Uhthoff’s phenomenon refers to a worsening of neurologic symptoms when the body becomes overheated. This can occur with a fever but also from exercise, hot showers, hot tubs and saunas, as well as simply from hot weather. The underlying mechanism is that heat can slow down nerve conduction and multiple sclerosis patients already suffer from impaired nerve conduction from the immune system’s attack on the myelin coating on nerves. Importantly, although patients feel worse, the heat is not actually further damaging the nerve and heat does not trigger the immune system to cause inflammation. Patients tend to quickly feel better once their body temperature returns to normal. 

Most patients do not experience heat sensitivity, but for those that do, the summer months can be especially difficult. Avoiding increased body temperature is ideal but often not feasible. Fevers should be investigated for the cause and Tylenol can be helpful in reducing fevers. Hydration with cold beverages can also be useful, as well as resting in shaded areas when outdoors or exercising earlier or later in the day when it’s a little cooler outside. Cooling vests and other equipment can also be used in some patients who are severely affected by the heat.