Heat sensitivity and MS (Uhthoff's phenomenon)

Heat sensitivity, also known as Uhthoff’s phenomenon, is the most common type of temperature sensitivity experienced by people with MS. It describes the temporary worsening of MS symptoms in response to an increase in temperature.

On this page, we look at when heat sensitivity was first recognised in MS, common triggers and the symptoms that are typically affected. We also look at some practical approaches that can help manage the effects of heat.

Heat sensitivity is a type of temperature sensitivity that is commonly experienced by people with MS. It describes the temporary worsening of MS symptoms in response to an increase in temperature. Around 6 in 10 people with MS report experiencing heat sensitivity.

It is also known as Uhthoff’s phenomenon, Uhthoff’s sign or Uhthoff’s syndrome – named after Wilhelm Uhthoff, a German eye specialist (ophthalmologist).

Uhthoff specialised in neurological conditions (neuro-ophthalmologist). In 1890, he recognised that some of his MS patients found that optic neuritis and other visual problems worsened while they were exercising but returned to normal after resting.

At the time, Uhthoff linked this to the exertion of exercise. But we now understand that the effect is actually caused by an increase in body temperature. We also now know there are other factors that can trigger Uhthoff’s phenomenon.

It has since been shown that other MS symptoms can also be affected by an increase in temperature, including:

• fatigue
• altered sensations
• weakness
• cognitive symptoms.

In the 1950s, the hot bath test was used as a diagnostic test for MS based on this phenomenon. This involved the person lying in a bath of warm water. If this caused or made existing neurological symptoms worse, it was taken as evidence that the person had MS.

By 1980, the hot bath test began to be replaced by other tests to diagnose MS. This was in part due to advances in imaging such as MRI. It was also deemed not specific enough and could lead to potential complications such as a drop in blood pressure.

Common triggers include:

• warm weather – especially humid conditions
• overheated or crowded rooms
• warm baths, showers, saunas and steam rooms
• exercise – raises the body’s core temperature 
• stress or anxiety – can indirectly raise your temperature
• fever or infections.

Hormonal changes can also lead to increases in body temperature or disrupt your body’s ability to regulate temperature. Some people find they experience heat sensitivity as a result of fluctuations in hormone levels during certain phases of their menstrual cycle, or because of the decline in oestrogen levels when going through perimenopause and menopause.

When your core body or skin temperature rises, people find that the following symptoms typically get worse:  

• fatigue
• blurred vision – especially if you’ve experienced optic neuritis before
• numbness and tingling
• weakness or balance issues – for example legs feeling heavy or weak
• cognitive symptoms such as concentration or memory.

Although symptoms can flare up very quickly, they should improve within a few hours once you cool down. For example, if you experience blurred vision while exercising, you should recover after a cool shower or rest.

The increase in symptoms in response to heat is not usually due to new MS damage. It is a temporary response triggered by an external factor. Once your temperature returns to normal, the symptoms should settle back down within a few hours.

Warm weather, having the central heating too high, vigorous exercise or a fever can all raise your core body temperature.

The following are some ideas of things you can try to help you manage the effects of heat and maintain a more stable core body temperature.

If possible, try to stay in cool environments. At home there are some things you can do such as:

• using blackout curtains to block heat 
• opening windows in the evening to cool down rooms in hot weather  
• using a floor or desk fan to help cool a room
• getting an air cooler if you can – this may not be as effective at cooling in humid conditions
• investing in air conditioning if you can – this can be costly and take up space.

There are also things you may be able to do if you are away from home and find that you are getting too hot:

• using a hand-held fan
• if possible, find an air-conditioned public space such as a shopping centre, cinema or library
• where possible, seek out a large stone building such as a town hall or church.

If this is not possible, try to stay in the shade and avoid crowded places. 

The following are some simple ways to help keep your body cool:

• spraying your wrists and face with a plant or hair mister filled with iced tap water – there are also cooling sprays available from high street pharmacists that can help
• running cold water over your hands, placing your feet in a bowl of cold water or splashing cool water over your face
• taking cool showers or baths
• if you use heated appliances, such as hairdryers, use them on a low setting.

There are also products that may help if you get hot in bed such as:

• a cooling pillow – these may be designed using specialised cooling materials or have cooling gel pads attached 
• gel pads that you can put on top of your existing pillow 
• bedding or a mattress made from breathable moisture-wicking fabrics like cotton or linen, or specialised cooling materials.

