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Malignant Hyperthermia

What Is It?

Malignant hyperthermia occurs when a rare, inherited muscle abnormality causes a severe and sometimes fatal reaction to a dose of anesthetics. Infrequently, extreme exercise or heat stroke, instead of an anesthetic, can trigger malignant hyperthermia in someone who has this muscle abnormality.

In people with the muscle abnormality, a gene mutation causes muscle cells to have an abnormal protein on their surfaces. The mutation does not affect muscle function significantly until the muscles are exposed to one of several drugs that can trigger a reaction. When a person with this condition is exposed to one of these drugs, calcium stored in muscle cells is released, causing the muscles to contract and stiffen at the same time, causing a dramatic and dangerous increase in body temperature (hyperthermia).

Medications known to trigger malignant hyperthermia are succinylcholine (Anectine), which is a muscle relaxant used during surgery, and certain inhaled anesthetics, including halothane (Fluothane), enflurane (Ethrane), isoflurane (Forane), methoxyflurane (Penthrane), sevoflurane (Ultane) and desflurane (Suprane).

Although malignant hyperthermia most often occurs during or after surgery, it can occur wherever anesthetic medications are used, such as in emergency rooms, dental offices, surgeons' offices and intensive care units. Symptoms of malignant hyperthermia usually occur within the first hour after a medication that can trigger the disorder is administered, although the symptoms can be delayed for up to 12 hours.

Most cases of malignant hyperthermia occur in children and adults younger than 30. It has been estimated to occur once in every 15,000 surgeries in children and less often in adults.

There are several gene mutations that can cause this muscle abnormality. This condition sometimes occurs in people who also have one of various forms of muscular dystrophy or another muscle disease associated with genetic mutations. People with such muscle disorders are thought to be at a higher risk of malignant hyperthermia.

The most common mutation, which causes about half of all cases of malignant hyperthermia, is transmitted in what geneticists call an autosomal dominant pattern. This means that a person with the mutation has a 50% chance of passing the gene to any of his or her children. It also means that each sibling of an affected person has a 50% chance of also having the gene.

Family members can have different levels of sensitivity to medications that trigger the problem. In some cases, reactions are mild, so a person may be exposed to high-risk medications several times before experiencing a reaction that is severe enough to be recognized.

The term malignant hyperthermia usually is used to describe only this problem caused by a gene mutation. However, some doctors also use the term when extreme hyperthermia (increase in body temperature) happens in a person without a gene mutation. Problems that can cause this to happen include:

• Heat stroke

• Drug overdose (especially MDMA, or ecstasy)

• Neuroleptic malignant syndrome, a rare condition in which fever and muscle stiffness are triggered by exposure to certain psychiatric medications