Abnormal perceptions may help cause heat sensitivity in MS: Study

Abnormal perceptions of discomfort and fatigue likely play a role in causing increased heat and cold sensitivity to extreme temperatures among people with multiple sclerosis (MS), a new study shows.

However, the results suggest that MS does not generally affect how the body regulates its own temperature.

“Our findings indicate that neuropsychological factors (e.g. discomfort and fatigue) could contribute to MS heat and cold intolerance in the absence of deficits in the control of body temperatures,” the researchers wrote, though they noted that “the underlying mechanisms of MS heat and cold intolerance remain unclear.”

The study, “Body temperatures, thermal comfort, and neuropsychological responses to air temperatures ranging between 12°C and 39°C in people with Multiple Sclerosis,” was published in Physiology & Behavior.

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The vast majority of people with MS experience heat intolerance, or an increased sensitivity to hotter temperatures, in which even a slight increase in body temperature — whether due to exercise, fever, or the weather — causes a worsening of symptoms. Some MS patients also experience a worsening of symptoms when it’s very cold.

“The adverse effects of heat and cold on MS have been known for over 100 years,” the researchers wrote. However, the team said, it’s still not fully understood why this temperature sensitivity occurs.

In particular, there has been little research on how MS patients’ subjective perceptions of their own comfort affect their response to different temperatures, and whether this corresponds to measurable changes in internal body temperatures.

“We still lack systematic investigations on the relationship between air temperature, body temperatures, and neuropsychological status in people with MS exposed to a broad range of heat and cold stress,” the researchers wrote.

As a first step toward filling this gap in knowledge, a team in the U.K. assessed, in a small study, how body temperature and self-reported comfort vary in response to changing temperatures. Their study involved 12 people with MS and 11 individuals without the disease, who served as controls. The controls were matched to the patients in terms of sex and age.

All of the participants took part two 50-minute trials — one for heat and one for cold — in a temperature-regulated chamber. In the heat trial, the chamber’s temperature was gradually increased to 39 C (about 102 F), while in the cold trial, the temperature was gradually lowered to 12 C (about 54 F).

In both trials, participants’ body temperatures were tracked with skin and rectal thermometers, and participants regularly reported on their own fatigue and comfort.

“We hypothesised that people with MS would present greater changes in body temperatures along with greater increases in thermal discomfort and self-reports of fatigue than healthy counterparts for a given change in air temperature,” the researchers wrote.

The results showed that body temperatures changed similarly, though minimally, among the MS patients and the controls. For both groups, there was a slight increase in body temperature during the heat trial and a slight decrease during the cold trial. However, there was no notable difference between the groups in the magnitude of these changes.

Measures of fluid loss, heart rate, and blood pressure similarly did not differ between the MS patients and the controls.

These data “indicate that individuals with MS do not appear to ‘heat up’ any faster than individuals without MS,” the researchers wrote, noting that the finding is “contrary to our initial hypothesis on body temperature responses.”

We believe that our findings provide evidence for a candidate, alternative mechanism for heat intolerance in MS, that is an MS-specific heightened sensitivity to thermal discomfort.

The team noted their findings “do not dispute the well-established … role” of small increases in temperature “triggering a worsening of MS symptoms.”

Instead, “we believe that our findings provide evidence for a candidate, alternative mechanism for heat intolerance in MS, that is an MS-specific heightened sensitivity to thermal discomfort.”

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But heat sensitivity shows in subjective experience of patients versus controls

Despite comparable changes in body temperatures, there were notable differences between the groups in the subjective experience of comfort at extreme temperatures.

Specifically, by the end of the heat trial, the vast majority (83%) of MS patients reported that they were uncomfortable or worse, whereas only about one-third of controls (36%) said they were uncomfortable or worse after the heat trial. Similarly, 87% of MS patients and 28% of controls reported feeling hot or beyond by the end of the heat trial. Rates of discomfort in the cold trial were similar in both groups.

“In support of our initial hypothesis on perceptual responses, our thermal sensation and comfort data indicated that our MS participants were more likely to experience thermal discomfort than their healthy counterparts for a given change in body temperatures, although this applied to body heating only,” the researchers wrote.

Self-reported mental and physical fatigue scores were similar among MS patients and controls when measured before the trials at room temperature. However, MS patients’ fatigue scores tended to worsen more substantially in both the heat and cold trials, with the most profound worsening generally seen for MS patients in the heat trial.

“These findings further supports our hypothesis that thermal stress, and the heat in particular, exacerbates fatigue levels (both mental and physical) in MS,” the researchers wrote.

Before and after each 5o-minute trial, participants completed a test of cognitive function. Results showed the MS patients tended to perform more poorly than the controls, but the results were not significantly impacted by temperature changes in either trial.

Altogether, these data suggest that MS patients experience an unusual sensitivity to temperature changes, particularly heat, that goes beyond the effects of changes in body temperature.

This may imply that MS can cause abnormalities in the neurological systems that the body uses to sense its own temperature and trigger feelings of temperature-related discomfort, the researchers said, though they stressed that further research is needed to fully understand the mechanisms at play.