You may want to avoid pillows and mattresses made from memory foam in hot weather as they can retain heat and be uncomfortable.

There are also wearable options when it comes to cooling products. Many are relatively inexpensive. They include:

• vests
• scarves
• wrist bands
• hats. 

The products work in different ways. Some contain a gel or other substance that can be precooled before you wear them. Usually by soaking in water. More expensive options may use active cooling technology that requires a battery.

If you are unwell with an infection or are feverish, treat this as you usually would. Drink plenty of fluids, especially water, to keep hydrated. Over the counter medications like paracetamol or ibuprofen, resting, lightweight clothing and cool showers can also help to bring your core body temperature back down.

Choose clothing that is loose and lightweight. Breathable fabrics like cotton, linen or moisture-wicking fabrics help. They allow better airflow and avoid trapping heat. Some people find light-coloured clothing a good option as it may reflect the sun and heat better than darker fabrics.

Eating can make you feel warmer as after you have eaten your body begins digesting the food to create energy. Hot or spicy foods, and warm drinks, can also cause you to warm up and sweat.

Drinking cold fluids such as iced water and eating iced lollies or frozen snacks can help you cool down. As can foods with a very high-water content, such as:

• salads – cucumber, tomatoes, iceberg lettuce and celery
• fruits – watermelon, melon, berries and citrus fruits
• vegetables – courgette, mushrooms, broccoli and cauliflower. 

Some of these options also have the added advantage of requiring little preparation and no need to use an oven.

Regular, moderate exercise is widely recognised as being beneficial for people with MS. It can help maintain good physical and mental wellbeing. The positive benefits of exercise mean that it’s important not to avoid it altogether. 

But many people find their MS symptoms get worse if they become hot when exercising. This is known as exercise-induced heat sensitivity. 

If possible try and find different ways of exercising that help you stay cool enough to still reap the benefits. Think about the type of activity you do and how you do it. Some activities will keep you cooler than others.

• Activities that focus on strength and resistance might be a better option than activities such as running. They tend not to raise your core temperature as much. 
• Swimming or other water-based exercises are also a good option. As well as reducing your risk of overheating, you may find that you can perform a greater range of movements than on land as the water supports your body weight. 
• Shorter bursts of interval-style exercise might be better than longer sessions.

We have more information on choosing the right exercise.

Preparation is key to helping you stay active without triggering symptoms. You could try:

• wearing loose, breathable clothing
• exercising at cooler times of the day, such as early morning or later in the evening
• going to an air-conditioned gym if you can. 

If you are outdoors, try sticking to more shaded areas where possible.

Another thing to consider is pre-cooling before you start to exercise. You might try:

• having a cool shower or splashing your face or wrists with cold water 
• drinking iced water – as long as you’re not worried about any bladder issues or access to toilets whilst you’re exercising
• using a cooling garment before and during any activity. 

Many cool vests are intended for use by athletes and are designed to be comfortable when active. Cooling hats, wristbands or neck wraps may also provide some relief.

Whilst exercising, take care to pace yourself and take breaks to avoid becoming overheated. If you do start to notice an increase in symptoms don’t be tempted to push through, as this is likely to be counterproductive. 

Once you’ve finished, make sure you cool down gradually and rehydrate. Give your body time to readjust. Symptoms should settle within a few hours if they are heat related.

Sometimes what appears to be heat sensitivity can actually be a sign of something else. You should check with your GP or MS team if any symptoms that get worse in the heat:

• are new issues or significantly worse
• last longer than they usually do
• don’t improve after cooling
• are accompanied by signs of infection or fever – infections can mimic or worsen symptoms and can trigger MS relapses.

As both heat sensitivity and MS relapses can cause symptoms to worsen, you may be concerned about the reason for any changes you’re experiencing. Read more about is it temperature sensitivity or a relapse?

There is such a lot of clothing and equipment that can help with heat sensitivity that it can be hard to know where to start. An online search will provide you with lots of options, but it can be useful to seek personal recommendations where you can.

You may wish to ask other people with MS what products they have found helpful or ask your MS team for advice. MS Trust Facebook peer support group has a large community that you could ask.

• Visual problems
• Ask the expert: heat sensitivity