Malignant hyperthermia (MH) is a rare, inherited, life-threatening crisis that occurs most often when a genetically susceptible person is exposed to specific anesthetic agents, such as volatile inhalation agents or the muscle relaxant succinylcholine. The condition is characterized by a rapid, uncontrolled increase in metabolism within the skeletal muscles. This hypermetabolic state quickly overwhelms the body’s ability to supply oxygen and eliminate carbon dioxide, leading to severe complications like muscle rigidity, dangerously high body temperature, and profound acidosis. Survival depends entirely on the speed and coordination of the medical team’s response to this urgent medical emergency.
Immediate Action: Recognizing the Crisis and Stopping the Trigger
The first step in managing a malignant hyperthermia crisis is the immediate recognition of early clinical signs, which often manifest before a significant rise in body temperature. The most sensitive initial sign is an unexpected, rapid increase in end-tidal carbon dioxide (\(\text{ETCO}_2\)), despite increased ventilation. Other early indicators include increased heart rate (tachycardia) and generalized muscle rigidity, particularly after succinylcholine use.
Once MH is suspected, all triggering agents must be stopped immediately. This involves discontinuing all volatile anesthetic gases (e.g., isoflurane, sevoflurane, desflurane) and the muscle relaxant succinylcholine. The patient should be hyperventilated with 100% oxygen at high flow rates (typically 10 L/minute or more) to counteract the rapidly developing acidosis. Anesthesia must be maintained using non-triggering intravenous agents, and the anesthetic circuit should be flushed or changed to prevent re-exposure.
Definitive Pharmacological Intervention
The definitive treatment for malignant hyperthermia is the immediate intravenous administration of Dantrolene Sodium. This medication is the only specific antidote for the crisis and must be given quickly, as a delay significantly increases the risk of complications. Dantrolene works directly on skeletal muscle cells by targeting the ryanodine receptor type 1 (\(\text{RYR}1\)) on the sarcoplasmic reticulum.
The drug uncouples the excitation-contraction mechanism by inhibiting the uncontrolled release of calcium ions from the sarcoplasmic reticulum. By blocking this excessive calcium efflux, Dantrolene halts the cycle of sustained muscle contraction and the resulting hypermetabolic state. The initial recommended dose is a rapid intravenous bolus of 2.5 mg/kg, repeated every five minutes until the signs of hypermetabolism resolve.
Older formulations of Dantrolene require reconstitution with a large volume of sterile water, which is time-consuming during a crisis. Newer, concentrated formulations, such as Ryanodex, contain 250 mg per vial and require only a small amount of sterile water, significantly accelerating preparation. Clinicians may administer a cumulative dose of up to 10 mg/kg if symptoms persist, though a lack of response at this level may suggest an alternative diagnosis.
Managing Systemic Damage and Temperature Control
Even while Dantrolene is working, aggressive supportive care is required to manage the severe systemic damage caused by the hypermetabolism. Uncontrolled muscle activity generates massive heat, necessitating active cooling measures to rapidly bring the core body temperature down below \(38^\circ\text{C}\). Cooling techniques include administering cold intravenous saline, placing ice packs on the neck, groin, and armpits, and using cooling blankets.
The intense metabolic activity produces severe metabolic acidosis, which must be treated with intravenous sodium bicarbonate to normalize the blood pH. Widespread muscle breakdown (rhabdomyolysis) releases potassium and myoglobin into the bloodstream. Hyperkalemia (high potassium) is treated with intravenous glucose and insulin, and sometimes calcium to stabilize the heart muscle.
To protect the kidneys from myoglobin-induced damage, a forced alkaline diuresis is initiated. This involves using intravenous fluids and diuretics to maintain a high urine output, typically greater than 1-2 mL/kg/hour.
Essential Post-Crisis Monitoring and Follow-Up Care
After the acute crisis is controlled, intensive post-crisis monitoring is necessary due to the risk of recrudescence, or a relapse of the hypermetabolic state. Patients are typically transferred to an Intensive Care Unit (ICU) and monitored closely for at least 24 to 48 hours. During this time, Dantrolene administration is continued (typically 1 mg/kg intravenously every four to six hours) to prevent the re-emergence of symptoms.
Monitoring includes frequent checks of core temperature, heart rate, blood gas levels, and serum markers such as creatinine kinase (\(\text{CK}\)) and potassium. Dantrolene may be discontinued once the patient achieves metabolic stability, provided \(\text{CK}\) levels are decreasing and there is no evidence of myoglobinuria. Following recovery, genetic testing is recommended to confirm the underlying susceptibility, often linked to a mutation in the \(\text{RYR}1\) gene. Patients and their families receive counseling regarding future anesthetic precautions and are advised to wear a medical alert identifier for Malignant Hyperthermia Susceptibility (\(\text{MHS}\)).