The brain gets warmer than we thought it did, says S.Ananthanarayanan.

The body temperature was long believed to be something unchanging, except in sickness. As there was evidence that the working of brain cells is sensitive to temperature, the conviction followed that the brain stays carefully temperature-controlled.

Nina M. Rzechorzek, Michael J. Thrippleton, Francesca M. Chappell, Grant Mair, Ari Ercole, Manuel Cabeleira, with Jonathan Rhodes, Ian Marshall and John S. O’Neill, at institutes and the universities at Cambridge and Edinburgh, UK, however, find that the brain stays a little warmer than the rest of the body. In their paper in the OUP journal, Brain, the team describe studies show that parts of the brain are warmer than others, and a regular rise and fall of temperature, over time, is a characteristic of the healthy brain

The temperature of the healthy body was an important constant in earlier times. Three centuries ago (1724), when the Dutch physicist, Daniel Gabriel Fahrenheit proposed a scale for measuring temperature, the lower fixed point, 0⁰F, was the temperature of ice mixed with ammonium chloride. But the upper fixed point was the average temperature of the human body. The Fahrenheit scale is now based on the freezing and boiling points of water, and this shifts the human body temperature to 98.6⁰F. The point, however, is that body processes are known to work at steady temperatures.

The way the body’s system controls temperature is by managing its surface temperature – when the body is warm, the blood vessels at the surface of the body grow wider, so that the body radiates the heat away. And sweat glands moisten the skin, for cooling by evaporation. And in cold weather, the blood vessels at the surface constrict, to keep the surface cooler, to lose less heat. But in sickness, when body temperature rises, or in very hot weather, when the body is not able to cool, a rise of 5-6⁰F, or 3-4⁰C, can make a person delirious and can lead to death.

Some animals that live in the hottest places regularly face temperatures of 45-50⁰C. This make temperatures run high, with danger to the brain. These animals have hence developed special strategies to keep the brain cool. The Arabian Oryx is an animal that lives in water-scarce deserts and its body temperature is known to touch 45⁰C, which no animal is known to survive. But the Arabian Oryx has evolved with a separate, ‘refrigerated’ blood supply for its brain. Vessels carrying blood to the brain pass through the nasal cavity, where carefully rationed moisture is allowed to evaporate and cool the blood before it reaches the brain!

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Despite the evident sensitivity of temperature as indicator and result of infection or injury, the paper by the team in Cambridge and Edinburgh says, we have little information about temperature patterns within the healthy brain. This is mainly because measuring temperature within the brain would invasive, the paper says, and what we know is thanks to brain-injured patients, where probes have been inserted. We hence go on with the assumption that the brain temperature “matches the body core.” Although it is accepted that the temperature of the brain and its relationship with the temperature of the body, as well as the sensitivity of brain tissue, are altered when there is injury, because of lack of information, “application of targeted temperature management in neurocritical care remains highly controversial,” the paper says.

There is evidence, however, that the temperature of the brain varies within a narrow range, over time and at different parts of the brain. It is known, for instance, that the brain temperature 1-2⁰C lower during sleep, when there is more blood flow to the brain. Direct measurements in animals have also shown that the brain core is warmer than the surface, the paper says, and that the temperature goes up and down just like body temperature, over the 24 hours in a day.

Abnormal temperature in the brain may prove to be powerful indicator of brain disorders, the paper says. And now, fortunately, it has become possible to observe, with fine resolution, the temperature in the interior of the brain, with the help of magnetic fields and radio waves. Nuclear Magnetic Resonance Spectroscopy is a method of detecting how strongly the atoms molecules within tissue absorb or emit radio waves of different frequencies. While this information helps create 3-D images of the interior of organs, the fact that the frequencies change with temperature also makes it possible to create, non-invasively, this is, with not surgery, or insertion of probes, a 3-D temperature model of the human brain.

The technique, the paper says, has been found useful as a research tool, but has not been applied to see how temperatures change, over time. In the current study, the team collected temperature data from two sources. One was existing data of over a hundred patients who had suffered brain injury and had undergone treatment and investigation. The second lot was a group of 40 healthy adults, whose brains were scanned using Nuclear Magnetic Resonance, morning, afternoon and late evening, over the course of a day. The individual characteristics of these volunteers, that is, at what time their brain activity peaked, and genetic and lifestyle differences were also factored in the analysis of brain temperature behavior.

The main finding was that the average brain temperature, at 38.5⁰C, was over 2⁰C warmer than the body temperature, as measured using a thermometer under the tongue. The highest temperature was, in fact, often higher than 40⁰C. And then, the temperature did not remain steady, but changed, over a range, about 1⁰C , through the day. And again, as one moved from one part of the brain to another. The patterns were also different for men and women, women’s brains being about 0.4⁰C warmer, and in the case of women, according to the stage of the menstrual cycle. And then the average temperature was lower with younger persons, although the range of variation was wider.

A significant feature of with the persons who had suffered brain injury was that the variation of the temperature, over the length of the day, was diminished in the patients who had the worst outcome. Of the 100 patients in this category, the paper says, only 4% of those who had the daily rhythm died in the ICU, as against 27% of those who did not display variations of temperature. Monitoring the pattern of brain temperature may hence be an important aid in the treatment of brain injury patients. “Using the most comprehensive exploration to date of normal human brain temperature, we have established ‘HEATWAVE’ – a 4D temperature map of the brain. The map provides an urgently needed reference resource against which patient data can be compared, and could transform our understanding of how the brain works,” says Dr Nina M. Rzechorzek, who led the study.

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