What Is Malignant Hyperthermia? The Anesthesia Reaction That Runs in Families — and How It Is Prevented and Treated

Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026

Quick Answer: Key Facts About Malignant Hyperthermia

Malignant hyperthermia is a severe reaction to certain anesthesia drugs that causes high fever, rigid muscles, rapid heart rate and breathing problems. It is caused by a gene change, most often in RYR1, usually inherited from one parent. Genetic testing or a muscle biopsy can check susceptibility. Treatment with dantrolene, cooling and intensive care usually resolves it within days.

Feature	Details
What it is	A severe reaction to certain anesthesia drugs, causing high fever, rigid muscles, rapid heart rate, and breathing problems. [1]
Why it happens	A gene change (most often in the RYR1 gene) that is usually inherited from one parent. [1]
Who is at risk	People with a family history of malignant hyperthermia, a suspected event during anesthesia, or rhabdomyolysis (muscle breakdown). [1]
How it is diagnosed	By symptoms and monitoring during and after anesthesia; genetic testing or a specialized muscle biopsy can check susceptibility. [2]
How it is treated	Dantrolene, oxygen, active body cooling, extra IV fluids, and intensive care monitoring. With treatment it usually resolves within a few days. [2]

What the research says

• The reaction is rare, and one gene explains most cases. A review reports that prevalence varies from 1 in 16,000 in Denmark to 1 in 100,000 in New York State. Mutations in RYR1 and CACNA1S were found in about 50% to 86% and 1% of susceptible individuals, respectively, and dantrolene is the only drug approved for treatment. (Yang et al., J Biomed Res, 2019) [3]

• Precautions based on family history worked in one children's hospital. In a retrospective study of 125 patients who received 174 anesthetics with malignant hyperthermia precautions, 59% had a personal or family history of the condition, and no malignant hyperthermia events occurred. Only 4% had genetic testing and none had a muscle biopsy, so most precautions rely on a careful history. (Carlson et al., Int J Pediatr Otorhinolaryngol, 2022) [4]

• In a mouse model, humid conditions worsened outcomes and dantrolene improved survival. In susceptible young male mice, survival fell during heat or isoflurane exposure in humid conditions, and dantrolene improved survival under every condition tested. In a related analysis of susceptible patients, humidity was associated with more malignant hyperthermia episodes in men (AUC 0.81). Most of this evidence is from animals. (He et al., Skelet Muscle, 2026) [5]

What Is Malignant Hyperthermia?

Malignant hyperthermia is a severe reaction to certain drugs used for anesthesia. During anesthesia, medicines are used to keep you unconscious and pain-free during surgery. In a small number of people, specific anesthesia drugs trigger a dangerous chain of events inside their muscle cells. [1]

The reaction typically includes a dangerously high body temperature, rigid muscles or spasms, a rapid heart rate, and other symptoms. Without prompt treatment, the complications can be fatal. [1]

The key thing to understand is this: the reaction is the result of a hidden genetic risk meeting a specific trigger. The genetic disorder that puts a person at risk is called malignant hyperthermia susceptibility, or MHS. In most cases, no signs or symptoms of susceptibility exist until you are exposed to certain anesthesia drugs. [1]

Figure 1: The three-step mechanism: a hidden gene change, triggering anesthesia drugs, and uncontrolled calcium release that sends muscles into overdrive.

What are the symptoms of malignant hyperthermia?

In most cases, there is no rash, no warning sign, and often no family symptoms at all before the reaction. When a reaction does happen, signs and symptoms may vary. They can occur during anesthesia or during recovery shortly after surgery. [1]

Symptom	What it means in plain language
Severe muscle rigidity or spasms	Muscles become stiff and locked, sometimes hard to move.
Rapid, shallow breathing	The body struggles to move air, causing low oxygen and high carbon dioxide.
Rapid heart rate	The heart beats much faster than normal.
Irregular heart rhythm	The heartbeat becomes uneven and unsteady.
Dangerously high body temperature	Body heat rises to unsafe levels very quickly.
Excessive sweating	The body sweats far more than expected during surgery.
Patchy, irregular skin color (mottled skin)	The skin looks blotchy instead of a steady color.

Rare triggers outside the operating room

In rare cases, people at risk of malignant hyperthermia have shown signs of a reaction after intense physical activity during excessive heat or humidity, during a viral illness, or when taking statin medication used to lower cholesterol. This is uncommon, but it is why a family history matters in more than just the operating room. [1]

Figure 2: The seven key signs of a reaction, and why the anesthesia team monitors for them continuously.

When Should You See a Doctor?

The single most important rule is simple: if someone in your family is known to be at risk of malignant hyperthermia and you need to have anesthesia, tell your healthcare provider and anesthesia specialist (anesthesiologist). Other drugs may be used instead. [1]

This applies to any surgery or procedure that uses anesthesia, no matter how small. It also matters to mention any family member who had an unexplained serious problem during or right after anesthesia, even if it was never given a name.

