The long-term effects of heatstroke on the body

This summer, huge swaths of the U.S. have already faced record-breaking heat waves. Heat kills more people than any other extreme weather event, and deadly heat waves are getting longer and hotter as the climate warms.

Staying cool – and informed – is essential. So we spoke with Thomas Clanton, a professor of applied physiology and kinesiology at the University of Florida and an expert in the effects of heat on the body, about how to recognize heat illness and the long-term effects of heatstroke.

Heatstroke is a medical emergency. If you notice signs of heatstroke in a person, call 911 immediately.   

What is heat illness?

It's a really broad spectrum. At the lowest end is heat exhaustion, and on the more extreme end we have heatstroke. The difference is really the presence of neurological symptoms in heatstroke.  Throughout the spectrum, mild to severe injury to liver, heart, kidney and muscle can be present. So, you can have heat exhaustion and you're probably still thinking pretty well, but you know you're hot. You try to get out of the heat and you're functional. However, heatstroke victims can go unconscious, lose motor control or become delirious, so their ability to respond is limited.

Clinically, a person would be diagnosed with heatstroke if they have a temperature above 40 degrees centigrade (104 degrees Fahrenheit) and also exhibit central nervous system symptoms.

What are some of the signs that someone needs to cool down?

Other signs that people notice include pallor (paleness) of the skin.  Whereas profuse sweating is a normal reaction to heat, at the extremes of heatstroke the sweat response doesn't work as well, and the skin can become dry. If you begin to notice these signs, get into the shade, drink plenty of water and move to a reclined position. If ice bags or wet towels are available, place them under the arms, on the neck and along the groin regions. If any unusual neurological symptoms develop, get medical assistance immediately.

A lot of times people in the “heat exhaustion” range may not know they are getting heat illness. I think that's one of the concepts worth emphasizing. Besides just feeling hot, an individual may feel a little “woozy” or just “not themselves.” When this occurs, and they are not well hydrated, they can move quickly to conditions of heatstroke. Heatstroke can develop rapidly and it often mistaken for just normal overheating and exhaustion, so it pays to be aware of the clinical symptoms and to act quickly.

Since people can’t always tell when they’re getting sick, does that mean it’s a good idea to have a friend along so you can look after each other?

Absolutely. A buddy can get help, call the ambulance, right? And you can help each other stay healthy. Coaches can play this role during workouts and team players up. I play golf in the summer, and my partner and I make sure each other drinks plenty of water and stays in the shade whenever possible. We bring wet towels for our shoulders. These techniques are really effective and pretty simple.

What do we know about the long-term effects of heatstroke?

We rightfully worry about people dying from heatstroke. But the evidence in the last few years is extremely good that some people who experience heatstroke may have medical consequences that can affect them the rest of their life.

The field has documented changes in the immune system of humans and animals years after a heatstroke. Heatstroke victims also have a greater frequency of developing chronic heart disease and kidney diseases later in life.

What causes these long-term effects of heatstroke?

In the animals I study, we see evidence of epigenetic changes that likely explain some of these long-term effects. Epigenetics is kind of cellular memory. So at a cellular level, cells have their own way of remembering if they've been exposed to severe stresses in the environment, which can help them respond over time by altering their cellular responsiveness. Cells imprint this memory by using enzymes that chemically tag their DNA. This memory is often helpful and can be adaptive, but can also be maladaptive if the stress is severe.

We certainly see strong epigenetic signals in the hearts, immune cells and skeletal muscle of mice one month after heatstroke. The mice look fine, their hearts look fine, but later they begin to develop metabolic disorders and other secondary effects. We believe that many of these epigenetic changes are maladaptive and make the animals less able to withstand additional stresses in their environment or to fight off other chronic forms of disease. Once we understand this in our animal models, we hope to develop approaches in humans that will help them ward off the development of these long term consequences to severe heat exposures.