Malignant hyperthermia (MH) is a condition that causes a number of physical changes to occur among genetically susceptible individuals when they are exposed to a particular muscle relaxant or certain types of medications used for anesthesia. The changes may include increased rate of breathing, increased heart rate, muscle stiffness, and significantly increased body temperature (i.e., hyperthermia). Although MH can usually be treated successfully, it sometimes leads to long-term physical illness or death. Research has identified a number of genetic regions that may be linked to an increased MH susceptibility.
Unusual response to anesthesia was first reported in a medical journal during the early 1960s, when physicians described a young man in need of urgent surgery for a serious injury. He was very nervous about exposure to anesthesia, since he had 10 close relatives who died during or just after surgeries that required anesthesia. The patient himself became very ill and developed a high temperature after he was given anesthesia. During the next decade, more cases of similar reactions to anesthesia were reported, and specialists began using the term malignant hyperthermia to describe the newly recognized condition. The word hyperthermia was used because people with this condition often develop a very high body temperature rapidly. The word malignant referred to the fact that the majority (70–80%) of affected individuals died. The high death rate in the 1960s occurred because the underlying cause of the condition was not understood, nor was there any known treatment (other than basically trying to cool the person's body with ice).
Increased awareness of malignant hyperthermia and scientific research during the following decades improved medical professionals' knowledge about what causes the condition, how it affects people, and how it should be treated. MH can be thought of as a chain reaction that is triggered when a person with MH susceptibility is exposed to specific drugs commonly used for anesthesia and muscle relaxation.
Triggering drugs that may lead to malignant hyperthermia include:
Once an MH susceptible person is exposed to one or more of these anesthesia drugs, they can present with a variety of signs. One of the first clues that a person is susceptible to MH is often seen when they are given a muscle relaxant called succinyl choline. This drug generally causes some stiffness in the masseter (jaw) muscles in most people. However, individuals with MH susceptibility can develop a much more severe form of jaw stiffness called masseter spasm when they receive this drug. They may develop muscle stiffness in other parts of their bodies as well. When exposed to any of the trigger drugs listed above (inhalants for anesthesia), people with MH susceptibility can develop an increased rate of metabolism in the cells of their body, resulting in rapid breathing, rapid heartbeat, high body temperature (over 110°F), muscle stiffness, and muscle breakdown. If these signs are not recognized, treated, or able to be controlled, brain damage or death can occur due to internal bleeding, heart failure, or failure other organs.
The series of events that occur after exposure to trigger drugs is activated by an abnormally high amount of calcium inside muscle cells. This is due to changes in the chemical reactions that control muscle contraction and the production of energy. Calcium is normally stored in an area called the sarcoplasmic reticulum, which is a system of tiny tubes located inside muscle cells. This system of tubes allows muscles to contract (by releasing calcium) and to relax (by storing calcium) in muscle cells. Calcium also plays an important role in the production of energy inside cells (i.e., metabolism). There are at least three important proteins located in (or nearby) the sarcoplasmic reticulum that control how much calcium is released into muscle cells and thus help muscles contract. One of these proteins is a "calcium release channel" protein that has been named the ryanodine receptor protein, or RYR. This protein (as well as the gene that tells the body how to make it) has been an important area of research. For some reason, when people with MH susceptibility are exposed to a trigger drug, they can develop very high levels of calcium in their muscle cells. The trigger drugs presumably stimulate the proteins that control the release of calcium, causing them to create very high levels of calcium in muscle cells. This abnormally high calcium level then leads to increased metabolism, muscle stiffness, and the other symptoms of MH.
The amount of time that passes between the exposure to trigger drugs and the appearance of the first symptoms of MH varies between different people. Symptoms begin within 10 minutes for some individuals, although several hours may pass before symptoms appear in others. This means that some people do not show signs of MH until they have left the operating room and are recovering from surgery. In addition, some individuals who inherit MH susceptibility may be exposed to trigger drugs numerous times during multiple surgeries without any complications. However, they still have an increased risk to develop an MH episode during future exposures. This means that people who have an increased risk for MH susceptibility due to their family history cannot presume they are not at risk simply because they previously had successful surgeries. Although MH was frequently a fatal condition in the past, a drug called dantrolene sodium became available in 1979, which greatly decreased the rate of both death and disability.
Pamela J. Nutting MS, CGC, Thomson Gale, Gale, Detroit,