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What Happens to the Brain During a Heatstroke?

  • Published26 Jun 2020
  • Author Michael W. Richardson
  • Source BrainFacts/SfN
Man and heat stroke
iStock.com/PraewBlackWhile

The body is a fine-tuned machine. Like any machine, it breaks down if the environment becomes too extreme. Our internal systems operate best at 98.5 degrees Fahrenheit (36.9 degrees Celsius). Under healthy conditions, the body manages this through thermoregulation. The brain and other organs collaborate to maintain the temperature. But when the body is exposed to extreme temperatures for a prolonged period, it can’t keep up.

High-temperature environments can lead to exhaustion, dehydration, and even fainting. Once the body reaches 104 degrees Fahrenheit (40 degrees Celsius), heatstroke can occur. A cascading series of bodily failures, heatstroke can cause dizziness, nausea, confusion, and cell death. If body temperature isn’t lowered quickly, there could be lasting damage to the brain and nervous system.

We spoke with Thomas Clanton at the University of Florida College of Human Health and Performance about heatstroke, its warning signs, how it affects the brain, and how to avoid it on a hot day.

How does the body regulate temperature?

The hypothalamus, located at the base of the brain, controls this function. It receives information from temperature sensors on the skin and other parts of the body to monitor temperatures and ensure everything runs smoothly. The hypothalamus regulates the body’s response to rising or falling temperatures. If body temperature drops, the hypothalamus makes muscles quickly contract and release. That’s what we experience as shivering. With rising body temperatures, that heat must be dispersed outside of the body. The hypothalamus signals sweat glands to produce sweat, creating a cooling effect as it evaporates. It can also affect blood vessels near the skin, so excess heat in the bloodstream can move to cooler air outside of the body through the skin.

What is heatstroke?

Heatstroke occurs after the body reaches an internal temperature of 40 degrees Celsius, or 104 degrees Fahrenheit and when there are clinical signs of loss of neural function and organ damage. While some effects may set in before reaching that temperature, we consider that the magic number. 

Exercise is an important contributor to overheating, especially for athletes playing in hot conditions. Active muscles create a lot of heat. That much heat can overcome the body’s ability to expel it. Passive heatstroke — which occurs when the body is at rest — mostly affects the elderly, especially during sustained heat waves.

At high internal temperatures, blood flow becomes a major issue. Heat and exercise make blood vessels dilate, so the heart works harder to keep blood flowing. Elevated heart rates, especially when you’re not exercising, can be a warning sign that body temperature is too high. Like any muscle, the heart can only work so hard before it’s exhausted. At the late stages, the cardiovascular system begins collapsing. Heart rate plummets, and blood flow slows. Without adequate blood flow to the brain, people may experience confusion or difficulty focusing, and light-headedness. Your brain is telling you to slow down and lie down, so the heart doesn’t have to work as hard.

The hypothalamus also slows the activity of certain organs to prioritize the heart’s and lungs’ energy needs. The gut, kidneys, and other organs not directly connected to respiratory or cognitive functions become less active as body temperature rises and blood flow can plummet, resulting in ischemia. In the most severe cases, these organs can be permanently damaged. Sepsis can result from severe heatstroke due to the loss of the integrity of the gastrointestinal tract during heat exposure and can cause many cases of fatal heatstroke.

How does heatstroke affect the brain?

If the heart is pushed too hard, it can stop effectively working, cutting off blood flow to the brain and other organs. Lack of oxygen to the brain causes fainting, a common side effect of heatstroke.

At 40 degrees Celsius, the blood-brain barrier begins to break down. This barrier separates the brain tissue from the bloodstream, keeping out unwanted particles and bacteria while allowing in the oxygen and nutrients the brain needs. At high temperatures, unwanted proteins and ions can build up in the brain, often causing an inflammatory response and negatively affecting normal functioning.

Also, high temperatures can cause cell death. As temperature rises, proteins can unfold, which can kill cells. Some cells are more susceptible than others, and this affects what brain regions are damaged. The cerebellum is one of the first to start failing. This area controls motor functions, among other tasks. Marathon runners suffering from heat illness may sway or collapse as their muscle control fails. The cerebellum has a high concentration of Purkinje cells, which seem sensitive to high temperatures, though we’re not sure why. At high enough temperatures, many different cells begin dying off.

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