Key reassurance: If you are at risk and do not have a serious reaction during your first exposure to certain anesthesia drugs, you are still at risk with future exposures. But other anesthesia drugs that do not trigger a reaction can be used instead. A known risk is a manageable risk. [1]

What Causes Malignant Hyperthermia?

Malignant hyperthermia can result when you have malignant hyperthermia susceptibility (MHS), a genetic disorder caused by a gene change, also called a mutation. The affected gene increases your risk of malignant hyperthermia when you are exposed to certain anesthesia drugs that trigger a reaction. The affected gene is most commonly inherited, usually from one parent who also has it. Less often, it is the result of a random gene change. [1]

Gene	How often it is involved
RYR1	The most commonly affected gene.
CACNA1S	More rarely affected.
STAC3	More rarely affected.

The RYR1 gene helps control how calcium moves inside muscle cells. Calcium acts like an on-switch for muscle contraction. In people with MHS, triggering anesthesia drugs cause muscle cells to release far too much calcium. The muscles contract continuously, generate enormous heat, and send the body's chemistry out of balance. That is why the reaction includes rigid muscles, a soaring temperature, a racing heart, and breathing problems all at once.

Who Is at Risk of Malignant Hyperthermia?

Your risk of having MHS is higher if someone in your family has it. [1]

Risk factor	What it means
A parent has the gene change	The disorder follows an autosomal dominant inheritance pattern: you only need to inherit one changed gene from a parent to be affected. If one of your parents has the gene change, you have a 50% chance of having MHS too.
Other relatives have MHS	Your chance of having it is also increased.
A past event suspected to be malignant hyperthermia	You or a close relative had an event suspected to be malignant hyperthermia during anesthesia.
Rhabdomyolysis	You or a close relative has had muscle tissue breakdown, which can be triggered by exercise in extreme heat and humidity or when taking a statin drug.
Certain inherited muscle diseases	You or a close relative has certain muscle diseases and disorders caused by inherited gene changes.

What Complications Can Malignant Hyperthermia Cause?

If not treated promptly, malignant hyperthermia can result in major complications. [1]

Complication	What happens
Rhabdomyolysis	A rare condition that causes muscle cells to break down.
Kidney damage or failure	Breakdown products from damaged muscles can overwhelm the kidneys.
Problems with clotting and bleeding	The body's clotting system becomes disrupted.
Death	Without prompt treatment, the reaction can be fatal.

How Is Malignant Hyperthermia Diagnosed?

Malignant hyperthermia is diagnosed based on signs and symptoms, monitoring during and immediately after anesthesia, and lab tests to identify complications. [2]

Susceptibility testing: how do you know if you carry the risk?

Testing to find out if you are at increased risk of malignant hyperthermia may be recommended if you have risk factors, such as a family history. Two tests may be used. [2]

Test	How it works	What it tells you
Genetic testing	A sample of your blood is collected and sent to a lab for analysis.	Identifies the gene change that shows you have malignant hyperthermia susceptibility (MHS).
Muscle biopsy (contracture test)	A small piece of muscle tissue is surgically removed for lab analysis, and the specimen is exposed to triggering chemicals to see how the muscle contracts.	Shows directly whether your muscle reacts abnormally to the trigger chemicals.

One practical note about the muscle biopsy: the test must be done on muscle tissue immediately after it is removed, so travel to a specialized muscle biopsy center is necessary. [2]

How Is Malignant Hyperthermia Treated?

Malignant hyperthermia can be prevented by using non-triggering anesthesia drugs in people at risk, and a reaction is treated in intensive care, usually resolving within a few days.

Prevention before anesthesia

If you or someone in your family has MHS, or you think you may be at risk, tell your healthcare provider and anesthesiologist before you get anesthesia. Drugs that do not trigger malignant hyperthermia may be used as part of your anesthesia instead. [2]

Immediate treatment of a reaction

Treatment	What it does
Dantrolene (Dantrium, Revonto, Ryanodex)	The key medicine. It treats the reaction by stopping the release of calcium into muscles.
Other medications	Correct problems with the balance of the body's chemicals (metabolic imbalance) and treat complications.
Oxygen	Given through a face mask or, in most cases, through a tube placed in the windpipe (trachea).
Body cooling	Ice packs, cooling blankets, a fan with cool mist, and chilled intravenous (IV) fluids help reduce body temperature.
Extra fluids	Additional fluids given through an IV line.
Supportive care	A hospital stay in intensive care for a day or two to monitor temperature, blood pressure, heart rate, breathing, and response to treatment.

During the intensive care stay, several lab tests are done frequently to check the extent of any muscle breakdown and possible kidney damage. A hospital stay is usually needed until lab test results start to return to a standard range. With treatment, malignant hyperthermia usually resolves within a few days. [2]

Figure 3: The six-part immediate treatment plan: dantrolene, other medications, oxygen, body cooling, extra fluids, and supportive intensive care.

What follow-up care is needed after a malignant hyperthermia reaction?

