Malignant hyperthermia (MH) or malignant hyperpyrexia is a rare life-threatening condition that is usually triggered by exposure to certain drugs used for general anesthesia, specifically the volatile anesthetic agents and the neuromuscular blocking agent, succinylcholine. In susceptible individuals, these drugs can induce a drastic and uncontrolled increase in skeletal muscle oxidative metabolism, which overwhelms the body's capacity to supply oxygen, remove carbon dioxide, and regulate body temperature, eventually leading to circulatory collapse and death if not treated quickly.

Susceptibility to MH is often inherited as an autosomal dominant disorder, for which there are at least 6 genetic loci of interest, most prominently the ryanodine receptor gene (RYR1). MH susceptibility is phenotypically and genetically related tocentral core disease (CCD), an autosomal dominant disorder characterized both by MH symptoms and myopathy. MH is usually revealed by anesthesia, or when a family member develops the symptoms. There is no simple, straightforward test to diagnose the condition. When MH develops during a procedure, treatment with dantrolene sodium is usually initiated; dantrolene and the avoidance of inhaled anesthesia in susceptible people have markedly reduced the mortality from this condition.

Signs and symptoms
The typical symptoms of malignant hyperthermia are due to a hypercatabolic state, which presents as a very high temperature, an increased heart rate and breathing rate, increased carbon dioxide production, increased oxygen consumption, acidosis, rigid muscles, and rhabdomyolysis.

The symptoms usually develop within one hour after exposure to trigger substances, but may even occur several hours later in rare instances.

Diagnosis
During an attack

The earliest signs are early masseter muscle contracture following administration of succinylcholine, a rise in end-tidal carbon dioxide concentration (despite increased minute ventilation), unexplained tachycardia, and muscle rigidity. Despite the name, elevation of body temperature is often a late sign. Other signs may include acidosis, tachypnea (in a spontaneously breathing patient), cyanosis, hypertension, cardiac dysrhythmias and hyperkalemia. Core body temperatures should be measured in any patient undergoing general anesthesia longer than 20 minutes.

Malignant hyperthermia is diagnosed on clinical grounds, but various investigations are generally performed. This includes blood tests, which may show a raised creatine kinase level, elevated potassium, increased phosphate (leading to decreased calcium) and—if determined—raised myoglobin; this is the result of damage to muscle cells. Metabolic acidosis and respiratory acidosis (raised acidity of the blood) may both occur. Severe rhabdomyolysis may lead to acute renal failure, so kidney function is generally measured on a frequent basis. Patients may also get cardiac arrythmias (PVCs) due to the increased levels of potassium released from the muscles during episodes.

Susceptibility testing

In those who have experienced an episode of MH, further testing is not usually useful, as even a normal test does not mean there is no risk of recurrence. The exception would be if it is unclear whether the initial attack was due to a different medical problem, such as sepsis.

Muscle testing

The main candidates for testing are those with a close relative who has suffered an episode of MH or has been shown to be susceptible. The standard procedure is the "caffeine-halothane contracture test", CHCT. A muscle biopsy is carried out at an approved research center, under local anesthesia. The fresh biopsy is bathed in solutions containing caffeine or halothane and observed for contraction; under good conditions, the sensitivity is 97% and the specificity 78%. Negative biopsies are not definitive, so any patient who is suspected of MH by their medical history or that of blood relatives is generally treated with nontriggering anesthetics, even if the biopsy was negative. Some researchers advocate the use of the "calcium-induced calcium release" test in addition to the CHCT to make the test more specific.

Less invasive diagnostic techniques have been proposed. Intramuscular injection of halothane 6 vol% has been shown to result in higher than normal increases in local pCO among patients with known malignant hyperthermia susceptibility. The sensitivity was 100% and specificity was 75%. For patients at similar risk to those in this study, this leads to a positive predictive value of 80% and negative predictive value of 100%. This method may provide a suitable alternative to more invasive techniques.^[19] A 2002 study examined another possible metabolic test. In this test, intramuscular injection of caffeine was followed by local measurement of the pCO those with known MH susceptibility had a significantly higher pCO (63 versus 44 mmHg). The authors propose larger studies to assess the test's suitability for determining MH risk.

Genetic testing

Genetic testing is being performed in a limited fashion to determine susceptibility to MH. In people with a family history of MH, analysis for RYR1 mutations maybe useful.

Treatment
The current treatment of choice is the intravenous administration of dantrolene, the only known antidote, discontinuation of triggering agents, and supportive therapy directed at correcting hyperthermia, acidosis, and organ dysfunction. Treatment must be instituted rapidly on clinical suspicion of the onset of malignant hyperthermia.

Dantrolene is a muscle relaxant that appears to work directly on the ryanodine receptor to prevent the release of calcium. After the widespread introduction of treatment with dantrolene, the mortality of malignant hyperthermia fell from 80% in the 1960s to less than 10%. Dantrolene remains the only drug known to be effective in the treatment of MH.

Its clinical use has been limited by its low water solubility, leading to requirements of large fluid volumes, which may complicate clinical management. Azumolene is a 30-fold more water-soluble analogue of dantrolene that also works to decrease the release of intracellular calcium by its action on the ryanodine receptor. In MH susceptible swine, azumolene was as potent as dantrolene. It has yet to be studied in vivo in humans, but may present a suitable alternative to dantrolene in the treatment of MH.

Research in mouse models suggests that azumolene "is likely uncoupling the efficiency of a Ca2+
-dependent RyR1 signal coupled directly or indirectly to the [store-operated calcium entry] machinery." There may be two different pathways of store-operated calcium entry: one, RyR1-reliant and the other, RyR1-non-reliant (see Disease Mechanism section above for RyR1 description). Furthermore, elucidating earlier ideas on the pathogenesis of malignant hyperthermia, researchers point out that it may be "as much a syndrome of exaggerated Ca2+
 entry as it is of exaggeratedCa2+
 release."

Azumolene has also been shown to be as effective as dantrolene at preventing and reversing contracture in in vitro experiments with human skeletal muscle.