Uhthoff's phenomenon or Uhthoff's sign is the temporary worsening of MS symptoms caused by an increase in temperature. It is usually applied to optic neuritis and other visual symptoms but can also refer to fatigue, pain, balance, weakness, bladder issues, cognitive or sensory symptoms.
The condition is named after Wilhelm Uhthoff, a German neuro-opthamologist, who first described it in 1890. He noticed that some of his patients reported that their visual problems got worse after exercising. At the time, Uhthoff linked the worsening symptoms to the exertion of exercise, but we now understand that the effect is caused by an increase in body temperature.
Increased body temperature could be caused by hot weather, central heating, an illness or fever, as well as from physical exertion. Studies have shown that a very small rise in temperature is enough to block or slow the conduction of a nerve impulse along a nerve that has suffered demyelination from MS damage. This can make your existing symptoms worse, but once your body temperature returns to normal, the symptoms generally improve.
You might consider temperature sensitivity if you notice your usual symptoms worsening. You may have an underlying infection you are not aware of, such as gum disease or a urinary tract infection. If you notice any new symptoms, you should mention them to your health team.
Uhthoff's phenomenon need not stop you exercising or exerting yourself physically, but you may need to think about staying cool while you do. Drinking cold drinks before and during exercise, exercising in cool water or while wearing active cooling clothing can all alleviate the issue. We have more details in our page on starting to exercise.