What is Malignant Hyperthermia?

Malignant Hyperthermia (MH) is a reaction to commonly used anaesthesia drugs where the body produces too much heat. If it is not recognised and treated in its early stages, MH can lead to death. The risk of developing MH is associated with a change in the genes that are responsible for controlling the release of calcium in muscle cells.

An MH reaction is a rare event, occurring in about one in every ten thousand general anaesthetics. Probably at least one person in every five thousand in the population is potentially susceptible.

Triggering anaesthetics do not necessarily cause an MH crisis every time someone at risk is given them. MH is hereditary; it is passed on through the family. It  can occur in every ethnicity. A person with MH has a 50% chance of passing that risk on to each child that they have.

There is no cure for the predisposition to MH. The management of MH involves avoiding the triggering agents during anaesthesia.
Since the 1970’s a drug has been available that can help in the treatment of MH. It is called dantrolene. It has been very effective in treating MH if used early in the developing syndrome. It is not a cure for MH and is only for use in emergencies as it does not provide complete resolution in a crisis.

MH management is therefore largely preventative. Non-triggering general anaesthesia with intravenous anaesthesia agents after purging the anaesthetic machine of all volatile residues is safe. Nitrous oxide gas is safe. Local anaesthetics are safe. Spinal, epidural and other regional (numbing) anaesthetics are safe.

The only drugs necessary to avoid in MH are volatile anaesthetic agents and a muscle relaxant drug called suxamethonium (succinylcholine). These drugs would only be administered by an anaesthetist or in a medical emergency by appropriately trained personnel. They do not exist in tablet form.

People with MH can still lead an active life with no restrictions on normal activity including sports, such as rugby, netball and running marathons. There is an association between exertional heat stroke and MH, so we advise against extreme sporting events in high temperatures.

How is MH Diagnosed?

The Muscle Biopsy

A specialised muscle biopsy which is called an In Vitro Contracture test (IVCT) is currently the only definitive test for MH. 

The IVCT is a highly specialised test and the muscle samples need to be taken to the MH lab within 15 minutes of the muscle biopsy in order for the tests to be reliable. Therefore the biopsy needs to be performed at a specialist testing centre.

The four geographical locations of MH laboratories for testing in Australia and New Zealand are:

• Melbourne, Victoria
• Westmead, New South Wales
• Perth, Western Australia
• Palmerston North, New Zealand

Children must be at least 10 years of age and weigh more than 30kg before they can be tested. In practice we usually wait until they are at least 12 years old.

The muscle biopsy involves taking a small sample of muscle from the outer part of the thigh. The biopsy is a surgical procedure through an incision (cut) in the skin which is usually 5–7 cm long.

The procedure is performed in the operating theatre and usually takes about 1 hour. A result is available within 2-3 hours.

A negative result means a person is not at risk of developing MH. For any future operations, no specific precautions relating to MH need be taken. Their children will not need testing or any special precautions. It is important to give the results of the muscle biopsy test to all treating practitioners so they are in no doubt about the risk of MH.

A positive result means a person is at risk of developing MH.

The Genetic Test

MH is a genetically inherited disorder. This means that it can be transferred from parent to child. In some families with MH, there is a change (variant) in their DNA (the blueprint of body makeup) and this can be identified with a blood test. There are many different variants that have been associated with MH and many variants that continue to be identified which may be associated. In some circumstances, genetic testing will be performed as the first investigation for an MH episode. In some families a DNA (gene) test can be used to identify those at high risk, and these people do not need to have the muscle biopsy. A negative genetic test does not exclude MH.

Identification of a causative mutation means a person is at risk of developing MH.