W. Dalton Dietrich III
by W. Dalton Dietrich III, PhD
I’ve been researching the role of mild hypothermia and hyperthermia on health outcomes for more than 35 years. What the basic and more recent clinical science tells us very clearly is that small variations in brain temperature can impact many secondary pathophysiological events that influence the early and long-term consequences of brain and spinal cord injury.
Strenuous exercise such as cycling or running can potentially raise the body’s core from its normal temperature around 37°C to a state of mild hyperthermia (about 39°C to 40°C, or similar to a mild/moderate fever). The brain heats up, too, and may be warmer than the rest of the body.
Using jugular vein temperature as a surrogate marker for brain temperature, studies have shown that the brain can still significantly be hyperthermic a full hour after ceasing extensive periods of exercise.
This is important because we have clinical evidence documenting the detrimental effects of elevated temperature levels on recovery after moderate to severe traumatic brain injury (TBI). The National Institutes of Health and Department of Defense have been interested in this research, because it means that civilians or soldiers who sustain a brain injury during active periods of strenuous activities in a hot climate might be susceptible to a slower recovery or more damage.
Recent preclinical research suggests that mild hyperthermia may also affect recovery from a concussion, or mild traumatic brain injury (mTBI). In a published study (Sakurai et al.), rats with normal temperature and those whose brain temperature had been raised to 39°C were subjected to a simulated mTBI. The rats with hyperthermia were more likely to have long-term cognitive problems and showed more neuropathological changes in the brain than the rats with normal temperature.
Do humans and rats react the same way to hyperthermia? We don’t know yet, but it seems reasonable to expect they would. The hippocampus, a region of the brain involved in learning and memory, has many similar functions in rats and humans, and learning and memory are both highly vulnerable to mTBI. Current studies in the laboratory are also examining the effects of brain temperature on other systems commonly affected by concussion, including vestibular function.