Malignant Hyperthermia
A life-threatening inherited reaction to certain anesthetics
Quick Facts
- Type: Inherited muscle (genetic) disorder
- Trigger: Certain anesthetic gases, succinylcholine
- Key signs: Soaring temperature, muscle rigidity, fast heart rate
- Emergency: Yes - treated with dantrolene and cooling
Overview
Malignant hyperthermia (MH) is a rare but life-threatening reaction that can occur in susceptible people when they are exposed to certain anesthetic medications during surgery. In affected individuals, these drugs cause a sudden, uncontrolled release of calcium inside muscle cells, leading to intense, sustained muscle contraction, a dangerously high body temperature, and a cascade of dangerous chemical changes in the body.
Susceptibility to MH is usually inherited and runs in families. People can carry the trait without ever knowing, because nothing happens until they are exposed to a triggering drug. When a reaction does occur, it is a medical emergency, but with rapid recognition and treatment using the drug dantrolene and aggressive cooling, most people survive and recover fully.
Symptoms
An MH reaction usually begins during or shortly after general anesthesia. The anesthesia and surgical team watch closely for early warning signs, which include:
- A rapid rise in the level of exhaled carbon dioxide
- Stiff or rigid muscles, sometimes starting in the jaw
- A fast or irregular heartbeat
- A rapidly climbing body temperature, which can become extremely high
- Rapid breathing and falling oxygen levels
- Dark or cola-colored urine from muscle breakdown
- Acid buildup in the blood and abnormal blood chemistry
Without treatment, the reaction can lead to organ damage, abnormal heart rhythms, and death. This is why MH is treated as an immediate emergency.
Causes
Malignant hyperthermia is caused by an inherited change in genes that control how muscle cells handle calcium. The most commonly involved gene is RYR1. When a person who carries one of these changes is exposed to a triggering drug, calcium floods the muscle cell, causing continuous contraction and massive heat and acid production.
The known triggers are:
- Volatile anesthetic gases such as halothane, sevoflurane, desflurane, and isoflurane
- Succinylcholine, a muscle relaxant used during anesthesia
Most other anesthetic and surgical medications, including local anesthetics and many intravenous agents, are safe for people with MH susceptibility.
Risk Factors
- A personal or family history of malignant hyperthermia
- A close relative who has had an unexplained severe reaction or death under anesthesia
- Certain inherited muscle conditions linked to RYR1 changes, such as central core disease
- A previous episode of unexplained high temperature, muscle rigidity, or dark urine during surgery
Diagnosis
During surgery, MH is recognized clinically from its characteristic signs, and treatment begins immediately without waiting for confirmatory tests. After recovery, or when susceptibility is suspected because of family history, several tests can be used:
- Genetic testing: A blood test to look for changes in the RYR1 gene and other related genes.
- Caffeine-halothane contracture test: A specialized test on a small muscle sample, performed at select centers, considered the most definitive test for susceptibility.
- Family screening: Relatives of an affected person may be evaluated, since the trait is inherited.
Treatment
An MH crisis is a medical emergency that requires immediate, coordinated action by the anesthesia team:
- Stop the trigger: The triggering anesthetic gases and succinylcholine are stopped at once and the surgery is concluded as quickly as is safe.
- Dantrolene: The antidote dantrolene is given intravenously to halt the abnormal muscle activity, and is the cornerstone of treatment.
- Cooling: The body is actively cooled with cold fluids, ice packs, and other measures.
- Supportive care: High-flow oxygen, correction of blood chemistry, treatment of abnormal heart rhythms, and protection of the kidneys from muscle breakdown products.
After a crisis, the person is monitored in intensive care because the reaction can return. People known to be susceptible can safely have future surgeries using anesthetic techniques and medications that avoid the triggers.
Prevention
The most important prevention is identifying people at risk before surgery:
- Tell your surgical and anesthesia team about any personal or family history of anesthesia problems
- Wear or carry medical identification noting MH susceptibility if you have it
- Encourage close relatives of an affected person to be evaluated
- For known susceptible patients, the anesthesia team uses trigger-free medications and keeps dantrolene immediately available
When to See a Doctor
Because MH happens in the operating room, the anesthesia team manages the emergency directly. Outside of surgery, speak with your doctor before any planned procedure if you or a blood relative has:
- A known diagnosis of malignant hyperthermia
- An unexplained severe reaction, high fever, or death under anesthesia in the family
- An inherited muscle disorder
If you have ever had a suspected reaction, ask about genetic testing and make sure all future surgical teams are informed so they can use a safe anesthetic plan.
Frequently Asked Questions
What triggers malignant hyperthermia?
In susceptible people, MH is triggered by certain inhaled anesthetic gases such as sevoflurane and by the muscle relaxant succinylcholine. Most other anesthetic medications, including local anesthetics and many intravenous drugs, are safe.
Is malignant hyperthermia inherited?
Yes. Susceptibility is usually passed down in families, most often through changes in the RYR1 gene. A person can carry the trait without symptoms until they are exposed to a triggering anesthetic, so family history is very important.
Can malignant hyperthermia be treated?
Yes. The reaction is reversed by immediately stopping the trigger, giving the antidote dantrolene, actively cooling the body, and providing intensive supportive care. With prompt treatment, most people recover fully.
Can I still have surgery if I am at risk?
Yes. People known to be susceptible can have surgery safely when the anesthesia team uses trigger-free medications and keeps dantrolene on hand. Always inform every surgical team about your risk.
How do I know if I am susceptible?
Suspicion is based on personal or family history of anesthesia reactions or related muscle disorders. Genetic testing and a specialized muscle contracture test can confirm susceptibility in people considered at risk.
References
- Malignant Hyperthermia Association of the United States (MHAUS).
- National Institutes of Health, Genetic and Rare Diseases Information Center. Malignant hyperthermia.
- MedlinePlus, U.S. National Library of Medicine. Malignant hyperthermia.
- Mayo Clinic. Malignant hyperthermia.