April 11, 2023 | News | Living with MS

What is Uhthoff’s phenomenon?

People with MS are often cautioned against heat exposure (e.g. hot bath, sauna) because it may worsen symptoms. This is called Uhthoff’s phenomenon. The name comes from a German ophthalmologist, Wilhelm Uhthoff, who noticed vision changes in people with MS after they had exercised. This led to the hot-bath test to diagnose MS, a practice only abandoned in the 1980s when laboratory tests and MRI came into use.

What causes the phenomenon is an increase in body temperature – as little as 0.5 degrees Celsius (or 1 degree Fahrenheit) in core body temperature. Becoming overheated from exercise can cause it, but more common culprits are hot baths or showers, hot tubs, saunas, sunbathing, hot meals/beverages and fevers (including those caused by infection). A less obvious cause is menstruation. During the luteal phase of menstruation (i.e. the two weeks before the start of a period), a woman’s body temperature increases about 0.3 to 0.7 degrees Celsius (about 1 degree Fahrenheit) – so within the range needed to trigger Uhthoff’s phenomenon. Oral contraceptives have varying effects on body temperature depending on the amount of progesterone (which increases body temperature) and estrogens (which lower body temperature) (Baker and colleagues. Temperature 2020;7:226-262).

Although over a century has passed since Uhthoff’s phenomenon was first described, the cause has not been determined. The most common theory is that temperature affects the ability of damaged nerves to transmit electrical signals. As evidence is a study that showed that higher temperatures slowed signalling in the optic nerve, an effect that was reversed when body temperature was lowered (Frohman and colleagues. Ann NY Acad Sci 2011;1233:313-319). However, other factors are likely involved since some triggers, such as stress or cigarette smoking, are not generally associated with a raised body temperature.

The main effect of Uhthoff’s phenomenon is a temporary worsening of MS symptoms, such as vision problems, fatigue, muscle weakness and pain (Panginikkod and colleagues. StatPearls, October 24, 2022). The onset of symptoms can seem like a relapse, prompting many people to call their MS clinic. However, this temporary worsening is not a true relapse – there is no inflammation in the brain and spinal cord – and will generally improve within a day. Lowering body temperature with a cool bath/shower, ice packs, air conditioning, cold drinks and cooling garments can help to ease symptoms. If the underlying cause is an infection (such as a urinary tract infection), it is important to get treated by a family doctor to lower the fever and clear the infection.

Uhthoff first associated his phenomenon with exercise – but that is not a good reason to avoid physical activity. If exercising outdoors in the summer, it is best to go when it is cooler rather than at mid-day. Be sure to remain hydrated. You can cool down afterward with a lukewarm shower.

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