Malignant hyperthermia

Malignant hyperthermia (MH) is a type of severe reaction that occurs in response to particular medications used during general anesthesia, among those who are susceptible. Symptoms include muscle rigidity, fever, and a fast heart rate. Complications can include muscle breakdown and high blood potassium. Most people who are susceptible to MH are generally unaffected when not exposed to triggering agents.

Exposure to triggering agents (certain volatile anesthetic agents or succinylcholine) can lead to the development of MH in those who are susceptible. Susceptibility can occur due to at least six genetic mutations, with the most common one being of the RYR1 gene. These genetic variations are often inherited in an autosomal dominant manner. The condition may also occur as a new mutation or be associated with a number of inherited muscle diseases, such as central core disease.

In susceptible individuals, the medications induce the release of stored calcium ions within muscle cells. The resulting increase in calcium concentrations within the cells cause the muscle fibers to contract. This generates excessive heat and results in metabolic acidosis. Diagnosis is based on symptoms in the appropriate situation. Family members may be tested to see if they are susceptible by muscle biopsy or genetic testing.

Treatment is with dantrolene and rapid cooling along with other supportive measures. The avoidance of potential triggers is recommended in susceptible people. The condition affects one in 5,000 to 50,000 cases where people are given anesthetic gases. Males are more often affected than females. The risk of death with proper treatment is about 5% while without it is around 75%. While cases that appear similar to MH have been documented since the early 20th century, the condition was only formally recognized in 1960.

The typical signs of malignant hyperthermia are due to a hypercatabolic state, which presents as a very high temperature, an increased heart rate and abnormally rapid breathing, increased carbon dioxide production, increased oxygen consumption, mixed acidosis, rigid muscles, and rhabdomyolysis. These signs can develop any time during the administration of the anesthetic triggering agents. Rarely, signs may develop up to 40 minutes after the end of anaesthesia.

Malignant hyperthermia is a disorder that can be considered a gene–environment interaction. In most people with malignant hyperthermia susceptibility, they have few or no symptoms unless they are exposed to a triggering agent. The most common triggering agents are volatile anesthetic gases, such as halothane, sevoflurane, desflurane, isoflurane, enflurane or the depolarizing muscle relaxants suxamethonium and decamethonium used primarily in general anesthesia. In rare cases, the biological stresses of physical exercise or heat may be the trigger. In fact, malignant hyperthermia susceptibility (MHS), predisposed by mutations in the skeletal muscle calcium release channel (RYR1), is one of the most severe heat-related illnesses. The MHS-associated heat susceptibilities predominantly affect children and metabolically active young adults, often leading to life- threatening hypermetabolic responses to heat.

Other anesthetic drugs do not trigger malignant hyperthermia. Some examples of drugs that don't cause MH include local anesthetics (lidocaine, bupivacaine, mepivacaine), opiates (morphine, fentanyl), ketamine, barbiturates, nitrous oxide, propofol, etomidate, and benzodiazepines. The nondepolarizing muscle relaxants pancuronium, cisatracurium, atracurium, mivacurium, vecuronium and rocuronium also are generally thought to be safe for patients MH.

There is mounting evidence that some individuals with malignant hyperthermia susceptibility may develop MH with exercise and/or on exposure to hot environments.