Posted tagged ‘hyperthermia’

Fever part 2: Why?

June 8, 2009

Normal body temperature is regulated in an area at the base of the brain called the hypothalamus. By the release of hormones and neurologic signals, the hypothalamus can instruct the body to raise its temperature (by gearing up heat-producing metabolic pathways), or lower the temperature (by shivering. sweating, and increasing blood flow to the skin.) Conceptually, we think that the hypothalamus has a very sensitive temperature “set-point”, and that it continuously monitors the temperature of the blood to make adjustments in the body, keeping the temperature as close to the set-point as possible.

Fever occurs when the hypothalamic set-point is temporarily changed. This can occur via a variety of mechanisms, often involving molecules called “pyrogens” than can be released by a variety of immune cells in the body. Pyrogens are also found as components of certain bacteria.

“Hyperthermia” occurs when the body’s temperature rises above normal, but it is different from fever. In fever, the hypothalamus’ set-point is altered, and the body “wants” to get to the new set temperature by using normal physiologic mechanisms. In hyperthermia, excess heating occurs because heat from the environment is so high and sustained that it overwhelms the body’s ability to cool itself off. Think of a football player in full gear in August—it might be 100 degrees out, and if dehydration occurs there won’t be any sweat left to cool the body off.  The hypothalamus wants to cool off, but there isn’t any way to do this. In hyperthermia, the body’s temperature can get so high that tissue damage occurs. The best way to avoid hyperthermia is to consume plenty of fluids, and to get out of the hot sun at the first signs of heat illness (often nausea, headaches, and warm & dry skin.)

Unlike in hyperthermia, during a fever there is no outside source of heat, and the body cannot heat up enough to cause itself damage. As long as a child or adult with a fever stays well hydrated, the fever itself will not cause any harm, though it can make the person feel lousy.

So what’s the point of a fever—why does the body have mechanisms in place to raise the set-point? There is evidence that fever may be beneficial in fighting at least some infections. Fever allows white blood cells to proliferate faster, and helps them migrate through tissue better. It also increases the virus-fighting ability of interferons, and may decrease the ill effects of some bacterial toxins. It’s also thought that an overly warm environment may be less hospitable to bacteria that have developed in a way to thrive best at normal body temperatures. However, despite these observations there isn’t strong data that fevers overall really change the outcome of most infections, at least in the developed world where general health, nutrition,  and access to health care is good.

If fever is at least theoretically helpful in fighting infections, should we be using medicines to reduce fevers? I think so. Fever does cause some tangible ill effects in people, especially children. It increases fluids losses (contributing to dehydration), and increases metabolic energy demands. More importantly, though, it makes people feel bad. People with fevers can be achy and miserable. In my mind, helping the child feel better trumps any theoretical benefit from allowing the fever to continue untreated.

Most commonly, fever is treated by using medications to lower the hypothalamic set-point back towards normal. This is how ibuprofen and acetaminophen work. Alternatively (or in addition), parents can try to help their children reduce excessive heat by using cool (not cold) packs, or bathing in tepid water (never cold water, and never alcohol.)

Fevers occur when the body decides to reset the temperature at a higher level, most commonly as part of fighting an infection. It may serve a useful purpose, though the evidence for a practical benefit in most circumstances is sparse. Ordinary fevers can’t hurt anyone, but if your child feels bad with a fever, using medicine to help him feel better is a good idea.