Succinylcholine is the skeletal muscle relaxant that can trigger malignant hyperthermia (MH). It is the only muscle relaxant classified as an MH trigger. All other muscle relaxants used in anesthesia, known as nondepolarizing agents, are considered safe for patients susceptible to this condition.
Why Succinylcholine Is the Trigger
Succinylcholine is a depolarizing muscle relaxant, meaning it works by mimicking the body’s own signaling molecule at the junction between nerves and muscles. In people who carry certain genetic mutations, this drug causes an uncontrolled flood of calcium from storage compartments inside muscle cells. That calcium surge forces muscles into sustained, intense contraction. The result is a massive spike in the body’s metabolic rate: oxygen consumption skyrockets, carbon dioxide production surges, and the body generates dangerous amounts of heat.
The key genetic target is the RYR1 gene, which encodes the calcium release channel on skeletal muscle cells. Mutations in this gene have been found in 50% to 86% of families identified as MH-susceptible, depending on the population studied. The condition is inherited in an autosomal dominant pattern, so a single copy of a causative mutation from one parent is enough to put someone at risk. Researchers have identified at least 10 specific causative mutations across different regions of the RYR1 gene.
Other Anesthetic Triggers to Know
Succinylcholine is not the only anesthetic agent that can set off MH. All inhaled halogenated anesthetic gases are also triggers. According to the Malignant Hyperthermia Association of the United States (MHAUS), the full list of triggering inhalational agents includes desflurane, enflurane, ether, halothane, isoflurane, methoxyflurane, and sevoflurane. In practice, an MH episode often involves exposure to one or more of these gases alongside succinylcholine, though either category can trigger a reaction on its own.
Nondepolarizing Relaxants Are Safe
Every nondepolarizing muscle relaxant currently in use is considered safe for MH-susceptible patients. These include vecuronium, rocuronium, pancuronium, atracurium, cisatracurium, and mivacurium. Unlike succinylcholine, these drugs block the nerve-muscle junction without triggering the calcium release mechanism that causes MH.
One large review of office-based plastic surgery found zero MH events across 23,000 general anesthesia procedures that used only propofol and vecuronium, both nontriggering agents. For patients known to be MH-susceptible, safe anesthetic plans typically combine a nondepolarizing relaxant with propofol, nitrous oxide, opioids, and other drugs that don’t interact with the calcium release channel.
How Common Is Malignant Hyperthermia
MH is rare. Reported incidence ranges from 1 in 10,000 to 1 in 220,000 general anesthesia procedures, with variation depending on the population, the triggering agents used, and how strictly cases are defined. The wide range partly reflects the fact that many susceptible individuals never receive a triggering agent and therefore never have an episode. Someone can undergo multiple surgeries without incident if nontriggering drugs are used each time.
What Happens During an MH Reaction
The earliest sign is usually a rapid, unexplained rise in the amount of carbon dioxide the patient exhales, which anesthesia monitors detect in real time. Heart rate climbs quickly. Jaw muscles may become extremely rigid, particularly after succinylcholine administration, a finding sometimes called masseter spasm. As the reaction progresses, body temperature can rise at a rate of 1 to 2 degrees Celsius every five minutes, eventually reaching dangerously high levels. Muscles throughout the body stiffen, and without treatment, the reaction can lead to organ failure.
The name “malignant hyperthermia” emphasizes the fever, but the temperature spike is actually a late sign. The metabolic changes, particularly the carbon dioxide surge and rapid heart rate, appear first and are more reliable early indicators.
How an MH Crisis Is Treated
The specific antidote is dantrolene, a drug that works directly on the calcium release channel to stop the runaway muscle contraction. Current guidelines from MHAUS recommend an initial intravenous dose of 2.5 mg per kilogram of body weight, repeated as needed with no fixed ceiling, until the patient’s heart rate and carbon dioxide levels begin to normalize. After the acute crisis is controlled, maintenance doses of 1 mg per kilogram every four to six hours are continued until all signs of the hypermetabolic state have resolved, which typically takes one to two days.
Speed matters enormously. Before dantrolene became available in the late 1970s, the mortality rate for MH episodes exceeded 70%. With prompt recognition and treatment, survival rates now exceed 90%.
Screening and Diagnosis
If you have a family history of MH or a personal history of an unusual reaction to anesthesia, genetic testing for RYR1 mutations is one path to diagnosis. A positive result for a known causative mutation confirms susceptibility. However, because not all causative mutations have been identified, a negative genetic test does not rule out risk.
The gold-standard diagnostic test is the caffeine-halothane contracture test, which requires a small muscle biopsy. A sample of living muscle tissue is exposed to caffeine and halothane in a laboratory, and the force of any resulting contraction is measured against specific thresholds. This test is only available at a handful of specialized centers, which limits its accessibility.
Preparing for Surgery if You Are Susceptible
If you know or suspect you are MH-susceptible, the most important step is telling your anesthesia team well before surgery. The anesthesia machine itself retains traces of inhaled anesthetic gases from previous cases, so it needs specific preparation. The standard approach involves removing or disabling the vaporizers that deliver volatile agents, replacing the breathing circuits, and flushing the machine with high-flow fresh gas for at least 20 minutes.
When time is short, activated charcoal filters placed on the breathing circuit can reduce residual volatile agent concentration to below 5 parts per million within about three minutes. These filters are maintained with a fresh gas flow of at least 3 liters per minute and changed after 12 hours. Even when regional anesthesia like a nerve block or spinal is planned, the machine should still be prepared in case conversion to general anesthesia becomes necessary.