Hyperthermia
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Hyperthermia is an elevated body temperature due to failed thermoregulation. Hyperthermia occurs when the body produces or absorbs more heat than it can dissipate. When the elevated body temperatures are sufficiently high, hyperthermia is a medical emergency and requires immediate treatment to prevent disability and death.
The most common causes are heat stroke and adverse reactions to drugs. Heat stroke is an acute condition of hyperthermia that is caused by prolonged exposure to excessive heat and/or humidity. The heat-regulating mechanisms of the body eventually become overwhelmed and unable to effectively deal with the heat, causing the body temperature to climb uncontrollably. Hyperthermia is a relatively rare side effect of many drugs, particularly those that affect the central nervous system. Malignant hyperthermia is a rare complication of some types of general anesthesia.
Hyperthermia can be created artificially by drugs or medical devices. Hyperthermia therapy may be used to treat some kinds of cancer and other conditions, most commonly in conjunction with radiotherapy.[1]
Hyperthermia differs from a fever in the mechanism that causes the elevated body temperatures: a fever is caused by a change in the body's temperature set-point.
The opposite of hyperthermia is hypothermia, which occurs when an organism's temperature drops below that required for normal metabolism. Hypothermia is caused by prolonged exposure to low temperatures and is also a medical emergency requiring immediate treatment.
Signs and symptoms
The normal human body temperature in a healthy adult can be as high as 37.7°C (99.9°F) in the late afternoon.[2] Hyperthermia requires an elevation from the temperature that would otherwise be expected. Such elevations range from mild to extreme; body temperatures above 40°C (104 °F) can be life-threatening.
Hot, dry skin is a typical sign of hyperthermia.[2] The skin will become red and hot as blood vessels dilate in an attempt to increase heat dissipation, sometimes leading to swollen lips. An inability to cool the body through perspiration causes the skin to feel dry. Fever, by contrast, frequently produces cool, damp skin.
Other signs and symptoms vary depending on the cause. The dehydration associated with heat stroke can produce nausea, vomiting, headaches, and low blood pressure. This can lead to fainting or dizziness, especially if the person stands suddenly.
In the case of severe heat stroke, the person may become confused or hostile, and may seem intoxicated. Heart rate and respiration rate will increase (tachycardia and tachypnea) as blood pressure drops and the heart attempts to supply enough oxygen to the body. The decrease in blood pressure can then cause blood vessels to contract, resulting in a pale or bluish skin color in advanced cases of heat stroke. Some victims, especially young children, may have seizures. Eventually, as body organs begin to fail, unconsciousness and coma will result.
Pathophysiology
Hyperthermia: Characterized on the left. Normal body temperature (thermoregulatory set-point) is shown in green, while the hyperthermic temperature is shown in red. As can be seen, hyperthermia can be conceptualized as an increase above the thermoregulatory set-point.
Hypothermia: Characterized in the center: Normal body temperature is shown in green, while the hypothermic temperature is shown in blue. As can be seen, hypothermia can be conceptualized as a decrease below the thermoregulatory set-point.
Fever: Characterized on the right: Normal body temperature is shown in green. It reads "New Normal" because the thermoregulatory set-point has risen. This has caused what was the normal body temperature (in blue) to be considered hypothermic.
A fever occurs when the body sets the core temperature to a higher temperature, through the action of the pre-optic region of the anterior hypothalamus. For example, in response to a bacterial or viral infection, the body will raise its temperature, much like raising the temperature setting on a thermostat.
In contrast, hyperthermia occurs when the body temperature is raised without the consent of the heat control centers.
Causes
Heat stroke
Heat stroke is due to an environmental exposure to heat, resulting in an abnormally high body temperature.[2] In severe cases, temperatures can exceed 40 C.[3] Heat stroke may be exertional or non-exertional, depending on whether the person has been exercising in the heat. Significant physical exertion on a very hot day can generate heat beyond a health body's ability to cool itself, because the heat and humidity of the environment reduces the efficiency of the body's normal cooling mechanisms.[2] Other factors, such as drinking too little water, can exacerbate the condition. Non-exertional heat stroke is typically precipitated by medications that reduce vasodilation, sweating, and other heat-loss mechanisms, such as anticholingeric drugs, antihistamines, and diuretics.[2] In this situation, the body's tolerance for the excessive environmental temperatures can be too limited to cope with the heat, even while resting.
