Malignant Hyperthermia (MH) is a hereditary disorder of skeletal muscle that classically presents as a hypermetabolic response to halogenated anesthetic gasses and/or the depolarizing muscle relaxant succinylcholine.
Patients who are genetically susceptible can have an MH reaction in response to triggering agents such as halogenated anesthetic gasses and/or succinylcholine and more rarely to stressors such as vigorous exercise and heat exposure. Nitrous Oxide and Xenon, although they are inhaled anesthetics, are not halogenated and have not been implicated in MH.
The exact incidence of MH is unknown. Studies demonstrate that MH occurs in about 1:100,000 in adults and 1:30,000 in children. The incidence of MH varies based upon geographic region. There are concentrations of MH susceptible families present in Wisconsin and the upper Midwest. The mortality rate is 3% to 5%, even when properly treated.
Other disorders are also associated with MH susceptibility. These include Central Core Disease (a rare, non-progressive myopathy characterized by hypotonia and weakness of proximal muscles) and King-Denborough Syndrome (rare myotonia associated with multiple distinct physical features).
Early researchers described rare episodes of stress-induced "awake MH." Later, Tobin described one of the most convincing cases of fatal, exercise-induced MH in a 13-year-old boy in whom the causative RYR1 mutation was present and was later found to be present in his family members.
MH susceptibility has also been described in other species, particularly in swine, where much of the early research into the disease and its treatment originated.
MH is an autosomal dominantly inherited disorder that is characterized by skeletal muscle hypermetabolism following exposure to halogenated anesthetics, depolarizing muscle relaxants such as succinylcholine, or, occasionally, physiologic stressors. The gene for the ryanodine receptor RYR1 is the primary site for mutations linked with MH, but other genetic loci have been identified, such as CACNA1S and STAC3, as causative for MH.
The uncontrolled release of calcium from skeletal muscle sarcoplasmic reticulum leads to sustained muscle contraction. The sustained muscle contraction produces a depletion of adenosine triphosphate (ATP), and dramatically increases oxygen consumption and the production of carbon dioxide and heat. The depletion of ATP stores leads to the membrane integrity failure and cell content leakages such as potassium, creatinine kinase, and myoglobin into the circulation.
Signs and symptoms of MH include tachycardia, tachypnea, hypoxemia, hypercarbia, metabolic and respiratory acidosis, hyperkalemia, cardiac dysrhythmias, hypotension, skeletal muscle rigidity, and hyperthermia. The earliest signs of MH are usually hypercarbia and tachycardia due to elevated carbon dioxide production. Fulminant MH reactions may have only a few of the usual signs, and it requires a high index of suspicion of MH to effect a timely and correct diagnosis and treatment. MH can occur at any time during the intraoperative and postoperative periods.
Susceptible patients can exhibit masseter muscle spasm. If signs of hypermetabolism such as metabolic and respiratory acidosis or an elevation in body temperature accompany the muscle spasm, a diagnosis of MH must be considered.
The gold standard in the laboratory diagnosis of MH is the caffeine halothane contracture test (CHCT), although genetic testing is rapidly advancing and may one day replace the muscle biopsy. The CHCT involves exposing a sample of live muscle fibers to halothane and caffeine to determine the muscle response to halogenated anesthetics. Genetic testing for mutations of the RYR1 or other associated genetic variants associated with MH is becoming increasingly more valuable as the testing improves. Testing can be expensive and is only available in certain centers. Therefore, when presented with a patient for urgent or emergent surgery who has a history suggestive of a close relative who has had an MH episode, anesthesiologists will usually provide a "non-triggering anesthetic," which is typically a variety of combinations of intravenous anesthetic agents. The differential diagnosis for MH can include many other, unrelated disorders such as neuroleptic malignant syndrome, pheochromocytoma, sepsis, thyroid storm, serotonin syndrome, or iatrogenic overheating.
The critical element in the treatment of MH is immediate dantrolene administration. Once an MH episode is suspected, all triggering agents must be discontinued and the patient hyperventilated with 100% oxygen with non-triggering anesthetic agents utilized for patient care and surgery should be ended as soon as possible. Dantrolene in a dose of 2.5 mg/kg must be administered intravenously as soon as possible, up to a maximum dose of 10mg/kg until the reaction subsides. In addition to administering dantrolene, attention also must be paid to correct hyperthermia, acidosis, hypoxemia, arrhythmias and preserving renal function. Arrhythmias can be treated with antiarrhythmics and renal function can be protected from possible acute tubular necrosis (due to precipitation of released myoglobin from the skeletal muscles) by maintaining a urine flow of at least 2 ml/kg/hr with furosemide.
