Malignant hyperthermia is seen mostly in swine, but it has also been reported in dogs (especially Greyhounds), cats, and horses. This syndrome is characterized by abnormally high body temperature, muscle rigidity, a very rapid and irregular heartbeat, increased breathing rate, bluish tinge to skin and mucous membranes, unstable blood pressure, fluid buildup in the lungs, impaired blood coagulation, kidney failure, and death.

Malignant hyperthermia is consistently triggered in susceptible animals by excitement, apprehension, exercise, or environmental stress. Giving certain anesthetics or specific drugs that affect the neurologic and muscular systems also consistently triggers malignant hyperthermia in susceptible animals.

Diagnosis is based on development of clinical signs in an animal that has been given an anesthetic agent or is participating in a stressful event. Signs can develop slowly or rapidly and include muscle stiffness, twitching, a rapid heartbeat, and an increased breathing rate. Animals that are not under anesthesia may show open-mouthed breathing and an increased breathing rate, followed by a temporary break in breathing. Blanching and redness of the skin followed by blotchy blue tinges can be seen in light-colored animals. Body temperature increases rapidly and can reach 113°F (45°C).

Many laboratory tests have been developed to help identify animals susceptible to malignant hyperthermia, but they are not useful for diagnosis of malignant hyperthermia in a sudden crisis.

Treatment and Prevention

Usually, malignant hyperthermia episodes come on suddenly and are very severe. If the condition is recognized early in an animal under anesthesia, supportive measures may be able to save the animal. Unfortunately, regardless of treatment, malignant hyperthermia is usually fatal.

Stress must be minimized to prevent malignant hyperthermia episodes in individual animals. If an animal that is suspected to be susceptible to malignant hyperthermia (or that has survived a previous episode) needs anesthesia and surgery, certain precautions should be taken. These include administering a drug called dantrolene 1 to 2 days before anesthesia and avoiding certain anesthetic agents. Certain local anesthetics are also safe to use. All procedures must be kept as short as possible because malignant hyperthermia happens most often when the animal has been under anesthesia for longer than 1 hour. Although these precautions cannot prevent malignant hyperthermia, they can reduce the chances of a crisis developing.

Whenever a case of malignant hyperthermia is suspected, owners of siblings and breeders should be notified if possible. However, malignant hyperthermia is not always linked to a pedigree line.

Last full review/revision July 2011 by George M. Barrington, DVM, PhD, DACVIM; Sharon J. Spier, DVM, PhD, DACVIM; Ivan W. Caple, BVSc, PhD, MACVSc, MRCVS; David L. Evans, BVSc, PhD; Jean A. Hall, DVM, PhD, DACVIM; Katharine F. Lunn, BVMS, MS, PhD, MRCVS, DACVIM; Donald C. Sawyer, DVM, PhD