What is heat illness?
Heat illness encompasses everything from mild heat cramps to life threatening heat stroke. These all occur as our core temperature increases and we become overheated, dehydrated, hyponatremic (low salt) or all of the above. Some authors have hypothesized that heat illness is a failure of our bodies built in protective mechanisms to fend off overheating, dehydration and hyponatremia. They hypothesize that if the system was working correctly, people would stop themselves from exercising, or cause themselves to exercise at a lower intensity, in order to avoid heat stroke or hyponatremia.

Heat cramps:
Heat cramps usually involve muscle cramping that can occur during or after intense or prolonged exercise in the heat. This is seen more frequently in the lower legs. Proposed causes include lack of acclimatization to the heat, low sodium, and dehydration. Treatment involves rest, cooling, massage and hydration. Typically fluids should be taken by mouth if the person is able to eat and drink, and can consist of any cold liquids that contain more than just plain water.

Heat exhaustion:
Heat exhaustion is a complex of symptoms that include fatigue, weakness, lightheadedness, sweating, muscle cramps and occasionally flu like symptoms. What separates this from heat stroke is a lack of significant change in a persons mental status (ie: they are alert and thinking clearly). Typically the body temperature is below 103’F. This needs to be recognized and treated quickly as this can progress to life threatening heat stroke. Athletes with heat exhaustion should stop exercising, cool off and hydrate. Again, hydration in this case can usually be done with drinking cold fluids. Return to racing or exercise that day should be avoided.

Heat stroke:
As stated above, what separates heat exhaustion from heat stroke is a significant change in mental status. These people are usually very confused, disoriented, or agitated with early symptoms. With severe heat stroke people can be in a come or have a seizure. Heat stroke is usually not seen with a body temperature below 105’F. This is a medical emergency and needs to be treated with rapid cooling under close supervision. These people will generally need fluid by vein. Anyone with a history of heat stroke is a risk for another episode.

Prevention:
Again, the exact cause of heat illness has not been clearly shown. It is recommended that athletes try to avoid heavy exercise in extreme heat conditions by exercising early or later in the day, thereby avoiding the hottest mid day hours. Maintaining adequate hydration is essential. There is no one formula for fluid replacement with exercise, especially in light of the serious issues with hyponatremia. The best way to measure fluid intake need is to compare a pre and post exercise weight. Based on this you can calculate how much you are sweating over an hour period, and as a result, how much fluid you need to replace. Weighing yourself regularly before and after a workout will help you properly assess you fluid needs. It is very import to acclimate to the heat. If you have an early season race in a hot climate and are coming from a colder area, it would be ideal to have at least 10-14 days to acclimate to the heat prior to competition. Also, don’t forget sun screen, sun glasses and a light hat to avoid the effects of sun damage.

Mark Snowise, M.D.

Amy Miller Bohn, M.D.
Clinical Assistant Professor
University of Michigan
Department of Family Medicine
1801 Briarwood Circle
Ann Arbor, MI 48108
(734) 998-7390