Despite having a similar core temperature to those without MS while resting and exercising, people with MS showed a significantly reduced sweat response which may in part explain the heat-related fatigue and other symptoms experienced by people with MS.
The majority of people with MS (over 80%) experience sensitivity to heat, which presents as heat-related fatigue and the worsening of previous MS symptoms such as vision complications, muscle control, bladder or bowel control and pain. As a result, people with MS may avoid exercise and physical activity, particularly in warm conditions, to avoid increasing their core temperature. However, this can prevent people with MS from obtaining sufficient levels of exercise, which has been shown to be beneficial in managing symptoms and improving the general well-being of those with MS.
The worsening of symptoms with increasing core body temperature is also known as Uhthoff’s Phenomenon and is traditionally thought to relate to heat slowing the conduction of nerve signals through areas where myelin has been lost. However, in a MS Research Australia-funded project, Associate Professor Ollie Jay and Ms Georgia Chaseling from the University of Sydney have been investigating whether heat sensitivity in MS might be a result of changes in the ability to regulate core body temperature in MS.
Associate Professor Jay and Ms Chaseling have been using more sensitive and accurate techniques to measure core temperature than used in previous studies, and looked at sweating and blood flow to the skin in people with and without MS at rest and during exercise in carefully climate-controlled conditions. In the first of two papers they showed that people with relapsing remitting MS (RRMS) do not show any difference in core temperature at rest compared to those without MS.
However, building on this study, they then went on to demonstrate that people with MS have a significant reduction in the sweat response while exercising in warm conditions compared to people who do not have MS. While people with MS showed a reduced sweat response, they did not show any impairment in another natural cooling mechanism, blood flow to the skin, compared to people without MS. Despite the reduced sweating, the overall increase in core body temperature as a response to exercise was surprisingly the same as the control group, suggesting that other mechanisms to maintain core temperature may be switched on in people with MS.
While more research is needed to further understand this different temperature regulation response in people with MS, the researchers suggest that damage to the central nervous system (CNS) in MS may result in the signals from the brain not reaching the skin’s cooling systems. The researchers also suggest that having to activate other body mechanisms to keep the core cool may place an extra burden on the body, contributing to the worsening of disease symptoms. Further research is needed to confirm this theory.
A better understanding of the causes and mechanisms behind heat sensitivity in MS will help experts develop the most effective evidence-based recommendations for people with MS to exercise within safe limits and find new ways to mitigate the effects of heat during physical activity.