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Formerly Associated With the Department of Anesthesia and Cardiovascular Research Institute, University of California, San Francisco, and the Veterans Affairs Medical Center, San Francisco, California 94941
Multiple sclerosis is a demyelinating disease of
the central nervous system, characterized by a relapsing and remitting
or, alternatively, by a steady and continuous course of
dysfunction. The hallmark of the disease is heterogeneity. How
the disease presents itself depends on the exact location and total
accumulation of lesions and on the impact of demyelination on nerve
conduction. Conduction maybe be enhanced, producing positive symptoms
like tic douloureux, paresthesiae, or tingling, or conduction may be diminished, producing negative symptoms like blindness and paralysis (7, 8). Diminished conduction may be total or partial,
with decreases in the maximum of frequency or velocity of conduction. During remission, which may last months or years, symptoms improve as
inflammation subsides, sodium channels migrate into the bare axon,
remyelination of the denuded site occurs, and, ultimately, conduction
resumes through the lesion sites. The deficit may also become permanent
as axons degenerate (9). During shorter periods lasting
hours or days, symptoms may also worsen, as conduction through old
lesions fluctuates; conduction is "highly insecure" and
subject to inflammatory factors like nitric oxide and temperature changes (5, 8).
Heat worsens and cooling improves negative symptoms of multiple
sclerosis, sometimes dramatically so. The underlying mechanism relates
to the influence of temperature on sodium channels and on current
necessary for depolarization of the axon. Increases in temperature
diminish the depolarizing current, whereas decreases in temperature
have the opposite effect (8). Sensitivity can be extreme,
and very small changes can have profound effects. Heating, for example
by radiation from the sun, can turn a limping gait into no gait at all.
Alternatively, hot air from a hair dryer can turn a hopeful morning
into an exhausting one. Cooling, on the other hand, by sitting in a
cool bath or shower, can turn the "disease off" (at least for a
little while) and give an individual back the freedom for exercise or work.
As you might expect, the impact of the disease on the thermoregulatory
system takes a myriad of forms, depending on lesion location and
severity. With any location and lesion load, however, the impact of the
disease depends on body temperature, worsening as activity is
diminished in more and more demyelinated axons. In addition,
thermoregulatory impact is worsened by lack of physical fitness and
lowered blood volume. The individual may be suddenly overcome. Getting
into a hot bath may seem a great way to relax, but then suddenly the
individual with multiple sclerosis is too weak to get out. Triggering
such a collapse of the thermoregulatory system explains sudden deaths
associated with sun exposure (1).
This editorial has been prepared to provide insight gained from living
with multiple sclerosis and its heat-related symptoms and from working
many years as a research physiologist. Discussions focus on three areas
where further research may have important implications to those living
and coping with the disease: thermoregulatory failure, the cranial
radiator, and exercise-induced heat.
Thermoregulatory system.
Recently, one study has assessed the thermoregulatory center and its
impact on multiple sclerosis (3). In this study of a
single subject with a history of multiple sclerosis and
thermoregulatory dysfunction, temperature thresholds were calculated as
the core temperature was slowly warmed to sweating and again as the
core temperature was gradually cooled to vasoconstriction and
shivering. It was found that the sweating threshold was a full 1°C
higher than normal and the vasoconstriction threshold was 2°C below
normal, making the calculated sweating-to-vasoconstriction range four times the normal amount. The shivering threshold was also lower than
normal (31.8°C), making the vasoconstriction-to-shivering range more
than twice the normal amount. Providing such information may sound
academic and simplistic; however, this may be an important point to
those with multiple sclerosis, altering them to the extent of
dysfunction, and to others like caretakers and insurance companies, alerting them to the hazards of increased body temperature and the need
for cooling devices.
Cranial radiator.
The brain is one of the most metabolically active tissues, generating
large amounts of heat. Dissipating this heat as well as heat absorbed
from the environment has been a major evolutionary hurdle, one that was
necessary to overcome for the continued development of the brain and
the evolution of humans. Simply put, the brain, like the automobile
engine, could not increase in size without the simultaneous development
of an adequate cooling system. The brain was constrained and could only
develop in parallel to its "cranial radiator" (2).
However, for many with multiple sclerosis, the cranial radiator is
simply not adequate.
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Exercise-induced heat. The final area where research could make a difference is exercise. The first report that vision deteriorated with exercise came in 1890 by the German ophthalmologist Uhthoff. Loss of vision was linked to block of the optic nerve and was associated with exercise and increases in body temperature (6). Subsequently, this phenomenon was expanded to include other negative symptoms as well, setting the stage for the long-standing recommendation given to many individuals with multiple sclerosis: "avoid exercise." However, there is no doubt that exercise programs are beneficial and for a number of reasons; besides the obvious ones, such as improved strength and reduced body fat, there are more subtle ones, such as diminished anger and depression (4).
There is a simple solution to this dilemma, made clear when I was swimming at a local recreational pool. I routinely swam 300-400 yards before fatigue limited my performance. At the end of the swims, getting to the shower without tripping was a serious concern. However, one day the heater broke. Water temperature fell 2-3°C per day, and with each day I felt stronger, faster, and more flexible. My distance swimming also increased. I was not alone. Picking up my daughter from her swim practice, I discovered that the heater in her pool had also broken: while the young swim team members balked at jumping in, one senior club member rejoiced. He, as you might have guessed, had multiple sclerosis. In both cases, we had come across the solution: using cool water temperatures to offset Uhthoff's phenomenon. These observations by themselves mean little, but a recent study confirms this simple solution (10). Further confirmation of these findings is important because keeping in good physical health is the best preventative treatment that can be offered to individuals with multiple sclerosis and the best advice for long-term, cost-effective care. Multiple sclerosis may someday be prevented or cured. However, today's research designed to find a cure or cause offers little solace to the 2.5 million people with the disease. These individuals must accept their disability and manage their symptoms. However, these symptoms would be easier to manage if those with the disease and others understood more about thermoregulation and its impact on multiple sclerosis.| |
ACKNOWLEDGEMENTS |
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D. G. Baker is an emeritus member of the American Physiological Society.
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FOOTNOTES |
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Address for reprint requests and other correspondence: D. G. Baker, 144 Richardson Drive, Mill Valley, CA 94941 (E-mail: dgbaker{at}itsa.ucsf.edu).
10.1152/japplphysiol.01251.2001
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This article has been cited by other articles:
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  A. T. White, S. L. Davis, and T. E. Wilson Metabolic, thermoregulatory, and perceptual responses during exercise after lower vs. whole body precooling J Appl Physiol, March 1, 2003; 94(3): 1039 - 1044. [Abstract] [Full Text] [PDF]
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