peripheral neurodegeneration’. So that could be part of the answer for some people with MS – heat might be affecting nerves outside the brain and spinal cord. 

Another factor might be psychological. My former PhD student Dr Aikaterini Christogianni looked into what’s called the ‘nocebo effect’ and heat. If you have MS and you know that heat is not good for you, you might notice the heat more, and be more anxious about it. That could have a real effect on symptoms you feel. 

The good news is, all this suggests that it helps to feel cooler and cool the skin as well as actually cool down body’s core. And that’s often easier to do. 

Do people with other conditions have similar effects from heat? 

Yes some other conditions are affected by heat a bit like MS. Parkinson's is one, and some forms of Alzheimer's and dementia. And people with more than one condition that’s affected by heat tend to be more sensitive to temperature changes. 

Parkinson’s quite often makes people sweat less. When sweat evaporates into the air, that’s a big part of how the body stays cool. So losing that makes it harder to keep cool. 

Sweat is controlled by the body’s ‘autonomic system’. That controls automatic things that we don’t need to think about. For example, keeping the body the right temperature. Lots of neurological conditions can affect that system, including sometimes MS. 

Unfortunately, the reality is that there’s not a lot of scientific evidence around temperature and health conditions. But it’s something we’re starting to learn more about.

Do we know why some people with MS are more affected than others? 

It’s not always easy to tell why one person with MS is affected by temperature more than another. Research suggests that sometimes it’s because of where the MS damage is in the brain. Especially if it’s in the hypothalamus (pronounced ‘high-poh-THAL-a-muss'). In fact, this has been most clearly shown when people are sensitive to the cold. One of the jobs the hypothalamus does is help regulate our body temperature. 

Some medicines people take for MS symptoms might make a difference too. For example, anticholinergic medicines, which are sometimes used for bladder issues, can reduce how much someone sweats. And that could mean it’s harder for their body to stay cool. 

But it’s hard to say how much difference all these things make, especially to an individual. There are probably lots of factors for everyone who’s sensitive to temperature changes.

MS and climate

Are people with MS affected differently by temperature in other countries?

Well, in 2022 we published an international research survey of around 800 people with MS. Most people who responded were from the UK, Australia, America, the Netherlands and Greece. People tend to report very similar effects from being hot or cold. But how they manage it might vary.

To take the example of Greece, you would think that people would be more often exposed to the heat, and a more intense heat too. But it really does depend. When somewhere is hot people might spend more time indoors, with spaces designed to stay cooler. They might be more likely to have air conditioning because the weather is so often like that. Anecdotally, I know that’s the case for my relatives with MS back in Sicily, where it’s very hot and humid.

So air conditioning or buildings built to stay cool are often practical solutions for people in those countries. It might not be so practical in the UK, where extreme temperatures are still less common.

We’ve talked a lot about heat. But what about the cold. How can these two things both affect people with MS? 

That's an interesting question. Because we often think of heat and cold as two different things, but actually we're talking about temperature. So hot and cold are just two ends to the same scale. 

We know our cells work well within a certain range – the environment that’s best for them. That’s called homeostasis (home-eeyoh-STAY-siss). So the body makes a big effort to keep within that temperature range. Either too hot or too cold means cells won’t work at their best. People with MS are often sensitive to either heat or cold. But in our survey we found for the first time that around a third of people said they’re sensitive to both. 

I think there could be a few reasons people are sensitive to cold. One could be mechanical. What I mean is that when muscles, ligaments and joints get colder they don’t work so well. That’s true for people with MS and people who don’t have MS. It’s why we warm up before exercise. But if you already have some difficulty with balance or stiffness from the MS, then the cold could make things noticeably worse. 

And as I mentioned before, if the hypothalamus part of the brain is affected by MS the body might not be able to control the temperature so well. In the heat, the body’s own ‘air con’ system doesn’t cool enough. In the cold, it doesn’t warm enough. 

So although it might seem like a contradiction that both hot and cold can affect someone with MS, really it’s because the body works best at a small range of temperature.

Cold water swimming

So the cold can be an issue for people with MS. But can’t very cold temperatures, like cold water swimming, sometimes help people with MS?

With the cold water swimming, I think that might help some people who are heat sensitive because it brings down their core temperature. A bit like athletes might use cold water to cool their core before they exercise. It gives you a lower starting point, so it takes longer for the core to get warm. In other words, this ‘pre-cooling’ might give you longer in the temperature bandwidth that you want. And that could mean more capacity for an activity or to go about your day.

I’m not sure there’s research yet to be certain that’s what’s happening with cold water swimming for people with MS. But it makes sense from what we know through lab studies and some research into athletes who train in water. It could usefully lower the core temperature. 

At the University of Portsmouth, they’ve recently looked at cold water and how it could benefit mental health. And of course some people enjoy the social side of cold water swimming, and the activity itself. If something boosts your wellbeing, that’s an important benefit too. 

How to keep cool

How effective and practical are different techniques for staying cool, or keeping warm? 

That's a question we've been battling in research. Because you might come up with the best thing since sliced bread. The perfect cooling technique. And then nobody does it because, for example, it’s not easy to go around with a huge ice vest when you go shopping. So the challenge is to find what works best in different situations. 

A lot of people with MS do find cooling vests with ice packs effective. They do work to keep the body cool. But for some people they’re most practical for ‘pre-cooling’ - getting the body cooler before activity so you get that extra bandwidth of ‘good’ temperature. 

Another thing that works to cool the core is cold water or ice slurry drinks. Basically an ice slurry is flavoured, crushed ice. We did some research with colleagues in Australia which suggested some people with MS could cycle for longer when they’d drunk cold water. Just like the bulky cooling vest, there can be drawbacks to cold drinks. For instance, people with bladder issues sometimes don’t want to take on all that liquid through the day. But if it suits you, it can certainly help.

In our survey we found three most popular strategies people with MS use for staying cool: air conditioning, electric fans or ventilation, and wearing light clothes. Obviously, air conditioning isn’t always available. So a lot of people use electric fans or other forms of ventilation instead. That helps our natural cooling mechanism. The breeze evaporates the sweat faster, taking heat from the body. Lightweight clothes also help with that. 

Try different things

So these are fairly simple techniques to stay cool, and for lots of people they’re the most practical. Remember, there might be real benefits from feeling cooler on the skin as well as from cooling your body’s core temperature. 

Perhaps a combination works best. For example, pre-cooling with a cooling vest, drinking cold drinks when you can, using a small electric fan or staying in the breeze, and choosing light clothes. When I say ‘light clothes’, that’s light colours as well as lightweight. This helps reflect heat from the sun. If you’re out and about and need something to make you feel cooler, a ‘cooling scarf’, ‘cooling hat’ or even menthol cream on the skin might help. 

Sometimes it’s worth taking precautions or checking things with your health care team. For example, if your MS causes changes in sensation, then ice or heat could damage your skin without you realising. And you might sometimes need to be especially careful about falling. For instance, will a cold shower stiffen your muscles and affect your balance for a while afterwards? And of course everyone should take sensible precautions for swimming. 

As for staying warm if you feel too cold? The most popular things in our survey were wearing layers of clothes, and keeping the house warm - with heating and by closing internal doors. Lots of people also find using hot water bottles, heat pads and electric blankets help. Often people also eat hot food and warm drinks. These can all help keep the muscles and skin warm, or help warm the core. Unfortunately, some of these can mean spending more on heating or electricity, so people should check if there’s government support for them to reduce those costs. 

We just don’t have a silver bullet for staying cool or keeping warm – no single solution for everyone. It’s about finding the combination of things that work best for you. Thankfully, lots of those are fairly simple things to try.