If you have experienced malignant hyperthermia due to certain anesthesia drugs, exercising during excessive heat and humidity could trigger another reaction. Talk to your healthcare provider about any precautions you should take. [2]

Three protective steps are also recommended after a reaction or a known family risk: [2]

1. Ask about genetic testing to determine whether you have the genetic disorder that puts you at risk of malignant hyperthermia.

2. Ask whether close family members should also consider genetic testing.

3. Wear a medical alert bracelet or necklace if you have MHS. This lets healthcare providers know of your risk, especially in an emergency when you may not be able to talk.

How can malignant hyperthermia be prevented?

Prevention comes down to one act of communication. Because the reaction only happens with specific trigger drugs, choosing different anesthesia drugs can avoid it. [1] [2]

Step	Action
1. Know your family history	Ask relatives about any bad reactions during or after anesthesia, and about unexplained muscle problems.
2. Tell your providers	Before any surgery or anesthesia procedure, tell your healthcare provider and anesthesiologist about your family history.
3. Let them adjust the plan	Your anesthesiologist can avoid the specific drugs that trigger the reaction and use alternatives.
4. Consider testing	Ask about genetic testing or a specialized muscle biopsy if you have risk factors.
5. Carry the information forward	Wear a medical alert bracelet and share the information with every new healthcare provider.

Talk with a healthcare professional. If someone in your family has had a serious reaction during anesthesia, or if you have a family history of malignant hyperthermia or unexplained muscle breakdown, contact a healthcare provider before any surgery or anesthesia. For more evidence-based health guides, visit Rinnit.com.

Frequently Asked Questions

Is malignant hyperthermia contagious?

No. It cannot spread from person to person. It develops when a person carries an inherited gene change, most often in the RYR1 gene, and is then exposed to certain anesthesia drugs that trigger the reaction. [1]

Is malignant hyperthermia the same as a fever from infection?

No. Despite the word "hyperthermia," the dangerous heat in this reaction does not come from an infection. It is produced inside overactive muscle cells after exposure to specific anesthesia drugs.

Can I know my risk before surgery?

Yes. If you have risk factors, such as family history, susceptibility testing may be recommended. Genetic testing on a blood sample can identify the gene change, and in some cases a specialized muscle biopsy (contracture test) is used. [2]

What happens if my family has the gene change?

If one of your parents has the gene change, you have a 50% chance of having it too, because only one changed gene is needed. Ask your provider whether close family members should also consider genetic testing. [1] [2]

Can people with this risk still have surgery safely?

Yes. Telling your anesthesiologist about the risk allows them to use anesthesia drugs that do not trigger a reaction. [1]

How is a malignant hyperthermia reaction treated?

Immediately, with the medicine dantrolene, which stops the release of calcium into muscles, along with oxygen, active cooling with ice packs and chilled IV fluids, extra fluids, and other medications. Patients may stay in intensive care for a day or two, and with treatment the reaction usually resolves within a few days. [2]

Conclusion

Malignant hyperthermia is rare, serious, and largely preventable once a family knows about it. The reaction hides behind a silent gene change, most often in RYR1, and only surfaces when specific anesthesia drugs are used. It produces a frightening combination of symptoms (rigid muscles, soaring temperature, racing heart, and breathing trouble), but it has an effective, well-established treatment centered on dantrolene, cooling, and intensive care, and it usually resolves within a few days when treated promptly.

The single most important step is paying attention to your family history. Any unexplained anesthesia problem in your family deserves to be mentioned before your next surgery, because that one conversation lets your anesthesia team choose drugs that keep you safe.

Related Reading

General Anesthesia: How It Works, Risks and Recovery explains how general anesthesia works during surgery.

References

1. Mayo Clinic Staff. Malignant hyperthermia - Symptoms & causes. Mayo Clinic. May 13, 2022. https://www.mayoclinic.org/diseases-conditions/malignant-hyperthermia/symptoms-causes/syc-20353750

2. Mayo Clinic Staff. Malignant hyperthermia - Diagnosis & treatment. Mayo Clinic. May 13, 2022. https://www.mayoclinic.org/diseases-conditions/malignant-hyperthermia/diagnosis-treatment/drc-20353752

3. Yang L, et al. The current status of malignant hyperthermia. J Biomed Res. 2019;34(2):75-85. PMID 32305961. https://doi.org/10.7555/JBR.33.20180089

4. Carlson KJ, et al. Management of patients susceptible to malignant hyperthermia: a surgeon's perspective. Int J Pediatr Otorhinolaryngol. 2022;159:111187. PMID 35660936. https://doi.org/10.1016/j.ijporl.2022.111187

5. He M, et al. Humidity impacts volatile anesthetic and heat responses of malignant hyperthermia susceptible mice. Skelet Muscle. 2026;16(1):10. PMID 41514332. https://doi.org/10.1186/s13395-025-00411-4

Medical disclaimer

This article is for educational purposes only and is not intended as medical advice. Always consult a qualified healthcare professional for diagnosis and treatment decisions.