Skip to Content

Malignant Hyperthermia

Medically reviewed by Last updated on Jun 10, 2019.

What Is It?

Harvard Health Publications

Malignant hyperthermia is a severe reaction to a dose of anesthetics. The reaction is sometimes fatal. It is caused by a rare, inherited muscle abnormality. Infrequently, extreme exercise or heat stroke can trigger malignant hyperthermia in someone with the muscle abnormality.

In people with the muscle abnormality, muscle cells have an abnormal protein on their surfaces. The protein does not affect muscle function significantly. That is, until the muscles are exposed to a drug that can trigger a reaction.

When a person with this condition is exposed to one of these drugs:

  • Calcium stored in muscle cells is released

  • The muscles contract and stiffen at the same time

  • There is a dramatic and dangerous increase in body temperature (hyperthermia)

Malignant hyperthermia usually occurs during or after surgery. But it can occur wherever anesthetic medications are used. This includes:

  • Emergency rooms

  • Dental offices

  • Surgeons' offices

  • Intensive care units

Symptoms of malignant hyperthermia usually occur within the first hour after exposure to the trigger medication. However, the symptoms can be delayed for up to 12 hours.

Most cases occur in children and adults younger than 30.

The muscle abnormality that can lead to malignant hyperthermia is caused by one of several genetic mutations. The most common mutation causes about half of all cases. A person with this mutation has a one in two chance of passing the gene to any of his or her children.

Family members can have different levels of sensitivity to medications that trigger the problem. In some cases, reactions are mild. A person may be exposed to high-risk medications several times before experiencing a recognizable reaction.

This condition sometimes occurs in people who also have muscular dystrophy. It also occurs with other muscle diseases associated with genetic mutations.


Symptoms and signs of malignant hyperthermia include:

  • A dramatic rise in body temperature, sometimes as high as 113 degrees Fahrenheit

  • Rigid or painful muscles, especially in the jaw.

  • Flushed skin

  • Sweating

  • An abnormally rapid or irregular heartbeat

  • Rapid breathing or uncomfortable breathing

  • Brown or cola-colored urine

  • Very low blood pressure (shock)

  • Confusion

  • Muscle weakness or swelling after the event

Many people who carry a gene for malignant hyperthermia will never develop symptoms.


Most people with malignant hyperthermia are not diagnosed until they have a serious reaction to general anesthesia. Doctors usually will suspect this condition if a patient develops typical symptoms, especially very high fever and rigid muscles.

Blood tests that show changes in the body chemistry hint at malignant hyperthermia. These include high levels of the muscle enzyme CPK (creatine phosphokinase) and electrolyte changes. Blood tests that show signs of kidney failure also can provide clues. If malignant hyperthermia is not recognized and treated quickly, the person's heart may stop during surgery.

If you experience most or all of the typical symptoms of malignant hyperthermia, your doctor may diagnose this condition without additional testing.

Expected Duration

With prompt treatment, symptoms should resolve within 12 to 24 hours.

However, if a severe reaction develops before treatment is started, complications may develop. These can include respiratory or kidney failure. These complications may not improve for days or weeks. Some damage may be permanent.


It is not practical to test for this condition in all people who are scheduled for surgery. However, some people should be tested before surgery. Or, they should avoid anesthetics that are known to cause this condition.

These include people with:

  • A family history of malignant hyperthermia

  • A history of heat stroke or hyperthermia after exercise

  • Muscle abnormalities that may be associated with malignant hyperthermia

If you do not have a family history of malignant hyperthermia, your first episode may not be predictable or preventable.

Once you have been diagnosed with the disorder, further episodes can be prevented. Notify your health care professional before you undergo any surgery or office-based procedure. That way, your doctor or dentist can avoid using succinylcholine or high-risk anesthetics.

You don't have to avoid surgery altogether. Many safe alternative anesthetics are available. If you know that you are susceptible to malignant hyperthermia, consider wearing a medical alert tag. This tag will advise health care personnel about your condition during an emergency.

If you plan to travel outside the United States, you can contact the Malignant Hyperthermia Association of the United States (MHAUS). MHAUS will help to determine whether doctors at your travel destination are aware of malignant hyperthermia and equipped to treat it. This is a sensible precaution because it is a rare disease.


As soon as malignant hyperthermia is suspected, doctors must act rapidly to treat the condition and prevent complications.

The first and most important step is to immediately stop giving the triggering medication and to stop the surgery. Doctors then give the drug dantrolene (Dantrium).

Dantrolene relaxes the muscles. It stops the dangerous increase in muscle metabolism.

Dantrolene is given intravenously until a patient has stabilized. Then, the medication typically is continued in pill form for three days.

Additional treatment can include:

  • Lowering body temperature with:

    • Cool mist and fans

    • Cooling blankets

    • Cooled intravenous fluids

  • Administering oxygen

  • Using medications to:

    • Control the heartbeat

    • Stabilize blood pressure

  • Monitoring in an intensive care unit

When To Call a Professional

Prior to any surgery, tell your surgeon, primary care doctor and anesthesiologist if you:

  • Have a family history of malignant hyperthermia

  • Have had one or more episodes of heat stroke or exercise-induced hyperthermia

  • Have muscle symptoms or a known muscle disease


An episode of hyperthermia can be life threatening. However, early treatment at onset of symptoms is usually successful. Once recognized and diagnosed, future episodes can almost always be prevented by avoiding known triggers.

Learn more about Malignant Hyperthermia

Associated drugs

Mayo Clinic Reference

External resources

National Institute of Arthritis and Musculoskeletal and Skin Diseases
Information Clearinghouse
National Institutes of Health
1 AMS Circle
Bethesda, MD 20892-3675
Phone: 301-495-4484
Toll-Free: 1-877-226-4267
Fax: 301-718-6366
TTY: 301-565-2966

National Organization for Rare Disorders (NORD)
55 Kenosia Ave.
P.O. Box 1968
Danbury, CT 06813-1968
Phone: 203-744-0100
Toll-Free: 1-800-999-6673
TTY: 203-797-9590
Fax: 203-798-2291

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.