Drugs
Some drugs cause excessive internal heat production, even in normal temperature environments.[2] The rate of drug-induced hyperthermia is higher where use of these drugs is higher.[2]
- Many psychotropic medications, such as selective serotonin reuptake inhibitors (SSRIs), monoamine oxidase inhibitors (MAOIs), and tricyclic antidepressants, can cause hyperthermia.[2] Serotonin syndrome often presents following exposure to multiple drugs. Similarly, neuroleptic malignant syndrome is an uncommon reaction to neuroleptic agents.[4] These syndromes are differentiated by the other associated symptoms, such as tremor in serotonin syndrome and "lead-pipe" muscle rigidity in neuroleptic malignant syndrome.[2]
- Many illicit drugs, including amphetamines,[5] cocaine,[6] PCP, LSD, and MDMA can produce hyperthermia as an adverse effect.[2]
- Malignant hyperthermia is a rare reaction to common anesthetic agents (such as halothane) or a reaction to the paralytic agent succinylcholine. Malignant hyperthermia is a genetic condition, and can be fatal.[2]
Other
Other possible, but rare, causes of hyperthermia are thyrotoxicosis and the presence of a tumor on the adrenal gland, called a pheochromocytoma, both of which can cause increased heat production.[2] Damage to the central nervous system, such as from a brain hemorrhage, a severe, uncontrolled epileptic seizure called status epilepticus, and other kinds of damage to the hypothalamus can also cause hyperthermia.[2]
Diagnosis
Hyperthermia is generally diagnosed in the presence of an unexpectedly high body temperature and a history that suggests hyperthermia instead of a fever.[2] Most commonly this means that the elevated temperature has appeared in a person that was working in a hot, humid environment (heat stroke) or that was taking a drug for which hyperthermia is a known side effect (drug-induced hyperthermia). The presence of other signs and symptoms related to hyperthermia syndromes, such as the extrapyramidal symptoms that are characteristic of neuroleptic malginant syndrome, and the absence of signs and symptoms more commonly related to infection-related fevers, are also considered in making the diagnosis.
If fever-reducing drugs lower the body temperature, even if the temperature does not return entirely to normal, then hyperthermia is excluded.[2]
Treatment
Treatment for hyperthermia depends on its cause, as the underlying cause must be corrected. Mild hyperthemia caused by exertion on a hot day might be adequately treated through self-care measures, such as drinking water and resting in a cool place. Hyperthermia that results from drug exposures is frequently treated by cessation of that drug, and occasionally by other drugs to counteract them. Fever-reducing drugs such as paracetamol and aspirin have no value in treating hyperthermia.[2]
When the body temperature is significantly elevated, mechanical methods of cooling are used to remove heat from the body and to restore the body's ability to regulate its own temperatures.[2] Passive cooling techniques, such as resting in a cool, shady area and removing clothing can be applied immediately. Active cooling methods, such as sponging the head, neck, and trunk with cool water, remove heat from the body and thereby speed the body's return to normal temperatures. Drinking water and turning a fan or dehumidifying air conditioning unit on the affected person may improve the effectiveness of the body's evaporative cooling mechanisms (sweating).
Sitting in a bathtub of tepid or cool water (immersion method) can remove a significant amount of heat in a relatively short period of time. However, immersion in very cold water is counterproductive, as it causes vasoconstriction in the skin and thereby prevents heat from escaping the body core.
When the body temperature reaches about 40 C, or if the affected person is unconscious or showing signs of confusion, hyperthermia is considered a medical emergency that requires treatment in a proper medical facility. In a hospital, more aggressive cooling measures are available, including intravenous hydration, gastric lavage with iced saline, and even hemodialysis to cool the blood.[2]
Epidemiology
The frequency of environmental hyperthermia can vary significantly from year to year depending on factors such as heat waves.
See also
References
- ^ Information from the U.S. National Cancer Institute
- ^ a b c d e f g h i j k l m n o p q r Fauci, Anthony; et al. (2008). Harrison's Principles of Internal Medicine (17 ed.). McGraw-Hill Professional. pp. 117–121. ISBN 9780071466332.
{{cite book}}: Explicit use of et al. in:|author=(help) - ^ Tintinalli, Judith (2004). Emergency Medicine: A Comprehensive Study Guide, Sixth edition. McGraw-Hill Professional. p. 1187. ISBN 0071388753.
{{cite book}}: Cite has empty unknown parameter:|coauthors=(help) - ^ Tintinalli, Judith (2004). Emergency Medicine: A Comprehensive Study Guide, Sixth edition. McGraw-Hill Professional. p. 1818. ISBN 0071388753.
{{cite book}}: Cite has empty unknown parameter:|coauthors=(help) - ^ Marx, John (2006). Rosen's emergency medicine: concepts and clinical practice. Mosby/Elsevier. p. 2894. ISBN 9780323028455.
{{cite book}}: Cite has empty unknown parameter:|coauthors=(help) - ^ Marx, John (2006). Rosen's emergency medicine: concepts and clinical practice. Mosby/Elsevier. p. 2388. ISBN 9780323028455.
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| Ungrouped skin conditions resulting from physical factors |
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External links
- Recognizing Heath Stroke, Institute for Good Medicine at the Pennsylvania Medical Society
- International Red Cross Information on Heat Stroke
- Hiking and Camping Note Book Heat Stroke Advice
- BBC Heat Illness News and Information
- Environment Canada's Heat Index (humidex) Chart
- Working in Hot Environments, from the United States' National Institute for Occupational Safety and Health (NIOSH)
- Excessive Heat Events Guidebook, from the United States' Environmental Protection Agency (EPA)
- Enhanced Home & Family Heatwave Preparedness
- Cold Water Immersion: The Gold Standard for Exertional Heatstroke Treatment
- Physiological Responses to Exercise in the Heat -- Chapter 3 of Nutritional Needs in Hot Environments by the Institute of Medicine of the U.S. National Academies (of Science) (N.B.: entire book is available in HTML format via this link)