What is Uhthoff's phenomenon in MS?

Uhthoff's phenomenon is the temporary worsening of existing MS symptoms when body temperature rises – from hot weather, exercise, fever or a hot bath. It happens because heat slows signal conduction along nerves that have lost myelin. Around six in ten people with MS are heat-sensitive. It is temporary and fully reversible on cooling, and does not cause new damage.

Does heat cause permanent damage in MS?

No. Heat-related worsening is temporary and reverses once your body temperature returns to normal, usually within a few hours of cooling down. It is not new damage or a true relapse – in some countries it is called a pseudo-exacerbation. So an off day in the heat is not a sign your MS is getting worse.

How can I stay cool with MS in Singapore?

Use air-conditioning and fans, have cool drinks and high-water foods, take cool showers, apply cool water to wrists, face and neck, and use cooling products such as vests or scarves. Wear loose, breathable clothing, stay hydrated, and time activity and exercise for cooler parts of the day. In Singapore's climate, cooling is a year-round habit.

Heat Sensitivity and Multiple Sclerosis (Singapore Guide)

Q: When should heat-related MS worsening be checked?

A worsening that does not settle after cooling and rest, keeps getting worse, or lasts more than about 24 hours, or any new unexplained symptoms, could be a true relapse or an infection (which can raise temperature) and should be reported to your MS team. Sudden severe or new symptoms need urgent assessment – call 995.

Written & reviewed by the BestDoctors.sg editorial team · Last updated 29 Aug 2026 · General information, not medical advice

Quick answer. Many people with MS are sensitive to heat: a rise in body temperature temporarily worsens existing symptoms such as fatigue, blurred vision, weakness and numbness. This is called Uhthoff’s phenomenon, and the crucial reassurance is that it is temporary and fully reversible once you cool down – it does not cause new damage or a true relapse. In Singapore’s hot, humid climate, heat management is a year-round issue, and simple cooling strategies – air-conditioning, cool drinks, cooling products, timing activity – make a real difference. MS can also cause other sensory symptoms and nerve pain, which are managed with your team. A worsening that does not settle on cooling should be reported.

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In a medical emergency, or for severe or sudden symptoms such as breathing difficulty, chest pain, fainting or stroke signs, call 995 immediately.

Why heat affects MS symptoms

Heat sensitivity is common in MS – around six in ten people are affected, according to the MS Trust. When body temperature rises, it slows or blocks the conduction of signals along nerves that have lost some of their myelin, which temporarily worsens existing symptoms. This is known as Uhthoff’s phenomenon. The most important thing to understand is that this is not new damage or new inflammation; it is a temporary effect on how already-affected nerves conduct signals. Recognising heat sensitivity for what it is helps take away some of the alarm when symptoms flare in the heat, because the flare will settle once you cool down.

What heat can worsen

The symptoms that commonly worsen with heat include fatigue, blurred vision – especially in people with a history of optic neuritis – numbness and tingling, weakness, balance problems, and difficulties with concentration and memory. The degree varies from person to person and with how hot you become. Triggers include hot and humid weather, exercise, a fever or infection, hot baths or showers, stress, and hormonal changes. Knowing your own triggers helps you anticipate and manage them. Because the worsening is temporary, planning around the heat – rather than fearing it – is the practical goal, and there is a great deal you can do to stay comfortable.

The key reassurance

The single most important message about heat sensitivity is that it is temporary and reversible. Once your body temperature returns to normal, usually within a few hours of cooling down, the symptoms settle back to their baseline. Heat-related worsening does not cause permanent or new damage, and it is not the same as a true relapse; in some countries this transient worsening is called a pseudo-exacerbation. Understanding this can be genuinely liberating, because it means an off day in the heat is not a sign that your MS is getting worse. It also means that cooling down is not just comforting but actively reverses the effect.

Staying cool in Singapore

Singapore’s persistently hot, humid climate makes heat management a year-round part of living with MS, and air-conditioning is a practical daily tool. Useful cooling strategies include staying in the shade or air-conditioning and using fans; having cool drinks and high-water-content foods; taking cool showers; applying cool water to the wrists, face and neck; and using cooling products such as cooling vests, scarves or wristbands, or cold packs. Wearing loose, breathable, moisture-wicking clothing helps, as does timing activity for cooler parts of the day and exercising in cooler conditions – early morning or evening, an air-conditioned gym, or a cool pool. Staying hydrated throughout is important.

Cooling around exercise

Because exercise raises body temperature, but is also beneficial in MS, a little planning lets you get the benefits while managing the heat. Pre-cooling before exercise – for example with a cool drink or a cool shower – can help, as can exercising in a cool environment, keeping sessions shorter or using intervals, and cooling down afterwards. Swimming in a cool pool is a popular option because the water keeps you cool as you move. The aim is not to avoid activity, which would mean missing out on its real benefits, but to make it comfortable and sustainable in a warm climate. A physiotherapist can help you plan this.

Other sensory symptoms

MS can also cause a range of altered sensations, from numbness and pins-and-needles to burning, itching, tightness or reduced sensation. Neuropathic, or nerve, pain – caused by faulty nerve signals – can feel burning, stabbing or aching. Some people experience the “MS hug,” a tight, band-like squeezing sensation around the chest, ribs or abdomen, caused by spasm of the small muscles between the ribs or altered nerve sensation, which can range from mild to distressing. These sensory symptoms are real and, while they can be unsettling, they are recognised features of MS that your team can help you understand and manage.

Managing sensations and pain

Sensory symptoms and nerve pain are managed with a combination of approaches, described here as options rather than instructions. Identifying and avoiding triggers, including heat, helps, as can loose or gently supportive clothing, stretching and gentle movement, breathing exercises, and applying heat or cold to an area. Psychological therapies such as cognitive behavioural therapy can help with coping with persistent pain, and a doctor may prescribe certain medicines for nerve pain or muscle spasm – these are decided and monitored by your team. Sensory symptoms and neuropathic pain should be assessed by your MS team, who can tailor treatment to you.

When to report a change

While heat sensitivity is temporary and reassuring, there are times to seek advice. A worsening that does not settle after cooling and rest, that keeps getting worse, or that lasts more than about 24 hours, or any new and unexplained symptoms, could be a true relapse or an infection – such as a urinary or chest infection, which can raise body temperature – and should be reported to your MS team for assessment. Sudden, severe or new symptoms, such as sudden vision loss or sudden severe weakness, need urgent assessment, so call 995. And chest tightness assumed to be the “MS hug” should not be self-diagnosed if there are any warning signs of a heart or breathing problem.

Sources

MS Trust – Heat sensitivity (Uhthoff’s phenomenon): mstrust.org.uk
MS Trust – The MS hug: mstrust.org.uk
NHS – Multiple sclerosis symptoms: nhs.uk
National MS Society – Heat and temperature sensitivity: nationalmssociety.org

General information only. This guide supports, but does not replace, advice from your own neurologist and MS care team. Never stop or change a disease-modifying therapy without medical advice. Report new or worsening symptoms that last more than a day to your MS team – but remember that an infection such as a urinary tract infection, or getting too hot, can also worsen MS symptoms temporarily, so these causes should be checked and treated. Call 995 for sudden severe symptoms, such as sudden weakness or drooping of the face, which should be treated as a possible emergency. Be wary of unproven, expensive “cures.” If you are struggling emotionally, you are not alone: the Samaritans of Singapore (SOS) are on 1767 and mindline.sg on 1771.

Heat Sensitivity & Multiple Sclerosis