Why MS symptoms get worse in hot weather — the science and how to protect yourself

June 6, 2026

Up to 80% of people with MS experience temporary worsening of symptoms in heat. Here is why it happens and how to stay safe during summer.

Why heat is MS patients' biggest summer enemy

In conversations with people in the MS community, this is one of the questions that keeps coming back. If you have multiple sclerosis, a sunny day is not always a blessing. For up to 80% of people with MS, even a small rise in body temperature can trigger temporary worsening of symptoms — a phenomenon first described by German ophthalmologist Wilhelm Uhthoff in 1890. He noticed that his MS patients' vision blurred after exercise-induced heat. Today we know this is not a true relapse, but a reversible nerve conduction block.

In countries like Serbia, where summer temperatures regularly exceed 35°C, heat sensitivity can significantly limit daily life. Understanding why it happens and having a concrete action plan can make the difference between hiding indoors all summer and living fully.

The science: why heat blocks nerve signals

Nerve fibers in the central nervous system are wrapped in myelin, a fatty insulation that allows electrical impulses to travel quickly and efficiently. In MS, the immune system strips away this myelin, leaving bare nerve axons exposed.

A normal myelinated nerve can tolerate temperature fluctuations. But a demyelinated nerve is exquisitely sensitive. When core body temperature rises by as little as 0.5°C — from sun exposure, hot showers, exercise, fever, or even emotional stress — the electrical signals travelling through damaged nerves slow down or stop completely.

Why exactly? Heat affects the function of voltage-gated sodium channels on the nerve membrane. In healthy nerves, these channels are densely clustered at the nodes of Ranvier, allowing saltatory (jumping) conduction. In demyelinated areas, these channels become scattered and inefficient. Rising temperature further destabilizes these channels, temporarily blocking conduction. This is called pseudo-conduction block — and it is completely reversible once you cool down.

> Key point: Heat does NOT cause new myelin damage. The worsening is temporary. But it can be frightening and functionally disabling in the moment.

Which symptoms get worse first

Heat affects every person with MS differently, but the most commonly reported worsening symptoms include:

Vision problems — blurred vision, reduced contrast, or temporary vision loss (often in the eye previously affected by optic neuritis). Fatigue — an overwhelming, sudden exhaustion that feels disproportionate to the activity. Muscle weakness — legs feel heavier, walking becomes harder, stairs become impossible. Numbness and tingling — especially in the hands, feet, or torso (the "MS hug"). Cognitive fog — difficulty concentrating, slowed thinking, word-finding problems. Balance and coordination — increased unsteadiness, worse ataxia, higher fall risk. Bladder urgency — increased frequency and urgency of urination.

The pattern often mirrors your existing symptom profile. If optic neuritis was your first MS symptom, heat will likely affect your vision first. If walking is your main challenge, heat will make your legs feel like lead.

Uhthoff phenomenon vs true relapse — how to tell the difference

This distinction is critical for peace of mind and correct medical action.

| Uhthoff phenomenon | True relapse |

|-------------------|--------------|

| Starts within minutes of heat exposure | Develops over hours to days |

| All symptoms improve within 24 hours of cooling | Symptoms persist >24–48 hours |

| No new symptoms — only old ones get worse | May include completely new symptoms |

| No fever (except if caused by infection) | Sometimes accompanied by fever or infection |

| Does not cause permanent damage | May indicate new inflammatory activity |

If your symptoms do NOT improve within 24 hours after cooling down completely, contact your neurologist. It may be a true relapse requiring treatment.

Practical strategies: how to survive a hot summer with MS

Pre-cool before going out

Cooling your body *before* heat exposure extends your functional window significantly. Drink ice-cold water, apply cold packs to wrists and neck for 10 minutes, or take a cool shower before leaving home. Some patients take a brief cold foot bath — cooling the extremities helps lower core temperature.

