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Hot topic - heat sensitivity

The heat is on for new cooling drug, says Feature Writer and MSer Ian Cook. AAAAAAian.jpg

If you suffer from heat sensitivity you will know it can be a big summer holiday spoiler and although other multiple sclerosis (MS) symptoms often get talked about, heat sensitivity doesn’t get the coverage it deserves. As a sufferer I find this fact surprising because the link between heat and MS has been known about for 130 years. Back in 1890 Wilhelm Uhthoff, a German neuro-ophthalmologist, noticed that some of his MS patients’ visual problems got worse after exercising and getting hot. This later became known as Uhthoff’s phenomenon.

Then, in the 20th century the diagnosis of MS involved something called the ‘hot bath test’ where patients were lowered into a bath of hot water to see if their condition worsened when they got hot. If it did they would be diagnosed with MS.

Sweat connection

More recently, in the early years of the 21st century, researchers tried to identify the exact mechanism through which heat sensitivity has an effect in MS. The first thing looked at was the fact that MSers overheat because we lose our ability to sweat as MS progresses. Normally adults can sweat between two and four litres per hour or 10–14 litres per day and sweat cools the skin as it vaporises in a process known as ‘evaporative cooling’. But in MS things don’t work so well. Research carried out in 2009 at Oulu University Hospital in Finland looked at sweating in 29 MS patients and compared these patients to 15 people unaffected by MS. The research found that MS patients sweated markedly less than people without the condition. After just 10 minutes of heating, sweating was significantly lower in the forehead, feet and legs of MS patients than in those of those who didn’t have MS, meaning MSers were overheating as they were unable to benefit from evaporative cooling.

Sweating in simple terms is a two-way process. Temperature receptors in the skin send messages through the nervous system to a part of the brain known as the hypothalamus where heat-sensitive nerve cells are located. These cells in return send messages to millions of sweat glands in the skin to
release sweat causing evaporative cooling. For a message to travel between the hypothalamus and the sweat glands the nervous system must carry these messages efficiently.

One of the key chemical elements involved in this process of efficient communication is sodium. As axons in the central nervous system heat up, the amount of sodium moving into the nerve increases in a process known as sodium loading. However in MS this process goes into overdrive and excessive
sodium makes it harder for messages to be sent efficiently up and down nerves to and from the sweat glands. This results in less sweating and overheating. 

Dr Mark Baker of Queen Mary, University of London is currently researching ‘sodium loading’ in axons. Dr Baker is looking for a drug or drugs that could target MS heat sensitivity by reducing the amount of sodium travelling into nerve cells when the temperature increases, allowing messages to muscles to be sent more securely and therefore better communication with the sweat glands and more sweating. iStock-992133438.jpg

Drug hope

One drug that is believed could have this effect is bumetanide. This drug is already used to reduce extra fluid in the body (oedema) caused by conditions such as heart failure, liver disease, and kidney disease so we know it’s safe to use. It is thought bumetanide might be effective in tackling heat
sensitivity because it reduces the amount of sodium entering cells. Dr Baker has been leading research into this area but says that bumetanide comes with major problems. It is poor at getting into the brain and nervous system and thus poor for accessing damaged axons. A side effect of bumetanide is
increased urination – something that would be unwelcome by many of us MSers who suffer urinary problems. Dr Baker says these facts are leading him to look for other drugs.

“We need to investigate other compounds that have much better brain penetration and we have plans to do this. We also think there may be another molecular mechanism causing sodium loading that is not affected by bumetanide and this is one of the things we are working on.

“Exploring this avenue may allow better pharmacological control of temperature-dependent symptoms, and in the longer term could provide a route to neuroprotection. So right far down the line neuroprotection is a massively exciting idea that means we may be able to protect axons and neurons from the worst effects of neuro-inflammatory disease and slow progression by reducing the energy expenditure in axons as well.”

Although it is early days in Dr Baker’s research, it is hoped that continuing work on heat sensitivity could lead to other drugs which would not have the side effects of bumetanide and may even have a neuroprotective role too. Until that day happens, MSers will remain reliant on the tried and trusted techniques for keeping cool such as fans, jackets or sprays. I, for one, will be hoping that it’s not too hot this summer, and I will of course also be using a fan, a spray and possibly other cooling aids in case, like last year, we have another long hot summer/ Meanwhile, the heat is on in the search for a new cooling drug.

This article originally featured in issue 122 of New Pathways magazine. For the latest treatment, symptom information and real-life stories, subscribe to New Pathways by clicking here