Malignant Hyperthermia

What Is It?

Malignant hyperthermia usually occurs when a rare, inherited muscle abnormality causes a severe and sometimes fatal reaction to certain common anesthetics. Infrequently, malignant hyperthermia can be triggered in a susceptible person by extreme exercise or by heat stroke.

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. Because the body's muscles all contract and stiffen at the same time, a dramatic and dangerous increase in body temperature (hyperthermia) occurs.

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 a surgery in the operating room, it also can occur in other places where anesthetic medications are used, such as 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.

Malignant hyperthermia can affect people in any racial group. Most cases 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 undergoing surgery.

There are several different 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 other muscle diseases that are 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 an affected individual has a 50 percent chance of passing the gene on to any of his or her children. It also means that each sibling of an affected person has a 50 percent chance of also having the gene.

People within the same family can have different levels of sensitivity to trigger medications. In some cases, reactions may be 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 name malignant hyperthermia usually is used to describe only this genetically programmed problem. However, some doctors also use the term when extreme hyperthermia (increase in body temperature) results without a gene mutation. Problems that can cause extreme hyperthermia in the absence of a gene mutation include: