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Under the microscope: heat sensitivity

Lynn Duffy

We’re on track for the hottest and driest July since records began. While some are basking in the sunshine, for people with MS the summer months can be hard. Over 60% of people with MS say that their symptoms get worse in the heat.

With symptoms like fatigue, blurred vision and muscle weakness all getting worse with changes in temperature, heat sensitivity can have a massive impact on someone’s life.

But what causes heat sensitivity in MS? And what can researchers do to help?

We put heat sensitivity under the microscope.

How science explains heat sensitivity

Human beings are highly sensitive to changes in body temperature. Even one or two degrees either side of 37°C can cause serious problems. To understand why temperature can have such a dramatic effect, we have to look at the science of the nervous system.

Our nervous system links everything that happens in our body, from how we move, to how we think and feel. To control all this, nerve cells are highly specialised at sending messages smoothly and quickly around the body.

For a message to travel from one end of a cell to the other, it requires an intricate balance of chemicals moving in and out of the nerve fibre. This delicate balance is disrupted by changes in temperature.

Researchers have found in healthy nerves, an increase in temperature of just a few degrees is enough to disrupt the signals sent along the nerve fibre. The reason for this isn’t fully clear, but one suggestion is that the heat affects the amount of sodium that moves in and out of the nerve. As the sodium levels change in the nerve, it becomes harder for a message to be sent.

How does this relate to MS?

Myelin helps messages travel along nerve cells quickly and smoothly. Damaging or losing this myelin, as happens in MS, means that messages slow down, become distorted, or don’t get through at all.

It’s been suggested that in MS, any increase in temperature, coupled with damage to myelin, makes it even harder for the nerve to send messages. So some people with MS find that their symptoms worsen when it gets warmer.

What research is happening to help?

Dr Mark Baker at Queen Mary, University of London wants to understand more about what changes in nerve cells when the temperature goes up.

Mark’s previous research showed that changes in sodium levels inside the nerve play a role in heat sensitivity. The team are now interested in understanding more about how sodium moves in and out of the nerve when the temperature changes. The researchers are currently looking at two existing heart failure drugs, amiloride and bumetanide. Either or both of these drugs may reduce sodium transport into the nerve.

It’s hoped that by reducing the amount of sodium moving into the cell when the temperature increases, the nerve will be better able to cope with the heat. This investigation will also give us a better understanding of the other ways sodium is getting into the nerve.

Being able to regulate the amount of sodium that gets into the nerve is an important area of neuroprotection research.

Mark’s work is still in the early stages, testing the drugs in rodent nerve cells in the lab. But, if the drugs work, it could be the first step to finding treatments that can help with the effects of heat sensitivity.

> Read more about Mark Baker's heat sensitivity research