Wear a cooling vest or neck wrap

Modern cooling vests use phase-change materials (PCM) or evaporative cooling. PCM vests maintain 15–20°C for 2–3 hours and recharge in a refrigerator or ice water. They cost 50–150 EUR and are worth every penny for MS patients in hot climates. Neck cooling wraps are cheaper (10–30 EUR) and cool blood flowing to the brain, which helps with cognitive symptoms and vision.

Time your activities strategically

Plan outdoor activities for early morning (6–9 AM) or evening after 7 PM). Avoid 11 AM–4 PM entirely. If you must go out during peak heat, limit exposure to 15–20 minutes with breaks in air-conditioned spaces.

Stay hydrated — but smartly

Dehydration worsens nerve conduction independently of temperature. Drink cold water or electrolyte solutions every 15–20 minutes during heat exposure. Avoid alcohol and excessive caffeine — both dehydrate and can worsen bladder symptoms. Some MS patients find that cold herbal teas or diluted fruit juices are easier to tolerate than plain water.

Create a cool home environment

Air conditioning is the single most effective home intervention. If AC is not available:

- Use fans with ice bowls in front of them.

- Keep curtains closed during the day; open windows at night.

- Sleep with a cooling mattress pad or gel pillow.

- Take cool (not cold) showers before bed to lower core temperature for better sleep.

Be careful with exercise

Exercise generates internal heat. In summer, shift to:

- Aquatic exercise in cool pools (26–28°C). Water dissipates heat 25 times faster than air and supports your body, reducing fall risk.

- Early morning yoga or stretching in an air-conditioned room.

- Shorter, more frequent sessions instead of one long workout.

- Pre-cooling with a cooling vest 30 minutes before activity.

Know your medication interactions

Some MS medications and common drugs can worsen heat tolerance:

Ampyra (dalfampridine) — can increase sensitivity to heat in some patients. Anticholinergic bladder medications (oxybutynin, tolterodine) — reduce sweating, impairing your body's ability to cool itself. Some antidepressants and antispasmodics — can interfere with thermoregulation.

Discuss summer adjustments with your neurologist if you take these medications.

What about hot baths, saunas and beach holidays

Hot baths, saunas, steam rooms and hot tubs are generally not recommended for heat-sensitive MS patients. Water temperature above 32°C can trigger significant symptom worsening within minutes. If you love baths, keep water at 30°C or below and limit duration to 10–15 minutes.

For beach holidays, plan carefully:

Arrive early morning; leave before noon. Use a beach umbrella or tent with full shade. Bring a portable cooler with ice packs and a spray bottle. Swim in the sea (cooler than heated pools) but monitor how you feel. Rest in an air-conditioned space between activities.

When to seek urgent medical help

Contact your neurologist urgently if:

- Symptoms do not improve within 24 hours of cooling down.

- You develop new symptoms you have never had before (e.g., new weakness, speech problems, severe vertigo).

- Heat exposure is accompanied by fever — infection can trigger both Uhthoff phenomenon and true relapses.

- You experience heat exhaustion symptoms: severe headache, nausea, vomiting, confusion, or fainting.

Heat sensitivity is one of the most common and most manageable challenges in MS. The key is preparation: know your personal triggers, invest in cooling equipment, plan your day around temperature, and never ignore warning signs. With the right strategies, summer can still be enjoyed — just with a little more planning and a cooling vest in your bag.

Sources

Frohman EM et al. Uhthoff's phenomenon in MS — clinical features and pathophysiology. NeuroRx 2005. Davis SL et al. Thermoregulatory dysfunction in multiple sclerosis. Med Sci Sports Exerc 2010. National MS Society — Heat & Temperature Sensitivity: https://www.nationalmssociety.org/. MS Trust — Managing heat sensitivity: https://mstrust.org.uk/. Multiple Sclerosis Society of Canada — Beat the Heat guide. This article is educational and does not replace medical advice. Consult your neurologist for personalized recommendations.