The management algorithm includes:
Better patient outcomes are associated with rapidity of diagnosis, rapid treatment with dantrolene, and prevention of the rapid rise in core temperature by using cooling measures.
After the patient has been stabilized, he or she must be taken to the intensive care unit for at least 24 hours for monitoring and to watch for signs of recrudescence. Patients at the highest risk for recrudescence are those with a large muscle mass or who have undergone at least 150 minutes of anesthetic exposure before triggering.
Despite having been performed frequently in the past, pretreating MH susceptible patients with dantrolene does not play a role in their care and should not be done. Instead, these patients should receive a non-triggering anesthetic.
The differential diagnosis for MH includes the following:
Complete recovery can occur if the signs and symptoms of MH are recognized early and proper treatment is started. Multiple organ failure and death can still occur, however, even with prompt treatment. The mortality rate is less than 5%.
The complications of MH include:
Family members of patients who suffer MH should be educated on the genetic aspect of the disease. A Malignant Hyperthermia Association of the United States (MHAUS) was founded in 1981 and should be used by affected family members to gain more information. MHAUS 24-hour hotline is (800) MH-HYPER or (800) 644-9737.
Dantrolene works by inhibiting calcium ion release from the sarcoplasmic reticulum. Its mechanism of action is through antagonizing the ryanodine receptors which lessens the excitation-contraction coupling of muscle cells.
Dantrolene is currently the only specific medication used for treating an MH crisis. Dantrolene is available as two different formulations: Dantrium/Revonto and Dantrolene Sodium/Ryanodex. These two medications differ in the amount of sterile water required to reconstitute each vial and the concentration of dantrolene present within each vial. Dantrium/Revonto is available in 20 mg vials that must be reconstituted with 60 mL of sterile water per vial. Dantrolene Sodium/Ryanodex is available in 250 mg vials that must be reconstituted with 5 mL of sterile water per vial. Regardless of which formulation of dantrolene administered, a dose of 2.5 mg/kg is recommended to treat an MH episode. Dantrolene is a highly lipophilic drug with low solubility in water, making the reconstitution of the drug in sterile water challenging. Ryanodex is a newer formulation that combines nanosuspension technology with a lyophilized formulation resulting in a much faster reconstitution of the drug, which can greatly improve the time to treatment and may be especially useful in locations where only one provider is available to treat the patient.
Keep in mind that additional doses of dantrolene might be necessary to adequately treat an MH triggering event, and a dose of 1 mg/kg every 4 to 6 hours is recommended for the first 24 to 48 hours after an episode of MH.
Dantrolene should not be combined with verapamil, as it may lead to hyperkalemia and hypotension.
All facilities where MH triggering anesthetics are administered are recommended to stock a minimum of 36 vials of Dantrium/Revonto or 3 vials of Dantrolene Sodium/Ryanodex along with other medications and rescue equipment needed to treat an MH crisis.
MH link to strenuous exercise, heat exposure or other causes of elevated body temperature:
MH susceptible patients are found to experience a metabolic crisis without exposure to triggering agents. Exposure of these patients to strenuous exercise, heat exposure, or high internal body temperatures (e.g., infections) may precipitate this crisis. Research is currently underway to evaluate this possible link.
Life-threatening laryngospasm is a much more common event than MH that can be quickly treated with low dose succinylcholine. Many office-based procedural facilities do not stock succinylcholine due to the risk of MH and the need to stock expensive dantrolene in case of an MH triggering event. The Society for Ambulatory Anesthesia offered an opinion on the dilemma by stating that succinylcholine could be stocked at these locations for emergency use only with the caveat that procedures should never be done on known MH susceptible persons in those facilities.
Patients and their families should be referred to the Malignant Hyperthermia Association of the United States (MHAUS) for information about this disorder and to receive follow-up from specialists in this area.
North American MH Registry:
The North American MH Registry of the Malignant Hyperthermia Association of the United States (MHAUS) is a database of information about patients and their families that have experienced MH episodes. Healthcare providers are encouraged to report MH and MH-like episodes to the registry.
Malignant hyperthermia is a rare disorder that is life-threatening. Because of the high morbidity, the disorder is best managed by an interprofessional team that includes a neurologist, intensivist, anesthesiologist and an internist. Even though dantrolene is recommended for treatment, good data supporting its efficacy are still lacking. The patient should be managed in an ICU setting with close monitoring.
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